AS Level CIE Geography: Rocks and Weathering

litosphere:

upper part of planet earth. colder and more rigid than below. ranging thickness

asthenosphere:

below lithosphere, boundary between the two 1300C isotherm

what does asthenoosphere do

divides colder, more rigid rocks from hotter more plstic rocks below. no change in composition

litosphere is divided into..

number of plates that move relative to one another.

plate tectonics:

lithospheric plates are in motion and thats responsible for the formation of major landforms.

continential drif:

scientific theory that earths continents move/drift relative to each other.

why are most ideas abt plate movements related to convectional 1movemen1s of material in earths matle

deep within earth, heat is being produced by radioactivity and some areas are hotter than others. at the hotter areas, the plstic rocks in earths mantle are thought to become lighter and rise causing convect1ion currents.

three mechanisms of plate movements:

ridge push, convection drag, slab pull

ridge push:

intrusion of magma 1into the spreading ocean ridges such as midatlanticridge propels plates apart

convection drag:

convection curets 1in plstic mantle drag overlying lithosphere. heat source and rising limbs are beneath oceanic ridges. heat is from radioactive decay in mantle. cooling and descending p1arts of cells are at subduction 1zones1.

slab pull:

cold, denser oceanic lithosphere1 sinks due to gravity into subduction zones at places s1uch as aleutian trench. thihs drags rest of plate with it.

main types of plate margin1:

convergent, divergenet, conservative, collision

convergent/destructive description and type of stress

where material is being destoryed or subducted and plates are moving together. COMPRESSION -> <-

divergent/constructive description and type of stress

where material is being adde1d nad plates are1 m1oving apart . tension

conservative description and type of stress

where pltes are slidin past one another with no material being added to or subducted from either side. shearing ->/<-

collision description and type of stress

where two continetnail plates meet and subduction has ceased. compression -><-

three types of convergent/destrcutive plate margins.

continential-oceanic convergent plate boundaries, oceanic-oceanic convergent plate boundaries, continential-continential collision boundaries

continential-oceanic convergent plate boundaries features produceed:

fold mountains, volcanic cones, ocean trenches.

fold mountains…

form the highest of the worlds mountain ranges. long, relatively narrow belts of mountains with parallel ridgees and valleys and main range is made up of series of ranges. flatter areas form plateaux

where do fold mountains form

where compression caused by plate collision has1 squeezed layers of 1rock.

normal relief fold mountains1:

when upfolds/anticlines form ridges and downfolds/synclines form valleys

where does normal relief fold moutain folding taatke plce

at great depths of earth where high temps and pressure makes rock behave as a plstic solid.

as well as fold mountain being uplifted…

material at depth is forced downwards leading to thickening of earths crust inmoutnain belt

accretionary wedge/prism:

deposited in adjacent ocean and trencha nd scraped up against leading edgeof continential plate and added to it

active volcanoes form..

high conical mountains in the ranges usually stratovolcanoes/composite cones made up of alternate layers of lava and ash made by explosive volcanic eruptions.

subduction:

at continential-oceanic plate margin , denser oceanic plate is forced beneath the less dense continential plate.

orogenesis/moutain building process:

oceanic plate absorbed into mantle and destoryed, subducted plate and overlying amntle partialy melted and smal pockets of magma gradually merge w each other and begin to rise to form volcanoes.

what happens of shore C-O

no wide continential shelf and ocean floor drops steeply into long narrow ocean trench parallel to fold mountains. trenches are bc of subduction.

benioff zone:

sloping zone of earthquake foci

O-O plate boundaries features:

strato-volcanoes, ocean trenches, accretionary prisms, benioff zone, island arcs and ocean trenches.

island arcs:

island chains w the same convexo-concave form amde of active strat-volcanoes but also by some sediments int he accretionarry prism.

how do island arcs form:

same way as two converging plates are both oceanic,rocks at edges of plates have same density

what partis subducted in O-O

always larger section of plate from ocean side becausegreater mass. heat flow is less than nomrla over trench w cold descneding slab and higher thannormal over volcanic island arc.

C-C features:

ridges, fold mountains, shallow intermediate foci

what is common at C-C

earthquakes because of compressional stresses.

divergent plate boundaries features:

oceanic ridges, most prominent topographic features, hidden beneath ocean surface

why do divergent ridges have higher relief than the rest of the oceanfloor

they consist of relatively hot thermaly expanded rock - area of high heat flow

why does ocean become deeper further from ridges

crustal material moves away from ridge and cools, contracts and becomes lower.

double central ridge separated by:

ridgevalley

transfrom faults/fracture zones

faults that continuouslyoffset the ridge in divergent

why does a central block fal between parallel fault systems in divergent

result of tension/crust stretching

continental drift evidence:

coastline fit, fossils/flora/fauna, fit of orogeninc belts, fit of rock types, paleocllimatic evidence.

why is coastline fit evidence

s. america and africa fit, wegeners theory.

why is fossils, flora, fauna evidence

similarities between plantsand animal fososils found on matching coastlines of s.a and africa, impossible that they swam across and ancestral/similar animals.

