5.c

Created by

arshia seth

Cards (169)

Describe the structure of the earth
- inner core
- outer core
- mantle
- crust
The inner core (3)
- solid iron and nickel
- under great pressure
- over 5400 degrees C
The outer core (2)
- molten iron
- movements within the liquid outer core generate the Earth's magnetic field
The mantle (3)
- consists of the solid lower mantle, the semi-molten asthenosphere and the hardened, cooler, upper section
- up to 5000 degrees C
- rich in silica, magnesium and iron
The crust (4)
- solid outermost layer
- silica-rich
- up to 1200 degrees C
- split into two types: continental (thick, less dense granite rock) and oceanic (younger, denser, thinner basalts)
What is the lithosphere (2)
- cool, brittle outer shell
- made up of the crust and the uppermost, rigid part of the mantle
What happens as you get closer to the core of the earth (3)
- increased heat
- increased pressure
- changing chemistry
Describe the role of convection currents (6)
- mantle convection is a fundamental part of plate tectonics theory
- previously, it was thought that the plates were moved by convection currents in the mantle
- now geologists believe that plate movement and mantle convection are mainly driven by cooling from above
- at subduction zones, slabs of cold ocean lithosphere become dense enough to sink into the mantle (downwelling)
- this causes the lithosphere to be stretched and thinned elsewhere, reducing pressure on the underlying mantle rock and causing it to passively upwell in response
- some upwelling is thought to occur in thin column-like plumes rising from the core-mantle boundary
What is a mantle plume
- a column-like upwelling of hot, solid material rising from the core-mantle boundary over millions of years
What is continental drift (3)
- Alfred Wegener's theory published in 1915
- the idea that rather than being fixed as geologists had long believed, the continents are actually moving
- proposed that the continents were once joined into a sigle supercontinent (Pangaea), which broke up and its pieces spread apart to become today's continents
Evidence for continental drift: matching coastlines
- similarity in continental coastlines either side of the Atlantic suggests they were once joined
Evidence for continental drift: fossil records (4)
- many examples of fossils found on either side of oceans but nowhere else
- eg. the reptiles Mesosaurus found only in southern parts of South America and Africa
- eg. Glossopteris, a fern found in all southern continents
- however, maybe the species evolved independently on separate continents, or swam to the other continents in breeding pairs to establish a second population? (Might not be possible eg. Mammals)
Evidence for continental drift: geological fit (2)
- there are similar mountain chains and rock outcrops on either side of oceans
- eg. the Appalachians have the same rock types and ages as mountains in Britain and Scandinavia
Evidence for continental drift: ancient glaciations (2)
- glacial deposits and striations formed 300 million years ago suggest a single ice sheet once covered Southern Africa, Australia, South America, India and Antarctica when they were joined in one land mass close to the South Pole
- glacial evidence is present in rocks on continents that are now located close to the equator
Explain the concept of sea-floor spreading (3)
- in 1931 Arthur Holmes suggested that the ocean floor is continuously recycled
- this is because it sinks into the mantle in some areas and is then replaced by new lithosphere elsewhere
- the magma rises (upwelling) to form new ocean crust
What are two pieces of evidence for sea-floor spreading
- palaeomagnetism
- age of sea-floor rocks
Explain how Palaeomagnetism is evidence for sea-floor spreading (4)
- iron-rich minerals in lava align with Earth's magnetic field, acting like tiny compasses frozen in time once the lava hardens
- shows that every few hundred thousand years, Earth's magnetic field reverses its polarity
- Vine and Matthews (1963) showed how rocks either side of the Carlsberg mid-ocean ridge mirrored each other in their striped pattern of normal and reversed polarity
- this indicates that new ocean floor is produced by volcanic activity at mid-ocean ridges, then spreads away on both sides as plates diverge
Explain how the age of sea-floor rocks is evidence for sea-floor spreading (2)
- gradual increases in age with increased distance each side of a mid-ocean ridge
- shows how newer rock is emerging from the ridge and the plate is sinking elsewhere
State the three ways in which plates can move
