16

Volcanoes as landscape forms

Ordinary, non-volcanic landforms are the resultsof erosion by wind, water, and ice. Erosion is anirreversible process which ultimately reduceseven the loftiest mountain range to a flat plain.Volcanic landforms by contrast, are the results ofopposing constructive and destructive forces.Constructive processes operate only while volca­noes arc active. This may be an extremely shortperiod-a matter of days or weeks-or ratherlong, with activity continuing intermittently overtcns of thousands of years. Paricutin, a common­or-garden basaltic scoria cone was born in aMexican cornfield on 20 February 1943. After ayear of activity it was 325 m high; when it finallysimmered into silence in 1952 it w3s410 m high.About 2 cubic kilometres of lava and tephra wereerupted during its nine years of activity. Bycontrast, Stromboli in the Mediterranean hasbccn erupting throughout history, but is still only981 metres above sea-level. Many rapidly con­structed volcanic landforms are not 'volcanoes'at all-the Valley of Ten Thousand Smokcs wasburicd undcr 15 cubic kilometres of ignimbrite inless than 60 hours.

When contemplating the impassive grandeurofa mountain range, one intuitively thinks of thedestructive processes of erosion as acting infini­tely slowly. Erosion is often conceived as theepitome of the slowness of geological processes.But this is a considerable oversimplification. In

arid environments such as the Atacama Desert orthe Moon's surface, erosion rates are indeedimmeasurably slow, but in others, such as thehumid tropics, they can be startlingly fast, evenby human standards. Catastrophic processessuch as avalanching accomplish in a fewmoments what it might take millennia to achieveotherwise. Whatever their rate, one thing iscertain: erosion starts work on a volcano as soonas it starts growing, even before its lavas cool.Erosion never ceases. A large volcano mayexperience several phases of rapid constructionin its lifetime, during which the rate of construc­tion exceeds the rate of erosion, but once eruptiveactivity wanes, erosion instantly gains the upperhand. On a large volcanic massif, erosion may beproceeding in one part, while new lava is beingadded to another.

All these variables yield volcanic landscapesthat are as richly diverse to analyse as they arepleasing to behold. Strangely, though, volcaniclandscapes have been little studied. There is abook on the subject, which remains an authorita­tive account although it was published almost 50years ago; onc which is still capable of supplyingvolcanologists with insights into why their volca­noes look the way they do. 'Volcanoes asLandscape Forms' was the title which C. A.Cotton selected for his book; his title is used forthis chapter to acknowledge his contribution.!

Volcanoes as landscape forms

- 16.1 Monogenetic volcanoes

341

16.1.! Scoria calles

A monogenetic volcano is the product of a singleeruptive episode. This may last a few hours, or afew years, but the essential point is that onceeruption has ce;.sed, the plumbing connectingthe vcnt to its magmatle source freezes over, sothe volcano never erupts again. Basaltic scoriacones are good examples of monogenetic volca­noes. They arc found in thousands all around theworld, in many lectonic environments, either ascomponents of scoria cone fields, like P.lricutinin Mexico, or as p•• rasitic vents on the flanks oflarger volcanoes-Etna has dozens, All over theworld. they have the same distinctive morpho­logy. They arc rarely more than lwO or threehundred metres high, and arc often asymmetri­cal: either elongated along a fissure. or else higheron Ihe side Ihat was downwind al the time oferuption. A breach on one side often marks thesile from which bva has flowed. A distinctivefeat ure is their sim pic geomet rical profile, def! nedby the angle of rest for loose scoria (Fig. 16.1). Allyoung scoria cones have side slopes elose to 3r.

In a statistical study of scoria cones, ChuckWood showed that 50 per cent were formedduring eruptions thaI lasted less than 30 days;95 per cent of them during eruptions that lastedless than one year. 2 Wood looked at the dimen-

sions of 910 scoria eoncs. and found that theirmean basal diameter was 0.9 km. In a sample of83 fresll scoria cones, he found some regulargeomctrical relationships: the heigll/ of Ihe cOileproved to beO.18 times the basal width, while thecrater diameter was 0.40 limes the basal width.This emphasizes a characteristic feature of scoriacones: lheir craters lire large in relation to I he si7.eof the edifice as a whole. Naturally, lheir crispprofiles soften with age, but Wood showed thatthe ratio of crater width to basal width changesremarkably lillie. Thus. scoria cones remaineasily recognizable, even after millennia ofweathering.

16. J.2 Mnors

Scoria concs arc the results of minor basalticeruptions taking place in dry conditions. Whenbasaltic mllgmas interact with water, the natureof the eruption is explosively different, prod ucingsllrlseya/l pyroclastic deposits (Section 6.4.1). Itis not necessllry for the eruption to lake placeunder water to produce explosive consequences-a water-bearing stratum (aquifer) in sedimen­tary rocks is all that is nceded. In the simplestcase, shallow phreatic explosions caused bymagma-ground-water interactions blastupwards through to the surface, forming large

Fig. 16.1 La Poruiia. northChile,:1 300m-high scoriacone. La Poruiia appearsyouthful, but may be manythousand years old. since it islocated in a hyper-arid pari ofIhe Alacama Desert. In Iheshadow al the foot of the cone.3 lrain on lhe Antofagasta-La1)3Z railro;.d provides scale.(Compare Ihe air phOlO inFig. 7.8.)

342 Volcanoes: a planetary perspective

holes in the ground. In the Eifel ;lrea ofGermany,eruptions of this kind formed 30 craters about akilometre across, now occupied by lakes. whichgave their name to the landform: maars. J\faurcr:Hcrs 3fC simple. circular depressions sur­rounded by low rims of ejected debris. Theirwalls arc steep-sided initially. but arc quicklyeroded away to gentle slopes. Since they 3fC bydefinition holes in the ground rather thanstructures buill up above it. maars typically fillwith water and arc thus manifested as lakes.

In his statistical study, Wood showed thatmuars arc Iypically small [eatures, most havingdiameters of about one kilometre. His preferredexamples arc from the Pinacate region of north­west Mexico, where eight young maars arebeautifully exposed in the Sonoran Desert. Theyarc circular to oval, with diameters between 750and 1750 metres and range in depth between 36and 245 metres.

In the desiccated heart of the Sahara Desert, animprobable place to look for magma-waterinteractions, there is an instructive //Iaar atMalha, in the Darfur province of the Sudan.About one kilometre in diameter and a hundredmetres deep, the maar was blasted through alayer of ubian sandstone, depositing an apronof ejl.'Ctcd debris around the crater, now wellexposed in the rim (Fig. 16.2). Conspicuous inthe debris arc rounded boulders of gneiss andgr..Illite up to a metre across derived from the

metamorphic basement underlying the sand­stone. Explosive activity thoroughly commi­nuted the relatively weak sandstone. while thegranites and gneiss were more resistant. At thebottom of the Malha maar is a small lake. fed by aseries of small springs seeping through the

ubian sandstone. Because they arc the onlysource of fresh water for thousands of squaTCkilometres, thescsprings are vital to thc people ofthe arc;:\. Their me.."lgre flow illustratcs how littlewater needs to be colllained in an aquifer forexplosive magmatic interactions to t"ke place.

At Malha and other mallrs, there is often only asmall proportion of magmatic material in theejecta. This can lead the unwary into grievouserrors of interpreta lion, si nee basi n-sha ped maarcraters have been mistaken for meteorite impactcraters when there is little obvious volcanicmalerial present, as at Jayu Khot:l in Bolivia

(F;g" 16"3)"

16.1.3 Tuff rillgs

A convenient. but not iron-clad distinction.between moars and tuff rings is that maars arcexcavated inlo the substrate, whereas luff ringsarc buill up above it (Fig. 16.4). And whereas"wars arc the results of shallow explosions, ofteninvolving scanty amounts of juvenile material,tulT rings contain an abundance of highly frag­mented basaltic scoria: they arc essentially ac­cumulations of surtseyan tephm. TulT rings arc

Fig. 16.2 Camels and goatsdrinking al the Malha /lllIlIr.

Darfur province, weslernSudan. White Nubiansandstone is exposed in thewall of the erater. while darkeroverlying material is ejccta.Animals in the foreground areclustered around small springswhich all rise at the samestrJtigraphic le\'el, probablythe contact between thesandstone and the underlyingcrystalline basement.

-"=,.".:"

."

•

. "

Fig. 16.3 Jayu Khot:l U1(/{lr on the altiplano of Boli\ia. iniliall) I1101l£h11O Ix: an impact cr:.uer. Tufa dcpo~ilS

from the high sl;l1ld of glacial Ltke Tauca are prcscnt around the cr:llcr. which lhcn.:fon: mll~l be more than10000 years old. Erosion has subdut:d the ejecta rim.

Fig. 16.4 A superblys}lllmclrical tun'ring, ncar theEn,I'AIe \'olc:lno. Ethiopia.(photo: 1·1. Ta1.idf.)

formed when magma comes ncar to the surfuccbefore being ex plosivcly fmgmcnlcd. Cerro Xico,only 15 km from the heart of Mexico City. is anelegant cX;lInpJc which form.:.'<! in the basin ofslwltow Lake Tcxcoco. before it was drained bythe Spanish in the sixteenth century. Superblycircular when seen from lhe air, Cerro Xico isfrustrating to photograph from the groundbecause it is so bro<ld <lnd flat, like all tuff rings.