why is fit of orogenic belts evidence

when atlantic coaslines fitted, blts of same group join up e.g. north appalachians,newfoundland,north brit andnorway

why is fit of rock types evidence

precambrian rocks on either side of atlantic join up when fitting coastlines

palaeoclimatic evidence:

fossils of tropical plantsin antartica = mustve been near equautor as this is theolny place it can grow. glacial depositsin e.g africa have glaciation evidence

sea floor spreading:

processof creation of new oceanic lithosphere at the oceanic ridges and the divergenceof new lithosphereon either side of the ridge.

how does sea floor spreading occur

partial melting of dark dense rocks called peridotite in upper mantle of asthenosphere beneath ridge axis in high temps and low pressure conditions so magma pockets form which colect and rise to surface and r eventualy extruded aong ride as dark coloured basalt lavas which cools rapidly in contact w sea water and develops spherica structures - pilllows. below this, some magma cools and solidifies to form rock types dolerite and gabbro. newly formed lithosphere cracks and diverges and plates on either side of ridge move slowly outward from it driven by mantle convection currents.

thickness, structure, composition, chemistry, density and age of oceanic crust

between five and tenkm, layered structure: sediment - basalt pillow lava - dolerite - gabbro, chem comp of basalt, rich in Fe and Mg, two.nine gcm, two hundred million year = oldest rock, solid

thickness, structure, composition, chemistry, density and age of continental crust

thirtyfive-ninety km, more complxstructre:igneous - sedimentary and metamorphic rock, similar to granite, rich in Si and Al, two.seven gcm, oldest fourthousand million years, solid

evidence for sea floor spreading

age of rocks of ocean floor determined by radiometric dating at crest basalts and sediments above are very young and get older away from ridge crest, thickness of sediment on ocean flloor increases w increasing distance from ridge as more time for sed to accumulate. older sedfound on sea bed at margins of ocean, direct satellite measurements of width of oceans show rate of ocean opening and closure, magnetic stripes [iron minerals in earths crust are magnetised byy earths mag field when rock is formed e..g iron rich baslt so tend to become aigned like tiny compass needles parallel to mag field palaeomagnetism/fossilmagnetism weak and permanent, and hotspots as volcanoes form over mantle plume then plate movement carries them away so age and dist apart of hot spot volcanoes can calc seafloor spread rate

palaeomagnetism:

earth acts as though it has bar magnet through centre. in passt there has been periodic geomagnetic reversl in dipoles of earths mag field as though bar magnent flipped. youngest rocks at ridge crest always have presendday/normal polarity/. stirpes of rock parallel to ridge rest alternate in magnetic polarity showing symetrical pattern on either side of ridge, stipes of unequal width, rate of spreading same but reversal no time interval

hot spots:

volcano in plate above amantle plane whihc is a stationary area of high head flow in mantle that rises from gerat depths and generates magma. e.g. hawaii and emperor seamounts/submarine volcano

weathering:

decay and disintegration of rocks in situ, involving physical, chemical nd biologica processes excluding the erosional effects of running water, rivers, sea, glaciers and wind and doesn't transport products away

regolith:

surface cover of loose unconsolidated material including aluvium, glacial deposits, wind blown sand, peat, scree and soil

Types of physical weathering:

freezethaw, heating and cooling/thermal fracture, salt crystal growth, pressure release/dilation,

freeze-thaw/frost shattering:

water trickles into cracks such as joints during the day andit freezes at night and expands by about ten % and dwidens the crack. the stress produced by this is greater than rock resistance so repeating freezing and thawin = disintegration and production of scree/talus and felsenmeer/block fields.

when is freezethaw active

not in winters in contental interiors where constant frost, must no. of freeze-thaw cycle

scree:

angular rock fragments that fall from cliffs in areas of freeze-thaw action, fals down gulleys nad is deposited as cone or fan shapes which ay coalesce with one another. comon in mountainous arious

shape and size of particles produced by freeze thaw is controlled by..

nature of rock, espeicaly lines of weaeness e..g joints, beddin planes and cleavage

thermal fracture is what + two types:

large diurnal ranges of temp in deserts cause rock annd minera to expand in day anc contract in night = disintegration. granular disintegration, block disintegration

granular disintegration rates of expansion and contraction vary between: three

diff minerals, diff axes of a crystal, crystas of diff size and diff coours as emit heat at diff rates.

granular disintegration:

complex stresses are set up in rocks = disintegration to produce minerall grains and great sand seas/ergs found in some deserts.

block1 disintegration:

produces rock fragments not minera grains and = features of stony deserts e.g. reg. scree slopes at foot of cliffs and boulder fields are also produ.

salt crystal growth:

ccurs when salt solutions in pores/joints of rocks crystallise which then expand and force rock apart.

most effective salts and conditions:

sodium sulfate magnesium sulfate sodium carboante and calcium chlorde. temps around twentyseven where temp flucs produce expansion rates of up to three hundred %

dilatation/pressure release/unloading:

affects areas whree ground surface lowered by erosion it reoves weight from deeply buried rocks e..g granite pluton formed at immense pressure several km below earth surface which leads to expansion of upper parts of granite and alows cracks to occur parallel to ground surface = pseudo-bedding planes. can also occur where horizontal pressure released by rock falls on cliff face = vert cracks = more rock1 fals.

