MODULE 2832 (B) THE ROCK CYCLE – PROCESSES AND PRODUCTS REVISION NOTES

2.1 The Rock Cycle

Candidates should be able to:

(a) Describe the rock cycle and define the processes which operate within it: (i) at the surface – weathering, erosion, transport, deposition and extrusion; (b) below the surface – burial, diagenesis, recrystallisation, metamorphism, partial melting, magma accumulation, crystallisation, intrusion and uplift.

You must be able to draw a fully labelled/annotated diagram showing the processes of the rock cycle. Weathering is the “in situ” breakdown of rocks at the Earth’s surface, doesn’t involve

transport. Erosion is the gradual wearing away of material at the Earth’s surface as it undergoes

transport. Transport involves sediment being moved from one place to another. The agents of

transport are wind, water, ice and gravity. Deposition occurs due to physical (commonly loss of energy) or chemical changes and

results in material being laid down on the Earth’s surface. One sedimentary bed is the result of one period of deposition.

Burial occurs as sediment is covered over as more layers of sediment accumulate on top (the overburden).

Lithification - the process of converting loose sediment into a sedimentary rock. Diagenesis - all the physical (mainly compaction) and chemical (mainly cementation)

processes that act on sediment during lithification. Extrusion occurs when magma is erupted as lavas/pyroclastics onto the Earth’s surface.

This forms a volcanic/extrusive rock. Intrusion occurs when magma cools and crystallises inside the Earth’s crust. This forms a

plutonic (deep-seated) or hypabyssal (shallow level/near surface) intrusive rock. Partial Melting is the process by which magma forms in the Earth’s mantle – different

minerals have different melting points so only part of the rocky/silicate mantle melts to form magma.

Magma accumulation is the process by which the magma collects within an underground magma chamber.

Crystallisation occurs when molten magma cools and solidifies into individual crystals/minerals.

Metamorphism is the process by which rocks in the Earth’s crust are changed by the effects of heat and pressure. The process is isochemical and involves solid state recrystallisation.

Recrystallisation occurs during metamorphism – it is a solid-state process by which existing minerals are changed/recrystallised into new crystalline metamorphic minerals.

Uplift is the process by which deeply buried rocks are returned to the Earth’s surface by tectonic forces.

The rates of geological processes are variable. Some are very slow and take tens of thousands to millions of years to complete, e.g. crystallisation of an igneous intrusion, metamorphism. Others may be very rapid, e.g. a volcanic eruption, earthquake, flash flood.

As the Earth is 4.5 billion years old, some rocks have been round the rock cycle several times.

The Principle of Uniformitarianism states “The present is the key to the past” and our assumption is that the same geological processes operated in the past, at the same rates, as they do today.

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(b) Describe the broad classification of rocks into igneous, sedimentary and metamorphic classes and their relationship to temperatures and pressures in the rock cycle.

Rocks are classified according to the way they form, i.e. a genetic classification. There are 3 rock groups: Sedimentary rocks - These are rocks formed at the Earth’s surface. Rocks at the Earth’s

surface are broken down by weathering and erosion. The broken down material is transported. When there is a physical/chemical change (e.g. a reduction in energy) the particles of rock are deposited. The particles are then buried and undergo lithification and diagenesis to make a clastic sedimentary rock, e.g. sandstone. Form at low temperatures and pressures.

Igneous rocks - Form from cooling and crystallisation of molten magma. There are two main types of igneous rocks: Intrusive (hypabyssal or plutonic) igneous rocks - these have a medium-coarse crystal size because the magma cooled slowly inside the Earth’s crust, e.g. granite; and Extrusive (volcanic) igneous rocks - these have a fine crystal size because the magma reached the Earth’s surface and cooled quickly, e.g. basalt. Form at high temperatures and pressures.

Metamorphic rocks - These are formed when rocks are subjected to heat and pressure (metamorphism) in the Earth’s crust. This causes their structure to change as minerals in the rock recrystallise in the solid state to form new minerals. There are two main types of metamorphism - Contact and Regional. Regional metamorphic rocks contain a foliation due to directed pressure, e.g. slate. Form at temperatures > 200°C and < 800°C and low to high pressures.

(c) Distinguish between the broad groups of igneous, sedimentary and metamorphic rocks using characteristic features including mineral composition and textures as seen in simplified drawings / photographs taken from thin sections (microscope work is not expected).

Igneous : Crystalline; Equigranular, porphyritic, vesicular, amygdaloidal, flow banded, glassy textures; Hard silicate minerals - quartz + feldspars + mafic minerals; May show cross-cutting relationships if intrusive; May have chilled margins and contain xenoliths; May be fragmental if pyroclastic.

Sedimentary : Grains (except evaporites); Bedding; Sedimentary structures – cross bedding, ripple marks, desiccation cracks, graded bedding, sole structures; May contain fossils; Matrix or cement; Quartz + mica + calcite + clay minerals + evaporites (some are soft).

