icelandic geology: an explanatory excursion guide based on a 1986 field meeting

Icelandic Geology: an explanatory excursion guidebased on a 1986 Field Meeting

Michael Hamlett and John F. Potter

BAMLETI, M. & J. F. POTIER. 1988. Icelandic Geology: an explanatory excursion guidebased on a 1986 Field Meeting. Proc. Geol. Ass., 99(3), 221-48. Iceland is probably theproduct of irregular diapiric upwelling from the lower mantle, beginning about 55 Ma,gradually producing a pile of sub-aqueous and subaerial lavas, the outcrop examples of whichdate from early Miocene (c. 16 Ma) to the present (most recent, 1984) (Vogt, 1974). Most older(Tertiary) lavas are in the east and west of the country, on the outer edges of the active riftingzones which mark the line of the mid-Atlantic ridge. This excursion guide examines particularlythe rifting zones in which there are many active volcanic centres. From these, lavas of a widerange of mineralogy have been extruded, often beneath ice or water, resulting in a wide varietyof rock types and landforms. Changes in sea-level and the removal of the almost total ice cover(about 10,000 years ago) have resulted in a geomorphologically young landscape. The highestareas (generally those above 1000 m 0.0.) remain covered by ice caps, from which glaciersencroach on to lower ground in many places. Geothermal water and hydro-electric power areavailable in abundance and these help to offset the relative paucity of other economicresources. The areas visited by the 1986 party are described and, where necessary, explained,with the aim of assisting any future travellers along the same route.

Michael Bamlett, Birkbeck College (University of London), Centre for Extra-Mural Studies, 26Russell Square, London WCIB 5DQJohn F. Potter, Farnborough College of Technology, Boundary Road, Farnborough, Hampshire GUI4 6SB

1. INTRODUCTION

A party of 26, organised jointly by The Geologists'Association and The University of London Department of Extra-Mural Studies, visited Iceland between23rd July and 2nd August, 1986. Regular air flightswere taken between Heathrow and Keflavik, andaccommodation on the first night and last two nightswas in youth hostels in Reykjavik. MB acted assecretary and JFP as leader. During the campingsection, designated camp sites were used (Fig. 1). Allhad toilet facilities, though in some these wereinadequate for the numbers using the site. All but one(Hveravellir) had washing facilities and some hadnatural hot water. Due to inclement weather, thenight at Nyidalur was spent in the tourist hut.

Roads in Iceland are usually marked by posts but itis rare for route numbers to be displayed. Sometimesplace names appear on finger posts but they are oftenpoorly maintained and may point in entirely thewrong direction! A local guide is desirable and acompass essential for anyone making their own way.Outside the few main towns there is very littlemetalled road and most road surfaces are stony, withthose of the interior regularly sprinkled withprotruding rocks and holes. There are also manyfords, some of them very deep.

The 1986 party stayed for 3 nights at the Reykjahlidcamp site (Fig. 1). It may be advisable for similargroups to arrange their programme so that they spendone night there, followed by one at Hl6daklettar, thena second at Reykjahlid. The itinerary described here

is that covered in 1986. In Reykjahlid, there is aheated public swimming pool and a small cinema with"Volcano Films", both of which are worth a visit iftime allows.

The Westmann Islands (Vestmannaeyjar) may bereached by a 25 minute air trip from the domestic airterminal at Reykjavik airport (not Keflavik), which ison the south side of the airfield. Extra time should beallowed to reach this terminal from the city centre(about 1 hour on foot or 10 minutes by taxi). The airticket can embrace a coach tour of Heimaey, thelargest island, and this is recommended if only oneday is spent here (See Day 9 and Fig. 32).

2. EXCURSION GUIDE

Day 1, Reykjavik to ThorsmorkLeaving Reykjavik and heading eastwards onHighway 1, the Ring Road, which circumnavigates thecountry, (Fig. 2) the first features of geologicalinterest are the pseudo-craters (or rootless craters) atRaudholar (The Red Hills) (Fig. 2, 1). Rootlesscraters are caused by the interaction of molten lavaand water. Because they frequently occur in coastallocations, they have been called littoral cones(Macdonald, 1972). Lava runs on to a waterloggedbeach or into shallow water, where it is chilledrapidly. Water trapped beneath the lava is changed tosteam under pressure. If there is sufficient pressure,steam escapes into the atmosphere, exploding out ofthe flow in several places, causing small craters to

221

222

ICELAND

MICHAEL BAMLETr AND JOHN F. POTTER

------66·n

---6S'n

~o 190Km

<D Key to daily mapsA Camp site(used by 1986 party)

Fig. 1. The excursion route and locations of the daily maps.

form, often very close together (Fig. 3). These cratershave only one eruptive episode; they are not directlylinked to a magma chamber. Twenty kilometressouth-east of Reykjavik, Blafjall (685 m) (Fig. 2,2),an example of a moberg hill may be noted to thesouth of Highway 1. The term moberg describes thecharacteristically yellow-brown fragmental rockscomprising brecciated pillow lavas and massivepalagonite tuffs, with possible glassy, basaltic inclusions. Palagonite is a devitrified, normally yellow,hydration product of basaltic glass, which acts as apartial cement for the fragmented rock (Walker, 1965;Hughes, 1982; Jakobsson, 1979).

Moberg rock is a variety of hyaloclastite which, inturn, is described by Honnorez & Krist (1975) as avolcaniclastic rock generated by non-explosive granulation of volcanic glass, which occurs when basalticlavas are quenched by contact with water. In Iceland,the contact with water occurred because lavas wereextruded beneath ice sheets (Noe-Nygaard, 1940).Since the ice cover in Iceland has been only partlyremoved over the past 10,000 years (Marner, 1976),

the products of sub-glacial eruption may be clearlyseen and have been described by Sigvaldason (1968).

About 3 km south, along Highway 39, thePleistocene moberg material can be examined within50 m of the road on the vaHey sides. On the valleyfloor, there are post-glacial, blocky, jagged andclinkery lavas (apalhraun). Successive flows such asthese tend to fill in the valleys and gradually envelopthe older moberg hills.

A substantial lava tunnel, Raufarholshellir (Wood,1971) occurs about 2 km north-west of the junction ofHighways 38 and 39, in the 5300 B.P. Leita lavas. Itis marked by a cairn on the north-east of 39. (Fig.2, 3). Here, the helluhraun flow solidified on itssurface, cooling rapidly in contact with the air. Thelower, inner cylinder of lava remained fluid andeventually vacated the zone beneath the hard crust,leaving a void. Various sizes of these structures can beseen in Iceland, but none as large or accessible as thisone. The roof has fallen in a number of places andaccess is possible to view the icicle-like forms hangingfrom the roof. The height of the void is about 3 m.

FIELD MEETING IN ICELAND 223

/~-route taken

....\ side road

AHe k I a

StongJ.,'~"6~"'-7

--t-; { ..;/ - --:"- .....

.. ,Y- 7'Walk

..c....I

,,-."""....

,,";:I

)Ii .... ··\ ......Ke Idur: ,-

'. "-".,,

}~..HvOI svollur..:

....~-.,.~ Thorsmork": ..... ,

....... ..,. .../ ?>-'~-o!'"

10(;&> iceo fue I station

~sea, lake

~ camp

A peaksettlement•Fig. 2. Map of route (Day 1).

Good examples of ropy lava, formed when the flowwas becoming more viscous with cooling, andencountering frictional resistance from the groundover which it was flowing, are noticeable on thesurface.

Re-joining Highway 38, and proceeding northeastwards, a 40 m raised beach is crossed near Hjalli(Fig. 2,4). This beach is one reflection of the generaluplift of the country caused by the isostatic recoverywhich followed the removal of most of the ice cover(Jonsson, 1957). Pleistocene hyaloclastites in theabandoned sea cliff show a form of delta beddingwhere they have built out towards the south-west,probably into the sea. The upper third of the cliff

o TY~iCal, horizo~tal ,sca~e 890

m

exhibits subaerial lavas which flowed over the delta(Fig. 4). About 4 km south-west of Hveragardi a(probably) post-glacial lava oversteps the older flows,draping itself down to the foot of the old cliff. Thelavas in this and other areas have been dated byradiocarbon dating of the peat preserved beneaththem (Kjartansson, 1973; Jakobsson, 1976).

Hveragardi is famous for its geothermally heatedgreenhouses, and steaming hot springs, the best ofwhich are visible just north of the river and village.Certain of these springs are known to spout. Thesefeatures are called goshvers in Iceland, as Geysir is anIcelandic proper name for a locality (see below, Day8).

Passing through Selfoss, again on Highway 1, aright turn along Highway 302, just prior to the bridge

Fissure eruption

Link to mag-rna

c ber

Lake or S8 a leve!

Water trapped beneath lava

changes to steam and 'erupts'

Fig. 3. The structure and origin of psuedo-(rootless) craters. Fig. 4. Lava delta and flow-foot breccia (after Jones, 1969).

224 MICHAEL BAMLETr AND JOHN F. POTrER

over the Thjorsa, leads to a broad waterfall(Urridafoss) at the head of the Thjorsa estuary. (Fig.2, 5). Here, situated between the rifting zones, theriver Thjorsa cuts through pre-glacial lavas and thefall is partially developed where basaltic dykesintersect the river course. The river has an opaque,grey appearance, caused by ice cap and glacierderived rock flour, carried in suspension.

