3.1 Plate Tectonics Flashcards
What is the structure of the earth?
Crust: A brittle outer layer of the planet 0-100km thick, broken into two types - continental and oceanic
Mantle - 82% of the earth’s volume, between 100km and 2900km deep, largely solid
Outer core: 2900-5100km deep, liquid due to melting
Inner core: 5100-6378km, solid due to enormous density and pressure
What are the two types of crust and their properties?
Continental crust: 35-70km thick, old or light with a density of 2.6gcm^-3 - granite as has silica and oxygen
Oceanic crust - 6-10km thick, young and heavy, 3.0gcm^-3, basaltic
What is the lithosphere/asthenosphere?
The crust and upper section of the mantle rom 15-200km deep is the lithosphere. it is the coldest part, and is mainly solid
The upper part of the mantle is below the lithosphere which is hot and semi molten.
How are plates distributed and what are some key ones?
They are distributed along plate boundaries kinda hard to explain icl.
The Pacific and NA plate collide in the North pacific
The mid Atlantic ridge is the boundary between the north American and Eurasian plates
The Caribbean plate collides with the South American and stuff
What is some evidence for continental drift?
Coastline fit - South Africa and Africa seem to fit which is further backed by other evidence.
Orogenic belts: The Atlantic coastlines share orogenic belts and fold mountains. The mountains of Greenland
Fossils, flora and fauna - India and Australia have similar species - South America, India and Africa all share species. Mosasaurs is founding both South America and Africa yet they could not possibly swim that far.
This relates to paleoclimatic evidence of ancient indicators like fossils species, similar rocks stored in glaciers
Fit of rock types
Changes in magnetic field - polarity changes at different places and at different times
Sea floor spreading
What is sea floor spreading? What is some evidence for it?
The creation of new lithosphere as rock melts in the upper mantle due to high temperatures and low pressures.
Magma ‘pockets’ build up and rise through cracks in the seafloor, spreading it apart. The magma spreads along the ridge and cools as basaltic lavas, forming ‘pillows’ below which magma solidified into dolerite and gabbro.
The new rock cracks and diverges either side of the ridge by convection currents.
Evidence - Age of rocks using their radioactivity, thickness of sediments, satellites, magnetic strips along the sea bed moving.
What causes plates to move?
Ridge push: the intrusion of magma into ocean ridges propels plates apart, spreading them outwards
Convection drag: currents in the plastic mantle drag overlying lithosphere outwards. The radiation sources are in the mantle which cause rising limbs beneath oceanic ridges, driving the rock to spread sideways as it is forced outwards. They cool and fall at subduction zones where it is heavier and cooler
Slab pull: as plates subduct beneath each other due to gravity it creates massive amounts of pull which drag the remaining rock with it.
What are hotspots?
Hotspots are plumes of lava formed away from plate boundaries, but usually nearby to them. The lava can rise vertically through the mantle from deep within the ground, causing melting of the asthenosphere and lithosphere, leading to volcanos known as seamounts.
This can lead to ridge push and sea floor spreading away form a boundary if the basaltic magma creates new lithosphere
Describe the ‘main’ destructive plate boundary
The continental-oceanic destructive plate boundary e.g. the Andes
The denser (3.0) plate is forced to subduct beneath the less dense (2.6) continental plate due to the force of gravity.
When they subduct, it creates an oceanic trench which runnels parallel to fold mountains due to the sinking of the land. Deep within the trench the plate continues to be pulled down from slab pull and is partially melting, due to the high heat. This form andesitic magma which is less dense due to hydrogen melting in the seawater.
This builds up in pressure and rises through cracks and fissures and cools, forming composite volcanos. The subduction may cause earthquakes. Along the zone of subduction, as it continues to subduct lower there is a line of earthquakes known as the Benioff zone due to the build up and pressure release of material being forced deep into the ground. This is because oceanic lithosphere is cooled due to hydrogen melting so can remain in the mantle for longer periods of time without melting and so can go deeper and cause pressure release.
The continental plate continues to converge into the trench and starts to fold over due to compression as it has nowhere else to go. This leads to the formation of cyclins and anticyclones, upward and downward facing strata in the rockface form by the deformation of the compressing rock. This occurs at great depth and pressure which force it up. Material is also scraped off and deposited, forcing it downwards into the oceanic trench which adds to the mountain. This forms an accretionary wedge/prism. The combined formation of fold mountains and volcanos is called orogenesis
What is the other type of destructive plate boundary?
Oceanic oceanic destructive plate boundaries such as the Caribbean
The older, denser oceanic rock is subducted and creates volcanos, subduction, Benioff zones and accretionary prisms exactly how oceanic and continental boundaries do.
The trench outer rise is a rise of rock just before the the oceanic trench caused by the plate folding before it subducts.
Partially melted plate rises, creating a volcanic island arc as it rises through cracks and fissures, forming andesitic strato-volcanos in an arc shape along the plate boundary
Oceanic trenches are also formed at the point of subduction creating long, narrow depressions which are the deepest section of the ocean.
An accretionary prism may also form underwater
What are collision boundaries?
When two continental plates collide, they are forced over one another due to the compressional stress forcing the plates to fold over, creating wide spread earthquakes but no volcanos. Fold mountains form in the same way as the other boundaries.
What are divergent boundaries?
Also known as constructive boundaries, this is when two oceanic plates moves away from each other. The loss in heat and pressure rises the basaltic magma which cools into rock, creating thermally expanded mountain ranges underwater and new lithosphere through sea floor spreading, as well as rift valleys within these mountain ranges.
In the centre of the ridge there are rift valleys as the ridge spreads either side of when the plates are dragged apart, in the form of new blocks which break and fall into the mantle. This is due to the intense pressure and tension of stretching of the rift valley and creates rifts in the middle.
The ridge can be offset by transform faults which can create cracking at right angles to transform faults due to the different spreading rates of the blocks.
What are conservative plate boundaries?
This is when two plates slide past each other at different speeds or in opposite directions. This creates devastating shallow foci earthquakes.