Metamorphic : Crystalline; Porphyroblastic, granoblastic textures; Types of foliation – slaty cleavage, schistosity, gneissose banding; Mainly hard silicate minerals - quartz + mica + garnet + Al2SiO5 polymorphs + calcite; May contain relict structures and deformed fossils; May form a metamorphic aureole/baked margin around an igneous intrusion.

(d) Describe the division of the geological column into eras and systems.

The Geological Column is the time framework used by Geologists. Geological time is divided into Eras and Periods/Systems.

Eras are the major subdivisions. The four Eras are: the Precambrian 4600-590 Ma; Palaeozoic (old life) 590-235 Ma; Mesozoic (middle life) 235-65 Ma; and Cenozoic (new life) 65 Ma - present.

The Eras are subdivided into Periods and the rocks laid down in a particular Period are known as a System.

The Periods/Systems are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary and Quaternary/Recent.

Angular unconformity – a plane surface representing a break in time when no deposition occurred and erosion may have occurred. The rocks below may be much older than the rocks above the unconformity. Beds below and above the unconformity may be very different (different environments of deposition) and will dip at different angles.

Sequence of formation: 1. Older sediments deposited horizontally; 2. Rocks uplifted, folded or tilted; 3. Erosion occurs → unconformity; 4. Younger sediments deposited.

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ERA PERIOD/SYSTEM TIME (Ma)Cenozoic Quaternary 2.5-0

Tertiary 65-2.5 Mesozoic

Cretaceous 135-65Jurassic 195-135Triassic 235-195

 Palaeozoic

Permian 280-235Carboniferous 370-280Devonian 415-370Silurian 445-415Ordovician 515-445Cambrian 590-515

Precambian   4600-590

2.2 Sedimentary Processes and Products

Candidates should be able to:

(a) Describe and explain the weathering processes producing soluble products and insoluble residues by chemical (hydrolysis, hydration, oxidation, carbonation), mechanical (exfoliation and frost shattering) and biological processes.

Weathering is a form of low temperature differentiation producing soluble products (ions removed in solution) and insoluble residues (clay minerals or scree).

Mechanical/Physical weathering (freeze-thaw, insolation, stress release, salt crystallisation) - results in changes in grain size, produces angular fragments, composition is unchanged. Freeze-thaw – Water enters cracks, freezes and expands by ~9%, exerts pressure on

rock → eventual failure. Requires daily temperatures to fluctuate above and below 0ºC. Produces angular fragments called scree.

Insolation – Alternate heating and cooling, causes rock to expand and contract, causes stress → failure. Common in deserts.

Stress release (exfoliation) – Overlying rocks removed by weathering, rocks “spring up” as stress is released, causes joints/fractures (unloading joints) to open up parallel to surface and fragments to break off. “Onion skin weathering” – common in granite.

Salt crystallisation – Salt water enters cracks, water evaporates, salt crystals grow exerting pressure on rock.

Chemical weathering (carbonation, oxidation, hydrolysis, hydration) – results in changes in composition of the rock, soluble ions are removed in solution leaving an insoluble residue of clay minerals. Carbonation (solution) – CO2 dissolved in rainwater forms a weak acid, which slowly

dissolves susceptible rocks, e.g. limestone. Oxidation – Reaction with oxygen in air, iron-bearing minerals oxidise → red colour.

Indicates weathering in terrestrial (land), arid environment. Hydrolysis – Reaction with water, e.g. feldspar → clay minerals (very slow!!). Hydration – Addition of water, e.g. anhydrite CaSO4 → gypsum CaSO4.2H2O.

Biological weathering - Roots in cracks physically prise the rock apart; humic acid produced by plants attacks rocks chemically.

(b) Explain the influence of gravity, wind, ice, the sea and rivers on the parameters of grain size, shape, roundness and degree of sorting and methods of transport (solution, suspension, saltation and traction).

Grain size - measured on the Wentworth Scale – the average diameter of the grains in mm. This scale is also used to name clastic sedimentary rocks. As the range of grain sizes found in nature is large a logarithmic scale is more practical than a linear scale. However, as the Wentworth scale is rather cumbersome it can be converted into the phi scale.

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The Wentworth scale:

Diameter (mm)

Phi Ø Sediment Name Rock Name

> 2Boulders, Cobbles, Pebbles, Gravel

Conglomerate (rounded grains)Breccia (angular grains)

2 -1Sand – very coarse

1 0Sand – coarse

0.5 1Sand – medium Sandstone - grain size is

0.25 2 between 2 – 1/16 mm.Sand – fine

0.125 3Sand – very fine

0.0625 (1/16)

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> 0.0625 Silt, Clay Siltstone, Mudstone,Shale, Clay

Roundness – the degree to which the sharp corners have been knocked off, based on the curvature of the corners. Visual estimation - compared to a chart - Very angular; angular; sub-angular; sub-rounded; rounded; well rounded.

Sorting – a measure of the range of grain sizes in a sediment, the degree to which the grains are the same size. Either a visual estimation or a mathematical calculation. Very well sorted; well sorted; moderately sorted; poorly sorted; very poorly sorted.