Returning to Highway 1, this road should beretraced for about 400 m, before turning north-eastalong Highway 30, then 32, and crossing an 8000year-old lava between the Thjorsa and Hvita rivers(Jakobsson, 1976). This is the Tungnaarhraun, a seriesof 11 flows from the Tungna volcano (Torfajokullcentre) in the eastern rift zone, which ran down aslope of 0.25° towards the south-west. The totalvolume of these lavas has been calculated as 35.6 km3

,

covering about 9000 km2•

Leaving the 'main road' approximately 8 km west ofBurfell (Fig. 2, 6) and proceeding north-eastwards upthe Fossa valley, an old farm building, excavated in1939, at Stong, may be visited. Pale-coloured

pumiceous ash-falls, blown north from the Heklavolcano, buried 16 farms in the valley in 1104 A.D.(Fig. 5). The Plinian (explosive) eruption ejected anestimated 2 km3 of ash which spread out in the wind tothe north.

Hekla has emitted 5 major, post-glacial, pumicerich eruptions, which have been numbered from themost recent (HI) downwards, as follows:

Hl.. l,l04 A.D. (2 km3 of rhyolitic ash)H3 2,800-2,900 B.P.H2 c.3.5oo B.P. (The Selsund pumice)H4 .4,000-4,500 B.P.H5 6,2oo-6,7ooo B.P.

The H numbers are now believed to be in the correctorder, having once been differently assessed. Manyroadside ditches in the affected areas reveal the pale,pumiceous ash bands from these eruptions. (Thorarinsson, 1944, 1967, 1979, 1981; Larsen & Thorarinsson, 1977).

Beneath the ash the Fossa valley floor is covered bythe fourth youngest of the Tungna flows (3800 B.P.),

o,Km

100,

HEKLA ash fall1104 A.D. (H 1)

isopachs in em.

(afterThorarinsson

1967)

Fig. 5. Isopaehs (in em) for the HI Hekla ash fall (after Thorarinsson, 1967).


and near the river pseudo-craters are visible. FromStOng there are distant views of a ridge of pinkrhyolite at Raudukambar 3 km to the north-west, andof Iceland's second highest waterfall (123 m), Haifoss,5 km to the north-north-east.

Returning to Highway 32, the Burfell hydro-electricdam (Fig. 2,6), completed in 1969 (output potential240 Mw), can be visited (Einarsson & Tomasson,1962). It is possible to drive along the top of a longearth dam which helps to channel the water of theThjorsa towards the generators. In so doing a 'trap' toprevent ice floes reaching the dam and also a siltpump to remove sediment from the floor of the lake,which would otherwise reduce its power generationcapacity, can be seen.

From Highway 26, an un-numbered road near theBurfell airstrip can be taken towards the east, and thenorthern slopes of Hekla. At a point where the roadreaches close (400 m) to the Hekla flows of 1970, awalk may be taken across an area of black, blown ash,to the lava front. This ash, redistributed from the May1970 and deposited (and redistributed) from theAugust 1980 and April 1981 pyroclastic phases coversan area which was fertile pasture in July, 1970.(Gronvold, Larsen, Einarsson, Thorarinsson &Saemundsson, 1983). In May 1970, four fissuresopened and the main pyroclastic phase lasted forabout 2 hours, producing 30 million m3 of tephra andfluorine-rich gases which were spread mainly to thenorth-west. This was succeeded by a lava phase,during which about the same volume of uniform basaltwas erupted, flowing north-east then north to halt atthis point. For information on the characteristics ofearlier Hekla lavas, see Tomasson (1967) andEinarsson (1950), and for eruption features, Thorarinsson (1968). At the 4-5 m high lava front the lavahad been particularly viscous and is now blocky. Raftsof lava, each with a chilled external surface and a'stalactitic' inner surface (where the cooled crustalrafts had been pulled away from the still plastic lava),are evident, as are the frequent tube-like extrusionswhere the once-plastic lava has squeezed out frombeneath the previously-cooled craggy surface.

Returning to Highway 26, and turning on toHighway 268, the drive south-west passes theheadquarters of The Land Reclamation Society ofIceland at Gunnarsholt. Soil erosion, particularly asthe result of wind redistribution of the light fractionsin the soils, is a major agricultural problem in Iceland.Farmers are allowed to maintain a herd of around 45sheep or 11 cows from which, apparently irrespectiveof the area of the farm, the government reckons areasonable living may be obtained. However, itappears that many try to keep more than this,particularly where the sheep are concerned. This hasresulted in over-grazing of pastures, and slopes havefailed as a result of the removal of the vegetation. Inmany of the valleys soil slumps and flows scar the

grassy slopes. One of the functions of the Society is totry to prevent this and to reclaim land for agriculturaluse.

Driving south-east, a brief visit can be made to thepreserved turf houses at Keldur (Highway 264) a smallhistoric settlement based on a cold water spring(Amason, 1976). Return next to the Ring Road nearHvolsvollur where the route eastwards crosses anextensive gravel outwash plain (coastal sandur).Highway 249 follows the Markaflj6t valley whichruns along the north side of the Eyjafjallajokull toTh6rsmork. Two glaciers extend northwards into thevalley and a track on the south of the road runs to theglacial tongue of Gigjokull, with its pro-glacial lake,L6nid, some 16 km east of the hill of Stora Dimon.This ash-coated glacier shows a good example of anice fall, with long, transverse crevasses. On the northside of the Thorsmork valley, from Trollabudireastwards, there are outcrops of an interglacialignimbrite which is related to a caldera collapse onTindfjallajokull (Thorarinsson, 1969). Because of theproblems involved in fording the rivers in this valley,the 1986 party did not attempt to visit these. Manyfords, some of them over 1 m deep, have to be crossedon the valley road. The valley itself contains a braidedriver and recently abandoned channels indicategreater amounts of water during the spring melt whenthe valley is full of water and impossible to cross.Complex ripple structures in the water-deposited ashmay be noted in the abandoned channels (Bluck,1974; Collinson & Thompson, 1981). The moreattractive camp site at Th6rsmork is that situated onthe north side of the valley and to reach this, the riverKrossa must be forded. The coach carrying the 1986party accomplished this with difficulty. The absence ofcomplete darkness encouraged most of the party toclimb the moberg hill north of the camp site to viewthe impressive Thronga valley beyond. On the climb,an example of periglacial stone stripes may be seen.

Day 2, Th6rsmork to LandmannalaugarJust over 1 km to the west-south-west of theTh6rsmork camp site, on the south wall of the valley,the narrow ravine of Stakkholtsgja may be examined(Fig. 6, 1). At its head (on its eastern tributary) athin, high, waterfall cascades meltwater directly fromEyjafjallajokull. The walls of the gorge reveal sectionsthrough cross bedded moberg material, with isolatedbasalt pillows, about 0.5 m in diameter, scattered inthe foresets of the brown, palagonitised, pyroclasticdebris. Fuller (1931) and Jones (1969) have describedthe manner in which this steeply dipping mobergmaterial accumulates as flow-foot breccia (Fig. 4).Larsen (pers. comm., 1987) has recorded exactly thissort of structure, which he terms 'pseudo-escarpment',from seismic profiles of the sea floor between Icelandand Greenland and again in East Greenland. Thesefeatures probably represent the earliest Tertiary rocks

MICHAEL BAMLETT AND JOHN F. POTTER226

N

fA

Hekla

0 25, ,Km

Fig. 6. Map of route (Day 2).

which developed above the Iceland hot spot andwhich have now been carried westwards from theiroriginal location, on the North American plate.(Moorbath, Sigurdsson & Goodwin, 1968).

The glacier snout to Steinholtsa (Fig. 6, 2) about2 km further west is hidden from view, but in 1967 ahuge landslip occurred onto the glacier surface, whichprovoked flooding from its proglacial lake. Boulders,some up to 3 m in diameter, litter the valley floor(Kjartansson, 1967).

Stora Dimon stands at the entrance to theMarkafljot valley; a small, moberg hill, isolated by250-300 m thick outwash plain gravels (Haroldsson,1981).

Proceeding south, and nearly 5 km after havingre-joined Highway 1, the 60 m Seljalandsfoss waterfall(Fig. 6, 3) descends the former sea cliff to the roadlevel. It is possible to walk behind the fall but the bestsection is in the cliff on the eastern wall. There are 3distinct lava units in this section. The lowest andthickest shows columnar-jointed basalt passing up intocurvilinear jointed basalt and finally into brecciatedpalagonitised hyaloclastite. A possible interpretationis that this flow was extruded into sub- or pro-glaciallake waters and the upper surface became brecciatedbeing directly in contact with the water, the columnarsection lost its heat mainly to the underlying rock,while the cooling surfaces in the middle section wereat a variety of angles.

Following Highway 1 to the east, the fall ofSk6gafoss (61 m) (Fig. 6,4) is visible from the road.Six kilometres east of this a track leading north acrossseveral flood channels and Sk6gasandur reaches thesnout of the S61heimajokull (Fig. 6, 5) after about5 km. Much of the pumice-rich palaeosandur wascreated during a j6kulhlaup of c. 1200 B.P. Manyjokulhlaups, sometimes called glacier bursts, are the

result of volcanic activity beneath the ice capsgenerating violent outbursts of meltwater when theactivity is sufficient (Thorarinsson, 1977).

The glacier is about 10 km in length from theN)'rdalsjokull to the north (Fig. 6). Meltwater depositfeatures downstream of the glacier snout showevidence for the retreat of this glacier. However, anactive push moraine has caused a recent roaddiversion and it appears that the ice front is nowadvancing. (P. Worsley, pers. comm.). Continuingeast along Highway 1, the vast gravel plain ofMyrdalssandur is reached.

