rocks and weathering Flashcards
describe the difference between continental crust and oceanic crust
oceanic plate is younger but more dense and thinner and made out of sima ( mixture of silicon and magnesium.
continental crust are older thicker but less dense and made out of granite or sial ( silica and aluminium )
describe a divergent plate boundary
oceanic and continental continental
Oceanic constructive: the convection currents lift up the lithosphere and creates a ridge, a fissure is created allowing magma to escape which rapidly cools and solidifies creating new crust which. is known as sea floor spreading.
Continental constructive: the connection current cause the plates to move away form each other buy due to the thickness of the plate there is no clean break ,only fractures are made and small faults which can create a rift valley as solidified magma builds up
describe convergent plate boundarys
Oceanic/Continental destructive: plates forced together and oceanic plate subducts since denser. In the Benioff zone, crustal melting occurs, and resultant magma forced through cracks – to form volcanoes. Subducting plate drags down crustal material to form an ocean trench.
Oceanic destructive: plates forced together, older plate subducts as is denser. Forced below and melts – producing magma chambers. Lower density magma rises through cracks allowing volcanic eruptions.
Collision: Both densities are lower than the mantle’s, so prevented subduction. Some subduction occurs as the lithosphere breaks free. Crust fragments are trapped in collision zone, cause deformation. Intense compression results in folding.
describe transformative plate boundaries
plates slip past each other with relative horizontal movement Lithosphere is neither created nor destroyed. though they produce very extensive earthquakes
what is weathering
weathering is the breakdown of rock in situ
what is the difference between physical weathering and chemical weathering
physical does not involve the change in the chemical composition of the rock which chemical does
describe all the physical weathering processes
Freeze-thaw: occurs in cold areas where ice forms as water freeze in cracks in rocks. Water that collects in cracks freezes and expands, cracks are forced open as this pressure exceeds rock’s resistance. Ice thaws, parts of the rock breaks free and the water penetrates deeper. Repeats until rock cut through.
Exfoliation/ heating and cooling: occurs in a hot desert with a large diurnal energy range (40°C to below freezing). Rocks heat via conduction, only outer layers expand as the rock is a poor heat conductor. At night outer layers cool and contract more rapidly than inner layers, creating stress. Non-uniform contraction stresses result in outer layers flaking off over time.
Salt crystal growth: physical disintegration due to fretting (saltwater penetrating) rock surfaces. Saline solution evaporates, leaving salt crystals. A temperature rises, salt expands – exerting pressure on the rock, causing disintegration
Dilation/pressure release process: Overlying rocks are removed by erosion (unloading). Underlying rocks expand as under reduced pressure. Fractures form parallel to the surface, producing pseudo-bedding planes. Deeper down, cracks are less prominent. If horizontal pressure is released, vertical faults develop – common on cliff faces.
describe all the chemical weathering process
Hydrolysis: acid water breaks down rocks
Hydration: certain minerals absorb water - allowing them to expand and change, producing mechanical and chemical stresses. Affects
Carbonation: acid rain breaks down limestone/chalk. Carbon dioxide dissolves in rainwater to make a weak carbonic (acid rain). Calcium carbonate in limestone reacts with carbonic acid to form calcium carbonate
factors affecting the type and rate of weathering ( 6)
climate - wet climate more chemical weathering and temperature affects the type of physical weathering, a hot wet climate menas the highest rate of chemical wethering as heat speeds up the rate of reaction.
rock type - the composition of rock can affect weathering as some rock material are more suseptible to weathering, porosity also plays a role as the more craks the easier it is for liquids to enter and therefore weather the rock
rock structure- igneous metamorphic and sedimentary rocks are formed differently with sedimentary being the strongest as it formed by constnat heating a cooling of rock creating resistance.
vegeation - intercepts precipitation andcan therfore deuce weathering of some slopes, thouugh the breakdown and decomposition of egation leads to biological weathering.
relief - affects temperatures and exposure.
Weathered material must be removed to allow the process to continue. If a slope is too steep, water runs away. If too shallow, no material is removed. Colder temperature occurs higher, so freeze-thaw is dominant at higher altitudes.
Human activity: increased weathering due to increased airborne chemical pollutants and acid rain. Vegetation removal reduces chemical/biological weathering (fewer organic acids).
what is mass movement
mass movement is the movement down a slope of wethered rock or oil as aresult of gravity.
what factores cause slope failure and what do they mean
increased shear stress - increase in the force on the slope
decreased shear strength - a decrease in the internel resistance of the slope to moement this can be through undercutting
types of mass movement
Heaves (soil creep): slow movement of material where soil particles are heaved to the surface by wetting, heating and freezing of water.
Slumps: weaker rocks such as clays ‘slump’ with a rotational movement along a curved slip plane.
Flows: more continuous, smoother form of slump. Occurs in deeply weathered clay, and if particle size is the same or smaller than a grain of sand.
Falls: occurs on steep slopes (>40°), with bare rock faces and exposed joints. Initial cause of rocks falling is erosion or weathering.
Slides: when an entire mass of material moves along a slip plane. Material holds its shape until hitting the slope bottom.
what is rainsplash
rainsplash is occurs when rain falls with sufficent intensity that it is ble to detach bare soil.This effect is most prominent on slopes with inclines between 33° and 45° at the start of a rainfall even, when the soil is loose.
what is sheetwash and rills
when water runs off as thin sheets of water on a steep slope disloging soil particles which the. move downslope with the water.. as sheetwash occurs the surfcse of the slope is lowered sightly overtime the flow path of the water erodes the surfce cretaing small channels known as rills.
impacts of human actvity on slopes (at least 6 )
Excavations: cutting into a slope and leaving loose excavated creates a slope too steep to have stability, and is therefore prone to failure. Excavation at the toe of a slope removes the supporting section of the slope.
Waste heaps: highly porous mounds of waste material from quarrying and mining leaves new, unstable steep slopes. Slips occur with soil saturation after rainfall.
Loading
Buildings: add mass to slope, increase slip likelihood.
Water: drainage routes disturbed by building foundations - excess water in soil increases mass and will lead to slips (also has lubricating properties). If saturated soils are subjected to an earthquake, liquefaction can occur (pressure between particles makes soil act as a liquid).
Vegetation removal: deforestation leaves land bare (increased surface runoff, more erosion). If roots die/are removed, material is no longer bound together, and stability is compromised.
Traffic vibrations: may trigger mass movements.
Footpath trampling: intensified localised erosion.
Construction on slopes: uses cut-and-fill technique (as described in excavation).