During erosion and transport the rock particles are subjected to abrasion and attrition. This results in the particles becoming smaller, rounder and better sorted. In addition, softer minerals are gradually destroyed.

Gravity and ice – very little influence on sorting, grain size, etc., so usually texturally and compositionally immature, poorly sorted, angular fragments.

Wind and water (high energy) – more efficient at rounding and sorting fragments, so usually mature depending on distance from source.

You must be able to plot histograms and cumulative frequency curves, calculate the coefficient of sorting, and be able to plot rose diagrams.

Solution – soluble ions are dissolved in water, load in solution cannot be seen. Suspension – fine, clay-sized particles are carried in the water column, gives water a

murky appearance. Saltation – sand and gravel sized particles are carried along the bottom in a hopping,

bouncing motion. Traction – coarse boulders sliding and rolling along the bottom, they are too heavy to be

lifted by the water. Make sure you can label areas of erosion, transport and deposition and interpret a graph

of particle size against velocity of flow. Clay sized particles require high velocities for erosion because they are flat and platy in

shape and cohesive. Clay sized particles require very low velocities/still water for deposition because they are

small, light and bouyant.

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(c) Describe the deposition in hot desert environments of wadi conglomerates, dune sandstones and evaporites in playa lakes.

Wadi conglomerates – a dry valley liable to flash flooding during infrequent heavy rainstorms. No infiltration or interception of water → runs rapidly over ground. Produces an immature, poorly sorted, chaotic mixture of gravels/boulders → breccia/conglomerate.

Dune sandstones – result of wind transport – very high energy. There are 4 main types of dunes: barchan (crescent); longitudinal, transverse, stellate. Produces a texturally and compositionally mature desert/aeolian sandstone. Medium sand grains (0.5mm), well rounded, high sphericity, well sorted – “millet seed sand”. Frosted grains, 100% quartz, iron oxide coating. Contains large scale/dune cross-bedding (> 1m scale) formed by sediment moving up the windward slope (stoss) by saltation, collecting at the top in the lee and cascading/avalanching down the lee slope. Lee slope (max. 35º) is steeper than stoss slope. As migration occurs successive lee slopes are preserved → foreset beds.

Playa lake mudstone and evaporites – temporary (ephemeral) lakes formed after rain in the desert. As the lake evaporates → fine-grained mud/clay is deposited, this dries out → mud cracks, followed by crystallisation/precipitation of a sequence of evaporite salts. As the water evaporates, the salts become more concentrated. A predictable sequence of evaporites is produced on the basis of solubility – least soluble first, most soluble last. Calcite → (dolomite) → gypsum → (anhydrite) → halite → K-Mg salts.

(d) Describe the deposition in deltaic environments of delta top (topsets) to form coal, sandstones of the delta slope (foresets) and shales to form offshore deposition (bottomsets). Understand deltaic deposition in cyclothems.

A delta forms where a river meets the sea and deposits its load. Heavier, coarser material (sand and gravel) is deposited first, closest to the river mouth. Finer sediment is carried further out to sea, before settling from suspension to form the prodelta. As the coarse material accumulates it becomes unstable and slumps down → sloping delta front/slope. Meanwhile, the delta builds outwards and upwards (progrades). As the delta progrades, the delta front sediments cover the prodelta, producing a coarsening upwards sequence. If the delta top emerges from the sea it may be colonised by plants (swampy environment) → coal.

This produces a distinctive sedimentary sequence: limestone (shallow marine) → shale (prodelta) - bottomsets → sandstone (delta front) - foresets → seat earth (palaeo-soil) → coal (vegetation colonising delta top) - topsets.

Deltaic sedimentation occurs as repeated cycles due to: subsidence of the delta (local/isostatic change); or rises in sea level (global/eustatic change).

Each cycle is called a cyclothem.

(e) Describe the deposition of clastic material formed in sediment-rich shallow seas, to form conglomerates, sandstones and mudstones.

Clastic material is derived from weathering and erosion of land. Main input is from rivers. Sediment becomes finer grained as distance from land increases. Pebbles and sand –

deposited in high energy areas – beach/near shore; silts and muds – deposited in lower energy areas – offshore on continental shelf.

The material is transported by wave and tidal currents. Sedimentary structures include: ripple marks, small scale cross-bedding, burrows.

(f) Describe the deposition of limestones formed in shallow seas to form oolitic and bioclastic limestones.

Covers a wide range of environments – reefs, sand shoals/carbonate sand banks, lagoons, deep shelf. Biological and biochemical processes are largely responsible for the formation and deposition of limestones. They usually form in tropical seas within 30° of the Equator – requirements include warm (>25°C), shallow (<200m, but most form within 10m of surface), marine conditions, with normal salinity and a lack of clastic terrestrial sediment input. A wide range of limestones are formed.

