Lithospheric Notesr

7/22/2019 Lithospheric Notesr

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Lithospheric Processes, Hazards and Management

Structure of the Earth

1. The Core

Inner and outer core, outer liquid about 3500km radius, inner solid about 1255km The core is heavy and dense, and nickel-iron alloy called Nife

2. The Gutenberg Discontinuity Separates core from mantle, zone of discontinuity Slowing of seismic waves in region

3. The Mantle3.1 Lithosphere, Asthenosphere and Mesosphere

Deepest solid layer is mesosphere, the semi-molten layer is the asthenosphere, thesolid layer above, including crust, is lithosphere

The rigid lithosphere is broken into plates which can move over the moltenasthenosphere

4. The Mohorovicic Discontinuity Between crust and lithosphere

5. The Crust Outermost layer of Earth. Continental crusts thicker than oceanic crusts, and often older

because of the impermanence and subduction of oceanic crust

5.1 Oceanic Crust (Sima) Made of basalt, or sima (silica and magnesium), so it is dense at 3gm/cm 3

5.2 Continental Crust (Sial) Granitic, or sial (silica and aluminium), less dense at 2.7g/cm3

Theory of Continental Drift

1. Essence of Theory All continents were joined 225 million years ago as Pangaea, which split into Laurasia

and Gondwanaland, and further drifting resulted in their present locations today

2. Supporting Evidence2.1 Continental Refits

Observation that the shorelines and continental shelves of several continents fiteach other, such as S America and Africa

2.2 Structural and Lithological Evidence2.2.1 Structural Evidence

Belts of structures such as fold mountains and shields should be traceablefrom one edge of a continent to a previously joined one

Caledonian Mountains in Scotland and Scandinavia can be linked to theAppalachians in the United States

2.2.2 Lithological Evidence Sequence of rock types, or stratigraphy, there is a high correlation of rock

types during the time when continents were supposedly joined

2.3 Palaeomagnetic Evidence2.3.1 Magnetic Field and Rock Magnetism

A rock gains its magnetism when it is iron rich and cools beyond Curietemperature (400-600C), aligning with the prevailing magnetic field


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If age of rock is known, rock magnetism can determine the position ofmagnetic poles at time of formation of rock

2.3.2 Polar Wandering Curve Collection of rock samples of varying ages, can determine location of poles

at different times. If positions for North pole are plotted through time for a

continent, a polar wandering curve is derived

These show that the poles appear to have moved greatly over time, but it isknown to be rather improbable.

Furthermore, each continent has its own curve Either each continent has its own poles, or the continents have moved

relative to each other

On a refit map, the poles all fall within the range of the actual poles2.4 Palaeoclimatic Evidence

2.4.1 Glaciation of the Southern Continents Tillites and striae are present on Southern continents, which are now close

to the equator, thus the climate would not be suitable for the formation of

such features

Also, glaciers moved inland from the ocean in Africa, S America andAustralia, which is impossible since glaciers move towards the ocean, unless

there was land there previously

2.4.2 Glaciation in Africa and South America Extensive glacier erosion in Africa and tillite deposition in South America

can be explained if they were connected previously

2.5 Palaeontological Evidence Fossil evidencefossils of certain ancient animals and plants are widespread and

found on many continents, which would indicate that they used to be joined3. Limitation of Theory

The mechanism for movement was unknown, but is now proposed to be the theory ofPlate Tectonics

Theory of Plate Tectonics

1. Seafloor Spreading1.1 The Sea-floor Spreading Hypothesis

Discovery of mid-ocean ridges and rift valleys and splitting due to tension. Also,ocean basins were found to be relatively young

Mid-ocean ridges were the locations for generation of new crust due to coolingmagma forming new crust where it diverged

1.2 Supporting Evidence1.2.1 Rock Magnetism

The earths magnetic field frequently reverses its polarity, so Vine andMatthews suggested that fossil magnetism at such rift valleys will have

alternating bands of normal and reversed polarity symmetrical on both

sides of the rift

Confirmed by magnetic survey of Reykjanes Ridge and other mid-oceanridges

1.2.2 Geothermal Heat Flow Generated by Earths interior, measured by thermistor probe


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Over mid-ocean ridges, temperature may be several times higher thannormal, which may be from mantle injection

1.2.3 Seismic Activity Distribution Mid-ocean ridges are centres of activity, key areas of volcanoes and

earthquakes

1.2.4 Dating of Volcanic Activity In Iceland, most recent volcanic activity occurs in a band down the centre,

with older volcanoes moving east and west

North Atlantic, farther from ridge are older islands, which can be explainedby sea-floor spreading

1.2.5 Pattern of Sedimentation Moving away from ridge, one should find older sediment on older crust Deep Sea Drilling project produced evidence regarding sedimentation

pattern

2. Subduction2.1 The Subduction Hypothesis

If sea-floor spreading is accepted, subduction accounts for the Earths volumestaying constant, since crust has to be remelted somewhere

2.2 Supporting Evidence2.2.1 Seismic Activity Distribution

Most intense seismic activities coincide with ocean trenches2.2.2 Distribution Pattern of Earthquake Foci

Benioffsexamination of the Kurile Trench shows that earthquake foci getdeeper further from the trench and towards the cordillera of island arcs

Termed the Benioff Zone, about 45 degree inclination. Line of disturbancecaused by passage of oceanic plate as it was subducted

2.2.3 Geothermal Heat Flow Geothermal heat is cooler over ocean trenches, indicating cold crust

descending and cooling the mantle

3. Mantle Convection Currents The earths plates move around the surface of the earth via convection. New crust

generated by upwelling magma at mid-ocean ridges, plates move away, carrying

continents, and at ocean trenches they are re-absorbed into the mantle

Decoupling of the lithosphere and asthenosphere Source of tectonic movement is the heat generated by residual cooling off of planet,

and the decay of radioactive materials in the core, releasing heat Rocks nearer to source are heated and become less dense and more buoyant than

surrounding rock, rising to base of lithosphere, moving laterally and releasing heat, and

then sinking to remix. This cycle maintains the convective motion

4. Limitations of the Theory Paradoxes: the possibility of an expanding Earth

Global Structural Landforms

1. Divergent/Constructive Plate Boundaries Zones of tension where plates split and are pulled apart, and new crust is formed Either 2 convective flows are dragging plates apart, or mantle plumes or hot spots cause

tensional stress, where doming and three-armed rifting occurs


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1.1 Rift Valleys Hot rising plume causes crustal stretching and formation of tensional cracks. Plates

move away from upwelling, broken slabs are displaced down, creating downfaulted

valleys called rifts or rift valleys

1.1.1 Features of Rift Valleys Large tracts of land may be broken up, and vertical displacement can

produce horsts and graben

Horsts are slabs of crust left upstanding, and graben are crustdownthrown by rifting