Diamond HC<ld. a much photographcd land­mark all thc island ofO<lhu (Hawaii) is another

343

tuff ring formed in prehistoric times (Fig. 16.5).For years it domin.llcd the cxotic p<llm-fringedsurfing beach at Waikiki. Now the beach isdominated by high-risc tourist hotels ... Dia­mond Head crater is about I kilometre indiameter, and for the most P:lrt its rim is about100 metres high. On its south-western extremily,however. Diamond Head itself rises no less t!l;ln232 metres. The emter's marked asymmctry wascaused by north-cast tradc winds, which musthave been blowing strongly during the eruption.

•

344

Fi~. 16.5 Diamond Head luffring. Honolulu. H:lwaii. Tradey,inds COluscd l11:tXll1ll1nl :Ish3ccumul;lIion do\\nwind.forllllng lhe high pomt on therim. furthest :I\\ay fromc:.llllCrJ

Vole'llloes: a planetary perspective

just as thc)' do through much orthe yem IOday.Ejccta inevitably accumulated much morethickly on the downwind side.

16.1.4 Tuff COI/es

Tulr cones arc smaller. stccper versions of tuffrings. composed of similar surtseyan tephra.Morphologically. they resemble scoria cones.Only a few kilometres from Cerro Xico, on thesallle lake bed near Mexico City. is a prominenttulT cone called EI Caldera (Fil:l. 16.6). It is notimmediately obvious why one eruption should

form a tuff ring. while anotller only a shortdistance aw:.y forms a cone. bUI it probably hasto do wilh the relative ;tmounts of water :Illdmagma. and the duration of the eruption. Tephrahas 10 be widely dispersed to fOfm a lulT ring.calling for a violent hydromagmatic eruption.with relatively high column and mass eruptionrate. Tuff cones may forlll from less violent. moreprolonged eruptions.

The tephra erupled arc hal and water satu­rated. so wet. muddy deposits are formed. Theirstickiness results in turT cones which develop

Fig. 16.6 El C'lldcra.;1250-m-high 1un' COliCconstructed above the bed offormer Lake Te~coco. nearMexico City. Morphologic:llly.it rescmbks a scoria cone. butis composed of surtscyantephra.

•

•

Volcanoes as landsca~ forms 345

startlingly steep slopes on erosion, sometimessteeper thall the 330 angle of rest of loose. dryscoria; sometimes so steep that it is barelypossible to walk up the dip slopes of the tephralayers. Koko crater, only a few kilometres fromDiamond Head on O:lhu. is an excellent exampleof a sleep tun" COIlC (Fig. 16.7). Koko crater issmaller than Di<ll11olld )'lead, only about 800metres in di,lIncler, but is much higher. 366metres. As in Diamond Head. the high point onthc rim is on lhe downwind side of the crater.

Unlike ordinary scoria cones, both inward andoutward dips arc commonly seen in lufT cones,tephra layers mantling the rim and both innerand ouler slor)(~s of the crater. Why does thetephra not simply slump off while still wet andmuddy? This is not clear. Some slumps do takeplace, but for the most part the steeply dippingtephra layers appear to have been sta ble from themoment they were deposited. Probably, lhe hOLwet pyroclastic material dried quickly. setting ashard as concrete. (Alteration of the glassy shardsby hydration to pa/agollile takes place rapidlyafter deposition, often obscuring original struc­tures.)

Eruptions which construct bOlh tun' rings andcones also yield pyroclastic surges which maytra\'c1 se\'eral kilometres (Section 11.1.1). How­ever. on the slopes of the cone or ring itself, it isoften not clear whether the nutteri,,1 was depos­ited by fatt or surge. In a thoughtful analysis of

the differences between tulT rings and cones.Wohlctz and Sheridan suggested that the mas­sive bedding found in tulT cones is due toemplacement in cool. wet conditions, less than100 c. l In contrast. the thinner-bedded depositsof tulT rings arc emplaced hOI and relatively dry.As increasing amounts of wa ler mix with magmato gcnerale surge blasts, increasing amounts ofsteam arc produced. but the steam is cooler andthercfore 'wetter'. Thus. as the water:magmaralio increases, wetter and stickier surges result.Wohlelz and Sheridan suggested that thereshould therefore be a regular progression ofminor basaltic landrorms, rrom scoria cones indry environments. through tulT rings whereground-water is present to tuff cones in openwater (Fig. 16.8).

16.1.5 Dia/remesSome of the IlIaW'.I' that turn up in odd geologicalcontexts around the world arc the surfaceexpressions of tlimremes. venical pipes blastedthrough basement rocks, which contain angularrock fragments of all sorts and conditions,4 Somefragments arc lumps lorn from the sides of thepipe at depth, others arc rrOlTl sh:Lllow levels;some may be of sedimentary origin-cven lumpsorcoal-and some arc ofobvious volcanic origin.Diatremes arc abundant in some regions-in theSchwabian Alps ;lrea of southern Germany,more than 300 have becn identified over an area

Fig. 16.7 Prolil..: view of Kokocrater. Oahu. I-hlwaii. Tradewinds blowing from righl 10

left arc responsible for Ih..:pronounced as)'mmelry of Ihis366-rn-high l\lIT COlle, whichshows good p;lrasol ribbing.

346

Scoria Cone

Volcanoes: a planclary perspective

lillie or...... water

Mechanical energy

Turf Ring

• ~m''''''

Pillow Lavas

Fi~. 16.M Effect of incrcasint; amounl.s of water on morphology of sm;lll basallic volcanic constructs. Internalstructures arc shown only schematically: slumps from S\L'Cp crater w"lIs arc observed in some lulT rings, whileinward and outward dips ;11'C characteristic of IlifT COIlt:S. After Wohlcl~.. K. 1-1. :Ind Sheridan. M. F. (1983).Hydrovolcanic explosions 11. Evolution of basaltic tulT rings and lufT cones. Am. J. Sci. 283, 385-413.

of about 1600 square kilometres. They are allbetween 15 and 20 million years old, and so theirsurface expression is 1l1uted. Some arc identifi­able as vague depressions, but mosl can only bepicked up through geophysical surveys.

At Kim bcrley in $OlHh Africa, a deep mine wascxc.l\,.llcd to probe thc limits of a large diatreme.This expensive undertaking was nol C<lrried outin thc spirit of scientific enquiry, but because thediatreme contained gem-quality diamonds.These were found in kimherlite, a mixed, brec­ciated rock containing:.l proportion of peridotite,derived from the mantle. where the diamondsoriginated. In the deepest pari of the mine, the

widlh oflhe carrol-shaped pipe is only 30 metres,while at the present surface levcl it is 300 mel res.Il may have been linked with :.l maar :.lbout 500mel res across at the original surface. SoulhAfrica is 110t now a region of active volcanism:from iSOlopic evidence, thc diamonds themselves'lppcar to be more than one thous:lnd millionyears old, whereas the kimberlites which broughtthem to the surface were eruptcd ninety millionyears ago, Erosion and subsequent redepositionof the diamantiferous rocks in the upper part ofthis diatreme and others like it gave rise to therich alluvial diamond fields of modcrn SouthAfrica and Namibia,

•

Volcanoes as landscape (arms 347

Fig. 16.9 Hyato Ridge. WellsGray Pro\'iocial Park. BritishColumbia. ~l 1()(X) J11 high IUraformed about IO())(} ye:us agoby subglacial eruption of albliolivine basah, now thicklyforested. Photo: courtesy ofCatherine l'liekson, CanadianGcoto1;ical Sur\'ey.

Fi~, 16.10 Detail ofa 'pillow' of basalt inhyaloclastitc nl3trix. showing quenched margins andradial jointing. formed by subgl3cial baS31t eruption.Hyalo Ridge /II)'a. British Columbi3. Photo;oourtcs)' of C:llhcrine Hickson, Can3dianGeological Survcy.

- 16.2 Polygenetic volcanoes

Polygenclic volcanoes arc thosc that havc cxperi­eneed more than one eruptive episode in theirhistory. Most ofthe world's volcanoes fit into thisrather loose category. but several different sub­groups can be identified, based on the numberand location of the venlS from which eruptionslOok place.

16.2.1 Simple cOlles

Simple cones are overgrown scoria cones: scoria

16,1.6 Ltmdforms reslf/li/lg from sub-glacialcrupfiolls

;\t the present day. permanent glacicrs arcconfined to the polar re~ions, a few sub-arctic icecaps such as those ill Iceland and Patagonia, andshrinking valley gl:Jciers on high mountainsaround the world. It is easy to forgel th,lI hugeareasoflhe Earth were covered by continental iccsheets which receded only II 000 years ago.When the ice receded, the steep sides and flat.lava-capped topS of subglacial VOlc;:lllQCS wererevealed (Section 6.4.3). In British Columbia,easily recognizable 'table mountains' developedby subglacial centr<ll vent eruptions are calledfrIyas; in Iceland. where eruptions from fissurescommonly produce elongate ridges. they arccalled lIlobl!rgs~ (Fig. 16.9-16.10).

cones which carried on erupling. They ha\'e asingle summit vent and radial symmetry. Thesearc the volcanoes whose graceful profiles adornso many calendars and postcards. There arcinnumera ble examples of splendidly symmetricalsimple cones around 'the world. MI. Mayoll(2400 III high) in the Philippines is often ciled asthe world's most beauliful vokano (Fig. 12.14).Licancabur (5916 m high) has a striking geo­metrical purity of line. dominating the oasis of

348 Volcanoes: a planetary perspective

Fi~. 16.11 Licancabur. north Chile. a 5916-m-highsimple cone. The dale of its I,ISI eruption is notkllOI...I, but appears to h:l\'c I:k.--en rca:1II. since lavason its l1ank~ are fl\.'Sh. There arc In\.-;\ ruins and asmall lake in its summit crater.