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Oolitic limestones are characteristic of higher energy environments such as carbonate sand banks/barrier beaches and often have a carbonate cement. Ooliths/oolites form by evaporation of lime-saturated seawater and precipitation of concentric layers CaCO3 onto a nucleus (sand grain, shell fragment, etc.), they become perfectly spherical as they roll backwards and forwards in the wave zone.

Fossiliferous limestones (biogenic and bioclastic) form in reef environments – usually formed of coral and algae, but with other carbonate debris bound into the structure. Reefs are wave-resistant frameworks and form in conditions of high energy where the water is well-oxygenated. They are subject to erosion by storms – which may deposit “wash-over” debris in the sheltered lagoons to the landward side of the reef.

(g) Describe the characteristic features of the primary sedimentary structures: cross bedding, ripple marks, graded bedding, desiccation cracks. Describe the environments in which they form and the processes of formation. Describe the uses of these sedimentary structures as way up and palaeo-environmental indicators.

These are primary features formed during the deposition of sediment. They are very useful as they provide information about: 1. Way up – are the beds the right

way up or inverted (up side down)? Beds may be inverted by folding. 2. Palaeo-current direction – some structures can be used to determine the wind/current direction at the time of formation. 3. Palaeo-environment – the environment in which the structure formed, e.g. a desert.

Cross bedding - Forms in high energy environments (wind or water). Requires flow in one direction (unidirectional). Sediment moves up the windward slope (stoss) by saltation, collects at the top in the lee and eventually cascades/avalanches down the lee slope. Lee slope (max 35) is steeper than stoss slope. As migration occurs all that is preserved are the successive lee slopes → dipping foreset beds. Way up – foreset beds are concave upwards, older beds are truncated (cut off) at top. Palaeo-current/wind direction – foreset beds dip in current direction. (Could measure and

plot on rose diagram). Palaeo-environment – given by scale. Large scale (dune) cross bedding >1metre = wind

blown (aeolian), desert. Medium scale = beaches, deltas, shallow seas. Small scale = rivers (fluvial).

Ripple Marks - Formed in a similar way to cross bedding. There are two types: Asymmetrical – formed by currents moving in one direction, i.e. unidirectional → rivers, deserts. Symmetrical – formed by currents moving to and fro, i.e. bi-directional → wave action, beaches. Way up – ripple crests point upwards. Palaeo-current/wind direction – asymmetrical ripple marks are steeper on down current

side; symmetrical ripple marks can only give trend at 90 to ripple crests. Palaeo-environment – given by shape (asymmetrical versus symmetrical).

Desiccation (mud) Cracks - Formed when wet fine-grained sediment (clay) dries out. Requires high rates of evaporation. As the sediment dries it shrinks and polygonal shaped cracks form. These are preserved when infilled by later, coarser sediment. Way up – cracks point downwards. Palaeo-environment – high rates of evaporation, arid, desert – playa (temporary) lake.

Graded bedding - Form when sediment settles out of water under the influence of gravity. The coarser (heavier) grains sink to the bottom first and the bed gets progressively finer grained upwards. Found in muddy sandstones (greywackes). Way up – bed gets finer upwards. Palaeo-environment – associated with turbidity currents – form when the current runs out

of energy.

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(h) Define lithification. Describe and explain the diagenetic processes of compaction and cementation.

Occurs during burial of sediment as the depth of overburden increases. Near surface process, source of energy is geothermal energy, upper limit is 200C, above this burial metamorphism occurs. Lithification – process of converting loose sediment into a sedimentary rock. Diagenesis – all the physical and chemical processes that operate during lithification. Compaction (main physical process) – grains move closer together due to weight of

overburden, porosity is reduced. Mudstones undergo more compaction than sandstones as mudstones are made of flat, platy minerals (clay minerals, mica) that can align under pressure, whereas sandstones are made of round quartz grains that merely rotate under pressure.

Cementation (main chemical process) – a crystalline cement (commonly quartz or calcite) is precipitated in the pore space from circulating pore fluids, porosity is reduced.

Other processes – (pressure) solution; recrystallisation; replacement; growth of new “authigenic” minerals.

2.3 Igneous Processes and Products

Candidates should be able to:

(a) Describe the intrusion of concordant and discordant bodies as both major and minor intrusions. Recognise and describe the characteristics of sills, dykes, transgressive sills and batholiths.

Major (plutonic) intrusions – Large batholiths with surface area >100 km2, emplaced at depth – magma rises as low density diapirs pushing the country rock aside. Produces coarse crystal-grain size. Commonly acid-intermediate composition.

Minor (hypabyssal) intrusions – Dykes – discordant, cross cut bedding. Sills – concordant, parallel to bedding. Both are intruded at shallow level/near surface depths. Produces medium crystal-grain size. Commonly basic composition.

Transgressive sills are mainly concordant but in places step up/cross cut and change level in the country rock.

Country rocks - rocks (often sedimentary) surrounding the igneous intrusion. These are older than the igneous intrusion.