1.1.2 The East African Rift Valley Extends from Jordan to Mozambique, 5500km. In central part, divides into

two branches, Albertine and Gregory rifts

The Kenyan rift valley exemplifies main features of rift faulting Simplest faulting results when two parallel faults allow valley floor to sink

between inward-facing scarps, producing bold and high fault scarps at 600m

or more

More commonly, a number a faults result in step faulting, and smaller faultsresult in grid faulting

Volcanoes also occur in rift valley due to crustal weaknesses, such asLongonot, Kilimanjaro

Lakes also occur where rifting goes below the water table, such as Naivashaand Malawi. Soda lakes as a result of sodium carbonate from magma and

volcanoes also occur, like Magadi and Natron

1.1.3 Merits of the Concept of Three Armed Rifting Tension caused by mantle plume explains plan of rift, which is three armed,

like the Rhine Rift Valley and the Red Sea Rift system, who both have failedarms in the Hess arm and the Abyssinian Rift.

Also, the uplift and tilt of horsts and graben is explained by mantle plumesbut not simple tension

1.2 Mid Ocean Ridges1.2.1 Features of Mid-ocean Ridges

Further spreading will cause rift valleys to lengthen and deepen into ocean Thousands of km long, hundreds of km wide, about 0.6-3km above seafloor Hot mantle decreases density due to thermal expansion, causing rocks near

ridge to elevate. As they move away, the cool and subside due to density

Central rift runs down middle of most ridges like in Red Sea, wheretemperature is higher and pillow like appearance of lava due to rapid

cooling underwater

Great transform faults, resulting is staggered path1.2.2 The Mid-Atlantic Ridge

Great submarine mountain chain1.2.3 Volcanic Islands

Atlantic islands located close to mid ocean ridges where is reaches surfaceof the sea, like the Azores, Ascension Island and St Helena

Majorly, Iceland, a tholeiitic basalt plateau. A rift valley, the CentralIcelandic Depression, lies down centre of island, coinciding with recentvolcanic activity


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Iceland grows outward from centre, so rocks get older further from fissure2. Convergent/ Destructive Plate Boundaries

Main stresses which occur are compressional. Depends on the type of crust involved Oceanic-oceanic results in the denser plate being subducted Continental-oceanic results in the oceanic plate being subducted Continental-continental results in folding since neither is dense enough to subduct much

2.1 Ocean Trenches Long narrow troughs in the ocean bed marking zones of subduction, Mariana Trench

is deepest at 11022m deep

Found where trench fringes a continent due to continental-oceanic collision, or inthe ocean floor as a result of oceanic-oceanic collision. The former has high

incidence of active volcanoes, and the latter has volcanic island arcs

2.2 Continental Volcanic Arcs Orogenesis occurs when sediments along coasts are compressed by folding and

faulting to form mountain chains such as Andes, Alps and Himalayas

Continental-oceanic, continental volcanic arcs may be formed Oceanic crust is bent and subducted, leading to partial melting of the water-rich

oceanic crust, magma formed less dense and slowly rises, which is usually andesitic

or granitic in nature, cooling and crystallizing greatly underground to give batholiths

Magma may migrate to surface, causing volcanic eruptions. When volcanoes havebeen eroded, the batholiths are exposed and observable

Faults occur in shallow zone of mountain, and deeper underground intensemetamorphism of rocks occur

2.3 Island Arcs May be produced by an oceanic-oceanic collision Formed by partial melting of plate and lithosphere along the Benioff Zone Lava ascends to form arc of volcanic islands, such as Japanese islands and Aleutians Lavas are dominantly andesitic, which have 15% more silica and 3 times potassium

oxide by weight than ordinary basalt

Composition of andesites vary in proportion to depth of Benioff Zone, with moreandesite the deeper it is

Heat produced for melting is caused by friction between the two plates2.4 Fold Mountains

Continental-continental collision causes crust to be fused together because neitheris dense enough to sink and subduct

They are pushed up to form mountain ranges such as the Alps and Appalachians Intense folding, faulting and buckling up of material. Deeper buried rocks maybe

more plastic due to higher temperatures, and thus only fold, not fracture

3. Transform Plate Boundaries Zones of shearing where plates slide past each other at transform or strike-slip faults Limited construction and destruction Zones of intensely shattered rock, forming narrow valleys on land and ridges on the sea

floor

3.1 The San Andreas Fault Major branches include Hayward and Calaveras faults Great length and complexity, thus named a fault system Offset stream channels and elongated ponds mark the fault and its movement


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Responsible for earthquakes along the fault, as segments either slip regularly orstore energy for years where rocks are more elastic, generating earthquakes of

varying intensities

4. Hot Spots Intraplate activity and large scale landforms are explained by mantle plumes instead of

plate tectonics

Regions where flow of geothermal heat is higher than average, commonly sites ofvolcanism and the lava is rich in alkali (Group I) metals

They remain relatively stationary4.1 The Hawaiian Chain of Islands

A stationary hot spot and a moving seafloor, a volcano can only remain in contactwith the hot spot for about a million years, after that the volcano will become

inactive

The Hawaiian islands provide evidencevolcanoes increase with age away fromHawaii

4.2 Other Hot Spot Activities Possibly in other areas like the Mid-Atlantic Ridge or Yellowstone National Park Exact role in plate tectonics is unclear

Seismic Activities

1. Causes and Characteristics of Earthquakes 95% of earthquakes are interplate at plate boundaries. Intraplate earthquakes are less

common

1.1 Deformation and Fracture of Rocks At the outermost layer of crust, rocks are strong but brittle. When plates move,

compression, tension or shear of rocks build up pressure, resulting in concentrated

releases of energy, forming faults.

May come in a single shock or series of shocks Friction at plate boundaries build stresses and strain, bending and deforming rocks.

When limits of deformation are exceeded, the rocks rebound, releasing energy,

producing earthquakes

1.2 Earthquakes and Faulting Fracture in a rock along which movement occurs is a fault. Movement along a fault

can be vertical or horizontal

Rocks above a fault is the hanging wall, rocks below is the foot wall Dip-slip faultsin a normal fault, hanging wall moves down. In a reverse fault, the

hanging wall moves up. The break in slope is a fault scarp. Normal is often divergent,

reverse often convergent

Strike-slip faultsleft-slip and right-slip, depending on direction. Most fault systems appear as a combination of fault movements

1.3 Focus and Epicentre Focus is the point where an earthquake releases the elastic strain by fracturing Can be shallow (70km), intermediate (70-300km) or deep (300-700km) At divergent and transform, normally shallow focus, but at convergent normally at

the Benioff Zone

Epicentre is the point on crust directly above focus1.4 Seismic Waves


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Seismic waves spread out from focus in all directions Body waves radiate in all directions, surface waves are vibrations trapped near the

surface of the earth

Primary pressure waves are longitudinal body waves travelling by compression andexpansion, while secondary shear waves cause ground to vibrate perpendicular up

and down. Primary waves are faster and thus felt first, but secondary waves are

more destructive because buildings can withstand little horizontal stress.