San Pedro de Alae.lIna in Chile (Fig. 16.11). InPeru. daily life in the city of Arequipa is carriedout beneath tile sublime curves of EI Misti (Fig.16.12). while in Guatemala. Agua and Fuegovolcanoes form an appropriate backdrop for thestrombolian eruptions of I>acaya. It is casy 10 be

over-impressed by the apparent simplicity ofsuch volcanoes. however. Subtle structural com­plexities oflen \Urn up on closer inspection. sothat the volcano should fonnally be regarded ascomposi/e-magnillccllt Mount Fuji is a case inPOlilt.

Simple cones arc characterized by rather smallsummit craters; often tiny for Ihe size of theedifice as a whole. and not much bigger than thatof a scoria cone-Mt. M,lyon's for example, isless than 200 m in diameter. Nestled withinLieancabur's sunllnit crater is a miniscule fresh­water lake, 90 melres by 70 mel res. At anelevation of almost 6000 m. it is probabl)' theworld's highest lake. but none the less a plank­tOllic fauna of considerable inlerest to biologistsmanages to exisl. Weak volcanic thermal emis,sions probably help to prevent the lake fromfreezing into a solid mass-stalwart divers haveexplored its depths during the world's highestaltitude dive.

In their summit regions_ simple cones arc oftenarmoured by lava nows (Fig. 16.13). Theseprovide mechanical strength. so vertiginouslysteep slopes, over 40. arc possible. as forexample on the 1157.m-high volcano St Eusta·tius in the Dutch Antilles. On Ihe mid-l1anks.interstralillcd scoria. l:tva. and talus eroded fromthe lavas predominate, so slopes case otT.

Fig. 16.12 EI Misli (5822 Ill)ri~s direel1y abo~e thesuburbs of Arcquip'l. Peru'ssecond city. preselHing anob~ious hazard to Ille CilY(rorcground). There have bccnmany eruptions since lheSpanish conquest. A lavadome was active in the summitcrater as recently as Ihe 1950s.Photo courtesy of F. Bullard.University of Texas.

Volcarloes as landsc<lpc forms 349

-

"-ig. 16.13 Acam:lrachi. northChile.;I 6046·m.high simplecone with S[lOCP slopesrc~ulling from extrusion ofla\'11 nows from (hI:: summitcr~tlcr. Edifice heigh! is only1200 m.

Although lav:I !lows may snake down to lowlc\'cls. Ihe lowest slopes aTC moslly construclCd oftalus c:.rried downwards by mass wasting. Thus.the elegant concave profiles of simple conesrcnCCI the interplay between erosion and erup­tion.

Gi"cn a single \cnt. the shape and height of asimple \olcano arc immutably controlled bygeometry: evcry additional increment of heighlrequires a huge additional increase in w/l/me.When a volcano reaches a height of more than2(X)() m. c:lch additional metre of height requiresthe eruption of lens of millions of cubic metres ofrock. And as height increases. so erosionbecomes more effective, though not in a simplegeometrical manner. So. for :lny given combin­ation oferuplion and erosion rales.the height ofa simple cone is self-limiting. As volcanoesbecome larger. approaching 3000 m. the volumeinCrelllelll required for each additional heightincrement is so huge that the required eruptionrates begin to exceed the geologically plausible.This explains why voleanoes on Earth rarelyexceed 3000 m in edifice height. (Note that theSIll/lilli' heights of many voleanoes are muchhigher. but these edifices are constructed onclev.lIed basements.)

These geometrical relationships can easily beappreciated by ex(X:rimenting with the formulafor the volume of a simple right cone:

where f'= volume. r= radius. and II = height.In reality. the shape of a 'conical' volcano such

as MI. Mayon is actually expressed by a morecomplex exponenti:.1 relationship of the forlll:

where IJ and M arc constants. lis volullle can befound by integr:lling this expression. Geologistswith a mathematical bent established theserelationships Illore than a century ago. but sinceIhe pioneering work of Milne6 and Becker. 7 thesubject has been largely ignored. so we still dono! understand many of the su btlet ies underlyingthe geometrical form of volcanoes.

A factor which further complicates consider­ation of the height of a volcano is that once avolcano grows large. it begins to deform under itsown weight. When volcanoes arc built on thinoceanic lithosphere, the mass of the volcano alsocauses the lithosphere to sag downwards into theasthenosphere. In the case of the large HawaiianvolcJ.nocs. this subsidence has been very con­siderable. According to J. G. Moore. as much asa half or two-thirds of the upbuilding of thevolcanoes may be offset by lithospheric subsi­dence. Thus. if the lithosphere were not so

350 Vo1c;:moes: a planetary perspective

llcxiblc. the Hawaiian volcanoes would be kilo­mel res highcr.~

16.2.2 Composite cOlles

Composite cones have had morc than onec...olutionary stage in their existence. but stillretain an overall radial symmetry. Throughouttheir complex eruption history. the locus ofaClh·ity has been csscllIially confined to a singlesile. Vesuvius is all example of a volcano wherean earlier edifice was wrecked by an eruption(AI) 79) and a younger one buill up in its place.such thai from sOl11e vantage points the ruins ofthe older edifice (MoniC Somma) are notobvious. and the VOIc;lIlO appears \0 be simpleand sYlllmetrical. II is surprising how extensivelya \olcano may be modified. yet still retain anoverall symmetrical sh;IJ>C. AI Parinacota inChilc. a hugc dcbris avalanchc cviscerated thcwc:.tcrn flank of thc volc,mo 13000 ycars ago(Fig. 16.14). So much reconstruction has takcnplace since then that no trace of the amphithcatreremains. Parinacota llppears to be a simplc cone.But thc hummocks and hollows of the vastavalanche deposit to thc west confirm that itshistory has been anything but simple.

An average composite cone is an edifice onlyabout 2 km high, but its structure may exhibit acomplex history (Fig. 16.15). Mt. Etna is anexample of a huge composite volcano. 3308 m

high. It has bro:ld radial symmclry. althoughthere ;Ire several summit vents and innumerableparasitic monogenetic vents. On its northernflanks, vents arc aligned along a rift-likc exten­sion. while on the eastern flanks ,I great amphi­theatre (the Valle deillove) takes a great bitc outof the edifice. From the ground. these featuresmake the volcano look distinctly complex, butfrom the perspective of space. the volcano takeson a symmetrical shape, and its identity as asingle conical edifice is obvious. All its manyvents ha,c tapped a single mantic sourcc.

Etna also exhibits conn~nicntlyanother prop­erty of many apparently simple major volcanoes:the edifice we see now is only the youngest of a$Cries constructed on more or less thc samc site.Present-day Etna (also called Mongibello).appears to havc been constructcd ovcr the last34000 years. but $Cyeral edifices cxisted prc­"jously. stretching back more than 100000 years.For example. between 80000 years and 60000years ago. an older volc<lno (Trifoglietto) existedslightly cast of the present Mongibcllo.

16.2.3 Compoulld uolcalloes

Nevado Ojos del Salado is a compound or/11l/fliple volcano, in thc tcrminology uscd byCotton. It is not an individual cone, but a massifcovcring all area of about 70 squarc kilomctrcs,formed of at least a dozcn young concs intermin-

Fig. 16.14 Large 11l0Ul1d~ inthe middle distance Oflhisphotography arc lorel!{/

blocks. cmplacc<l by acatastrophic debris avalanchetaking placc aboul 13000years ago on the 6348 m highParinacota volcano, northChile (background). No tr,lCCof an avalanche SC:lr fCm:linson thc conc. Pomcrapcvolcano is at thc left.

Vokano<,s as hllldscape forms 351

Fig. 16.15 Vcnical airphotograph of Ceborucovolcano. Mexico. an excellentexample of a cOl11posil~

volcano with a ("llnpl~x

history bUI an imJividualidcntit)·. M;lI\~ dilfercntcpiwdes of activit} areidentifiable. including nestedcalderas and 1:Iva-l1ow fields ofdifferent age:. Ccboruco is21~ III high. ;lIld itsoutermost C;Lldcr;t ;tbout -l kmin diameter. It last erupted in1872.

gled with domes and craters. Remarkably. thedetailed anatomy of this. the world's highestvolcano. has never been studied. so it is notcertain exactly how many componenlS make upthe massif. or how their magmatic plumbingsystems arc interconnected. Allhough the sum­mit of the massif is very high (6887 m) theindividual cones arc not especially large: b<lselevel in the area is around 4500 melres, so thevolcanic cdilice is not Illllch Illore llwn 2000 III

high (Fig. 2.4).Nevada Ojos del Salado's many vents do not

seem to rellect any obvious tectonic control. Outmany compound volcanoes do-they commonlyform elongate massifs. with many cones. vents,and craters aligned to form a ridge. probably thesurface expression of a dyke. Aucanquilcha inChile is a fine example. Topographically. it formsa ridge extending about 10 km in an cast-westdirel,;tion. :llld composed of several individualcones reaching over 6000 m (Fig. 16.16). [n this

case, as in many others, there is e\'idence foractivity migrating through time along the ridge.