Contact - the junction between the intrusion and the country rock. Xenoliths (foreign rocks) – fragments of the surrounding country rock caught up and

trapped in the magma – these are older than the igneous intrusion.

(b) Explain the origin and nature of chilled and baked margins and metamorphic aureoles.

Chilled margins of finer crystals develop around the edge of igneous intrusions because the outer part of the intrusion cools faster than the centre. Most likely to represent the original composition of the magma.

Metamorphic aureole - zone of altered/recrystallised/metamorphosed rocks around a large igneous intrusion. Major igneous intrusions (batholiths) contact metamorphose the surrounding country rocks. The size of the aureole depends mainly on the volume of magma emplaced, the composition of the magma and the surrounding country rock.

Baked margins - minor igneous intrusions (dykes and sills) are not large enough to produce metamorphic aureoles, instead a thin baked margin is formed in the surrounding country rock. Produces fine-grained, recrystallised, hard rock.

(c) Distinguish between intrusive and extrusive igneous rocks. Explain the differences between rocks formed by these methods - sill and lava flow.

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Sill (intruded - must be younger than overlying rock) - medium-grained; two baked margins top and bottom; xenoliths may occur at top and bottom; top surface is regular; veins and stringers of sill may occur in overlying rock; may transgress from one bed to another; may contain cumulate layering.

Lava flow (extruded - must be older than overlying rock) – fine-grained/glassy; one baked margin at bottom only; xenoliths will be found at bottom only; may contain vesicles, flow banding; could be pillow lavas; may have an irregular, scoracious, reddened weathered top surface; fragments of eroded lava may occur in overlying rock; underlying rocks represent palaeo-land surface – may be eroded, weathered surface; may have associated pyroclastics.

Both may show columnar jointing and could have porphyritic textures.

(d) Describe how crystal grain size is related to rates of cooling of volcanic, hypabyssal and plutonic igneous rocks. Know how vesicular and porphyritic textures are formed.

Rapid cooling at (or close to) surface (due to large temperature difference between magma and surroundings) produces fine crystals – little time for crystals to grow.Slow cooling at depth (due to insulation) produces coarse crystals – atoms and ions have more time to diffuse through magma and attach themselves to growing crystals.Crystal size indicates rate and depth of cooling: Volcanic rocks = <1mm – fine-grained - rapid cooling – extrusive. Hypabyssal rocks = 1-5mm – medium-grained - shallow-level, near surface – intrusive. Plutonic rocks = 5-30mm – coarse-grained - slow cooling - deep-seated – intrusive.

Equigranular – mosaic of equidimensional crystals – formed by cooling at a constant rate. Found in volcanic, hypabyssal & plutonic rocks.

Porphyritic – Coarse crystals (phenocrysts) set in a groundmass of finer crystals – formed by cooling at two different rates/depths. First, slow cooling at depth forms large/coarse crystals, then the magma rises to a higher level/the surface and more rapid cooling occurs producing the finer grained groundmass. Found in volcanic, hypabyssal & plutonic rocks.

Vesicular – when lava is erupted onto the surface vesicles form as bubbles of gas escape from the magma, indicates rapid cooling under low pressures. Vesicles may become oval/elliptical as the lava flows over the ground. Only occurs in volcanic rocks.

Amygdaloidal – vesicles are infilled by later, secondary crystals/minerals (e.g. calcite). Only occurs in volcanic rocks.

Flow banding – As lava flows along the surface, the crystals become aligned in the direction of flow. Only occurs in volcanic rocks.

Glassy – Lava cooled so rapidly that there was insufficient time for crystals to grow → volcanic glass (obsidian). Only occurs in volcanic rocks, particularly those erupted under water.

(e) Describe volcanic processes - basic magma at constructive margins and hotspots, intermediate and acid magma at destructive plate margins. Explain why volcanoes form at plate margins.

Igneous rocks/magmas are subdivided into acid, intermediate, basic and ultrabasic - based on silica content. The % silica, mineralogy and colour of the rocks are all related to composition. The crystal size is independent and is related to rate/depth of cooling. All igneous minerals are silicate minerals - therefore, the % silica is not the same as the % quartz. Acid rocks – silica content >66%; rich in orthoclase (K-feldspar), contains quartz and

plagioclase (Na-feldspar), less than 30% mafic minerals (usually mica); therefore light-coloured.

Intermediate rocks – silica content 66-52%; 2/3 plagioclase:1/3 orthoclase feldspar, contains ~10% quartz, 30-60% mafic minerals (usually amphibole); therefore medium-coloured.

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Basic rocks – silica content 52-45%; 50% plagioclase (Ca-feldspar), 50%+ mafic minerals (usually pyroxene); therefore dark coloured.

Ultrabasic rocks – silica content <45%; 100% mafic minerals (olivine and pyroxene); therefore very dark coloured. Rarely occur as volcanic rocks.

Magma viscosity – the extent to which the magma flows is controlled by silica content. Acid magmas (high silica content) have a high viscosity, don’t flow; Basic magmas (low silica content) have a low viscosity and are fluid.

