trying to get unit 5 in my brain Flashcards
thought i would love it (i was wrong)
what makes plates move
Convection currents takes place in the mantle
heat form the core heats the mantle which therefore rises
this current cools down as it comes closer to the surface of the earth
as a result it moves in a horizontal direction along the bottom of the crust
when the current cools down more the convection current descends and goes towards the core
divergent boundaries
at a divergent plate boundary 2 plates move apart from each other
it can be 2 oceanic plates or 2 continental plates
as the plates move apart, faults are formed into ehich rising magma enters
it cools and creates new crust
under oceans this is sea floor spreading and creates mid ocean ridges
ridges are elevated mountain ranges on the ocean floor that consist of rock that is hotter and less dense than the older colder plate
as the plates separate rising hot magma comes to fill the gap and rises to create a ridge
spreading rates are not the same throughout the worlds mid ocean ridge system
slow spreading ridges are fed by small, discontinuous magma chambers
fast spreading ridges are fed by large continuous magma chambers
how people found sea floor spreading
the earths magnetic field has different polarity in the different periods of time
lava cooling on the sea floor acquires the polarity of the earths magnetic field at the time of cooling
convergent plate boundaries
a convergent boundary is formed where one plate moves underneath the other one
this could be an oceanic plate sub-ducting under a continental plate as it is more dense or an oceanic plate which is older and denser sub-ducting under another oceanic plate which is younger
subduction zones
the area where one plates moves underneath another
the denser plate is pushed underneath
the oceanic crust remains cooler and therefore denser than the surrounding mantle for millions of years so subduction continuues and the crust is destroyed
as the earth has not grown, the amount of subduction balances against the new crust created at divergent boundaries
the plate subducting tends to dip at an angle between 30 and 70 degrees
the older the crust the steeper it dips
oceanic trenches
ocean trenches are formed at subduction zones
they are long narrow depressions in the ocean floor
most in the pacific
trenches are usually symmetrical with the steepest side towards a landass
benioff zones
a narrow zone of deep earthquake foci at a subduction zone
the zone extends from the surface of the ocean trench down
the deeper earthquakes occur further from the subduction zone
island arcs
chains of volcanic islands on the continental side of an ocean trench
associated with subduction zones
normally formed when oceanic crust subducts below ocean crust
formation of island arcs
the descending plate starts to heat up ans at a depth the rocks are leted to form magma and rise toward the surface
eventually this magma makes its way up into the overrising plate where they add material to the crust and build volcanoes above it
if the upper plate is oceanic the volcanoes pile up until they poke through the surface of the ocean and form an arc
types of weathering
mechanical: the action of the physical environemnt which breaks down rock into smaller pieces
chemical: the action of a chemical found in nature that breaks down rock or changes its substance
freeze-thaw weathering
all rocks have cracks and joints in them which water gets into
the water will freeze and expand when it turns to ice
it exerts a pressre
this exceeds most rocks resistance and the rocks cracks expand and pieces of the rock around it rack
when the water melts these pieces are broken off the rock
this process repeats again and occurs in environemnts where temperatures fluctatuations are common
exfoliation
found in hot desert with a large diurnal range
rocks heat up during the day and the outer layers epxand
rocks then cool and contract at night
that causes the peeling or exfoliation of the rock to occur
salt crystallisation
occurs when a saline solution gets into the joints of rocks
when the water evaporates salt crystals are left behind and as temperatures rise the salt expands and exerts a pressure on the rock
this causes the joints to expand and disintegrate
the more porous a rock, the more susceptible to salt crystallisation
pressure release
pressure release occurs when overlying rocks are removed by erosion
the underlying rocks then expand as the pressure above them is released
this causes cracking and fractures parallel to the surface
with increasing depth the cracks become smaller
carbonation
occurs only on rocks with calcium carbonate in them
rainfall that falls to earth is always slightly acidic as the water has combined with carbon dioxide to make a weak carbonic acid
the mineral calcium carbonate in the rock reacts with the carbonic acid in the water to form calcium bicarbonate
calcium bicarbonate is soluble and is therefore removed by the peroclating rain water which breaks down the rock
hydrolysis
occurs only on rocks with the mineral feldspar in them
acidic water reacts with the feldspar to form kaolin, silicic acid and potassium hydroxyl
the acid and hydroxly are removed by the water solution leaving kaolin behind as an end product
hydration
the clay minerals in shales and mudstones absorb water
these minerals expand as they absorb water
the volume of the shales and mudstones can then increase
this causes cracking and breaking down of the rock
oxidation
occurs only on rocks containing iron minerals
dissolved oxygen in both the soil and the air react with the iron minerals in rocks to oxidise and become iron oxide
this iron oxide remains in soils or rocks as a reddish brown coating or in waterlogged areas can be removed in solution
factors affecting the rate and type of weathering
climate
rock type
rock structure
how climate affects weathering
the wetter and more humid the climate, the deeper and more intense the weathering becomes
how rock type affects weathering
minerals are the chemicals that form the rocks
some minerals are more resistant to weathering than others
grain size
the bigger the grains or crystals that make up the rock, the faster the rate of weathering
this is because they have a large void space and therefore high permeability so water can enter them more easily
lines of weakness
natural cracks in rocks are lines of weakness
they allow water to penetrate the rocks and increase the effects of weathering
these joint patterns create differential resistance within the same rock type