Love waves are surface waves that cause horizontal shearing, and Rayleigh waves,or ground roll, travel like ripples. Love waves are generally faster, but Rayleigh more

destructive

Surface waves are slower but more destructive than body waves because theyinduce resonance in buildings

1.5 Global Patterns of Earths Seismicity1.5.1 Divergent Boundaries

Narrow belts of shallow-focus earthquakes coinciding with crests of oceanicridges at divergent boundariesless than 70km deep, small magnitude

Crests of oceanic ridgesnormal faulting, basaltic magma intrusions. Also,vertical faulting, associated with ridge topography

Shallow focus at transform faultsno volcanic activity1.5.2 Convergent Boundaries

Widespread and intensesubduction zones, inclined at moderate to steepangles, focuses as deep as 700kmcan be brittle at that depth

1.5.3 Intraplate Seismicity Not associated with known faults or historical activityresult from crustal

stresses e.g. uplifting of mountains like Himalayas

Built up stress by plate moving vertically while moving over asthenosphere2. Earthquake Magnitude and Intensity

2.1 Intensity and the Mercalli Scale Intensity is the strength of shaking by an earthquake at a location, determined by

effects on people, structures and the environment

The Modified Mercalli Scale measures damage and human perception of anearthquake by using descriptors

It is not a measure of an earthquakes size or energy, but rather its perceptibleeffects and damages, thus useful for comparing effects

Dependent on variations in population density, building materials and methods anddistance from epicenter

Useful in ranking earthquakes before technology was available to measure them, aswell as creating isoseismal graphs

2.2 Magnitude and the Richter Scale Magnitude refers to the absolute size of and amount of energy released by the

earthquake, using amplitudes of the seismic waves

The Richter Scale is used to measure magnitude3. Effects of Earthquakes

3.1 Ground Motion Passage of seismic wave through surface rock layers and regolithdamage and

destroy buildings3.2 Tsunamis


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Series of large waves created by abrupt displacement of water. Long period, crestsare very high and troughs very low. Troughs arrive at shore first, causing sea level to

fall and exposing the seabed

Generated when sea floor abruptly deforms and displaces the overlying water,especially submarine earthquakes at subduction boundaries

Boxing Day Indian Ocean Tsunami in 2004230000 in 14 countries died3.3 Landslides and Liquefaction

At convergent plate boundaries, steep slopes are prone to landslides when shaken. Also, soil layers may liquefy, causing mudflows. Liquefaction is particularly

dangerous as soil in a suspended state cannot bear any load, causing structures built

on it to collapse

Landslides in Gansu Province, December 16, 1920, killed 180000 1964 Niigata, El Salvadors land is tephra, consolidated pyroclasts

3.4 Fires Fires caused due to fracturing of gas pipes, ruins of wooden and other flammable

materials. Exacerbated due to blocked streets and damage water supplies

Tokyo 1923, lunchtime. Wooden fuel, typhoon created fire storms, water main werebroken

San Francisco, 1906700 deaths and $400 million in property damage due to fires3.5 Factors Affecting Damage

Natural phenomena only become natural hazards when humans are affected Several factors: population density, prediction abilities, geology and topography,

magnitude, preparation, governance and economic ability, building design, time of

earthquake

4. Managing Earthquakes4.1 Earthquake Forecasting and Prediction

4.1.1 Earthquake Forecasting Identifies areas prone to earthquakes and man-made structures vulnerable

to damage from earthquake shaking. Can be used to develop building codes

and response plans

Less precise, long term, based on seismic gaps4.1.2 Earthquake Prediction

Calculating likelihood of an earthquake of a certain magnitude in a giventimeframe. Scientists monitor earthquake precursors.

Animal behaviour: Suspicious animal behaviour before onset of earthquakes1975 EQ in Haicheng. Animals 7.5 times more likely to go missing a weekbefore an earthquake75% accuracy

Tiltmeters: earthquakes are accompanied by tiny tilts of the earths surface,so these are used to measure variations and changes in slope

Seismic monitoring: use of seismographs e.g. Global Seismic Network.Singapores Meteorological Services Division has 7 seismic stations

Recurrence Intervals: Average times between ruptures are recurrenceintervals, used to calculate probability. Seismic gaps are areas which are

likely to break badly in future e.g. 1975 research in Los Angeleseight

major earthquakes since 565, spaced at intervals of 55 and 275 years


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Foreshocks: many large earthquakes are preceded by foreshocks. Past dataallows calculations whether small foreshocks will result in a large mainshock

later, such as the San Andreas fault

4.1.3 Problems Associated with Earthquake Prediction There is currently no foolproof method Animal behaviour failed to predict the 1976 Tangshan earthquake, no

foreshocks nor precursors

Recurrence intervals and foreshocks are only averages, not precise e.g.Parkfield California, predicted in 1993 but only struck in 2004

4.2 Mitigating Earthquake Hazards4.2.1 Building Design

Isolated-base technologyflexible support placed between structure andfoundation, counteracting movement of seismic waves and preventing

resonance

Work well in new buildings, but most structures in earthquake prone zoneswere built before such techniques were developed

4.2.2 Hazard Mapping Show hazards from earthquakes that experts agree could occur Useful in identifying areas prone to liquefaction, landslides and ground

shaking, in order to set insurance, develop safety codes and identify safe

locations

4.2.3 Controlled Earthquakes Pumping water into ground under high pressure to release pressure and act

as lubricantold oil wells in Colorado and South Africa

However, the magnitude might not be able to be controlled, and can resultin more damage

4.2.4 Evacuation Measures Earthquake drills educate population, properly designed warning system, so

evacuation is possible.