The type of volcano (shape/form), style of eruption and volcanic products are all related to the magma type and viscosity. This is in turn related to the plate tectonic setting.

Basic magmas form by partial melting of the ultrabasic mantle at constructive margins and hot spots. Fluid – volatiles/gases can escape → gently effusive eruptions of lava.

Basic volcanoes - gently effusive, fissure and shield volcanoes, main products are lavas and gases, occur at constructive margins and hotspots.

Acid and intermediate magmas form by partial melting of the subducted slab at destructive margins, magma picks up silica as it rises through the crust. Viscous – volatiles/gases can’t escape → highly explosive eruptions of pyroclastics and gases.

Acid - intermediate volcanoes - highly explosive, central vent volcanoes, main products are pyroclastics and gases, occur at destructive plate margins.

(f) Describe the products of volcanoes: gases, pyroclasts and lavas of basic, intermediate and acid composition. Explain the distribution of volcanic products around volcanoes.

Magma – molten rock below the earth’s surface – a molten mixture of silicate minerals, water and other volatiles.

Volcanoes produce three main products: gases (gas), lavas (liquid) and pyroclastics (solid).

Lava – molten magma erupted onto the Earth’s surface from a volcano. Most lavas are basic in composition (fluid magma). Submarine lavas take the form of pillow lavas, e.g. at mid ocean ridges. Subaerial lavas take the form of pahoehoe (ropy lava) or aa (blocky lava), e.g. Hawaii. Basic lava flows may contain vesicles and flow banding. Acid lavas (high viscosity) are rare and tend to form acid domes, e.g. Mt. Pelée. Acid lavas may be glassy and contain flow banding.

Gases – Main gases are steam (50-80%), CO2, N, Sulphurous gases – SO2 and H2S. E.g. Lake Nyos, Cameroon, erupted 100 million m3 of CO2 gas in 1986, killed ~2000 people. Viscous, acid magmas tend to trap gases that cannot escape from the magma → violent, highly explosive eruptions.

Pyroclastics – solid fragmental material ejected from a volcano. Mainly acid composition. Classified by size – ash < 4mm; lapilli 4-32 mm; blocks and bombs >32 mm. When lithified – ash and lapilli → tuff; blocks and bombs → agglomerate.

Isopachytes – lines on a map joining points of equal thickness of pyroclastic material. Thickness decreases away from the vent. Particle size decreases away from the vent. Isopachytes may have an elliptical shape - wind blowing the material or a lateral eruption.

Other products – pyroclastic flows (nuée ardentes), lahars.

(g) Describe the characteristics of: submarine; fissure, shield and strato-volcanoes and explain the type of volcanic activity at each. Know about caldera formation, pyroclastic flows and geysers.

Volcano – a hole (vent) or crack (fissure) in the ground through which volcanic products escape. The vent or fissure is connected to an underground magma chamber via a conduit or pipe. A vent may contain a crater and there may be parasitic vents/cones on the flanks of the volcano. Submarine volcanoes – commonly fissures at the mid ocean ridges, tend to erupt gently

effusive, basic pillow lavas. May get Surtseyan (phreatomagmatic) eruptions if rising magma comes into contact with sea water → superheated to steam.

Fissure volcanoes – cracks; gently effusive Icelandic style eruptions; basic pillow lavas; mainly occur at mid ocean ridges/constructive margins, e.g. Iceland, Mid Atlantic Ridge.

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Shield volcanoes – central vent volcanoes; gently sloping/low angle sides with a very large base; gently effusive Hawaiian style eruptions; basic lavas (pahoehoe and aa if subaerial); mainly occur at hot spots, e.g. Hawaii (Mauna Loa).

Strato-volcanoes/Composite cone volcanoes - central vent volcanoes; classic steep-sided cone shape, more explosive eruptions ranging from Strombolian → Vulcanian → Vesuvian → Krakatoan/Plinian (exceptionally violent); intermediate to acid composition; typically alternating lava flows and pyroclastics build up the cone; mainly occur at destructive margins, e.g. Mt. Vesuvius, Popacatapetl.

Acid domes – steep-side dome composed of acid lava; liable to very violent eruption, e.g. Mt. Pelée.

Pyroclastic flows (nuée ardentes) – “glowing ash cloud” – a hot (~600°C), turbulent cloud of ash and gas that flows down the flanks of a volcano at great speed (>100 km/h); exceptionally dangerous Peléan style eruption; produces a characteristic rock called a welded tuff/ignimbrite; a feature of acid-intermediate volcanic eruptions, e.g. Mt. Pelée, Mt. St. Helens, Mt. Pinatubo, Mt. Vesuvius (Pompeii was destroyed by one!).

Lahar – volcanic mud flow, e.g. Nevado del Ruiz (Columbia), Mt. Pinatubo. Caldera formation – top of the volcano is blown off in an exceptionally violent, cataclysmic

eruption leaving a very large crater (> 10 km diameter); as the magma chamber beneath the volcano empties further subsidence takes place (caldera collapse), e.g. Krakatoa, Yellowstone.