16.2.4 Vo/cmlo complexes

Compound. or multiple. volcanoes have anindividual identity. On a map. one could I>ointtothe volcano and say there. Volcano complexesa re so jumbled that the best one can do is to dra IV

:I line around them. Form:lIly. lhey e:1I1 bedefined as extensive assemblages or spatially.temporally, and genetically related major andminor centres with their associated lava nowsand pyroclastic rocks. More practically. onecould get losl in the confusing (angle of similar­looking flows and cones that make up a volcanocomplex. Cordon Punta Negra in Chile is abeautiful example. where at least 25 small coneswith well-developed summit craters arc presentover an area of some 500 square kilometres.None of the cones is more than a few hundredmetres high, and some of the older ones arc

352

Fi~. 16.16 Auc:tnquilcha.north Chile. Vicw of northp:lrl oft!'c lO-km-longcaSI west-trending compound\·olcano. which docs notpossess ob\ iOlls \ okanomorphology. Actin: fum:ITolcsha\'c deposited sulphur whichis CXIr:tctcd from the "orld'shighcsl mine al the summit ;1\

o\"cr 6000 m. PJ:1Il1 in thefOfCGround IS :11 :tn clc\:Jlionof about 5CXXl m and is p;trl ol"orld's highl"l>l pcrm:lIlcntl)inhabited scttlement.

Vo1c'lIlocs: a planetary pcrspcdivc

almost buried under a confusion of lavas. It isdinicult to tell which lavas came from whichcone.

Volcano complexes like Cordon Punta Negrarepresent a form of -dislribul(,.'(r volcanism. Iferupted frolll a single vent. all the magma foundill a complex would make a decent large COile, butrather Ihan erupling through a single conduit,!iCveral closcly spaced conduits were active moreor less contemporaneously. It remains 10 deter­mine the length of time required to create acomplex like Punta Negra, Ihe range of composi­tions present, :llld how these V:l ried through lime.

Scoria cone fields. like those of Central Mex-

- 16.3 Shield volcanoes

Whereas cones arc either slraighl sided (scoriacones) or eOIlC:lVC (most large cones), shic1dvolcanoes arc conpe,\" upwards. And while conesmay be ralher steep. somelimes I'e:lching above40" in lheir summil regions. shields arc genllysloping. often less thall 10'. Simple and compo­site cOlles include lavas. pyroclastics. and talus,but shields arc constructed almost entirc1y oflavas. Finally. rocks with compositions rangingfrom b:IS:lllic to rhyolitic turn up in cones. butshields arc almOSI exclusively basaltic. Whereaserosion and mass wasting pl;ly importanl parts in

ico. represent another type of distributed volca­nism, In an area near the centre of the MexicanVolcanic Belt. directly south of Mexico City. onehundred and forty-six Quaternary scoria coneswere counted within an area of about 1000square kilometres, :1 cone density of 0.15 persquare kilometre. Basal diameters varied fromO. t to 2 km. OJ For years, many of the buildings onIhe campus of the University of Mexico havebeen renowned for their e:wberanlly colourfulmurals. It is less well known that the campus iscanst rueted on Ihe la va l1eld from one of the moslrecenlly aClive cones in the field. Cerro Xitlcerupted only 2400 years ago.

shaping growing COlles, the gcomclry or shieldvolcanoes is dictated only by the rheology ol'thelavas or which they arc m:ldc. Thus. a youngshield volcano is a ralher subtly shaped con­struct. with a gelltly swelling prolile, madeentirely or basalllavas. Whereas ,trtists respondreadily 10 the sweeping. uplifting curves of a Fuji­like cone. the bland profile of a shield volcanoalTers less inspir;l\ion.

16.3.1 Hawaiiall shields

Mauna Loa and Mauna Kea arc shield volca-

Volcanoes as landscape forms '53

noes in a class of their own, rising nearly ninekilometres from the noor of the Pacific. MaunaLoa reaches 4169 Illetres above sea-level. II has atOlal volume of about 40000 cubic kilometres. ahundred limes greater than a lyrical compositeconical volcano like Mt. Fuji. Despite its hugeproportions, Mauna Loa is an unpretentiousvolcano. ils smoothly arching whale-back profilemore reminiscent of the gelltle contours of theDorsel downs than a mountain of alpine altitude(Fig. 2.26). Only when the snowline defines thesummit region wilh sharp whiteness unexpectedin a tropical island is the magnitude ofils edificeapparent.

While Maunas Loa and Kea are the largestand youngest shields in the Hawaiian Islands.there arc many others of difTerent ages anddegrees of erosion. Each of the eight majorislands represents one or more dissected shields.gelling progressively older westwards. A youngshield has gentle slopes of only 2 3 at its base.steepens slightly to about 10 in its middle slopes.and then llallens off again in the summit region.Each shield is composed of myriads of individual11ows, many of them compound /la/we/we 110ws.averaging only a few melres Ihiek. While eachmajor shicid probably had a summit caldcra inwhich some acti\'ity was focused. a characteristicfeature of the Hawaiian shield volcanoes is thaithey arc elongated along rift zones. from whichmost lav,ls were erupted. Dykes propagalinglaterally from the cenlralmagma chamber carrybasalt magma laterally until it emergcs from aparasitic \'en! on the nanks orthe vokano. Thesedyke-fed rift zones arc prominent topographicfcaturcs, extcnding for tcns of kilomct res, and arcmarked by many small spaller eOlles. pit eralers.and l1ssures. Kilauea. the smallest but mostactive shield at present. has experienced almostcontinuous lav3 effusion since 1983 from thePu'u 0'0 vent. 17 kilometres frol11 the summitcaldera. During the period 1969-74, activity wascentred all the Mauna Ulu ('growing mountain')vent. 10 kill along lhe rirt. In 1955.1a\'as spewedout from:l \'ent25 km :lIang the rift. while in 1960a major outburst engulfed lhe village of K:lpoho.almost at sea-le\el, and nearly 30 km distantalong the rift.

When a lypk:,i1 rifl eruption begins, lava Ilows

spread rapidly over the surface. at a rate ofaboul50 cubic metres per second. burning their waythrough oliia rain forest and marijuana plan­tations indifferently. Once the eruption is wellestablished. a large fraction of the lava (pcrh:lpsas much as 80 percent) flows through lava tubes.This dramatically extends the distances lhe nowscan reach, enabling lavas initialed high onKilauea's cast rift to debouch into the sea.extending the coastline. Apan from creatingopportunities to photograph sizzling red lavaand white steam clouds against an azure ocean.these tube-fed nows account for the gentlysloping profiles of shields: most of tile eruptedvolume ends up a long way rrom its point oforigin, adding to the nanks of the \'okano. rat hcrthan the summit region. Eruptions of moreviscous magmas, which do not flow in tubes. leadto steeper, conical volcanoes.

Maunas Loa and Kea arc similar in altilude.But whereas Mauna Loa is activc (it erupted in1984). Mauna Kea has not eruptcd in historictimes. One reason why astronomers have beenwilling to risk building huge :md hugely expen­sive lelescopeson Mauna Kea is thaI Mauna Keaappears to be 1110re mature. This conclusion isbased on the presence on its upper nanks ofabundant small scoria cones and lavas of alkaliccomposition: markedly different from the tho­leiites making up IllOSt of the volume of thevO!c:lno (Section 3.2.1). Morphologically. thesealkalic lavas somewhat resemble andesites. inthat the nows arc thicker and chunkier thanordinary bas•."tlts. In the summit region of MaunaKea where a small ice-cap existed during the laSIIce Age. glaciers scoured the surface of the flows.creating topography more reminiscent of lhehigh Andes than a tropical OCC:ln island volcano.

Mauna Ke.. has reached what is termed the"alkali cap' stage in its e\'olution: a Slage whichthe older shields on the Hawaiian Islands havealso reached (Fig. 16.17). Unfortunately for lheastronomcrs. reaching thc alkali cap stage docsnot nccessarily mean the vokano is extinct-o",the neighbouring island of MauL Haleakalavolcano. also a mature shield, erupted as recentlyas 1790. Should an eruption take place onMauna Kea. the damage done to astronomicalrescarch programmes would be incalculable.

354 Volcanoes: OJ planetary perspective

1 Deep submarine stage

Shillld ,'ole,1II0

Sea len",

~" le,"d

Ashrone

ExplOSIOn debris

.'i\.;- . '';'

"Ii'

/--~\"\-\~c':l~;-/r----

--------" ...............- ..........................

/ ,/ ,

/ ,/ ,

2 Shallow submarine stage

3 Sub..1erial shield·building stage

4 Caldera slage

V \:

Fig. 16.17 Scoria (,.'Qncs cluster (background) 314000 m at the ~ummil of Mauna Kca. J·bwaii.During Ihe Icc Age. a sm'lll icc-cap <XX:upicd thesummu. Iwulders in Ihe foreground show c\'idcnccof glaciallransporl:ltion. Photo: courh.:sy of P.~"ouginis-Mark.