Twenty kilometres to the north-west, the volcanoKatla nestles beneath Myrdalsjokull. Katla's activityappears cyclic and the last major eruption was in 1918,when the ice sheet was penetrated, giving rise to anextensive jokulhlaup, which resulted in a majorredistribution of the coastal gravels and an addition tothe sandur in the form of the Vik Deposit (Jonsson,1982). Since 1300 A.D. the coastline has advancedseaward by more than 3 km as the result ofjokulhlaups. To the north-east, Vatnajokull envelopsthe Grimsvotn volcano (Gronvold & Johanneson,1984). There, a permanent subglacial lake ismaintained by high geothermal temperatures. Thewater escapes fairly regularly and causes jokulhlaups(Rist, 1955; Thorarinsson, Einarsson & Krartansson,1960, p. 148).

Approaching the braided estuary of the Kudaftj6t,Highway 1 turns northwards, passing a large group ofpseudo-craters at Herj6lfsstadii. One of these cones ofpyroclastic material has been sectioned due to erosionby a distributary of the river and no sign of 'plumbing'within the cone is evident. At the bridge over theLeira river, near Hrifunes (Fig. 6, 8) moberg rocksinclude a clear section of basaltic pillow lavas in theriver bank.

Leaving the Ring Road at Uthlid (Fig. 6, 9),Highway 208 proceeds north along the western side ofthe Skafta valley. At Buland, (Fig. 6,10), the F22 (Fis the prefix for mountain roads) is taken towardsEldgja. At many vantage points prior to Buland theLaki fissure lavas (1783-4 A.D.) may be seen in theSkafta valley, smoothing the topography of the valleyfloor and infilling earlier gorges to a thickness of up to200 m. These are the largest set of recent flows inIceland (covering 550 km with an estimated volume of12 km3

). They issued from a fissure line at Laki, whichmay be traced from Grimsvotn towards Katla. Rich influorine and sulphur dioxide gases, the eruptions in1783 killed 50% of Iceland's cattle and 75% of thesheep and horses, thus decimating the country'sagricultural economy and leading to extensive faminein which a fifth of the population died (Williams,1985; Sigurdsson, 1982; Sigvaldason & Oskarsson,1986).

Just about 1.5 km before the Eldgja fissure isreached, the F22 track passes through a small area


of sub-aerIal phase

SPATTER I rom I.re fountainS

Fig. 7. Diagram of the structure and origins of Eldgja fissure,

stream beds. Of the several camp grounds atLandmannalaugar, the best, providing bathing in hotand cold pools, is also the farthest up the valley anddirectly beneath the 15 m high front of the rhyoliticLaugahraun flow.

Day 3, Landmannalaugar area and on to NyidalurSince the last inter-glacial period, the Torfajokullvolcanic centre has erupted rhyolitic lavas covering anestimated 450 km2

• From the camp site, it is possibleto walk alongside the post-glacial Laugahraun, notingthe older rhyolitic, mainly explosive, tephra depositswhich form brightly coloured green, yellow and redhills to the south. Following the valley along thesouth-west side of the flow, unusual columnarrhyolite from this older material may be seen in alarge block adjacent to the stream, and in the cliffbeyond. A further 500 m upstream, the rhyoliticLaugahraun lava flow is seen overlying a terracedglacial till. Exploration of the surface of the flowreveals specimens of banded rhyolite, including somewhich has been squeezed just prior to solidifying andwhich appears rather like a large discharge oftoothpaste. There are also numerous pitchstonefragments.

The rhyolite lava source at Brennisteinsalda is closeto a fumarole area, where good, though fragilecrystals of sulphur may be seen below a red andyellow rhyolitic moberg slope. The return to the campsite is across the lava surface, which is covered with anetwork of paths. Around the camp site there are anumber of hot springs issuing from the lava base andseveral of the 1986 party made use of the bathingfacilities before leaving the area.

About 4 km north-north-east of Landmannalaugar (Fig. 9, 1), and adjacent to the road, is theNordurmimur-Stutur cinder cone, lying within a0.75 km diameter 'caldera-like' structure (Fig. 8).Early explosive activity constructed the tephra wallsand the next stage was a basaltic eruption from whichthe lava (Nordurnamshraun) filled, and then overflowed from, the crater in the south-west comer,spreading out over the valley of the Tungnaa to theeast. Finally, the cinder cone developed in thesouth-west comer of the earlier lava lake area.Tephra 'bombs' and a small lava tube may be seen onthe south and west slopes of this cone.

The F22 road to the north skirts Frostastadavatnand, at the north-eastern comer, an un-numberedtrack may be taken to the lake-filled explosion craterof Lj6ttipollur (the ugly pool) (Fig. 9, 2) formed duringthe sixteenth century. The F22 should then be takennorthwards to its junction with Highway 32 near theSigalda hydro-electric dam and power station (Fig. 9,3). This diverts much of the water which wouldnaturally go over Sijolddufoss which, as seen from theroad, is therefore much depleted. There is an exampleof pillow lavas in a moberg sequence near the junction

LAYERED PYROCLASTIC DEBRIS --"'"JoIII"~

---------

-- ....

-------.....

-------- - _ PRE-ERUPTION SURFACE ----...:;::----__ ~ - (not seen) - _

---

which once impounded molten, post-glacial pahoehoelavas. The lava lake drained away to leave a low«2 m high) ring of cooled lava terraces over which,on the north side, a small waterfall occurs (Fig. 6, 11).

An un-numbered side track enters the Eldgja fissurebut the most spectacular portion to the north-east,must be explored on foot. The Eldgja fissure is about5 km to the north of the Laki fissure line; they bothfollow the regional NE to SW rifting trend. Eldgja'srecent eruptive activity occurred in the period930-934 A.D. when most of the 8.2 km long, 600 mwide and 140 m deep fissure was created. The earlyphases of activity were subaqueous and Surtseyan(explosive), lining the fissure sides with palagoniticbreccias and welded lava lumps (Fig. 7). Subaerialeffusive fire fountaining followed, with extensivebasaltic flows from the fissure towards the south,(particularly from the north-eastern end) andcovering about 700 km2

• Whether the fissure incorporates any graben-like structure cannot be determined,but at a late stage much of the fissure floor wascovered with molten lava within which a number ofsmall spatter cones were created. The walk along thefissure passes these cones and eventually reaches thedouble waterfall of Ofaerufoss (Fig. 6, 12). Theeffusive lavas lined the top and inside of the fissure inthis vicinity (they dip steeply south-east into thefissure at about 35°) and the differential erosionbetween the softer, underlying Surtseyan tuffs andbreccias and the lavas, has left a remnant basalt archacross the top of the lower fall. The climb, to cross thenatural 1.5 m wide bridge, affords excellentphotographs.

The F22 track from Eldgja to the camp site atLandmannalaugar (which, on 25th July, 1986 had onlybeen open 4 days) provides still more exciting scenery,as the colourful rhyolitic 'areas of KirkjUfell areskirted. Much of the drive is likely to be betweenbanks of snow (up to 4 m high in 1986) and along

228 MICHAEL BAMLETT AND JOHN F. POTTER

o!

Acid extrusives

(Tertiary & Pleistocene)

1

Km

2,

Acid extrusives


Nyidalu r .......,'"

Hofsjok ull ,",'",.

-"1.$- /..•

fb-"O I'<" 7('"~ --'"0" (

Q' "o ,/-

l',.,: ,"t4..........-

;1I "..\Jl,.

" ....,..". ..,.,..,

l";~..... ~"'/ ,,,,-

"f3(

~~ a ¥I ·.....----''------''-K-m.........- .......---...

I,, 2

....rrLandmannalau ar

Day 4, Nfidalur to Reykjahlid (Myvatn)On the east side of the F28, about 15 km north ofN)'idalur, a group of surface polygons occur on a flatsection of desert. (Fig. 10, 1). The polygon marginsare shallow ditches about 0.7 m wide and themaximum diameters are between 25 and 30 m.Washburn (1956) and Thorarinsson (1964) have

of roads below the dam, in a small tributary valley(Fig. 9, 3).

Highway 32 continues north-eastwards betweenThorisvatn and the Kaldakvisl river, until the roadcrosses the river gorge, on the sides of which are fourbasaltic flows of upper Pleistocene age (Fig. 9, 4).Each lava flow clearly shows cooling from above andbelow and hence a development of columnar jointsfrom both of these surfaces. The road then becomesthe F28 and the stony desert, the Sprengisandur, isentered. The desert is at about 650-700 m abovesea-level and is covered by coarse, gravel to cobblesize debris which is too large for the frequent strongwinds to remove. Angular fragments are the result ofsevere frost weathering.

The camp site at Nyidalur (= the new valley,discovered as recently as 1845) includes two touristhuts. In the valley (sometimes called Jokuldalur)which is 800 m above sea-level and runs south of theTungnafellsjokull and south-east of the F28, thediversity of plants is exceptional for a high valley insuch a hostile environment. There are also largeperiglacial polygons and, on the northern wall,igneous dykes.

Mixed tholeiite / alkali -rich lavas

rr-;;IrrL!:.-...t:J Rhyolitic lavas

Fig. 8. Geological sketch map of the Landmannalaugar andNordurnamshraun area.


reviewed such features and Rapp & Annersten (1969)provide a possible explanation of their origin.

Twenty kilometres to the north is the legendary'centre of Iceland', alongside the 'dead' lake ofFj6rdungsvatn (Fig. 10,2). About 12 km north of thisis the main watershed of the country a little south ofthe junction of the F28 and F78 mountain roads (Fig.10,3).

Further examples of former ice activity may benoted alongside the road above the Skjalfandaflj6tvalley, to the east. Roches moutonnees (e.g. Fig. 10,4)with striations on their stoss surfaces and pluckingfrom their north-facing lee sides indicate northwardice movement.