Haicheng 1975, buildings evacuated several hours before earthquake.Successful because a variety of signals were monitored

4.3 Earthquake Response Response efforts occur in stages: search and rescue, immediate relief such as

medical attention, shelter and food, reconstruction, recovery and long term

development

Immediate relief: food and water, hygiene and disease, shelter, medical care,communication, crime (looting) and psychological support

Extrusive Volcanism

1. Components of Volcanic Eruptions1.1 Lava Flows

Magma is molten rock beneath earths surface. Magma is less dense thansurrounding rock, moving towards surface, upon reaching is called lava

1.1.1 Types of Magma Three distinct types, depending on their silica content: basaltic (50%),

andesitic (60%) and rhyolitic (70%)


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Finer grained rocks have lesser time available to crystallize because theycool at the surface (basalt, andesite and rhyolite)

Coarser grained rocks cool underground and thus produce larger crystals(gabbro, diorite and granite)

Basalt, being fluid, limited time for crystallization. Rhyolitic magma is moreviscous, flows less readily and has more time to crystallize. Thus basalt and

granite are more common than gabbro or rhyolite

1.1.2 Types of Lava Less fluid magma usually solidifies underground, intrusive volcanism Fluid magma more likely to make way to surface to form lava flows, such as

basaltic magma forming pahoehoe and aa flows

Underwater, most are pillow lavas1.1.3 Pasty LavaHigh Viscosity

Restricted to continental edges and strings of islandsCarribean, Japan Piles up around vent as lava dome, made of rhyolite

1.2 Pyroclasts Pulverised rock and lava, deposit of pyroclasts is tephra

1.2.1 Types of Pyroclasts Ash (64mm) Ash falls occur when ash ejected into atmosphere settles over wide area.

Ash flow are clouds of ash and gas flowing along land surface

Bombs are twisted, globular shapes which cooled while being ejected,blocks are angular pieces of rock ripped from volcano during eruption

Sorting of materialheavier material falls closer to volcano1.2.2 The Generation of Lahars/Mudflows

Large composite volcanoes can form mudflows or lahars When ash and debris become saturated with water, such as snow or ice

melt due to eruption (Mount St. Helens in May 1980, 30km/h), lahars can

form destroying homes and infrastructureup to 100km/h

1.3 Gases1.3.1 Composition of Volcanic Gases

Largely water vapour, then carbon dioxide, sulphur and nitrogen1.3.2 The Generation of Nuee Ardente/Pyroclastic Flows

Formed when hot, incandescent gases combine with rocks and ash Due to the hot gases, they travel extremely fast due to being almost

frictionless, up to 200km/h, found 100km from source2. Types of Volcanic Eruptions

Can be mild or violent, depending on the nature of the magma2.1 Magma and Viscosity

Viscosity depends on silica content of magma. Rhyolitic magma is thus viscous andforms short, thick flows but basalt is more fluid and travels longer up to 150km

Higher temperature = lower viscosity and longer flows The greater the gas content, the more fluid magma is

2.2 Magma and Nature of Eruption Depends on viscosity and gas content of magma At higher pressure, more gas can be dissolved in magma. As magma rises up,

pressure is largely reduced, allowing gases to be released


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These gases form bubbles. Fluid magma allows gas to escape readily, but viscousmagma impedes the escape of gas. Thus, fluid magma eruptions are less violent, but

viscous magma collect gases as bubbles which increase in pressure, resulting in

more explosive ejections

Furthermore, viscous magma is likely to clog up vents, such as in lava domes,building up even more pressure

2.3 Types of Eruption Basaltic lava tends to form shield volcanoes and runny lava. Eruptions such as

Icelandic (basalt plateau), Hawaiian (shield, runny flows) and Strombolian (explosive,

frequent gas explosions of runny lava) are attributed to basaltic lava.

Rhyolitic magma tends to form composite volcanoes with viscous lava. Eruptionsinclude Vulcanian (violent, viscous lava with many pyroclasts), Vesuvian (more

violent, powerful blasts of gas) and Plinian (most explosive, greating great clouds of

gas and debris and pyroclastic flows).

3. Features of Extrusive Volcanism3.1 Shield Volcanoes

Basaltic lava form broad, domed structures called shield volcanoes Average surface slope of a few degrees, normally less than 10, but wide base over

100km in diameter

Small percentage of pyroclastic material, largely successive layers of basaltic lava,which form thin sheets over large distances, such as Mauna Loa and Mauna Kea

Convex slope, since it flows readily at summit but as it cools, becomes more viscousand so slope angle increases near the base

3.2 Composite Volcanoes Stratovolcanoes are formed by relatively viscous andesitic or rhyolitic magmas Large and symmetrical, concave slope formed by alternate layers of lava and tephra Sorting of pyroclastic material with more bombs and blocks near summit, gradually

sorting to ashes

Steep summits and flatter bases due to this sorting, concave profile May form lava domes, plugging the central vent Mount Mayon and Fujiyama, Vesuvius, Pompeii

3.3 Cinder Cones Volcanic peaks consisting of pyroclastic cinders Pyroclasts accumulate around vents after being ejected by eruptions Form small, steep-sided cones of about 33 degrees depending on angle of repose of

materials Parasitic cones on or near larger volcanoes, often in groups. Many form within

calderas of larger volcanoes, final stage of activity

Wizard Island in Crater Lake, Oregon, formed after Mount Mazamas summitcollapse to form caldera

3.4 Basalt Plateau Largest amount of volcanic material is exuded from fissures in the earth Very fluid basaltic lava, successive flows building lava plains (Deccan Plateau). Can

flow up to 150km from source

Basalts are resistant to erosion while surrounding rock may not, and thus can formplateau basalts

3.5 Calderas


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Circular depressions in volcano summit, normally composite Formed when summit collapses into empty magma chamber below after an

eruption (Crater Lake, Oregon)

4. Volcanic Hazards Lava flows are a hazard to property, confined to the slopes only. At frequently active

volcanoes, lava flows are generally well understood by residents

Ash fall, extending 1000km away, can bringing total darkness, suffocating animals,smothering plant life and preventing machinery use

Pyroclastic flows and mudflows, greatest hazard, developing rapidly, up to 200km/h5. Volcanic Hazard Management

5.1 Prediction of Volcanic Eruptions First outburst of activity can be predicted, mostly fluid eruptions impossible to

predict subsequent direction or intensityKilauea, Hawaii, Nov 1959 forecasted

Viscous magma still cannot be predictedNevado del Ruiz, Columbia, Nov 1985,killed 20000 in heated mudflows

5.1.1 Land Deformation Measurement Ground deformations around volcanoes due to underground movements of

magma. Mt St Helenstiltmeters 0,5-1.5m a day preceding eruption

Tiltmeters successfully predicted Kilauea5.1.2 Seismic Activity Monitoring

Magmas can apply stress to rocks, fracturing them. Such earthquakes occurat depths of less than 10km, low magnitude

Volcanic tremorslong period vibrations indicating resonance (predictedMt Redoubt in Alaska on 2 Jan 1990), and regular vibrations indicating

origin and nature of magma

Not all activity associated with volcanismcan indicate cessation of activity5.1.3 Geomagnetic and Geoelectric Effects

Volcanoes contain ferromagnetic materials, changing local magnetic field.Magnetism reduces with temp, decreasing field may indicate rising magma.