Howc\'cr. the volcanologic.ll record suggests thateruptions arc cxcc<::dingly infrequent-the lastone appears to have been about 4500 years ago.Thus. although there is a slight risk. the incom­parable viewing conditions provided by a highmountain in the mid-Ilacific easily justify it.None the less. if an eruption were to take place.astronomers would inc\'itably take a somewhatjaundiced view of their volcanologicalcolleagues ...

During their active lifetimes, construction ofHawaiian shield volcanoes keeps ahead of ero­sion (Fig. 16.18). At anyone location on avolcano, the inlervals between successive sets oflava nows may be long enough for soil horizons10 develop. bUI there is relatively lillIe laterallransport or materi'll. Once activity slows, deepcanyons arc rapidly incised, leading 10 somestartling lopography on lhe older islands. OnHuleakala (Maui) late-stage seoriu cone erup­lions have taken place within a vast summitamphitheatre cxcava Icd by erosion. Even duringtheir active lives, the morphologies of shields canbe drastically modified by l:llldsliding. Kilauea'sentire soulh-cast flank is slumping slowly into theocean, slipping down along a series of great faultscarps or paiL Some catastrophic collapses mayalso take place. geller.tting huge tsunamis. Fortu­nately, none has occurred in historic times(Section 13.1.4).

355

9 Atoll stage

16.3.2 Galapagos shields

Clustered on the Equ;ltor 1100 km west ofEcuador, the Galapagos Islands arc belterknown for their contribution to Darwin's ideason the origins of species t han for their volcanoes.None the less, the volcanoes occupy :l hot-spolselling similar to, but more complex than, theHawaiian volcanoes, Each island is either ashield VOIC:lllO or a coalescence of scveral shields,each 45-80 km across, which risc about 1500 mabove sca-Ievel. In detail. the Galapagos shieldsdiffer from those of Hawaii in three ways: First,they lack the gentle, wh:lle-back profiles ofMauna Loa, but instead have profiles tradition­ally likened to upturned soup-plates. with a

Each major Hawaiian volcano is an enormousshield. flut individual rirt eruptions may them­selves construct small, gently sloping lavashields. Mauna Ulu. in the Volcanoes NationalPark was born in 1969, Innumerable pllllOeiloelavas welled up and over the rim of the vent.spreading out to form an apron of anastomosingflows. In less than a year. a shield almost 100metres high and a kilometre in diameter hadgrown. Subsequently. its summit crater wasoccupied by a small lava pond which persisted for,several years. Pahoehoe lava erupted fromMauna Ulu ~twcen 1969 and 1974 ultimatelyreached the coast, covering almost 45 squarekilometres.

Fig. 16.18 Stages in the morphological evolulion ofHawaiian shield volcanoes. (After Macdonald, G.A.. Abbon. A. T" and Peterson. F. L. (1970).Volcanoes ill/he Seu. Univ. Hawaii Press. Honolulu.517pp.)

"'.le\'el

.'

--""'- ~--L '

_...---- ..............- ............... /'0.. .... __ .....

• ~ 5e~

.",'''''-, •. "«.., ",'..•<, ,,' .~

,'",.;' ... i"", 't, " 't

Cinder (ones'5;:::~"~':t!;~',"12'",

Volcanoes as landscape forms

5 Postcalder.l stage

6 Erosional stOlge

7 Stage of reef growth

~<.~ ~ ~.,..:,~~vdFringing cora) r..'Cfs

(n.....h b..'Comc wide ifupw.,rJ growth.1CC1Jln 1'" nk'S ~il1 king of islJ nd)

356

Fi~. 16.19 Volcano Cumbrcs.Isla Fernandina. G:J.I:lpayos.shoy,ing the im'Crlcd ~up­

plate profile characlcri~ljc orGalapagos ~hiclds. Cumbrcs is1250 metres high. Photocourtesy I'clerMouginis-Mark.

Volcanoes: a planetary perspective

marked change of slope from gentle to steep(> 10 ) on the mid-flanks. and flatfish tops (Fig.16.19).

Second. whereas the active Hawaiian shieldsMauna Loa and Kilauea have summit calderasscveral kilollletres across. these are shallow, lesslhan two hundred llletres deep. On the Galapa­gos Islands. by contrast. summit calderas arespectacularly deep. That on Fernandina, forexample. is Illorc than SSO In deep.

Third. Ihe Galapagos volcanocs arc morencarly radially symmclric~l than the Hawaiianshields. While there is some evidence for dykeintrusion, linear rift zones like lhose of Kilaucaand Mauna Loa ;Ire subdued. Surrounding thesummit calderas arc prominelll sets of circumfer­enlial fissures. These circumferential fissures arcalmost uniquc lO thc Galapagos. On Earth, a fewterrestrial volcanoes exhibit comparable fissures,for example Deception Island (Antarctica) andNiuafoou AlOli ('Tin Can Island': Tonga), butthe best analogues arc on Mars.

Is is not clear what factors arc responsible forthe marked differences in topography betweenthe Hawaiian and Galap'lgos shields. One widelyhcld hypothesis is that their internal architectureis dilTerent, with ring dykes in the Galapagos andrectilinear dykes in the Hawaiian shields. Insome ways, the Galapagos shields resembleovergrown seamount volcanoes.

16.3.3 lee/aI/die shields

Iceland's shield volcanoes arc modest in size. butelegantly symmetrical. Twenty have been con­structed in post-glacial times. They arc topo­graphically subdued and arc usually only a fewhundred metres high. Some have slopes of aslittle as 1°, renecting the low viscosities of thebasaltic lavas involved. They rescmble the smallHawaiian lava shields such as Mauna Ulu.Skjalbreidur. the classic Icelandic shield, hasuniform slopes of 7_8°, is 600 III high, and has adiameter of about 10 kill. 1ts total volume is onlyabout 15 cubic kilometres, a mcrc pimple com­pared with Mauna Loa. Georgc Walker hassuggested that the Icelandic shields were buill upquickly, by almost continuous cruplion of thin­pahoehoe basaltic lavas irom the central vent.Some of these fluid flows wcre :lble to lravcllongdistances over gentle slopes. A flow from Trolla­dyngja may have tr'lvellcd more th'lIl 100 kmover a 10 slope.

Small, flat lava shields of Icelandic type arccharacteristic of many fluid bas'lll provinces,such as those of the Snakc River 1'laill. 1O Theyprovide important clues to the morphologies to

be expected in the source regions of extraterres­trial basaltic volcanism, for example on thesmooth plains of Mars (Section 18.5.3).

Volcanoes as landscape forms

- 16.4 Volcanic landforms resulting from erosion

351

16.4,7 Singes il/ Ole erosio/l of Wiles

The topography that il volcanic cone displays asil succumbs to the depredations of erosiondepends on the climate. and what it is made of.Scoria cones arc esscntially heaps of looselypacked, porous pyroc1asls, and therefore theyabsorb water like sponges. Even under condi­tions of heavy tropical rainfall. water soaksimmediately into a scoria cone rather thanrunning olT. gi\ ing erosion little chance of takinghold. Scoria cones thus remain rccognizitble formillennia. In cones which contain a high propor­tiOll of welded spatter, and in t ulT cones where thetephra arc fine grained or muddy, porosity isllluch less, and runolf is enhancL-d. Once runolTgcts under way, it radically resh:lpcs the volcano.On a symmctrical cone, the first stage in thisprocess is the developmcnt of parasol ribbilly:evenly spaced V-shapcd radial gullies scpar;l\edby ridges (Fig. 16.20).

It is unusual for parasol ribbing to remainintact for long. For one reason or another.usually reflecting subtle variations in originaltopography. 'master' gullies begin to prevail.capturing the heildwaters of lesser gullies. andcutting rapidly downwards into the hear! of thevolcano. Youthful volcanic materials arc oftenpoorly consolidated, and so th is process can t;1 keplace amazingly rapidly. especially in tropicalconditions. After the great K rakatau eruption of1883.thc Dutch geologist Verbeck reported thatonly two months after the eruption, gullies 40me/res deep had been cut into the pyroclasticdeposits. Even in the more temperate conditionsof the US". deep gullies had been cut in the M t.

St Helens debris avalanche deposit within a fewweeks of the eruption of May 1980, Within a fewyears, gullies tens of metres deep in placespresented formidable obswcles to movement.

When two or three master gullies arc active ona coniCal volcano, widening and deepening

Fi~. 16.20 Parasol ribbing on sUrlsc)'an luff·ring. west of Lake 'Abhe, Ethiopia. Photo: 1-1. Ta7.icff,

358 Volcanoes: a plancl'ary perspective

themselves, there will inevitably come a pointwhen the heads of two gullies intersect. Thisisolatcs a triangular, llat·surfaccd facet of theoriginal cone. known as a p/aue:e. a termoriginally :ldopted by the French geographer E.de Martonne (Fig. 16.21). Their distinctivetriangular shapes also earned plalleze,~ the morecolloquial term 'lla t-irons', As erosion cOJltinues,I"alle:es get whittled away.l:lvas on the llanks oflhe \'olcano become progressively degraded, andils summil Icvel is reduced. Ultimately, all that isleft of a volcano is a gently rounded hill. Table16.1 shows onc possible sequence of stages in theerosional history of a cone.