Approximately 30 km farther on, a short side roadto the east leads to Aldeyjarfoss, on the Skjalfandafljot river. The fall is over 20 m high columnarbasalts, above which are curvilinear columns and thenpalagonite (See Day 1 for explanation). On top of thegorge to the east a post-glacial lava, which hasdiverted the river, completes the section, while on thewest the former rocky river bed, upon which visitorsare able to tread, is covered by pot-hole like erosional

Nordurnamur- Stutur cinder cone

Palagonites

Basaltic lavas

River alluvium & fluvio-glacial deposits

Tephra, mainly rhyolitiC, and pumice

(unless otherwise labelled)

Lakes

~

[[[]

~

E;;;J~~

~

230 MICHAEL BAMLETI' AND JOHN F. POlTER


features termed 'cups and kettles' in Icelandic. Thefarm at Myri, where the mountain road gives way toHighway 842, is the first for some 240 km.

About 14 km farther north, on the western side ofthe road and just north of the Sandvik bridge andsouth of the petrol station, a small track leads to theStoruvellir farmstead, behind which a steeply-graded,hanging tributary valley exhibits a section of lavaswith inter-bedded tillites, as follows (thicknesses notnoted):

Basaltic lavas, some palagonitised.Conglomeritic tillite with sandy matrix.Basaltic lavas, with zeolites & chalcedony.Palagonitised basalt lavas.Late Tertiary basaltic lavas.

A basic dyke is seen at the foot of this section, cuttingthe two lowest layers. The series dips at approximately 30° to the west and successive lava flows havefrequently reddened and baked the tops of theautobrecciated as it flows beneath. The main zeolite ischabazite (Ca[AI2Si40d.6H20). There is also a verysoft, brown mineral which, like the zeolites and thechalcedony, is found in the vesicles of the basalts. It isprobably saponite, one of the smectite clays of themontmorillonite group.

Cross the Sandvik bridge and use Highway 844 toprogress north, along the Bardardalur, a valley whichmarks the division between the Tertiary andPleistocene lavas to the west and the post-glacialTrolladyngja flows to the east. Godafoss lies near thejunction of Highways 1 and 844/842. It has cut a gorgein a 5,000 year-old Trolladyngja lava, which camedown the Skjalfandaflj6t valley.

To reach the Myvatn depression, follow Highway 1to the north-east into the Reykjadalur valley, passingthe hot springs at Laugar (Fig. 10, 6) and then followthe Ring Road to the western extremity of Myvatnlake where it is joined by Highway 848. Some 7 kmnorth-east of this junction, between the lake and thethe foot of the moberg hill Vindbelgjarfjall (529 m),there is a cluster of craters (Fig. 10, 7). Because this isa conservation area with much ornithological interest,please avoid climbing fences to examine geologicalfeatures. There is at least one accessible crater, whichhas a double rim, indicating that it is unlikely to be apseudo-crater as these would not normally experiencemore than a single eruptive event. 'Breadcrust bombs'may be found here. They are tephra fragments whichwere formed from large lumps of partially cooled,viscous lava, rich in gas, which were thrown out fromthe crater. The outer surfaces cooled rapidly as thefragments flew through the air, forming glassy skins,but the interiors retained their heat. The containedgas came out of solution and expanded, forming avesicular internal structure which, because of theexpansion, cracked the skin. It is unlikely that thesewould be found in association with pseudo-craters.Thus, this double-rimmed crater at least, and byassociation some of the adjacent ones, are more likelyto be spatter or tephra craters connected with theLaxa lavas.

These lavas have two main flow series. The olderone, from the Ketildyngja lava shield to thesouth-east, about 3800 B.P., dammed the formerdrainage channels from the shallow depression and ledto the formation of Lake Myvatn. The younger Laxalavas were erupted about 2000 years B.P. from the

I a vas

Tro IladyngiaA

1460m

lava s

1

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G d f I \ ~,Reykjalid;,,\o a oss\ '. ~\~ f&:~

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15 .' - •... .;0-

\ !'\ Myvatn\

0': \St6ruvellir ... 1

'1'\" Post -glacial

\

~I

Myri .")--Aldeyjarfoss

~g~:t~:IY: i:I:~~~:~:::!l ~;,I

/I

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A

Threndslaborgir-Lt1dentsborgir crater row which lieson a north-south trend line 6 km to the east of thelake. The most recent event of a lage scale whichaffected the lake's immediate area, took place inAugust 1729, when a lava flow associated with a

~ 0 solid 0 0 cool 0 lava 0 c~"-_o--,,,---_

~ flowing molten lava ~

\'0\\\\\\\\\\'\\\\\\\\\\\\B < c15 m

lengthy period of activity around Leirhnjt1kar (Fig. 21,10) (the Myvatn or Krafla fires, May 1724 to August1729) entered the northern margins of the lake. Thesurface of this flow may be examined on the northernshores of the lake below the camp site at Reykjahlid.The 1729 lava is a pahoehoe type and abounds incollapse structures where the molten material has runout from beneath a solid, cooled crust leaving a voidinto which the crust has subsided (Fig. 11). There arealso examples of lava tumuli (Fig. 12). These are smallmounds up to 5 m high, formed when the viscous,cooling lava surface buckled and usually cracked alongthe crest of the bulge (Fig. 13). In a few cases thesummit is marked by a cork-like structure, which mayhave been ejected when the upheaval took place.

c4 tmmai

Fig. 11. Explanation of lava collapse structures. The moltenlava flows out from beneath the solid crust, leaving voids insome places when the lava cools. Overlying slabs thencollapse into these spaces.

Day 5, Reykjahlid-Grimsstadir-Asbyrgi-TjornesHusavik- ReykjahlidThe main industry at Reykjahlid, other than that ofcatering for tourists, is the production of diatomite.Diatoms are microscopic unicellular algae which maylive in either marine or freshwater conditions. Theyare free-floating and have siliceous tests. About10,000 living species are known. The waters of LakeMyvatn are shallow (maximum depth 4 m) and are

Fig. 12. Tumulus in 1729 pahoehoe (helIuhraun) lava, showing septate cracking of cooled surface; near Reykjahlid, Myvatn(see also Fig. 13).

232 MICHAEL BAMLE'IT AND JOHN F. POTTER

LAVA TUMULUS A. Lava pressure b.reaks cool crust

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B. Hump in surface beneath lava c. Pressure ridge origin

Fig. 13. Some possible methods of formation of lava tumuli.

Jbkulsa 11

ATolle istareykjabunga

Dettifoss


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warmed by many hot springs. They therefore providean ideal environment from which the diatoms mayextract silica. When diatoms die, their tests sink to thelake floor, where they accumulate at the rate of about1 mm per year. Suction pumps and a pipe transferthem to a manufacturing plant just east of Reykjahlid,where, with the aid of energy from geothermal steam,they are dried and refined into diatomaceous earth(Fig. 14, 1). This is transported to Husavik viaHighway 87, and thence to West Germany.

The Bjarnaflag geothermal power station is about7 km north-north-east of the plant and provideselectricity for it and the suction pumps.

The whole area of east Reykjahlid and the ridgebeyond it, traversed by Highway 1, is very active withhot springs, and steam vents are present in manydomestic gardens as well as in open country. TheNamafjalJ ridge exhibits a clear north-south alignedgraben as well as examples of pillow lavas,palagonitised basalts and fumaroles. At Namaskard,on the flanks of the ridge, boiling mud and manysteam vents may be seen. Sulphur was once mined inthis area (Figs. 14, 2 & 21, 3).

Very high temperatures have been recorded fromthe upper crustal layers here, reaching 290°C at depthsof only 1.8 km. Sulphurous crusts on some of the mudpools are very thin and the pale coloured sectionsshould be avoided by visitors. The whole areaadjacent to the car park should be treated withextreme care.


Fig. 15. Map of the features of geological interest in theDettifoss-Hafragilsfoss area.

Continuing eastwards along Highway 1 and crossingthe many NNE-SSW trending rifts and crater rows,the next site of interest is the 'drive-in' crater ofHrossaberg, about 30 km east of Reykjahlid and southof the Ring Road (Einarsson, 1965). The walls of this400 m diameter crater were formed by Surtseyanexplosive eruption about 12,000 years ago. They are

cinder

---f-+--l--+-;jI7t- II ne of se c t ion

Det t i foss 44m

HIGHWAY

864

not fOINsea Ie

built of pyroclastic fragments (up to 0.3 m indiameter), water-laid tuffs and accretionary lapilli (upto 20 mm in diameter). These lapilli are thought tohave formed by small ash/lava particles sub-aqueouslygathering finer ash about them in 'snowball' fashion.

The view from the top of Hrossaborg to thesouth-west shows the lava shield of Ketildyngja(939 m) and the tuya mountains of Burfell (953 m)(Fig. 14, 9) and Blatjell (1222 m) (See Day 8 and Fig.26).

Leaving Hrossaborg and returning to the mainroad, the Jokulsa a Fjollum is crossed on a bridgenear Grimsstadir (Fig. 14, 10), where a left turnshould be taken onto Highway 864. In 28 km, take aside road on the left to Dettifoss car park. This, themost powerful waterfall in Europe, is 44 m high andhas a mean flow of 220 m3 per second. In maximumspring snow melt this rises to around 1500 m3 persecond. In immediate post-glacial times, it is likelythat the melting of the overall ice cover resulted inoccasional flows of 0.5 million m3 per second. The30 km long gorge of the Jokulsa, together with the drygorges in the lower course (see below), remain asevidence of this tremendous erosive power and of theamounts of water involved. Ideally, the transportshould leave the party at the Dettifoss car park anddrive about 1 km northwards to another car parkbeside the Sveinar cinder cone and above Hafragilsfoss, leaving the party to follow the eastern bank oftke river between the two.