Field may increase due to piezomagnetism as pressure and stress exerted

Resistivity of subsurface layers of volcano. Telluric currents may indicatenatural conduits for magma movement

5.1.4 Gases Analysis Analysing gaseous constituentsrestricted by need to analyse instantly

5.2 Volcanic Hazard Mitigation Look at measurement of slopes to indicate buildup of magma, seismometers and

seismographs (long period event, resonance, compression, Bernard Chouet),

analysis of gas activity and content (Williams, fumaroles, but may not be accurate

due to clogging of vents)

Hazard Management: Evacuation (Mount Pinatubo), planning beforehand, diversionof lava flows and mudflows (Sakurajimas drainage channels to divert lahars but cost

a lot of money, Icelands cooling of lava flow to solidify, requires a lot of water)

5.3 Response to Volcanic Eruptions Lava flows are likely to follow existing valleys, can be diverted or cooled Eruptions cannot be contained or directedevacuation considered. Need adequate

morgue facilities, local emergency facilities for burns or lungs damaged by hot ash,apparatus for emergency workers and civilians like face masks


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Local guidelines for heavy ash fall following Mount St Helens in 1980. Sweep ashfrom roofs, authorities equipped to measure levels of toxic gases, analyse particle

size. Alternative sources of drinking water located. Transport may not work due to

ash fallcan also interfere with radio and TV transmissions.

Classification of Rock Types

Rock cyclemovement of material through space and time as it is transferred and transformed from one

type of rock in one location to other rock types and places.

1. Igneous Rocks Solidification of molten magma or lava. Most common rock type. Crystal of minerals, often silicates. Crystallisation of these materials causing rock to

solidify

Nature of rock dependent on mineral content and rate of crystallisation1.1 Intrusive vs. Extrusive Igneous Rocks

Intrusive rocks were formed under the surface, and mostly have large crystals(phaneritic) due to slow cooling and crystallization underground (thousands to

millions of years). Granite. Coarse texture.

Extrusive were formed above surface from cooled lava. Microscopic crystals(aphanitic) due to quick cooling. Basalt. Fine texture.

2. Sedimentary Rocks Distinguished by strata present in rocks, separated by bedding planes.

2.1 Types of Sedimentary Rocks2.1.1 Sedimentary Rocks of Mechanical Origin

Rocks where the constituent material has been derived from elsewhere andtransported as solid particles to the ultimate site of deposition

Detrital/clastic rocks Loose clasts or organic material undergo diagenesis, causing clasts to bind

together, and in the process of lithification turn into rocks

Diagenetic processes occur by compaction (buried under great pressure)and cementation (minerals crystallizing in pore spaces, cementing clasts

together)

Transporting agents of wind and water tend to sort particles by size, so sizeis a subdivision for clastic rocks

Small (2mm,conglomerates, breccias)

2.1.2 Sedimentary Rocks of Chemical Origin Precipitation from a solution of dissolved salts, of chemical origin Formed close to site of deposition and mixed with detrital sediments E.g. evaporatesformed by evaporation of salts in shallow seas (anhydrite,

gypsum, halite) and lithified

2.1.3 Sedimentary Rocks of Organic Origin Formed by accumulation of organic matter remains, such as fossils Limestonefrom remains of sea creatures subjected to diagenetic

processes, calcium carbonate and calcareous rocks

3. Metamorphic Rocks


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Changing mineralogical composition or physical structure of rock via high pressure andtemperature

Uses tectonic forces, such as plate movement, to compress and heat rocks. Happens inthe solid state

3.1 Contact vs. Regional Metamorphism Contact metamorphism only involves extreme heat, not pressure. Grade of

metamorphism depends on distance from heat source.

Chemical content does not change, but can be altered, such as water composition orrecrystallisation e.g. limestone to marble

Regional metamorphism involves both temperature and pressure, and often occurat convergent plate boundaries

Gradual increases in heat and pressure can lead to metamorphic gradation Recrystallisation perpendicular to compressional force can lead to layered

appearance, or foliated texture

Weathering

The in situ breakdown of rock by natural agents. Response by rocks at surface to low temperatures,

pressures, and the presence of air and water. Denudation of the landscape.

1. Geometry of Rock Breakup1.1 Block Disintegration

Breaking down of rocks into large blocks, common in rocks with well developedbedding planes or joints intersecting at right angles

Concentrated weathering at secondary joints leads to large, angular boulders, suchas limestone

Commonly the first stage, followed by other modes of weathering1.2 Granular Disintegration

Rock is broken down into numerous smaller fragments into grains, common incrystalline rocks like granite and sedimentary rocks like sandstone

Grains separated along the original crystal or grain boundaries1.3 Exfoliation

Detachment of concentric slabs from the rock mass, leaving behind smallerspheroidal bodies. Also known as spalling.

Thickness depends on processinsolation weathering leads to smaller layers,pressure release leads to thicker layers

1.4 Frost Shattering Disintegration of rock along new surfaces of breakage to produce highly angular

fragments with sharp edges. Irregular because they do not break along defined

planes of weaknesses

1.5 Spheroidal Weathering Rock rounded from an initial block shape, as a result of uneven weathering on the

rock surface, with edges and corners being eroded more rapidly

2. The Processes of Weathering2.1 Physical Weathering

Uses mechanical force to break up the rock, often depending on temperaturefluctuations to produce stresses, thus superficial and occurs only near surface

2.1.1 Pressure Release / Dilatation


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Breaks down rock through exerting physical stress. Can lead to sheet jointsor exfoliation

Regolith above rock removed by erosion, resulting in lesser pressure andexpansion of rock, potentially fracturing it

Can result in sheet joints, aiding the weathering process In extreme situations, exfoliation occurs forming exfoliation domes,

followed by block disintegration into smaller rocks

Rocks formed at great depths are particularly susceptible Can occur on a micro scalegranular disintegration

2.1.2 Freeze-Thaw Weathering / Frost Shattering Water from precipitation enters joints and beddings in rocks. Upon freezing,

expands about 9%, exerting pressure

Closed system generates pressure which can easily exceed rock tensilestrength

Repeated stress with each cycle of thawing and freezing can lead tocryofracturing over time

Can occur on a smaller, granular scale when water penetrates pore spacesand freezes into ice crystals, such as in chalk

Requires oscillation about freezing point, such as a wide diurnaltemperature range in Alpine environments

Moisture content of rock is importantif not saturated, freeze-thaw haslesser effect since the pores can absorb water

2.1.3 Insolation Weathering Disintegration of rocks due to expansion and contraction through heating

and cooling, effective in large diurnal temperature range (deserts)