In regions wilh different climatic regimes, ilwill take dille rent periods of time for a volcano topass through thesc sllccessivc stages. In thehyper-arid conditions of the Cetl\ral Andes itlllay lake several million years to reach stage 3: intropical areas such as Indoncsia, it may take onlytwenty thousand years. 11

Necks lIlItl dykes Even after a volcano has beendeeply dissected. anatomically distinct land­forms may remain. Feeder vents through whichbva reached small composite volcanoes arc oftenpreserved long arter lhe rest of the volcano hasdisappeared, surviving as massive pill.lrs of rockmore resistant La erosion than the lavas andpyroclastic rocks of the cone. Travel postersoften feature the oddly situated churches in theAuvergne area of France, which are perched onvolcanic necks or purs, protruding above thecountryside as sleep, craggy eminences. Recogni-

i

I I

Fi:;. 16.21 Form:lIion of plalle:('s or 'fbI-irons' on a\·01c.1nic cone subjected 10 gullying erosion.

tion Ihal pll),S such as the Rocher Saint Micheland the rocks surrounding them were of volcanicorigin played an important role ill the history ofgeology <lnd science as a whole. As ArchibaldGeikie put it in 1897:

To France. which has led Ihe way in so mall)'departmenls of human enquiry. belongs Ihe merit ofha\'ing laid Ihe foundations ofthc systematic studies ofancient volcanoes. Her group of Puys furnished the

Table 16.1 Stages in the erosional history of a volcanic cone

Stage MOI'phologieal forms

2

J

4

5

Fresh, young cones, pristine lava flows and summit cralers. No glacialmorames

Small gullil."S on flanks: lavas and summit crater discernible but degraded:cone still sharp, moraines present in glaciated a~s.

Indh'iduall:I\'a flows barely visible; no crater; well-established gullies:constructional surfaces dwindling: pltme:es initiated.

No 13\'3s visible; dl-.::pl)' incised gullies: pllllre:es, but original surfaces left.Considerable relief. Major U·shapcd valleys in glaciated areas

]J;trdy recognizable: low relief; radial symmetry main clue to volcaniconglll.

Volcanoes as landscapc forms 359

earliest inspiration in this subject. and havc cvcr sinccbeen c1a5~;e ground to which the geological pilgrim hilSmade his way from all parts of the world.

Nicolas Desmarest (1725-1 SIS) and GeorgePoulelt Serope (1797-1876) were able to demon­strate that several episodes of eruption anderosion had taken plaee amongst the volc."lnoesof the Auvergne. Scrope used these observations,seemingly trivial to us today. to emphasizc thecontinuity of geological processes. thus helpingto overturn thc prevailing 'catastrophisr view ofthe history of the Earth. Influenced by religiousbelief in thc Creation. and particularly in thegreat Deluge. catastrophists had explainedmountain belts and the gorges that incise them asthe products of single-short lived evcnts. 11

In Ilrit •• in. Arthur's Seal. a famous Scotlishlandmark which looms over the city of Edin­burgh, was the site of a Carboniferous volcano.now exhumed. Eroded necks and vents form thchighest points. while on the n'll1ks outlines ofsome of the original lava flows Can still be pickedoul. In the United States. Ship Rock in cwMexico jags up in an astonishing pinnacle 430metres above the desert surface. Dykes radiatingout from the centrc arc beautifully displayed.standing up swrkly likc walls. Ship Rock owes itssoaring. perpendicular architecture to a series of\'crticaljoints in the breccia-filled volcanic ncck.a fcature common to other necks around thcworld. Interesti ngly. lhe finesl necks and puys allseem to have been formed from .~mall, probablymonogenetic volcanoes. When large compositevolcanoes arc eroded, compar<lblc necks are notrevea lcd, prob:l bl y because Iheir cores a re deeplyaltered and decayed by hydrothermal activity.

In New Mexico. lhe dykes around Ship Rocksland up as walls in lhe desert because they 'Iremore resistant lhan the rocks around them. (InEnglish, lhe word dyke ilself originally meantwall; dykcs in Holland arc walls to keep the waterout,) When dykes arc less resistant th'lll the rocksthey cut, the opposite happens. In the north-westHighlands of Scotland. ancient gneisses morethan 1700 million years old arc cut bya swann ofdykcs about 56 millioll years old. associated withthe Tertiary volcanic centres on Skye .llld Mull.On land. mallY of these dykes ha ve been erodcd

into parallel-sidcd depressions; wherc they cutthe coast line. long, dccp grooves result. Thcse a resuch COlllmon features of the coastline that theyhave acquired their own name in the local Gaeliclanguage: sloe....

Dykes arc often thought of as rather minorigneous phenomena. Most arc indeed r.lIhersmall, less than a metre wide, There arc manyhuge exceptions. though, In the Terti:lry dykesW:lrm of north-west Britain. t he Cleveland dyke(about 100 metres widc) can be traced disconti­nuously from its source in the Hebrides to thcNorth Sea coast of Yorkshire. a distancc ofalmost 400 km. Calculalions of ratcs of injectionand cooling suggest that Ihe dyke must havc beenil1lrudcd very quickly. zipping across lorthernEngland in less than 5 days. The Cleveland dykeis only one of a swarm. all of broadly the s.1meage. which formed in response to rifting movc­ments rclated to the opening of the NorthAtlantic Ocean.

16.4.2 Erosiotl of laun flows

Lava flows havc superlatively rough surfaccs. sothcy arc natural traps for wind-blown dust. Theirglassy outer skins also break down mpidly whenexposed to watcr and air, Thus, soils quicklyform in crevices. allowing plants to colonize theflow. Obviollsly. the rate of this process dependscritically on local climate, so it may varydramatically even on a single volcano. New lavaflows on the wct, trade-wi nd side of t he HawaiianIslands arc overgrown by rain forests in only afew decades. whereas those on the parchedleeward side remain prominent for centuries. Inregions where higher forms of plant life fail tothri ve, lichens colon ize the su rface of flows. Someof the lichens on the lavas or lhe Cr<lters of theMoon laV3 field, Idaho. arc so exuberantlycolourful that they almost distntct attenlion fromthe J<lvas-almosl. Even in the most arid areas.aceum ula lion of wind-blown dust su btly changessurface features of lavas. producing a generallightcning in albedo. significant in rcmote-sensedimages which 'sec' homogenized pixels coveringtens of square metres.

Basalt plate:lux the world ovcr arc composedof lavas of similar physical properties. Thus. thestepped topography of basaltic Iral'S is similar

Volcanoes: a planct<ny perspective360

the world over (Section 2.5.2). whether in Ihedismal, rain-sodden moors of1he Hebrides or Ihegrandeur of the Drakcnsbcrg Escarpment.Ikcausc their lavas and the scoriaccolls horizonsbelween thelll arc so porous. surface drainagedocs not easily become established on lavaplateaux. Waler seeps instead downwardsthrough the nows. mo\·jng laterally when itrcaches impcrmcabk. clay-rich horizons.

In the Snake Ri\cr I'Jain (Idaho). the SnakeRivcr itself rccci, cs no ael ualtributarics from theexpansive lava plain it euls through. but its flowis sUbsl:ullially increased by springs soaking outfrom the lava and scoria horizons in the ~nyon.

A consequence of this is that erosion of the basaltplain takes place by :;(lI'P;lIg. rathcr than streamflow. producing deep. steep. dead-end canyons.terminated by olcoves or lllllpllitileatre:;". Head­ward erosion of these alcovcs takes place assprings seeping out from the lavas undermine orS<lp the cliff line. so tlwt it e\'entually collapses.Removal of this talus is C<luS(.x! by chemicaldisintcgration and solution of the basalt bywater. and partly by mechanical transport whenthe now ratc is grcat cnough. A typical example isthe Blue Lakcs alcovc. which heads a shortcanyon tlm.'C kilomctres long and 100 metresdccp. Some Hawaiian shield volcanoes displaysimilar alcovc-headed vallcys, probably aboformed by sapping.

Spring sapping of lava platcaux may secm tobe of less than riveting interest to volcanologists.It descrvcs our allention beeausc similar pro­cesses milY havc playcd a key role in shapinghuge areas of lhe su rface of Mars. Areas of basaltlavas such as Ihe Snake River Plain provideexeellcnt opportunitics 10 explore these.!.l

CO!III11I1(/l'joilllil1q Basalt lavas like thosc in lheSnake Rivcr Plain oncn exhibit spectacularcolumnar jointing in clifTs ;tnd c:Jl1yons. Two ofBritain's best-known l;tndmarks. lhe Giant'sCause",:ly in Antrim ;lnd Fingal's Cave on theisland ofSwfTa, originated when the Brito-Arcticor Thuleiln b;.salt province was formed (Fig.16.22). Doth arc famous for the elegance of thethousands of polygon:ll columns that stand outin cliffs and on shore. In California, the Dcvil'sPost Pile is another popular natural landmark,

Fi~. 16.22 I)art of the Giant's Causeway. Antrim.Northern Ireland. where ;L thick columnar jointedbas:.1t Ilow is exposed on Ih;: sea t;oasi.

high in the rOTests of the Sicrra Ncvada. From adistancc, the columns in these eroded lavasresemble the pipes of a great cathedral organ.