From the Dettifoss car park, the walk along theeastern bank of the Jokulsa (Figs. 15 & 16) affordsexcellent views of both Dettifoss and the Hafragilsfoss(27 m). In the sides of the gorge, sets of columnarjoints in the inter-glacial lavas are displayed (see Day3, Kaldakvisl gorge). North of Hafragilsfoss, apost-glacial volcanic fissure crosses the gorge, withstrato-volcanoes on either bank which have been

approx 800 m -

350mPOST- GLACIAL LAVA

(Sveinarborgir)

T I LLI TES

Fig. 16. Geological cross section of the Jokulsa gorge north of Dettifoss.


sectioned by erosion. It is possible to examine thevolcanic 'plumbing' on the eastern wall, although theclimb down to (and up from!) the lower level may betoo dangerous for older or less athletic members of agroup. The feeder dyke (with horizontal columnarjoints) spreads out into lava flows at the base of the6000 year-old Sveinar cinder cone (Fig. 17). Thecolumnar joints in the lavas may also be examinedmore closely. Frequently, these columns are markedby horizontal 'chisel' marks, evidence of stages inshrinkage during the cooling process (Fig. 18). In oneplace, the joints are disrupted by a vertical shearplane movement which clearly occurred prior tocomplete solidification (Saemundsson, 1978).

From the top of the red, oxidised cinder cone, twoadditional gjas are visible running parallel (NNE toSSW) to the regional trend and that of the Sveinarfissure of which the cone forms a part. One of the gjas

Fig. 17. Feeder dyke to lavas at the base of the 6000 year oldSveinar cinder cone; north of Hafragilsfoss (east side of theJokulsa a Fjollum gorge (see also Figs. 15 & 16). Earlier,interglacial lavas, showing vertical, columnar jointing are cutby the feeder dyke.

displaces the complete section of lavas, but the otherseems to pre-date the inter-glacial lavas seen in thegorge (Namakura, 1970). Fresh water springs issuingin the bottom of the gorge mix two contrasting waterbodies with vivid blue and black colours.

Return then to Highway 864, from which it ispossible to note the rifting trend again in thetopography of the land to the north-north-east. Thisis a northward extension of the Burfell fissure system,which originates to the east of Myvatn.

Twenty five kilometres northwards is the junctionwith Highway 85. A dry gorge on the left (south) ofthe road is probably one of the former courses of theJokulsa (see Fig. 19), abandoned when the watervolume diminished. The river now occupies the mosteasterly of the three gorges. The third, most westerlyone, is named Asbyrgi and its head may be reached bytaking an un-numbered road (first on the left (south)after the fuel station, about 3 km west of the 864/85

Fig. 18. Detail of vertical columnar jointing in theinterglacial lavas above Hafragilsfoss, on the east side of thegorge of the Jokulsa a Fjollum. Evidence of stages in thecooling process is shown by the horizontal 'chisel' marks.


Asby rg i

X1,2 Sites of rivercapt ur e / divers ion

Fig. 19. Map of the former routes of the Jokulsa Fj611um.

junction) for 4 km to the south, where there is a carpark. There are two sites to visit here; first, the site ofthe former waterfall, now just a trickle, and plungepool, now occupied by a small lake. The cliffs aboveshow thin, post-glacial, mostly olivine theoleiite lavasfrom the Stora Viti (Theistareykjabunga) shieldvolcano, some 20 km to the south-west (Fig. 14). Thesecond is the 'viewpoint', signposted from the pathamong the trees. Here, the structure of the gorge maybe seen, with the Y-shaped plan clearly visible. Thereare also some interesting examples of 'bottomstructures' among the lavas, with the overlying flowbases being moulded into the shapes of the ropysurfaces of the underlying pahoehoe. The canyonwalls are up to 100 m high and it is known from tephrastudies (Thorarinsson, 1979) that at least some of theJokulsa waters flowed this way until 2500 B.P.

Leave the gorge by the same route and follow

Highway 85 west for about a further 1 km, whereanother un-numbered track may be taken towards thesouth for about 12 km to the camp site and car park atHl6daklettar (Echo Cliffs) (Fig. 14). Transport maybe left here and the party should take the pathtowards the river. About 8000 years B.P., a volcanicfissure opened up along a NNW to SSE trend acrossthe northward-flowing Jokulsa, which at the time, isbelieved to have been carrying a large volume ofmeltwater from the retreating ice caps. Thus, theeruptive materials reacted with the water andexplosive activity ensued. The resultant pyroclasticmaterial prevented the easy eruption of lava, whichwas contained beneath a blanket of tephra, resultingin a very irregular lava surface. The basalt cooled,forming columns at 90° to the cooling surfaces at thebase of the pyroclastic blanket. Much of this blankethas now been eroded revealing the irregular and oftenspectacular cooling structures, some of which appearas radially-arranged columns. The feeder dyke isexposed in the wall of the canyon but is not easilyaccessible from the west side.

Return to Highway 85 which should be followedwestwards across the northern side of several lavaswhich emanate from Theistarekyabunga volcano.These have been eroded by the sea while the land wasat a relatively lower level, before post-glacial isostaticelevation (for chronology, see Einarsson, Hopkins &Doell, 1967). The raised platform and abandoned cliffline are very clear, as are the screes of wind-blown ashtrapped at the foot of cliffs by onshore winds andeddies, in the vicinity of Gardur (Fig. 14, 4).

Between Asbyrgi and Lon, the Theistareykjabungalavas, dipping at 0.75° to the north, cover many of theNNE to SSW trending fissures and fill the Axarfjordurtrough which runs northwards out to sea. This activelyrifting trough forms the continuation of the midAtlantic ridge (as the Jan Meyen Ridge in the ArcticOcean). The alignment is believed to be dextrallyoffset by a NW to SW transform fault in Axarfjordurbay (Bjornsson, Johnson, Sigurdsson, Thorbergsson& Trygvason, 1977; Saemundsson, 1974).

At L6n, the numerous NNE to SSW fault scarpsmark the margin of the Axarfjordur trough as theroad climbs onto late Tertiary rocks of the Tjornespeninsula. A cliff top vantage point allows views tothe south along the trough as far as the Krafla area(NE of Myvatn), and east, along the coastline oflagoons and bars to the distributaries of the Jokulsa.

Some 20 km from L6n, are coastal sections aroundBreidavik (Fig. 14, 6). If time allows, it is worthexamining these in detail as they demonstrate acomplex sequence of glacial advances and retreats.Einarsson et at. (1967) describe 10 till sequences fromthis area, some almost certainly only the result of localadvances. The party was unable to examine all thecoastal sections, but the unconformity on the west sideof the bay at Breidavik could be seen from the coach.

236 MICHAEL BAMLETI AND JOHN F. POTIER

TABLE 1. The Tjomes succession

SEQUENCE TYPICAL ROCKS SEEN AT

Later lavas Normal magnetised basalt lavas with tillites

Valadalstorfa lavas

Breidavik Beds

Reverse magnetised basalt lavas

Tillites, thin lavas & cold water marine sediments

Valadalstorfa

Breidavik

UNCOMFORMITY

Cardium beds(Early Pleistocene)Mactra bedsTapes beds(Late Pliocene)

Furuvik Beds

Tjomes Beds

Tillites, normal & reverse magnetised basalt lavasMainly littoral and marine

fossiliferous sedimentsshowing evidence ofincreasing climatecooling up thesuccession

Furuvik

Barmur,Hallbjarnarstadakambur

Late Tertiary Plateau Basalts Husavik

This is an important break in the Tjornes succession,which may be simplified as shown in Table 1.

The detail of the fossiliferous Barmur sequence maybe examined in an unstable, but not dangerous, cliffsection by the road leading down to the harbour atsheltered inlet near HalIbjarnastadir. (Fig. 14, 7).

Fossils recorded by the 1986 party from theHallbjarnastadir section (Fig. 20), included gastropods(including Hinia (Nassa) reticosa) and the bivalvesArtica islandica, Cardium groenlandicum, Corbulomya complanta, Macoma sp., Spisula (Mactra)sp.and Tel/ina sp. Some larger shells were lined ininternal cavities with minute quartz crystals. Descriptions of the fossiliferous Tjornes Beds are given byBardarson (1925), Norton (1977), Einarsson (1963)and Buckland, Perry, Gislason & Dugmore (1986).The Tjornes Beds fossils in this exposure indicate ageneral cooling of the environment as the firstPleistocene glacial advance occurred about 2 millionyears B.P.

Seven kilometres beyond this site is the port ofHusavik (population 2500). Highway 85 may then befollowed to its junction with Highway 87, which maythen be taken back to Reykjalid.

Day 6, The vicinity of Reykjahlid, Krafla and theobsidian ridgeImmediately south-east of the main road junction inReykjahlid, a footpath leads south to the dilationfissure of Storagja (Fig. 21, 2 and Fig. 22). Byfollowing the safety rope into the fissure, it is possibleto bathe in waters which, in 1986, were at atemperature of 35°C. The fissure displaces lavas whichpre-date the H5 Hekla ash of 6200-6700 B.P. Otherlavas to the north and south, which are less than 2900years old, are almost undisturbed. Soil profiles nearhere reveal Hekla ash layers H5, H4 and H3 (see Day1, above) overlain by a 120 mm thick basaltic ash fromHverfjall (Fig. 21, 4), 3.5 km to the south-east, whicherupted about 2500 years ago.