Rock is poor conductor of heat, so heating is confined to surface. Sharpthermal gradient develops, with surface expanding more than within,

causing stresses to develop

If stresses exceed strength, sheet joints form leading to exfoliation of thinlayers (since limited to surface)

Can result in granular levelsif grains are made of different colours, darkercolours expand more, such as darker mica than sandstone within granite

Efficacymay not be very significant since exfoliation is on a very smallscale. Also, Blackwelder and Griggs experiment showed that water is more

significant (no change even with 100C of changes for 244 years)

However, laboratory results do not reflect real conditions, such as stressfrom rocks surrounding, or not really 244 years of weathering experienced

2.1.4 Salt Weathering Physical weathering, although chemical reaction is involved When water within rock is saturated with salt, salt will crystallize and exert

pressure on rock

This process, repeated over time with salt crystals growing, can split rocks Often cause honeycombing patterns Important in arid environments, as groundwater is brought to surface by

capillary action, evaporating and leaving salt behind

Salt inflicts thermal expansion or by wetting and crystallizing


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Coastal deserts are susceptible due to availability of salt water and hightemperatures

2.2 Chemical Weathering Breakdown of rocks by altering chemical composition of minerals by water, oxygen,

acids. Can occur at great depths due to infiltration and percolation of water

Thus dependent on availability of water. Produces fine grained regolith2.2.1 Solution

By soil moisture and groundwater. Quartz can dissolve in water to give silicain solution

2.2.2 Carbonation Weakened by carbon dioxide dissolved in rainwater. Calcite (limestone)

reacts with carbonic acid to form calcium and calcium bicarbonate, which

can dissolve in solution with water

2.2.3 Hydration Affects rocks which can take up water, absorbing water into minerals. Can

cause expansion of a mineral. Iron oxide is hydrated by water to give

hydrated iron oxide

2.2.4 Hydrolysis Reaction with pure water. Feldspar reacts with water to give kaolinite as an

eventual end product

2.2.5 Oxidation Reaction with oxygen from soil or atmosphere. Rusting of iron with oxygen

to give iron oxide

2.3 Biological Weathering Any weathering carried out by living organisms or their by-products Biomechanical weathering: carried out by plant roots prising and breaking rocks

apart, common in urban areas. Opens passageways to allow for water and other

forms of weathering

Biochemical weathering: by plants, organic matter creating organic acids to carryout chemical weathering and chelation

2.4 Classification of Weathering Most rock disintegration is affected by complex interplay of all three processes,

difficult to truly distinguish them

Operate in conjunction to assist each other3. Climate and Weathering

Determinants of the rate and type of weathering: rock characteristics, climate,geological structure, vegetation and soil cover, level of water table, topography of local

areas, mans activities. Climate is one of the most important.

3.1 Weathering in Different Climatic Zones Much weathering depends on water and temperature Differences in precipitation and temperature thus has effects on weathering in the

different morphoclimatic regions

3.1.1 Weathering in the Humid Tropical Regions High temperature results in faster chemical weathering (vant Hoff: speed

of reaction increases 2.5 times with rise of 10C)

High precipitation also results in more chemical weathering (solution,hydration, hydrolysis etc.)


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Dense vegetation and organic matter helps chemical weathering, almost 4times as rapid in humid tropics than temperate regions

Physical weathering limited due to masking effect of thick regolith coveringsurface

Uniformly high temperature does not support it either Results in thick layer of regolith due to deep chemical reaction. Regolith

removal is slow due to vegetation, so there is a build-up of regolith

3.1.2 Weathering in the Seasonally Humid Tropical Regions Due to heavy seasonal rainfall, chemical weathering is also rapid, but not as

much as the tropics

Less dense vegetation also means easier removal of regolith and thusthinner layer, which can expose the basal surface of weathering

3.1.3 Weathering in the Hot Arid Environment Physical weathering dominant due to high range of diurnal temperature,

especially insolation and salt weathering

Only some chemical weathering, due to sources of moisture such asinfrequent rains, dew and fog

In general, low rates of weathering and largely superficial, so regolith is veryshallow. Furthermore, no cover or organic material means it is removed

without time to accumulate

3.1.4 Weathering in the Temperate Regions Chemical weathering is only moderately active due to moderate

temperatures and rainfall

Physical weathering can play an important role through freeze-thawweathering in winter, but it rarely goes to any great depth

3.1.5 Weathering in the Glacial Regions Abundant snowfall and low temperatures oscillating around 0C means

freeze-thaw weathering is dominant in glacial regions

In summer, rain falling can dissolve carbon dioxide and oxygen to formweak acids

3.2 Peltiers Diagrams Shows relationship between climate and both types of weathering

3.3 Strakhovs Diagram Precipitation and average temperatures correlation with basal surface of

weathering (divides weathered from unweathered rock)

4. Rock Characteristics and Weathering While on macro scale, climate differences affect weathering, on a local scale,

weathering is more influenced by rock type

4.1 Rock Strength and Hardness Harder rocks are more resistant to physical weathering, depending on minerals

making up the rock and strength of cementation between minerals

Older rocks are normally harder since they undergo more cementation andcompression

4.2 Chemical Composition Affects resistance to chemical weathering, determines if minerals are susceptible or

not Limestone prone to carbonation due to being mainly calcium carbonate


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Sedimentary rocks may have resistant clasts, but not resistant cement May affect physical weathering, such as different coloured minerals affecting

expansion and contraction, leading to granular disintegration

4.3 Rock Texture Coarse or fine grained. Coarse grained rocks allow for chemical weathering to

reduce coherence, and large pore spaces allow for high primary permeability,

trapping water for chemical and frost weathering

Numerous boundaries between fine grains increases surface area for chemicalagents, speeding up chemical process

4.4 Rock StructureJoints and Beddings Selective weathering along lines of weaknesses in rocks, high secondary

permeability allowing water to easily penetrate, increasing surface area for both

chemical and physical weathering

5. Other Factors Affecting Weathering Topography affects weathering, as steep slopes aid to remove regolith Altitude affects weatheringabove the tree line, temperature is suitable for freeze-

thaw, but too high is not because of lack of oscillation around 0

Aspect affects weathering, like whether it is on north-facing or south-facing sides6. Landforms Associated with Weathering

6.1 Scree/Talus Slopes and Block Fields Commonly associated with freeze thaw weathering. Screes or talus are made of

angular fragments of rock accumulated at the bottom of steep slopes

Can form blockfields on gentler slopes6.2 Exfoliation Domes

Formed by exfoliation of massive rock, like granite, due to pressure release andunloading, sheeting