Most lava columns arc hcxagonal, thoughfour-, /lve-, sevcn, and eighl-sidcd examples alsooccur. Each column is lerminated by either asmooth concave or a convex surface. Thesesurfaces arc joint plancs, sometimcs called hal/­alld-socket join IS, which divide the columns upinto innumerable stacked tablets, like piles ofpoker chips (Fig. 16.23). Both the columnsthemselves and the curved joinl SUd,lceS :lrccaused by contraction and fracturing in tilecooling flow. In detail. a flow may have one ormore sets of regular columns. separated by anirregularly fractured layer. The regular colulllnsconstitute a colOl/nade. and the irregular layer anemablatllre. 14 Columnar jointing is a consistentfeature of some large lava flows. and can be

•

Volcanocs as landscape forms 361

Fig. 16.23 Smoothl)· cun-oxl joints form the surfacesof the columns or the Gi:ml·S C:luscY. ay. Conc:I\'eand com'ex surfaces :lppear equally abundant.

tr:lced for mallY kilometres in the 110\\ls of theColumbia River Plateau.

IIwertetl relief Obedient to the laws of physics.lavas flow down valleys. sometimes filling themcompletely. Distinctive landforms result if thelavas arc more resistant to erosion than theunderlying rocks in which the valleys were cut, asoften happens when lavas flow over sedimentarystrata. The v:llley-filling lava forms a thick.resistant mass. while the sediments on either sidearc more rapidly removed. In the fullness of time.the original valley will be expressed as a resistantridge of lava stllnding above the surroundingsediments, in which new valleys have been cut. InIlritain, a beloved example of i,n'ersioll of,.e/iefisfound on the obscure Hebridean island of Eigg.where a Tertiary rhyolite lava which filled anolder valley now forms a 400 metre high, fivekilometer long ridge. This ridge. the famous'Scuir of Eigg' dominates the tiny island. other­wise renowned only because its nearest neigh­bour is Muck. (Fig. 16.24). Inversion of relief canbe seen on many volcanic cones. where flowswhich originally Ilowed in gullies cooled to formthick. confined wedges. After erosion, the lavasend up as resistant caps to spurs and plane;es onthe dissected volcano.

16.4.3 Erosioll of pyroclastic deposits

In the short term. the topographical effeets of aheavy ash fall arc straightforward: tephra blan­kels the pre-cxisling surface. smoothing and

,, ' , .f t.•

Fig. 16.24 On thc small Hcbridcan island of Eigg.Scotland. a rhyolite la\'a flow which original!) filleda ri\'cr valley cut in older b:ll>:llt la\as is nov.presen'cd as the Scuir of Eigg. an imposing cr:lg. Itis a d:lssic example of volcanic in\'crsion of relief.From an 1897 skctch by Archibald Geikic.

subduing the previous humps and hollows.Where large-volume pyroclastic flows arc con·cerned. involving thousands of cubic kilomelresof magm:t. lhe pre-exislillg topography may becompleldy buried, leaving :111 unbroken plate:tUmany thousands of square kilometres in ex lent.This situation docs not last long. Unweldedpyroclastic deposits are soft and friable. and sothey arc rapidly eroded. Deep gullies arc cuI at:lIlastonishing ralC as soon as the first rains fall.Within a few years. :1 dislinctive lopogmphyemerges. Flat-lopped relics of the original pla­teau remain. separated from one another by adendrilic maze of narrow. vertical-sided gullies.(Wadis. canyons. quebradas. gulches. or gorges.depending on local idiom.) These gullies arcoften so deep and steep thai they arc mere slits.less than a metre wide and ten or 1110re deep. soconstricted that they arc dillicull to walk along{Fig. 16,25).

Intensive gullying or this kind is common insoft rocks ofall kinds. nOI only tephra. It happensdistressingly quickly as a result of overgra7jng orclearing of timber in areas subject to occasionalheavy rainstorms. Development of extremelysteep (vertical) sides of gullies cuI in pyroclas1icrocks is an expression of the fact that lhey arcincoherent. structurcless deposits. Unlike sedi­mentary deposits. unweldcd pyrocl:lstic rocksoften lack layering. and because t hey arc unlil hi­lled, cannot form boulders. All they can do is

362 Vokanocs: a planetary perspective

Fig. 16.25 An eruption about 3000 years ago formed the Dcriba Caldera. I):.ar(ur Pro\'jncc.....'estern Sudanand ilS apron of pyroclastic fall and flow deposits. Erosion of the soft pyroclastic deposits has yielded adendritic maze of Sleep. deep gullies.

form vertical clilTs, with scree slopes of loosepumice beneath them. If the deposit is welded orshows vapour phase alteration {Section 10.5,1),lhel1 boulders form, allowing a talus slope todevelop al the foot of the cliff.

Where welding or vapour.phase alteration arcpresent, cooling joints and fractures also de­"clop; usually. but not always. perpendicular toIhe original surface. On erosion. elegant sets ofcolumnar joints are sometimes exposed: not ascommonly as in basalt lava plateaux, but morepleasing because the rocks are a warm, reddishochre colour, rather than drab basalt bbck (Fig.14.17). Vert iC:lI jointing accentuates the verticalcliffs developed in pyroclastic rocks. Where

ignimbrite plateaux are eroded in arid env!n­ments, box canyons are common. As the edge ofthe plateau reccdes.l1at-topped islands ofignim·brite arc left, lheir walls rising as steep, high. andforbidding as castle ramparts above talus slopesof fallen boulders. These mesas and billies makeappropriate backgrounds to Western movies.

Jointing in ignimbrites is often so perfettlydeveloped that the fracture surfaces look and feelartificially clean. They are so smooth that theyprovide irresistible temptations for people toexpress themselves in yrafilli. In both north andsouth America, prehistoric Native Americansused ignimbrite joint surfaces to draw pctro­glyphs showing animals that Ihey hunted, and

VolcoJnocs as landscape forms

-363

absiraci mystical symbols (Fig. 16.26). Twen­tieth-century Americans usc 1he same surfaces formore rudimentary gralnti.

l'{mhmys In 1he Cenl ral Andcs. ignimbritcs arcexposed over huge areas at high altitudes.Prccipil;l1ion is slight in the region. taking placemainly inlhc fOrtn of snow which ablates ratherthan melling. Surface runoff is Ihcrcforc minimal.so erosiOJl by nowing water is inconsequential.By contrast. fierce winds blow from the north­west for much of the year. As a consequence.much erosion takes place through aeolian pro­cesses. producing a wi nd·scu!ptcd topography ofyardaugs and dcnationary hollows. }'lmllmy.~arclong. wind-eroded ridges. somewhat resemblingupturned canoes. their prows facing the prevail­ing wind (Fi~s. 16.27-16.28). Andean ignimbrites

Fig. 16.26 Pre·Columbian ]>Ctroglyphs on smoothjoinl surract.'li on the 9-million-}"ear-old Sifonignimbrite. Rio Loa valley. north Chile. Llamas andgeometric:ll symbols arc abundant.

,

--'

Fig. 16.27 SPOT s..1tellite image of j'llrJOIIgs

developed in llle 4, r-million-year-old Alan:lignimbrite on the frontier bctWl-Cli Chile andArgentina. )'(m/(mgs arc pointing into lh.: prevailingnorth-westerly wind. Image is :lbout 4 km across.Courll-sy of CNES.

arc silicic. and so they contain generous quanti­ties of quartz phenocrysts. On erosion, lheytherefore liberate the agents of their own destruc­tion: quartz grains freed from their matrix andpicked up by the wind rapidly abrade theremaining rock. Absolute rates of erosion h3venever been determined. but the ·half·life· of anignimbrite subject to aeolian erosion in thecentral Andes may be about a million years: aftera million years, only half the initial volumercmaIllS.

Why worry about aeolian erosion rates ofignimbrites in somc obscure part of SouthAmerica? One reason is that some planetaryscientists believe that huge areas of Mars arccovered by ignimbrites. There is no doubt thatthere arc enonnous areas of superb rOrt/lIIlgs onMars. But how did these Jtardallgs form? Andwhat are they eroded in? Are they ignimbrites. oran older aeolian deposit? To help ,l11swer thesequestions. terrestrial j"ardollgs must be betterunderstood. The high. dry. cold Andean plateauprovides an exccllent tcrrestrial counterpart tothe evcn drier and colder Martian deserts. sofunher studies of Andean j'llr(/al1{Js may throwsome light on an important aspect of Martian

Wiy,mms lIIul fem rocks From C.tpp;ldocia inTurkey to Los Alamos in New Mexico. erosion ofpyroclastic rocks consistcntly yields wigwams ortent rocks: conical pinnaclcs or spires of dazzlingwhite rock thatray be tens of mctres ill height

..