chemical weathering processes:

hydrolysis, hydration and dehydration, carbonation,

Biological weathering processes:

organic action-vegetation roots

factors influencing weathering:

climate, rock structure, rock texture, rock composition

hydrolysis:

when mineral is broken down by reaction w water, important in silicateminerals formingmost rocks esp minera feldspar and occurs in acid conditions. LOOK AT REACTIONS IN TXTBOOK

hydrolysis texbook reaction produces what:

clay residue and various solutions, and otherclays from silicate materials eg montmorillonite which are removed in groundwater and can be found i analysis of river water.

hydration/dehydration:

wetting and drying can cause addition/removal of water from mols of some minerals = expansion or contraction = disintegration. calcium sulfate minerals anhydrite and gypsum affected LOOKA T TEXTBOOK FORULA.

carbonation:

process affects carbonate minerals making up limestone e..g cacite, calclium carbonate where rainwater containing weak acid/carbonic acid which forms as rain absorbs atmopsheric co2. carbonic acid then attacks carboate materials. calcium hydrogen carbonate removed in solution and washed away downw rives and muddy insoluble impurities in limestone = clay residue. responsible for karst.

carbonation equations in textbook

yes h2o+co2=h2co3 and h2co3+caco3=ca(hco3)2

carbonation accelerated/wewathering of limestone by

pollutatnts e..g sulfur dioxide and nitrogen oxides

1vegetation rootss:

bioloigica weathering can have phsyical effect on rocks in wedging of tree roots so where soil is shallow, seeds and roots find their wayw to natural bedrock cracks and as seeds germinate and roots grow cracks widen and deepen = break up bedrock.

o1ther bio effects assisting chemical processes:

release of humic acids by decaying vegetation encouraging hydrolysis, release of carbon dioxide by plants = carbonation, blanket of vegetation traps water and encourages variety of chem process.

how does climate influence weathering

determines which weathering processes occur and at what rate

how does rock structure influence weathering

weaknesses e.g. joints/bedding planes/cleavage allkow water penetration to increase both physical and chem effects. weaknesses also control sizze and shape of weathered fragments e..g spheroidal weathering of dolerite , formation of tors in granite and scree slopes

how does rock texture influence weathering

coarsegrained rock weather faster than their fine grained equivalent because weathering of one minera in rocks tend to weaen fabric of rock to greater degree. igneous rocks have greater resistance to physical disintegrtion than sedimentary rocks as greater strength of interlocking crystalline textures in comparison w granular ones.

rock composition influence on waethering

minerlas which form at highest temps are least stable at surface temps meaning minerals in dark coloured rocks e..g basalt weather faster than pale e..g granite

most susceptible minerals in baslt three and granite one:

olivine, plagioclase feldspare, pyroxene. biotite

least susceptible minerals in granite [none in basalt]

orthoclase feldspar, muscovite, quartz

mass movement:

the downslope movement of rock and weathered debris by gravity alone. doesn't include erosive agents e.g. runnign water/glaciers.

mass movement types:

reeps and solifluction, debris flowsa and lahars, slope failures:sslides and fails

creep:

slow, donwslope movement of unconsolidated material and soft rocks , rarely more htan one - two cm per year

how do creeps form:

clay rich material liable to plstic flow on saturated thick surface deposits ons teeper slopes or by pressure from overlying cap rocks or human construction. freezing and thawing of water in surface layers of clay/soil rpoduce heave expansion of water on freezing = bulging of suface paralele to slope, on thawing aterial drops verticaly. weting and drying = clay expand and contract = heave.

solifluction:

reep in areas of permafrost where waterlogged sumer ocnditions accelerate the creep, ay involve viscious flow

creep responsible for:

terraacettes[smal ridges across hillsides] and convexo-concave rollint landscapes

flows/mudflows:

mudflows involving rapid movement of rock and weathered debris mixed w water down valeys and dont involve shearing and are turbulent, structureless mix of sediment and water.

flows linnked to:

steep slows, narrow valleys, removal of veg and construction projects, thick regolith, heavy rainfall, slope failure or slide trigering flow, earhtquakes or traffic vibration [liquefaction]

lahars:

mudflows on slopes of active volcaones

landslides:

single dramatic events when a section of a hillside become unstable, shears away and moves downhill. shear stresses in slope exceed shear strength of rock/soil. can be natura and human

factors leading to slope failure:

slope angle, geological structure, rock type, amt water present, removal of toe of landslip, loading of head of slope, vibrations

slope angle - slope failure

steeper slope = more insstability

geolical structure - slope failure

fractures e.g. bedding planes dippig out of sloope = more slippage chance

rock type - slope failure

veertical cliffs can stable in some rocks but clay slopes unstable on slope angles less than ten degrees. layers of imper rock trap water above = lubricated layers

amt of water present - slope failure

water increases weight of soil/rock. pore waterpressure decreases w shear sstrength of material w saturated clays ebing particularly unstable. haevy rain or snow emlt = slope fail