Minor activity: Fumarole – vent from which volcanic gases escape. Geyser – a thermal spring that produces intermittent eruptions of hot water, formed when

groundwater is heated by volcanic activity.

(h) Appreciate the social and economic effects of volcanic activity and the danger to life and property of different types of volcanic activity including climatic change. Describe the advantages which volcanic activity can bring. Describe methods for the prediction of volcanic activity: historic pattern of activity, changes in ground level, changes in gas composition and precursor earthquake tremors. Describe the methods of risk analysis: extent and path of lava flows, blast damage, ash falls, pyroclastic flows and lahars using hazard maps.

Primary volcanic hazards – pyroclastics, gases, lava flows, pyroclastic flows. Secondary volcanic hazards – lahars, landslides, tsunamis, fires. Social impacts – concerned with human beings and their relationships – injury, death,

bereavement, stress, homelessness, lack of fresh water, famine, disease, collapse of whole communities.

Economic impacts – concerned with financial/monetary matters – mainly losses -building/property damage, loss of production, destruction of crops/farmland, disruption to communications/infrastructure, job losses, insurance claims; some gains – construction opportunities.

Long term impacts – Climatic change – major volcanic eruptions have been implicated in natural climatic change – volcanic debris/dust is thrown into the upper atmosphere and prevents solar radiation reaching the Earth, also SO2 gas aerosols produce a white coating on particles in the atmosphere (reflect solar radiation) → Global cooling of 1-2°C from average.

Long term advantages of volcanic activity – produces fertile soils (ash rapidly breaks down releasing P and K); exploitation of geothermal energy; formation of mineral deposits (e.g. gold, silver, copper, sulphur); tourism.

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Prediction A successful prediction must pinpoint – spatial occurrence (where), timing (when) and magnitude (how big). Based on precursor phenomenon (small events that occur before the main event). Prediction of volcanic eruptions has been relatively successful.Methods of prediction: Past eruptive history and periodicity of individual volcanoes. Ground swelling/inflation as magma chamber fills – measured with tilt meters. Harmonic tremors – increase in seismic activity (earthquakes) prior to eruption, due

pressure of rising magma. Changes in composition of fumarolic gases (HCl, SO2, S and Cl). Rise in ground and groundwater temperatures. Changes in Earth’s magnetic field strength. Changes in seismic wave behaviour – can be used to detect areas of liquid (i.e. rising

magma). Remote sensing techniques – infra red imagery from satellites can detect changes in heat

flow, SAR (synthetic aperture radar) can detect ground swelling.Risk Analysis Can be carried out by hazard risk mapping – delineating areas at risk from the likely paths

of lava flows, blast damage, ash falls, pyroclastic flows and lahars. Tephrochronology can be used – Tephra (pyroclastic) deposits can be mapped out and

dated to indicate areas at risk. However, hazard maps can be misleading, e.g. Mt. St. Helens, 1980 – eruption was

predicted, but the lateral blast was not!

2.4 Metamorphic Processes and Products

Candidates should be able to:

(a) Explain thermal (contact), regional and burial metamorphism in relation to varying temperatures and pressures.

Metamorphism = process by which rocks are changed by heat and pressure. The rock undergoes solid-state recrystallisation without melting. This is a very slow process. Metamorphism is isochemical – nothing is added or taken away, the minerals simply recrystallise into new metamorphic minerals. If the rock is subjected to directed pressure a preferred orientation of the minerals develops at 90º to the pressure – a foliation.

Limits of metamorphism – 200-800ºC. Below 200ºC = diagenesis. Above 800ºC = partial melting (anatexis).

Types of Metamorphism: Thermal/Contact metamorphism – Caused by heat from a cooling igneous intrusion. Local

scale – produces a metamorphic aureole. High T (200-800ºC), low P (2-3kbars). No directed pressure, heat only → random texture, unfoliated rocks.

Regional Metamorphism – Associated with mountain building (orogenesis) at destructive plate margins/continental collision zones. Large scale – 100s of km in extent. Low to high T, low to high P. Directed pressure/compressive stress → foliated rocks.

Burial Metamorphism – In sedimentary basins – an extension of diagenesis. Low T (but > 200ºC), medium-high P.

Dynamic/Dislocation Metamorphism – Occurs along faults – rocks are crushed by movement/pressure → mylonite. Very high P.

Factors Controlling Metamorphism – Temperature, Pressure (load, directed/compressive, pore fluid), Parent Rock, Time.

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(b) Describe the relation of parent rock composition and the environment of metamorphism to the mineralogy of the resulting metamorphic rock: (i) mica and clay minerals in slate from shale; (ii) mica and garnet in schist from slate; (iii) quartz, feldspar and mafic minerals in gneiss from schist; (iv) calcite in marble from limestone; (v) quartz in quartzite (metaquartzite) from sandstone.