South-east of Reykjahlid, structural movements,some occurring as recently as 1977, may be examinedin a strip of country of which the Grj6tagja fissureforms the western boundary. (Fig. 21, 3). The tracksouthwards from near the diatomite factory leads toGrj6tagja across an active, fissured and subsiding areawith fumaroles. This represents the rifting zonebetween the Eurasian and American plates. An ironbar, cemented over one fissure close to the track in

Sketch of clift section at HaltbjaroastadjrEO NNE

Cardjum groenlandjcum zone ~nJ..a;~~117·"·

MORAINE

~~~~zone

Fig. 20. Sketch showing the fossil zones in the cliff section at Hallbjarnastadir.

FJELD MEETING IN ICELAND 237

Fig. 21. Diagram (not to scale) of the area east ofReykjahlid (Day 6). 1= Reykjahlid campsite; 2 = Storagja;3 = Grjotagia and spreading graben; 4 = Hverfjall; 5 =Namafjall hot springs; 6 = Krafla geothermal power station;7 = Viti crater; 8 = Krafla peak; 9 = Shallow graben;10 = Leirnjukur; 11 = 1984 lava; 12 = Hrafntinnuhryggurrhyolite ridge; 13 = Dimmuborgir; 14 = Ludent; 15 = Windblown ash etc. from very recent eruptions.

recent years has broken and had, in 1986, partedabout 45 mm, to give a crude indication of thespreading rate. Here and elsewhere in Iceland, thespreading rate is monitored by sophisticated laserequipment, the locations for which may be seenadjacent to this rifting zone (Decker, Einarsson &Plumb, 1976; Gerke, 1974).

Grj6tagja is about 1.5 km from the main road. Therift margin shows a maximum vertical displacement ofabout 4 m and forms a fault scarp within pre-H5 lavas(>7000 B.P. ). There is a horizontal dilation normalto the strike of the fault and, where the water table isrevealed beneath the tilted blocks of lava which markthe fault, bathing was possible until 1977. In 1986, thetemperature in the two linked pools was 52°e. About100 m north of the pools, younger lavas (about

2000 B.P.) from the western slopes of Namafjall (Fig.21, 5) cover the fault scarp.

From the Ring Road, the next interesting area lieswest of the Krafla volcano, where the belt of riftingand geothermal activity seen around Grj6tagjacontinues northwards. Take the road northwardstowards the Krafla geothermal power station (built1975-77) (Fig. 21, 6). It is not as efficient as wasanticipated, mainly because of the geologicallydisturbed character of the area. Twelve, 2 km deepboreholes have been drilled, to penetrate the lower oftwo aquifers in which temperatures of up to 340°Chave been recorded. Only five of these boreholes wereproductive in 1986, generating between 6 and 7megawatts instead of the predicted 35. The mainimpediment to efficient production is the disturbanceof the aquifers by tectonic and magmatic activity,which has rendered several of the wells unusable.Well casings have been corroded by CO2 and S02 gaspulses at times of heightened geothermal activity(Smarason, 1981).

Drive past the power station and take the right forkto the car park near the Viti crater. (Fig. 21, 7). Thislargely explosive vent was active during the 1724-29eruptions; and is one of a number which havebreached the western rim of the 9 km diametercaldera wall of Krafla (Fig. 21, 8).

Return to the car park at the north end of thepower station road. From here, a path leadsnorth-eastwards across a shallow graben, with clear,small faults on either side. (Fig. 21, 9). (Tryggvason,1980). At least five successive, partially overlapping,lava flows dating from 1975 or later may be seen onthe walk towards Leirhnjukur (= clay or mud hill, Fig.21, 10). The most recent lava dates from 1984 (Fig.21, 11). South of these lavas, sulphurous springsproduce bubbling mud pools (Fig. 23). Beyond, theLeirhnjukur crater row extends to the north-northeast, displaying both scoria and spatter cones; theactive fissure zone finally reaching the north coast atAxarfjordur (Saemundsson, 1974).

Leaving vehicles in the car park, walk towards thepower station before taking a track up the main sidevalley to the east. This passes close to noisy, ventinggeothermal boreholes and violent fumaroles. Beyondthese, and about 2 km east of the power station is theHrafntinnuhryggur rhyolite ridge (Fig. 21, 12). Thisnorth-south fissure ridge is thought to have beenextruded beneath an ice sheet, the top surface of theextrusion being rapidly chilled (Walker, 1965). Theobsidian occasionally exhibits lithophysae (hollows)which are lined with cristobalite (Deer, Howie &Zussman, 1966, p. 340). The hazardous, brittle, glassycharacteristics of obsidian must be emphasised andspecimens must not be hammered and should behandled with great care.

From the ridge, views to the south embrace a widerange of volcanic forms of glacial and post-glacial ages

HIGHWAY 1••'.' 5

N

t


Fig. 22. View southwards along the Grjotagja fault scarp with tilted block, in pre-H5 lavas (over 7,000 years old); east ofMyvatn.

and of both rhyolitic and basaltic composition. Thisvariation in lithology over a relatively small areareflects the rapid changes in the character of theigneous activity in Iceland (Jakobsson, 1979; Gibson& Gibbs, 1987). On the immediate slopes of therange, a number of post-glacial explosion craters arepresent. Acicular lava fragments (Pele's tears andhairs), propelled in showers from the 1975-84Leirhnjukur crater row eruptions, can be found inribbons and patches on the ground (Fig. 21, 15).There are also examples of patterned ground andwind-formed ripples in the ash close to the line ofelectric cables from the power plant towards the east.

Return to the transport and retrace to Highway 1,driving west to the main road junction in Reykjahlid.Here, turn south on to Highway 848 and, in about4 km, take the side road to Dimmuborgir (the blackcastles) (Fig. 21, 13). This 'park' is a group of lavapillars, which are the remnants of a once fluid lava,most of which has drained away. The younger Laxalavas flowed westwards into pre-historic Myvatn fromthe Ludentisborgir-Threndslaborgir fissures. Steam,formed by the mingling of lava and lake water, ventedthrough the fluid lavas in an irregular pattern, cooling

pillars of basalt but, before aU the lava had solidified,the remaining fluid lava drained out of the areathrough a channel which can still be seen. The10-20 m high pillars display two features related tothe lava drain out: (a) uneven, lava-smeared sides tothe pillars were created as highly viscous lava pulledaway from the solidified sections, and (b) 'tide marks',in the form of lava rings around the pillars, representbrief pauses during the process of lava withdrawal.The attitude of the pillars suggests that the area wasslightly domed immediately prior to the lava drainout.

Alternative theories regarding the origins ofDimmuborgir have been suggested by Barth (1950)who suggested that the pillars are the remains of alava lake that drained back into a subterraneanreservoir, by van Bemmelen & Rutten (1955) whoargued for an origin based on eddying in a turbulentlava flow, and by Rittman (1938) who suggested thatDimmuborgir is an eruption centre. On examinationof the evidence on and around the site, we are veryfirmly convinced of our first explanation (see alsoPerkins, 1983).

Access to the volcanoes of Ludent and Hverfjall


Fig. 23. Hot springs (foreground), the 1984 lava flow, andgeothermal venting from Krafla power station boreholes(background); near Leirhnjukur, Krafla.

(Einarsson, 1965) and the fissure rows ofLudentisborgir and Threngslaborgir may be gained byfootpaths from Dimmuborgir. The explosive tuff ringof Ludent (Fig. 21, 14) (482 m) is the oldest of thesestructures (about 9000 B.P.). On its northern slopes, athick, viscous dacite flow of 3800 B.P. may be seen.Hverfjall (Fig. 18, 4) is a typical tephra cone, formedin a brief explosive phase about 2500-2700 B.P. Bothcones afford views from their summits of theLudentisborgir- Threngslaborgir crater rows (see Day4). From Dimmuborgir, it is a short (4 km) drive tothe Reykjahlid camp site.

Day 7, Reykjahtid-Akureyri-VaramalidLangamyri-HveravellirLeave Reykjahlid on Highway 848. Just past theschool at Skutustadir, between the road and the lake,there is a group of pseudo-craters, which are so closetogether that there can be little doubt about theirorigin (Fig. 24, 1). Follow Highway 848 to its junctionwith Highway 1, which may then be followed back to

Godafoss (see Day 4, Fig. 24, 2) before continuing onHighway 1 through the Lj6savatn valley (TheLightwater Pass) (Fig. 24, 3). This valley has the samesouth-east trend as the Husavik displacement (seeDay 5). Adjacent to the road, are several ditcheswhich display examples of Hekla and other ash falls(see Day 1). Many rock slides and landslips inpre-glacial basalts can be seen on valley sides. Nolonger is the scenery influenced by post-glacialvulcanicity. To the west, some 4500 m of late Tertiarylavas have been modified by glaciation and manyassociated geomorphological features prevail'. Duringthe Pleistocene, the valleys in this area were filledwith glacier ice. Subsequent ice melt left the highervalley slopes un-supported and landslips are, therefore, common (Fig. 25).