(Fig. 16.29). These can form in two ways. When apyroclastic plateau is dissected by dendriticdrainage, two branches of the complex systemcOlllmonly intersect each 01 her, isola ling a blockof the deposit which is then trimmed into apinnacle. More often. wigw:lms are formed likeearth pillars: where the edge of a pl:lteau is beingeroded away, resistalll blocks of lava or otherrocks within or on lap of the deposit (pcrh:lps lagbreecias) prolec-t the underlying deposit fromerosion. while the surrounding material israpidly carved :lway. For a while, the protecting

=

Volc<1l10<'s; a plaJletary perspective

geology. One intriguing problem is this: ifignimbrites exist on Mars. they are morc likely tobe of mafic composition than silicic. If they arcmafic, they will lack quartz. What, then, providesthe abrasive material to help the M:lrtian windsculpt such outstanding ym'dmlgs'!

Fi~. 16.28 Prow of a rlirt/lmB50 km south of those in Fig.16.27. Aeolian eros;ol1 is mostintense within about one llletreof the ground; thus theignimbrite is mpidly undercut.and boulders faJlto Ihe valley"oor. "here Ihey arc rapidlyabraded awa}'.

ril.:. 16.29 Tent rocks formedby erosion of the BandelierTufT (ignimbrite) on the nanksof the Valles caldera. NewMexico.

364

Volcanoes as landscare forms 365

boulder remains perched precariously on top ofthe pinnacle. but eventually it topples, leavingonly the pinnacle itselr. As in other pyroclasticerosional phenomena, formation of wigwamsdepends on the deposit being homogeneous,lacking marked horizontal or verlical variationsill strenglh.

Fumarole mOil/ills and rjd{Je.~ Ignimbrites arehot when first deposited. maybe even close tom;lgrnatic temperatures. A consequence Mthis isthaI aner they arc emplaced, they sit and stew intheir own magmatic volatiles. and in any steamthat may be liberated from underlying ground­water. Apart from causing generalized vapour­phase alteration. this process may ca usc localizedell'ccts which have striking morphological conse­quences on erosion. Where the ignimbrite isbroken up by \'erliealjoinls. hot nuids moving upthe joints cause alteration and deposition of silicaalong the joints. This may a!Tect lhe ignimbritcaround thejoinl for a few centimetres or as muchas a metre. resulting in formation of (II'mol/fell

joims. On erosion. armoured joints. origilwlIyfractures in thc rock. stand lip as resistant walls.often sevcral metres high. In pl;lccs in Chile, thesewalls arc so steep and thejoinling so regular thatfrom a distance the weathered ignimbrite lookslike a ruincd city. Where hot Iluids escape to thesurface via pipes ralher than joints. armouredcflim/lep are produced.

Where more extensive ignimbrite ground­water reactions took place, steam blasts maybreak through to the surface of the ignimbrite,producing fumarole vents similar to those in theValley ofTen Thousand Smokes. On erosion. themost common expression of these fossil fumar­oles is in myriads of shallow depressions or pils,better seen on iterial photographs than on theground. but in some circumstances, swellingmounds of blisters arc exposed. These positiverelief features arc probably the result of erosionfCvealing differences in hitrdness around thefumarole at dep1h within the ignimbritc, and arcnot original surface reaturcs lS (Fig. 16.30).

rig. 16.30 'Fumarole' moundsand ridges dc\'clop.'d on IheSUrf;ICC of thc CarCOlcIgnimbritc, Rio Loa valley,north Chile. Rq;ul;lralignmcnt of Ihc ridgcssuggcsls Ihal Ihcy \\credC\'clopcd abovc joints in themain body of the ignimbritc.

- 16.5 Topographical by-products

So fa r. we ha ve considered volcanoes in isolation.BUI voleanoes can also cause drast ic topographi­cal changes to their surrounding landscape,

usually by interfering with local drainage. On :l

small scale, lava flows commonly block valleys,impounding lakes behind a lava dam. This can

366 Volcanoes: a planetary perspective

have senous consequences if the dam shouldsuddenly fail. releasing large volumes of waler.An instructive example is Sab,lncaya vOIe'lIlO inPeru, Lavas from the north flank of this volcanoonce dammed the mighty Majes C:ll1yon. morethall twO thousand mclres deep, and one of thegrandest in the world. A substantial lake mustha\'c existed for a time. because nat-lying sedi­melllS deposited in the lake arc preserved behindthe dam, which :llso forms a prominent nick­point in the canyon profile. AI some unknowntime, the dam was breached: there is no lake atthe present day.

Also in the Andes. a large debris avalanchefrom !}arinacola volcano 13 500 years agoblocked the existing drainage to the Pacific,resulting in the formation of Lake Chungara.10 km across. which at 4550 m is the highest lakeof respL"'Ctable dimensions outside Tibet (Fig.\6.\4).

011 a larger scale, Lake Van (70 km across) illArmeni:t is said to have been ponded by lavasfrom Nemrut volcano. And on an even largerscale in central Africa, Lake Kivu (looded part ofthe Western Rift valley when it was empoundcdby construction of the volcanocs of the Virunga10untains about five million years ago. For­

merly, drainage was northwards to join the Nilevia Lake Albert, When the volcanoes intcrposedthemsclves. thwarted rivers initially drained intoand filled Lake Kivu. which cventually ovcr­(lowed. draining southwards at the southern endof the rift via the Ruzizi River into LakeTanganyika. Lake Tanganyika itsclfis connectedvia the Lukuga Rivcr to the great Congo (Za'ire)River. Thus, the Kivu drainage was switchedfrom the ile and the Mediterranean \0 theCongo and Atlantic l6 (Fig. 16.31),

Any map of rivers around the Rift is. of course.of only ephemeral value. When Haroun TazielT

N l'driltNilt N

o 100 '200 300 «JJ "'". . . . o 100 200 300 400 "'".. ..

UlkeKiVIJ

Virul\g;lMOl,lnl~;n~ "'~

f./lk,. Albert

l.ilkeVictorill

l.ilkl!Edward

I)rain;lgefroml.a ko::

l:,nganyika

LIl~'e

Tll1l8a1ryika

l.ilkcTIlrr81lrryika

(a) (b\

Fig, 16.31 (a) Sketch of the Nile drainage through the Western Rift before eruptions which built the VirungaMountains. I>onding Lake Kivu. At that time, drainag.e from L:tke Tanganyika nowed northwards via theNile into the Mediterrane:.n. (b) After formation of the Virung:t Mountains. Lake Ki\·u formed, the RuziziRi\'cr drained southwards. and Lake Tanganyika drained via the Lukuga into the Congo and the Atlantic.

Volcanoes as landscape forms 367

visited Lake Kivu in 1948, lava from the Kiturovolcano was streaming steadily into the lake,reshaping its northern shores, and providing abonanza to local boatmen in the form of shoals ofparboiled fish. Future eruptions along the riftwill eventually reshape drain:lge patterns onceagaill.

On the very largest st'ale, the topographical

- Notes

I. Calion, C. A. (1944). Volctmoes as landscapefM/IIS. Whitcombc and Tombs, Christchurch,New :Ualand. 415 pp.

2. Wood. C. A. (1980). Morphomctric cvolution ofscori:l concs. J. VolcmllJl. Ceollwrm. Res. 7,387--413.

3. Wohlctz. K. Ii. and Sheridan. M. F. (1983).Hydro\'olcanic explosions II. Evolution of basal­tic tufT rings and tufT cones. Alii. J. Sci. 2.83,385--4ll

4. Lorenz, V. (1986). On the growth of maars anddiatremes and its relevance to the formation of tufTrings. BII/I. Volc(JtlQl. 48. 265-74.

5. Jones. J. G. (1969). Intraglacial vole-.lIloes of theLaurgavatn region. southwest Iceland. Q. J. Ceo/.Soc. UJIlu. 124. 197-211.

6. Milne, J. (1878). On the form of \·olcanoes. Ceo/.Mag. 5, 337-45.

7. lkcker, G. F. (1885). The geometrical fonn of\'olcanic cones. Am. J. Sci. 30, 283..JJ3.

8. Moore, J. G. (1987). Subsidence of the Hawaiian

swell above a mantle hot-spot may inlluencetopography over a region thousands of kilo­metres in extent. Radial drainage patterns incisedas a result of hOI-spot uplifts have been mapped011 several continents. If we were to include thetopography of the se.:l-lloors, we could concludeth.lt most all the world's landscape is of volcanicongm.

Ridge. In Volcanism in Hawaii. US Geol. Surv.Prof. Pap. 1350, pp. 85-100.

9. Martin del 1)07.7.0, A. L. (1982). MonogenelicVUICilllism in Sierra Chiehinautzin, Mexico. /JIII/.VOICUIlO/, 45, 9-24.

10. Greeley. R. (1977), Volcanism ofthe EaSlern SlwkeRit:er Plai/!. /Jallo. NASA CR 154621. 308 pp.

II. Oilier. C. D. Volcanoes. Australian N:uionalUniversity Press, Canberra, 177 pp.

12. Scrape, G. P. (1858). Thc geology of the extinctvolcanol:s of Central France. John Murray. Lon­don.

13. Pieri, D. (1980). Martian valley: morphology,distribution. age and origin. Sciellce 210, 895-7.

14. TomkiefT, S. T. (1940). Basalt lavas of lhe Giant'sCauseway. !JIIll. Volc. 6, 89-143.

15. Sheridan, M. F. (1970). Fumarolic mounds andridges of the Bishop Tuff, California. Gool. Soc.Amer. Bull. 81. 851--68.

16. King. L. C. (1942). SOli/I! Africa" Scenery,pp. 153-4.