6.3 Limestone Pavements Formation by chemical weathering along joints. Limestone surface exposed to reveal

joints, which are enlarged via carbonation, forming clints and grykes

Mass Movement

1. Initiation of Mass Movements Mass movement is downslope movement of weathered materials in response to

gravity

Dependent on shear strength and shear stress of slope1.1 Shear Strength vs. Shear Stress

Depends on instability on hill slope when equilibrium has been disturbed Safety factor is measured by ratio of resistance against movement to force trying to

enact movement (shear strength against shear stress)

Speed of movement depends on how much stress exceeds strength1.2 Factors Affecting Mass Movement

1.2.1 Gravity, Slope Angle and Shear Stress Gravity induces movement downslope, depending on angle of slope and

weight of regolith

Angle of repose is when stress = strength, when friction balances gravity Regolith is pulled down faster on steeper slopes

1.2.2 Nature of Slope and Shear Strength


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Sand and gravel slopes generate friction between particles Silt and clay slopes depend on cohesion and water Rock slopes depend on internal strength of solidification and crystallization Bedding planes or joints of weakness might focus failure on these areas

1.2.3 Role of Water Water increases stress while decreasing strength Rainfall can saturate soil, reducing cohesion due to pore pressure between

pore spaces. It also lubricates, reducing friction. Also increases weight

A bit of water is still necessary for maintaining cohesion in clays1.2.4 Role of Triggering Mechanisms

Earthquakes can trigger mass movements by vibrating and shaking regolithmaterials loose, reducing friction

Undercutting of slopes can also trigger2. Classification and Types of Mass Movement

2.1 Carson and Kirbys Classification of Mass Movement Plots mass movements along a continuum, flexible classification according to speed

and moisture content

2.2 Mass Movement Processes2.2.1 Soil Creep

Slow but widespread and highly effective. More material is moved by creepthan any other means. Creep is faster in dryer, colder areas (10mm/year)

Gravity creep and soil heave. Gravity creep occurs when soil particles aredisturbed by flora and fauna, which then move downslope because of

gravity. Chain movement of particles continues until initial movement is

absorbed

Soil heave happens because of expansion and contraction, due to heatingand cooling, wetting and drying or frost action.

Expansion heaves at right angles to surface, but contraction is affected bygravity to give a net downslope migration

2.2.2 Solifluction Common in periglacial areas. Waterlogged soil slides slowly over the

impermeable permafrost, resulting in solifluction lobe

2.2.3 Fall Occur on steep slopes where angle of friction is greatly exceeded. Slope

made of hard rocks which are able to maintain high angles

Weathering allows detaching of rock to fall due to gravity. Falls until itreaches its angle of repose

2.2.4 Slide Sudden and rapid, occur at high relief and unstable slopes. Triggering action

is usually needed

Mass slides down a shear plane until it shatters at the bottom. Slide planescan be lubricated or selectively weathered

Landslides occur in sands or clays, due to buildup of groundwater,increasing stress while decreasing friction strength

Common when weak layers support heavier ones above2.2.5 Slump


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Occur in weaker rocks than slides, have a rotational movement along acurved slip plane, resulting in terraces and a flow at the bottom

Occur where moisture is concentrated at base of water soaked clay rich soil When the lower toe becomes mobile due to water, after heavy rain, it flows

away, resulting in material slumping away from the top

2.2.6 Flow Soil moisture content is high, rapid form of movement. Flow is greatest at

surface and decreases to zero at the bottom

Internal deformation under its own weight, dependent on saturation ofwater, like clay, so fine particles are prone to flows

Earthflows are linear movements of moist clay rich regolith. Slower thanlandslides, a few feet/hour, day, or even month

Mudflows are more rapid and less viscous, occur in areas with sparsevegetation, and is thus quickly saturated

2.2.7 Others Debris and snow avalanches are also possible

3. Human Activities and Mass Movement3.1 Human Induced Mass Movements

3.1.1 Undercutting and Mass Movement Undercutting a previously stable slope, such as building roadways on

mountainous terrain

As a result, landslides are more common, and slopes are more saturated 12 September 1995, Kulu, Himachal Pradesh, India, landslide killed 65

people due to undercutting of slope

3.1.2 Construction and Mass Movement Clifftop buildings increases the stress on slopes, increasing instability Holbeck Hall Hotel in Scarborough, 5 June 1993, rainfall plus hotel caused

the ground to slump

3.1.3 Deforestation Due to Population Pressure Cities like Hong Kong and Rio de Janeiro expanding onto marginal land to

accommodate population pressure. New roads and buildings built on

deforested steep slopes, which reduces stability

4. Managing Hazards Associated with Slope Failure Worldwide, landslides have caused average of 7500 deaths/year and US$20billion per

year from 1980-2000. Landslides increase as more people settle in less suitable areas

most deaths occur in LDCs.4.1 Predicting/Preventing Slope Failures

4.1.1 Landslide Hazard Maps Avoid building in places prone to landslidesGIS can used to make debris

flow and landslide hazard maps, prescribing restrictions in land use like road

building, timber harvesting, housing subdivisions

4.1.2 Controlled Development Pre-construction assessment: Study of area before construction, geologic

feasibility report. Modify landscape as little as possible

Controlled development: Controls on hillside development through zoninglaws (affluent countries: hillside properties for scenery, congested cities:


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slums on hillside like favelas) Should not be any construction steeper than

27 degrees

4.1.3 Slope Monitoring Monitoring: extensometers and tiltmeters to detect micromovement on

slopes

Modelling: Computer modeling to simulate scenario of mass movement,useful in land use planning, hazard planning and evacuation plans

4.2 Mitigating Slope Failures4.2.1 Improving Drainage

Drainage: Proper drainage required to prevent saturation of soil and reducewater pressure, such as outlets and culverts

4.2.2 Improving Vegetation Cover Vegetation: Should be left in natural state to enhance drainage and increase

stability by removing water through evapotranspiration. Can also stabilize

slopes.

4.2.3 Construction of Retaining Structures Retaining structure: Holds back earth and stabilizes soil and rock from

downslope movement, such as gabions and walls

Debris catch or dam: Structures used to catch falling material and trap flowdebris, such as wire netting, dams and barriers

4.2.4 The Use of Weights For slopes overloaded at the top, add load to the lower part of the slide to

resist movementpile heavy boulders on toe to increase stability. Angle

can be changed by removing slope top, adding weight to base, remaking

entire slope with lower angle.

4.3 Responding to Slope Failures Search and rescue, provision of medical care, food, shelter, water, long term

recovery. Like earthquakes and volcanoes.