Near Hals (Fig. 24, 4), the road enters a formerlake basin which now drains westwards into theEyjarfjordur. On its slopes, two terraces may be seen,at 217 m and 88 m O.D., which probably mark formerice-dammed lake shorelines. This valley, theFnj6skdalur, is occupied by the river Fnj6ska, one ofthe few rivers in Iceland with substantial meanderdevelopment in its lower course (Nordahl, 1983).Highway 1 has recently been re-constructed betweenHals and Svalbaro, and the new sections, especially onthe eastern slopes of Eyjafjordur, expose late Tertiarybasalts, dipping at up to 20° towards the east. Theyare unconformably overlain by almost horizontalbasalts which are about 5 million years old. A newroad bridge has been built across the head of theEyjarfjordur, making use of a gravel bar and aconsequent narrow section of the Eyjafjara river. Theolder part of Akureyri is built on a 50 m high kamecreated by meltwater from Glerardalur to thesouth-west. Two kilometres north of the town centre,a cluster of roches moutonnees occurs on either side ofthe road. Leaving the Eyjarfjordur, Highway 1 turnssouth-west along the Oxnadalur, in the walls ofwhich, Tertiary lavas, often faulted and with dykes,may be viewed from the road. There are also more ofthe landslips mentioned above. Sella~.dsfjall, a sidevalley near the watershed between Oxnadalur andNorduradalur, is an example of a trough end valley(Fig. 24, 6) which has been ice-filled but is without acirque at its head. The effects of solifluction (soilcreep) are evident on many valley sides.

The Nordura valley receives several fast-flowingtributaries, mainly from the north. One of these, nearFremrikot, is the river Valagilsa. (Fig. 24, 7). Thegorge north of the bridge shows Tertiary lavas withboth aa and pahoehoe characteristics. The aa flowshave enveloped trees, now weathered out to holes.Cobbles and gravel in the river channel includefragments of Tertiary lavas derived from the north.Vesicles in these lavas are often filled with secondaryminerals including green celadonite (a zeolite, Deer etat., 1966, p. 396), phacolite (twinned form of



Day 8, Hveravellir-Gullfoss-GeysirThingvellir-ReykjavikLeaving Hveravellir, there are clear views of the1000 m tuya, Klalfell (Fig. 26, 1) and the shieldvolcano, Strytur (840 m), 4 km to its north, as thetraveller moves first east, then southwards on the F37track. Tuyas have their origin in explosive sub-glacialeruptions, when the lavas are rapidly palagonitised inthe meltwater. This type of process continues withinthe lake impounded by the surrounding ice, whichgives a steep marginal slope to the lavas. Eventually,the lava/palagonite pile breaks the water surface anda sub-aerial effusive lava is erupted. Because this isnot palagonitised, it forms a weathering-resistant cap

r25

I

Km

chabazite), phillipsite and chabazite (zeolites, Deer etaI., 1966, p. 393 et seq.; Walker, 1960).

The ring road continues into the Heradsvotn valley(Fig. 24, 8) where the braided river flows on a massiveinfill of sediments in a glacially deepened gorge. Thisis crossed at Varmahlid and the road then follows theconvenient col between Hellufell (908 m) (Fig. 24, 10)and Kaldbakur (965 m) (Fig. 24, 9) before descendingto Langamyri in the Blondudalur (Fig. 24, 11). As thisvalley is approached, the Tertiary lavas may beobserved dipping to the south-west at about 200

towards the western Icelandic zone of active rifting.Progressing to the west, the dip decreases gradually asthe lavas become younger.

The sequence of the roads taken from here mayvary, as there are likely to be frequent changes as theBlOndudalur hydro-electric scheme is developed. The1986 party followed Highway 1 as far as Hunaver,turned south on to Highway 731 past Langamyri, thencrossed the river near the fuel station to Highway 732.At Langamyri, oxidised tops of the the lavas show upclearly on the valley sides. Near Eyvindarstadir, thehydro-electric scheme's construction works involvedthe group in several road diversions towards the F37mountain track. Just prior to joining this track the

_.- :-,- ....-- ... \ .EYJafjordur ... I t'

: - I \/.i..,}"" r.... \! :

,*' .\ ~ .:.~-_ ....3..... .' ,' .....,,~

AK.0 REYR.! ": ~Y'L;,' r')' 0 \: ,:2':0 \. Reykjahlid

,./ '>:; :;\ ~'IiIA.\...5 /. /',," r"

Varmahlid : 8 : Ij .....,,::;.~, ?"';';0(~",~:.(.~:-j-y.:~-~:..\ j~~ 1-'-1···.~...Langamyri ~ 7 / 6

" 10 \, ,. .... --- ... -'"\11 " ,/ / dalur,"'. 'rl'iOrdura

, '12F37 '.

Track,~_~+H.lI..el:.aILJilllLJli.Ols.ill- ---l entrance to the Gilsa valley is crossed (Fig. 24, 12).

.. Here the lavas, still dipping at almost 200 are\ inter-bedded with glacial moberg material, (indicating

\ a younger age) and also intersected by dykes.\ Travelling south, views of the ice caps of" Hofsjokull, Hnltfell and Langjokull may be had from: the F37 as it crosses the stony desert plateau (see\ above, Day 3). The side road should be taken to reach" Hveravellir camp site, located near the head of a

\'i shallow valley where hot springs supply the valley: stream. It lies just north of the main Iceland\ watershed and at the eastern margin of the western\, rifting zone." Where hot water reaches the surface, mini terraces\ of geyserite (siliceous sinter, SiOz.nHzO) are formed

L ~lo.-l::iJLe.r.;;l..lI..elliJ:.....aKrIL..!::::~~__'='~~~ as the silica comes out of solution when the watercools. These terraces, which are only a fewmillimetres high at each step, are formed when theeffusion of hot water is non-explosive. Geyser-typeeffusions tend to build bee-hive shaped structures(Francis, 1976, p. 296) although, as will be notedbelow (Day 8), this is not always so. Collinson &Thomson (1981) draw attention to the role of algae inremoving the silica from solution. Each terrace marginpossesses a low rim of silica created rather in themanner of river levees, where evaporation at themargin of the terrace pool is greatest.


Fig. 25. Landslip in Tertiary basalts, near Baegisa, west of Akureyri.

on the moberg pile. When the ice melts, an oftenisolated, steep-sided, flat-topped mountain is left. Thewidespread occurrence of these tuyas in Icelandenables glaciologists to determine the former thickness of the Icelandic ice cap; for the base of thesubaerial lavas (the table-top) marks the approximatetop of the past ice cover (Fig. 27).

It is important to check the compass regularly here,

as there are several small and unsigned side tracks.The desert here was until recent years littered withfrost-weathered grettistoks, rock slabs broken intothin slices by the freezing of water which accumulatedin the cleavage planes which, in turn, are due to thepresence of abundant sheet silicate minerals (such asclay minerals) in the rock. However, these haveproved popular with Icelanders as ready-made



1 AQUATIC EFFUSIVE PHASE 2 EXPLOSIVE (TUFF) PHASE 3 SUBAER IAL EFFUSIVEPHASE

4 ADVANCED SUBAERIALEFFUSIVE PHASE

5 ICE SHEET MELTS

lake

Flow-loot breccia

tu If

Pillow lava

Earl ier lavas

Fig. 27. Tuya (table mountain) formation (vertical scale exaggerated).

material for paving and flooring, so the 1986 party wasfortunate to find an example close to the road.

Twenty eight kilometres from Hveravellir a road onthe left (east) may be taken towards the rhyolite tuyaof Kerlingafjoll and the twin, ice-covered peaks ofSnaekollur (1477 m) and Svarthyrna (1158 m). Fluvioglacial deposits at Gygjarfoss and near Asgardsfjall,and fumaroles and vividly coloured rhyolites nearSnaekollur are the attractions here (Fig. 26, 3,4).

Returning to the F37 and continuing southwards,examples of shield volcanoes may be seen at Baldheidi(771 m) and Leggjabrj6tur (= leg breaker, which ispart of Solkatla peak 1026 m, on the eastern edge ofthe Langjokull ice cap) to the west of the road (Fig.26, 5). Following the Jokullfjall, the F37 then climbswestwards round the slopes of the Blafell tuya(1204 m) to a point where there is a huge cairn at thelocally highest point on the road. From here, thesouth-western 'lowlands' come into view, and to thewest the basaltic moberg fissure row on the southeastern margin of the Langjokull, the Jarlhettur,is visible. East of this cairn, interglacial lavas, tillites,foreset-bedded meltwater deposites and palagonitictuffs may be examined in the lower western slopes ofBlafell. South of the Jarlhettur junction and the fordacross the Sanda river, opposite the airstrip, arefurther good examples of roches moutonnees (Fig. 26,6) with striations trending at c. 2650 magnetic(Kjartansson, 1955).

The mountain road ends at the waterfall of Gullfoss(= the golden falls, 31 m) on the Hvita river. Theaverage discharge/flow rate is 118 m3/sec rising to amaximum of 2,000 m3/sec in times of flood. The riverhas cut through both aa and compound pahoehoeflows, the former with included tree trunks andpalagonitic material, all of which may be seen abovethe fall on the western side of the gorge. The fall itself(Fig. 28) is controlled by NNE-SSW and NE-SWfaulting, and is over a basalt which protects layers oflake-deposited siltstones with cryoturbation, currentbedded sandstones, and tillites. The recession gorge ofthe fall is 2.5 km long and must have been erodedduring the last 10,000 years.

From Gullfoss, Highway 35 should be taken toGeysir (see note Day 1). The Great Geysir rarelyspouts now, but, with artificial encouragement, isreserved for special occasions. However, Strokkur(= the churn), about 100 m to the south-west hasbeen revived by careful re-boring of its pipe. It spoutsat erratic intervals of up to 5 minutes and up to 40 minto the air, with an enormous bubble before thespout. The Great Geysir has a pipe 22 m deep and2.5-3 m wide, in the centre of a shallow geyseritedome 18 m across (Fig. 29). At the base of the pipe,the water temperature is well above normal sea levelboiling point but is prevented from boiling by theweight of water above it. Thorkelsson (in Barth, 1950)suggests that 'boiling' is the result more of reduction


Fig. 28. Sketch section of the stratigraphy at Gullfoss.

in pressure than of increase in temperature. Thewaters of the hot spring area are rich in dissolvedgases and these begin to be liberated, as bubbles,well below the surface. Being in contact with water,they become saturated with water vapour. When the

o 100, ! •

m

~Rlver & lake

, Fissure zone

rt: Road