- convection currents (mainly responsible for 'hot spots')
- ridge push (plate is pushed by the weight of a mid-ocean ridge)
- slab pull (the weight of the descending plate causes the plate to sink)
Name the three types of plate boundary
- convergent (collision and destructive)
- conservative
- divergent / constructive
How does a convergent (collision) plate boundary move 
- plates move towards each other (no subduction)
How does a convergent (destructive) plate boundary move
- plates move towards each other (denser plate subducts)
How does a conservative plate boundary move 
- plates move alongside each other
How does a divergent / constructive plate boundary move
- plates move away from each other
Features of divergent / constructive plate boundaries (4)
- sea-floor spreading as plates are pulled apart
- Mid-ocean ridges (long chains of submarine volcanic mountains- warm, thin lithosphere over an area of upwelling)
- transform faults break mid-ocean ridges up into segments (no volcanic activity takes place here but earthquakes can occur)
- ridge push (plates side down either side of the ridge due to gravity)
Describe what a divergent / constructive plate boundary look like
Give an example of a divergent plate boundary
- mid Atlantic ridge
State the three different types of convergent (destructive) plate boundaries (3)
- oceanic-continental
- oceanic-oceanic
- continental-continental
What happens at oceanic-continental convergent (destructive) plate boundaries (4)
- denser oceanic plate subducts beneath continental plate due to gravitational force
- rigidity of plate allows it to bend elastically under stress, resulting in a deep ocean trench
- sea-floor sediments scraped off oceanic plate create an accretionary wedge (adds new material to the continent, fold and thrust mountains are created)
- subducting oceanic lithosphere is subjected to increasing temperature and pressure- making it more dense and increasing its capacity to pull (slab-pull)
Explain the features of oceanic-continental convergent (destructive) plate boundaries (3)
- less dense magma rises through the overlaying continental lithosphere, causing igneous activity and forming volcanoes eg. Those in the Andes on the Pacific Coast of South America
- earthquakes are common due to faulting in the subducting slab
- faulting and fracturing occur in the Wadati-Benioff zone
Describe what an oceanic-continental destructive plate boundary looks like
What happens at oceanic-oceanic convergent (destructive) plate boundaries (3)
- when the two oceanic plates meet, the slightly older and therefore denser one will subduct under the other, creating a trench
- features are very similar to that of oceanic-continental margins
- as the plate descends, partial melting of the mantle wedge in the plate above causes magma to rise to the surface and form chains of volcanic islands (island arcs)
- large earthquakes are extremely common
Describe what an oceanic-oceanic destructive plate boundary looks like
Give an example of an oceanic-oceanic convergent (destructive) plate boundary (2)
- the Pacific Ocean is largely rimmed by either convergent or conservative plate boundaries
- about 80% of all earthquakes and many volcanoes are located in these regions
What happens at continental-continental convergent (destructive) plate boundaries (2)
- when two continental plates converge, little if any subduction takes place because the two plates have similar densities and continental lithosphere is not dense enough to sink into the mantle
- instead, the plate is folded and faulted into mountain chains and intense pressure causes metamorphism
Explain the features at continental-continental convergent (destructive) plate boundaries (3)
- no volcanoes
- often violent earthquakes
- fold mountains
Give two examples of continental-continental convergent (destructive) plate boundaries
- the Himalayas (India colliding with Eurasia)
- in Europe, the collision of the African and Eurasian plates over the past 40 million years has created the Alps
What happens at conservative plate boundaries (4) 
- plate movement is not smooth but jerky and sporadic
- frictional forces lock sections of the boundary together along faults, building up strain energy
- when the plates finally slip, the energy is released as earthquakes
- there is no volcanic activity
Give an example of a conservative plate boundary (3)
- the San Andreas Fault
- between the Pacific and North American plates
- although they are moving in the same direction, they are doing so at different rates, causing earthquakes
What builds up in response to stress and causes an earthquake
- strain energy