3.1 Plate tectonics Flashcards
What are the different layers of the Earth?
Inner Core
Outer Core
Mantle
Asthenosphere
Lithosphere
Crust (oceanic and continental)
In Our Minds Are Little Characters
What is the inner core?
- solid
- iron/nickel
- very hot due to pressure/radioactive decay
- this heat responsible for Earth’s internal energy
What is the outer core?
- semi-molten
- iron/nickel
What is the mantle?
- mainly solid rock, high in silicon
What is the asthenosphere?
- semi-molten layer
-allows convection currents to occur - this layer is responsible for the movement of tectonic plates
What is the lithosphere?
- broken up into plates
- madeup of brittle crust and top part of the upper mantle
- coolest/most rigid part of the Earth
What is continental crust?
- solid rock (granite)
- older, thicker rocks
- LESS DENSE
- CANNOT sink/be destroyed
What is oceanic crust?
- solid rock (basalt)
- younger/created at constructive plate margins
- denser but thinner
- can sink and be destroyed at subduction zone
How do tectonic plates move?
- Alfred Wegner first to suggest continents drifted but could not account for how
- lots of evidence to prove they shifted location but wasn’t until discovery of ocean rides that people believed him
- in 1930s able to explain how plates were moving: CONVECTION
- core’s temp around 6000C**, causes **magma to rise in the mantle, as it reaches the top of the mantle it cools to around 1000C
- this cooler magma is denser, so sinks back towards the core and is reheated and begins to rise again (convection current)
- this constant movement of magma causes the tectonic plates, which are floating on top, to be moved along
- this movement very slow - few cm a year
What is ridge push?
- constructive plate margin (plates moving apart)
- magma rises as the plates move apart.
-The magma cools to form new plate material. - As it cools It becomes denser and slides down away from the ridge under the push of gravity.
- This causes other plates to move away from each other.
What is slab pull?
- at subduction boundary, one plate is denser and heavier than the other plate
- the denser, heavier plate begins to subduct beneath the plate that is less dense
- the edge of the subducting plate is much colder and heavier than the mantle, so it continues to sink, pulling the rest of the plate along with it
- destructive plate margin
What happens at destructive/convergent plate boundaries?
continental and oceanic
- denser oceanic plate subducts below the continental
- friction in the subduction zone causes major earthquakes (area known as the BENIOFF ZONE
- plate subducting leaves a deep ocean trench
- rocks are scraped off the descending plate and can accumulate over time at the edge of continental plate where they are compressed and uplifted to form an accretionary wedge and the continental crust folds to create young fold mountains, e.g the himalayas
- oceanic crust is melted as it subducts into the asthenosphere
- extra magma created causes pressure to build up
- pressurised magma forces through weak areas in the continental plate
- composite volcanoes form
- example of ocean trench:
- Mariana Trench
- 10,994 metres deep
- located east of the phillipines
What happens at destructive/convergent plate boundaries?
oceanic and oceanic
- heavier plate subducts leaving an ocean trench
- fold mountains will also occur
- heavier, melting subducting oceanic plate rises through the thinner, lighter oceanic plate above it, upwelling magma and forming island arcs
- e.g Caribbean islands
What happens at destructive/convergent plate boundaries?
continental and continental
- subduction of oceanic crust draws continental masses together
- as the two continental masses meet, neither will be subducted
- instead they collide forming fold mountains
- as neither plate can sink into denser rocks below
- instead they are crushed, crumpled and forced upwards, usually folding in the process
What happens at constructive/divergent plate boundaries?
Oceanic and oceanic
-tectonic plates moving apart
- creates a gap between the plates where magma from the mantle rises to fill it
- as the magma cools and solidifies, it forms new oceanic crust
- sea floor spreading
- over time, the continuous spreading of the oceanic crust widens the gap between the plates and contributes to the formation of mid-ocean ridges
- mid Atlantic ridge
What is the evidence for sea floor spreading?
Paleomagnetism: study of rocks that show the magnetic fields of the Earth
- as new rock is formed and cools the magnetic grains within the rock align with the magnetic poles
- our poles switch periodically
- each time these switch the new rocks being formed at plate boundaries align in the opposite direction to the older rock
- on the ocean floor either side of constructive plate boundaries, geologists observed that there are symmetrical bands of rock with alternating bands of magnetic polarity
- evidence of sea floor spreading
What happens at constructive/divergent plate boundaries?
continental and continental
- any land in the middle of the separation is forced apart, causing a rift valley
- volcanoes form where the magma rises, as instead of forming new oceanic crust magma rises through instead
- eventually the gap will most likely fill with water and separate completely from the main island
- the lifted areas of rocks are known as horsts whereas the valley itself is known as a graben
- example: East African Rift Valley
- transform faults connect the end of one plate boundary to the end of another plate boundary
What happens at conservative plate boundaries?
- between any crust
- parallel plates move in different directions and at different speeds
- no plates destroyed so no landforms created
- when these plates move, a lot of pressure is built up
e.g SAN ANDREAS FAULT:
- 10,000 earthquakes per year
- marks boundary between North American and Pacific plate
What are hotspots?
- areas of volcanic activity that are not related to plate boundaries
- hot magma plumes from the mantle rise and burn through weaker parts of the crust
- this can create volcanoes and islands
- the plume stays in the same place but the plate continues to move, which sometimes causes chain of islands (such as Hawaii)
What is the evidence for Continental drift theory?
MISMATCH OF CLIMATE AND SEDIMENTARY DEPOSITS:
-e.g coal derived from plants that grew/decayed in hot climates yet found in UK
FOSSIL EVIDENCE:
- similar fossils reserved in rocks of same age in different countries
GEOMETRIC FIT OF CONTINENTS:
coastline Western Africa and South America fit together
GEOLOGICAL MATCH AND CONTINUITY:
- similar rock types found on other sides of the world
What is sea floor spreading?
- process that occurs at divergent plate boundaries
- as plates separate, magma rises from the mantle and fills the gap, creating new oceanic crust.
- the magma cools and solidifies forming a new sea floor
- over time, this process continues, pushing the older crust away from the plate boundary
- this results in formation of mid-ocean ridge
How do ocean trenches form (8)?
- they are deep, elongated depressions in the ocean floor
- they are formed through subduction
- when two tectonic plates collide, one plate is forced beneath the other into the Earth’s mantle
- as subducting plate sinks into the mantle, it creates a trench on the ocean floor above it
- the downward force causes the overlying crust to bend and form a steep sided trench
summary of tectonic landforms and their associated boundaries
-
Destructive/convergent:
- ocean trenches: e.g Mariana Trench
- fold mountains, e.g himalayas
- island arcs (oceanic-oceanic)
- steep compositive volcanoes, e.g Mount St Helens -
Constructive/divergent:
- shield volcanoes, e.g Eyjafjallajokill, Iceland
- mid-ocean ridges, Mid-Atlantic ridge
- rift valleys (continental): -
conservative/transform:
- transform faults: plates slide past each other causing fractures/shallow earthquakes, e.g San Andreas Fault
4.Hotspots:
- volcanoes and island chains
how do rift valleys form
- constructive plate boundary
- crustal tension: continental crust is stretched causing it to thin and fracture
- tensional forces create normal faults, where the blocks of earths crust move vertically
- subsidence: block of land between two faults drops down relative to surround crust forming a graben (rift valley), uplifted blocks on either sides = horsts
- e.g east african rift valley