1

IHINCjYEIIIB

water temperature approaches boiling point, theproportion of water vapour in the ascending bubblesbecomes very high and some of the hot foamoverflows at the surface. This so relieves the pressureon the heated water at the base of the pipe that it'flashes' into a vast volume of spray which surges up toform the spout.

Continuing 30 km on Highway 37 to Laugarvatn,Highway 365 should be taken along the southernslopes of Kalfstindar (Fig. 26, 7). About 8 km afterthe junction, 200 m from the road is a cave(Laugarvatnshellir) (Fig. 26, 8) in the moberg, inwhich one of the inhabitants of Reykjavik was born in1922. A narrow valley immediately east of the cavedisplays large, flattened pillow lavas (bolstraberg)alternating with moberg material in a characteristicsubaqueous extrusion sequence. Highway 365 shouldthen be followed to Thingvellir.

Here is a vivid example of the summit graben of themid-Atlantic ridge continued north-eastwards fromthe Reykjanes ridge of the northernmost Atlantic(Aronson & Saemundsson, 1975). The graben is about5 km wide and the boundary faults may be traced forabout 25 km, from the south-western end of

sinter

conglomerate

GULLFOSS

-

Fig. 29. Diagrammatic section (not to scale) through theGreat Geysir system (after Barth, 1950). Fig. 30. Sketch map of the Thingvellir area.

244 MICHAEL BAMLE'IT AND JOHN F. PO'ITER

Thingvallavatn north-eastwards to around Uigafell(Fig. 26, 10) adjacent to Highway 52. The Americanplate is on the north-western side, the Eurasian plateon the south-eastern side. For further details see Figs.30 & 31. Associated with the 40 m high fault scarps oneither side of the graben, there are a number of gjas.Those on the 'Eurasian' side may be visited adjacentto the north-east corner of the lake and those on the'American' side in the vicinity of the Althing site (ofIceland's ancient parliament) north-west of the lake.Highway 36 links the two locations, running across the9000 year old pahoehoe lavas from the classic shieldvolcano of Skjaldbreidur (1060 m) 25 km to thenorth-east (Fig. 26). Normally, the obvious directroute across the graben is closed to traffic but it ispossible to go farther north to discover a route across.

The north-eastern end of the lake has subsided some70 m during historic times including, more recently,1 m during 1789. The graben widened by 2 cm during1970-3 (Decker et al., 1976). Volcanic activity hasalso been restricted in recent times, with the doublecrater tephra cone of Sandey Island in the lake beingthe most recent at around 2000 B.P., being also theapproximate age of the Nesjahraun lavas whichencroached on the lake from the south-west (Fig. 26,9). The graben continues, in muted form, to thecoast, containing the solfataras at Hengill andHveragardi (see Day 1). The route to Reykjavik,along Highway 36 to Mosfellssveit and Highway 1,avoids the graben margins and crosses Tertiarybasalts, upon which remnant traces of raised beachesat c. 40 m are superimposed.

Some Icelandic volcanic & plate margin featlJres

(modified.from Walker(1964).

Fig. 31. Summary diagram of the main structural and volcanic features seen during the Field Meeting. 1 = Ridge due tosubglacial fissure eruption. Hyaloclastite moberg material; 2 = Tuya-subaerial spread of lavas over 1; 3 = Post glacial lavafield, partly burying I-possibly pahoehoe with lava tumuli and collapse structures; 4 = Non-eruptive fissures--some normalfaulting accompanying dilation movement; 5 = Rotationally faulted and displaced block; 6 = Dykes at sites of fissureeruptions; 7 =Spatter cones along the line of post-glacial fissure eruption; 8 =Cinder cone on same fissure as 7; 9 = Lavachannels by which lava was conveyed away from the eruptive fissure.


1km•

A -1973 lavas

Fig. 32. Sketch map of the island of Heimay, WestmannIslands.

Day 9, The Westmann Islands (Day flight fromReykjavik)The oldest rocks on the Westmann Islands do notexceed 10,000 years but the main attraction forvisitors are the results of the 1973 volcanic eruptionsof Eldfell on Heimaey (Thorarinsson, Steinthorsson,Einarsson, Kristmansdottir & Oskarsson, 1973).Heimaey hosts Iceland's largest fishing fleet and has apopulation of c. 5000. During the 1973 eruptions theisland was first completely evacuated before returningvolunteers saved much of the town from burialbeneath ash and lava during ensuing months. In all,more than 300 houses were engulfed in lava andpartially buried property is a reminder of thecatastrophe. The oldest part of Heimaey is the rangeof moberg hills which shelter the town from thenorth-west (Fig. 32). At its western extremity thecrater of Herj6lfsdalur forms an arena for the annualnational festival and a camp site. On the seaward sideof this crater, columnar basalts may be seen. To the

south and south-east of the town rise the volcanoes ofHelgafell (226 m) and the new Eldfell, respectively.The 1973 activity commenced with a series of up to20, incandescent fountaining vents along a nearly 2 kmlong north-south fissure through the position of theEldfell cone. Within 24 hours, activity centralised intoeffusive, Strombolian fountains in the vicinity ofEldfell and, within a month, a 200 m scoria cone hadbeen constructed. A thick mantle of pyroclastic debriswas deposited on the town (Self, Sparks, Booth &Walker, 1974), causing fires and considerable damage.Ultimately, basaltic lavas broke through the NNErim of the cone and advanced north towards the townand harbour and eastwards into the sea. Theirprogress towards the town was eventually halted after6 million tonnes of sea water had been pumped ontothe lava and many homes had been lost. A 40 m highcliff of c1inkery lava now marks the extent of the newKirkjubaejahraun (Fig. 33). In the 5 months ofintense activity, 1.5 km3 of ash and 240 million m3 oflava were erupted.

The cone of Eldfell may be climbed. The slopesyield excellent breadcrust, twisted and fluted bombs,and the peak has a number of fumaroles encrustedwith native sulphur. From the cone, some benefitswhich have ensued are apparent. The harbour mouthhas been narrowed by the new lavas and this affordsgreater protection; the lavas still contain considerableheat and boreholes into it provide the island with hotwater; the ash has both building and potentialagricultural values; finally, an important touristindustry has developed.

The flight between the islands and Reykjavikaffords views of the volcanic island of Surtsey(erupted, 1967) and the coastal scenery and sandurs ofsouth-west Iceland. The waters of the River Thjorsacharged with glacially eroded debris, spread impressively into the sea and the flow patterns of the morerecent lava fields are clearly discernible.

Day 10, ReykjavikThose seeking an alternative to shopping can take ashortish walk (3.5 km from the city centre) to the lowcliffs of Fossvogur. Here, an interesting section offossiliferous Holocene and late Pleistocene depositsmay be examined (Fig. 34) east of Reykjavik cityairport.

The storage tanks for the..city's natural hot waterstand on a low hill at Oskjuhlid, west of theSkogarhlid road. From here, a descending series ofpost-glacial and inter-glacial sediments .. may beinvestigated. Walking south-east from Oskjuhlid,perhaps 5 m of fossiliferous post-glacial marinesiltstones are poorly exposed in trenches, yieldingsmall gastropods and various bivalves, includingAstarte and Mactra. The underlying cliff section on thenorthern banks of Fossvogur is well displayed and


Fig. 33. Front of 1973 Kirkjubaejahraun lava partly engulfing a house; Heimay, Westmann Islands.

Fig. 34. Sketch map of Oskjuhlid area and Fossvogurcoastline.

reveals the following sequence:

Buff-yellow sandstone, with included basaltboulders up to 0.2 m diameter, towards thebase c. 2.0 m

Cross bedded glacial outwash sands withbasalt pebbles c. 1.0 m

Tillite with basalt boulders up to 1.0 mdiameter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.15 m

Irregular base.Buff-yellow sandstone with bivalves in life

positions and occasional basalt boulders upto 0.2 m diameter (mixed debris from meltof floating ice?) up to 2.0 m

Irregular base with boulders up toO. 15 mdiameterCryoturbated, finely laminated silt-

stone seen for 0 to 0.5 mReykjavik Grey Basalts poorly shown

On the eastern side of the Reykjavik peninsula, eastof Ellidavogur inlet, slightly older beds, including athin lignite, of the penultimate interglacial period, arevisible.

FIELD MEETING IN ICELAND

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248 MICHAEL BAMLE'IT AND JOHN F. POTIER

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GLOSSARY

Apalhraun-Blocky lava flowBolstraberg, bolstrahrahraun-Pillow lavasDalur-ValleyDragar-Direct run-off streamDyngja-Shield volcanoFell, tiell-Mountain, peakFoss-WaterfallGjti-Cleft, canyon, gorge, narrow valleyGeislasteinar-ZeolitesGoshver-Hot water spout, gusherHelluhraun-Smooth or ropy lava flow, pahoehoeHraun-Lava flow or group of flowslardfreaedi-Geologylokular-Debris-laden stream from glacier or ice caplokull-Ice sheet, ice cap or glacierLettar-CliffsLindar-Spring water streamMoberg-Yellow brown fragmental rocks (see text).

Type of hyaloclastiteReyk-SmokeSandur-Stony plain. May be valley, coastal or

tundra/desert.Shellir-CaveTuya-Table-topped mountainVatn-LakeVik-Harbour, inlet, port

NOTES ADDED IN PROOF

A 1988 visit revealed the following importantchanges:-a) The petrol station near the Sigalda power station

(Day 3) is now closed.b) The Leirhnjukur area is currently forbidden to

visitors and the car park made inaccessible (Fig.21, 9, 10, 11).

icelandic geology: an explanatory excursion guide based on a 1986 field meeting

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