porosity and permeability
porosity is a measure of how much of a rocks volume is open space
this can be between grains or within cracks or cavities of the rock
permeability is a measure of the ease with which water can move through a porous rock
the more permeable the rock, the more it can be weathered
slope
refers to an inclined surface or hill slope
it is the area between a watershed and the base of the slope
slope form is the shape of the slope in cross section
slope evolution is the development of slopes over time
slope processes are the activities acting on the slopes
the slope system
slopes are open systems
inputs are energy from the sun and mass
outputs are energy and mass
regolith is the unconsolidated material on top of the rock
some factors occur on the outside (exogenetic) or inside (endogenetic)
climate slope control
humid areas-slopes are rounder due to chemical weathering
arid areas–slopes are more jagged due to more mechanical weathering
geological structure and rock type slope control
faults create lines of weakness and therefore instability in a slope and can create steep valley sides
rock type can affect a slopes vulnerability to weathering
vulcanicity can cause igneous intrusions which when exposed create steep slopes
regolith and soil slope control
deep regolith can mean slope instability
regolith with high percentage of sand can cause slope failures
aspect slope control
aspect is the direction a slope faces
south slopes were often in shade
north slopes faced south and therefore had cycles of freeze thaw increasing weathering and altering slope
vegetation slope control
vegetation can decrease overland runoff and reduce erosion of a slope
vegetation can increase landslips as build up of soil between trees deepens regolith and therefore increase potential for failure of the slope
mass movements
large-scale movements of the earths surface that are not accompanied by a moving agent such as a river glacier or ocean wave
mass movements occur because of slope failure when gravity acts to move the material downwards and the resistance of the slope is not strong enough
falls
movements of rocks/debris under the effect of gravity
flows
movements of a mass of soil or rock that contain a significant amount of water
heaves
slow movements of material up to the surface of a slope and down slopes
slides
an entire mass of material moving along a slip plane
classification by speed and water content
wettest fastest movement is a mudflow
slowest driest movement is a soil creep
soil heave/creep
very slow and small scale process
occurs mainly in winter
individual soil particles are pushed to the surface by wetting, heating or freezing of water
talus creep
slow movements of fragments on a scree slope
rock falls
falls occur on steep slopes
on bare rock faces where joints are exposed they are more common
normally causing by weathering and once the rocks are detached they fall under the influence of gravity
if the fall is short it produces straight scree
if it is long it forms a concave slope
slides
occurs when an entire mass of material moves along a slip plane
they occur where there is a combination of weak rocks and steep slope and undercutting
as the slide moves along the slip plane it tends to retain its shape and structure until it hits the bottom of the slope
slip planes occur at fault lines along a bedding plane and at the junction of 2 layers
rock slides are when a huge volume of rock moves together
landslides include rock, stones and soil
slumps and flows
slumps occur on weaker rocks
this is often rotational along a curved slip plane
clay absorbs water and becomes saturated and exceeds its liquid limit
it then flows along a slip plane
this can be due to the undercutting at the base of a cliff
flows are more continuous and less jerky
avalanches
rapid movements of snow down a slope
they are common in mountainous areas
new snow can fall over older snow
or partially melted snow moves
they occur on slopes over 20 degrees
they also occur on north facing slopes where the lack of snow limits snow stability
causes of mass movements
likelihood is its safety factor
this is the relative strength/resistance of the slope compared with the force that is trying to move it
for slope failure to occur there needs to be a reduction in shear strength or an increase in shea stress
factors increasing shear stress
removal of lateral support
removal of underlying support
loading of a slope
transient stresses
lateral pressure
factors reducing shear strength
changes of rock structure
weathering
changes in porewater level
removal of vegetation which previously bound the soil
borrowing of animals, decay of plant roots
the human impact
quarrying
mining
pollution
acid rain
dumping of material
terracing
cuts steps into the profile of a slope
can also stabilise an entire hillside
the steep slopes between the steps can become less stable
buidling a dam
can destabilise the slopes surrounding the reservoir as they are cut to create it
it changes their shape
loading the valley with water can also destabilise slopes
road cutting
create an artificial valley with steep slopes on either side
these can be unstable
deforestation
can cause soil erosion and less binding of soil leading to slides or flows
can increase CO2 levels in the atmosphere increasing the carbonation of limestone
adding lighting
encourages the growth of plants and can increase biological weathering
soil erosion
soil is eroded away changing the shape of the slope and the area is destabilised leading to slides and flows
foundations
material is removed to make a flat area to put solid concrete
changes slope form
mining
surface mining leads to the removal of overburden to reveal the mineral below
this changes the height of a slope
underground mining can lead to small depressions appearing on the surface of the land
this can lead to instability and collapse
quarrying
excavating large areas to use the rock creates dips in the landscape devoid of soil and vegetation
slopes either side of quarries can have landlsides
urban areas
increased temperatures in urban areas have led to increased chemical weathering rates
heaps of material
after material is removed during mining and quarrying it is often piled into a heap
these can be very unstable as they are unconsolidated masses
global warming
CO2 additions have increased temperatures leading to increase rates of chemical weathering
carbonation relies on CO2 and so this will increase also
release of gasses
high rates of acid rain and cry acid deposition increase corrosion of buildings and structures