Limestone and the Karst Landscape

1. Limestone1.1 Formation of Limestone

Calcite deposition in deep-sea conditions, calcareous remains of plant and animals Skeletons filled with mud and precipitates, diagenetic processes form limestone

1.2 Types of Limestone Importantly, carboniferous limestone forms landforms, but not oolite

1.3 Characteristics of Limestone Carboniferous limestone and dolomite

1.3.1 Chemical Composition Limestone at least half the rock contains more than 50% carbonate

minerals, with calcite as most common, pure limestone is at least 90%

calcite

Physically resistant to weathering, but chemically unstable, carbonation Landform associated with limestone is probably solutional and found in

humid or temperate regions

1.3.2 Structural Control Low primary permeability and high secondary permeability


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Selective weathering at joints, not uniform weathering, will occur.Carboniferous limestones secondary permeability is much higher than

oolite, explaining landform formation in carboniferous and not oolite

Older the rock, lower primary and higher secondary, due to increasedlithification with age

1.3.3 Weathered End Product Leaves behind little impurities after weathering because most is removed in

solution, but still leaves behind insoluble calcite which will blanket the

surface, preventing further erosion

Leaves behind little regolith, in contrast with granite2. Karst Landforms

2.1 Enclosed Depressions Common in karst areas, enclosed basins where precipitation is drained internally by

subterranean conduits. Extent of depression depends on rainfall, thus determining

types of landforms in temperate and humid

2.1.1 Dolines Medium sized closed depressions Percolating rainwater causes selective weathering at fissures and bedding

planes, especially where groups of fissures are

Rate of solution becomes greater due to increased surface area, voidcreated and subsides, producing depression

Throughflow directed towards base of hollow, enhancing solution andcontinuing cycle

2.1.2 Uvalas A combination of dolines into areas with sub-basins and uneven floors,

increasing size but decreasing number of depressions2.1.3 Cenotes / Collapse Dolines

Depressions with circular, smooth-walled vertical shaft, develop wherewater filled cave just below ground

Fractured rock weakened by percolating rainwater and upward solution ofcave water, collapsing and producing a shaft

2.1.4 Cockpit Karsts Same as dolines, but torrential nature of rainfall in humid tropics causes

surface flow and weathering via surface gullying along joints, causing

elongated depressions along joints which eventually interconnect, forming

deep irregular cockpits separated by cones2.1.5 Tower Karsts

Further weathering of cockpits until base level of erosion is reached,clogging up the floor with impermeable calcite. Weathering then forms the

cones into tower karsts

2.2 Karren Features Microsolutional features which form on exposed limestone surfaces Clints and grykes, limestone pavements, exposed plain of limestone caused by

glacial erosion. Blocks and grooves created by joints are accentuated by chemical

weathering

Karren forms, such as spitzkarren in tropical and Mediterranean areas whereprocesses have operated for a long time in high rainfall


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Grykes bounding clints are widened and deepened by erosion2.3 Drainage Features

2.3.1 Karst Gorges Karst landscapes may contain major rivers, due to being in early stages of

karstification when fissures are not yet developed enough to absorb rivers

Valleys eroded by streams are gorges, with vertical cliffs and scree slopes Well-jointed limestone maintains verticality by falling when weakened, no

other mass movement results in no gentle sloping

2.3.2 Swallow Holes, Dry Valleys and Blind Valleys Rivers flowing on limestone will eventually create openings in rock bed due

to selective weathering, creating a swallow hole which leads into

subterranean drainage

Downcutting of valley will stop downstream, forming a dry valley and maycreate a cliff due to reversal of gradient

Upstream, vertical erosion continues, downcutting the blind valley Successive sinks cause headward migration of stream and developing new

blind valleys

Streams may eventually reemerge as resurgent streams2.4 Caves

Limestone caves are subterranean stream networks, carved out by the water itchanneled

Groundwater dissolves rock along joints and bedding planes, forming large cavities Cave conduits are formed due to low primary and high secondary Migration of water table can cause caves to form at different levels Speleothermsdepositions of calcite. Water supersaturated with carbon dioxide

will cause crystallization of calcite, forming stalactite on ceilings, and water drippingonto ground forms stalagmites, eventually joining to form columns

Granite and Associated Landforms

1. The Formation and Characteristics of Granite1.1 Formation of Granite

Intrusive igneous rock formed from solidification of rhyolitic magma underground Solidifies to form batholiths, plutonic features Exposed after denudation of landscape Especially prone to pressure release as a result

1.2 Characteristics and Weathering of Granite1.2.1 Chemical Composition

Quartz, feldspar and other minor minerals Prone to chemical weathering of hydrolysis: Feldspar -> kaolinite clay Forms gruss, residual debris, within which are embedded corestones due to

block and spheroidal weathering

1.2.2 Rock Texture Phaneritic, large crystals

1.2.3 Rock Structure High secondary permeability due to shrinkage joints and sheet joints as a

result of cooling and pressure release

Selective weathering along joints, block disintegration and spheroidal


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2. Granite Landforms2.1 Landform Development in the Humid Tropics

High temperatures and rainfall cause rapid chemical weathering, resulting in deepregolith of saprolite

2.1.1 Model of Deep Weathered Layer Ruxton and Berrys time dependent model of weathering Mature stage: Zone 1 is residual debris, Zone 2 is residual debris with

corestones, Zone 3 is corestones with residual debris, Zone 4 is partially

weathered rock

2.2 Landform Development in the Seasonally Humid Tropics Thinner regolith than humid trops, due to lesser chemical weathering during

drought period, as well as lesser vegetation to prevent surface runoff from removing

the regolith layer, may expose basal surface of weathering

2.2.1 Tors Small hills or heaps of boulders rising abruptly from surface Exposed by stripping to basal weathering surface e.g. Zimbabwe, Dartmoor

2.2.2 Inselbergs Steep sided isolated hills Ruwares are incipient inselbergs, with smooth convex surfaces. In

etchplains (land surfaces with more than one phases of deep weathering

followed by removal of regolith), pluvial periods cause dominant selectvie

weathering where joints are numerous. Interpluvial periods cause surface

wash to strip regolith due to degenerating vegetation. Undulating basal

surface of weathering exposed the ruware. With repeated cycles of pluvial

periods, ruware becomes higher

Bornhardts are the next stage, where heights can exceed 300m with aconvex summit with rock slabs due to sheet joints because of pressure

release, but the rock dome is otherwise very durable

Blocky inselbergs resemble tors, where rectangular jointing is prominent,with selective weathering giving is similar appearance to tors

Castle koppies are degraded, old inselbergs subjected to weathering. Low,irregular hills

2.3 Landform Development in the Temperate Regions2.3.1 Temperate Tors

Dartmoor. Probably due to previous climates, when warmer, more tropicalclimates were experienced by current temperate areas

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