Parent rock – original rock prior to metamorphism. Metamorphic grade – intensity/degree of metamorphism the rock has suffered. Low –

medium – high grade. Can be determined by the texture (crystal size and type of foliation) and mineralogy of the metamorphic rock.

Index mineral – a metamorphic mineral that is stable over a particular temperature and pressure range – indicates the metamorphic grade, e.g. mica, garnet, Al2SiO5 polymorphs (andalusite, kyanite and sillimanite). The metamorphic grade can be mapped using index minerals. Always mapped by first appearance of the mineral (may persist at higher T & P).

Isograds – lines on a map joining points of equal metamorphic grade.

Regional Metamorphism – Large-scale metamorphism (> 100 km2). Result of the high pressures and temperatures generated at destructive plate margins/continental collision zones → mountain building (orogenesis). Directed pressure/compressive stress → foliated rocks.

Regional Metamorphic Textures Types of foliation – slaty cleavage; schistosity; gneissose banding. Porphyroblastic texture (large new minerals that grow during metamorphism) may also be

found in any of the above.

Regional Metamorphism of ShaleRich in clay minerals (Al-rich), resultant rocks also Al-rich. Low grade = slate → fine grained (<1mm), crystalline, slaty cleavage (microscopic

alignment), dark coloured, harder than shale. Mineralogy – typically mica, clay minerals and chlorite.

Medium grade = schist → medium grained (1-5mm), schistosity, variable colour. Mineralogy – typically mica and garnet (may be porphyroblastic).

High grade = gneiss → coarse grained (>5mm), gneissose banding (mm to cm scale segregation of light and dark coloured minerals). Mineralogy – typically quartz and feldspar in light bands; mafic minerals (biotite, etc.) in dark bands.

The Barrovian ZonesGeorge Barrow, 1912 – mapped a sequence of regionally metamorphosed shales in the SE Scottish Highlands. He was able to map out zones of metamorphism using index minerals and isograds. The metamorphism occurred during closure of the proto-Atlantic Ocean (Iapetus).

Low grade Medium grade High gradeTexture Slate

fine-grained, slaty cleavage

Schist medium-grained

schistosity

Gneisscoarse-grained,

gneissose bandingIndex

mineralchlorite biotite garnet staurolite kyanite sillimanite

Regional metamorphism of sandstones and limestones Produces the same products as contact metamorphism, i.e. metaquartzite and marble. The rocks are monominerallic and made of equidimensional minerals that cannot align under pressure. In the case of hard, round quartz grains – these merely rotate under directed pressure.

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(c) Describe thermal metamorphism by heat from an igneous intrusion to form different grades of unfoliated rocks within a metamorphic aureole. Understand the factors controlling the width of contact aureoles and metamorphic grade.

Thermal/Contact metamorphism – Result of heat from cooling igneous intrusion. Heat only, lack of directed pressure → unfoliated rocks with random texture. Minor intrusions – dykes and sills – insufficient heat to greatly affect country rock →

baked margin. Major intrusions – batholiths – retain heat for 10,000s - millions of years → metamorphic

aureole in country rock. The closer to the intrusion the higher the temperature – may result in a zonation of metamorphic rocks.

Metamorphic aureole = zone of altered/ recrystallised/ metamorphosed rocks around a large igneous intrusion, up to 10 km wide. Temperature decreases with distance from contact.

Factors controlling the width of the metamorphic aureole – Volume of magma/intrusion; Composition of country rocks; Temperature of magma; Composition of magma – acid magmas contain more volatiles; Depth of intrusion – temperature difference.

Contact Metamorphic Textures: Sugary/Granoblastic texture – randomly oriented mosaic of equidimensional crystals. Porphyroblastic texture – large new minerals that grow during metamorphism.

Products of Contact Metamorphism:

Limestone (calcite) → Marble. Calcite grains/fossil fragments recrystallise to calcite crystals. Fossils are destroyed. Produces a randomly oriented mosaic of equidimensional interlocking calcite crystals → sugary, granoblastic texture.

Sandstone (quartz) → Metaquartzite. Quartz grains recrystallise to quartz crystals. Sedimentary structures are destroyed. Produces a randomly oriented mosaic of equidimensional interlocking quartz crystals → sugary, granoblastic texture.

Shale → Spotted slate, Andalusite slate, Hornfels. Shale is rich in clay minerals (Al-rich) → new Al-rich minerals form. As shale contains a mixture of minerals several new products are possible depending on distance from contact (temperature). Spotted slate (low grade) – furthest away – rock only partially recrystallises, relict textures

(sedimentary structures, fossils) are present. Spots of biotite/graphite form from organic matter in shale.

Andalusite slate (medium grade) – higher temperatures, spots converted to new minerals → randomly oriented andalusite (Al2SiO5) porphyroblasts form.

Hornfels (high grade) – closest to contact. Shale is completely recrystallised to form hard, medium- to fine-grained, crystalline rock with equidimensional crystals – granoblastic texture. Minerals include sillimanite, quartz, feldspar and pyroxene.

© D. Armstrong, 2004

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