Plate tectonics Flashcards
Info about earth’s continental crust:
•30-70Km thick (35)
•Thicker
•Older
•Less dense
•Composed of granite
Info about earth’s oceanic crust:
•6-10Km thick (6)
•Thinner
•Younger
•More dense
•Composed of basalt and gabbro
Info about the earth’s mantle:
•Radius of 2900Km thick
•1000-3500 °C (3000 °C)
•84% of earth’s volume
•Composed of silicates of iron, magnesium and sulphur
•Top part of mantle is solid - Lithosphere
•Bottom part is malleable - Asthenosphere
Info about the earth’s outer core:
•Radius of 2200Km
•Temperatures around 4000-5000°C
•Moves around inner core creating a magnetic field
•Composed of liquid iron and nickel
Info about the earth’s inner core:
•Solid, dense ball of metal
•Radius of 120Km
•Composed of iron and nickel
•Temperature above 6000 °C
What is slab pull?
At destructive plate margins, the denser oceanic plate is forced to subduct under the less dense continental crust. The sinking of the plate edge pulls the rest of the plate towards the boundary
What are convection currents?
Earth’s mantle is hottest close to the core, so lower parts of asthenosphere heats up becoming less dense and slowly rises. As it moves towards the top they cool down becoming more dense and slowly sink creating circular movements of semi-molten rock causing drag at the base of the tectonic for movement
What is ridge push?
At constructive plate margins, magma rises to the surface and forms new crust, heating the surrounding rock which expands and rise above the surface of the surrounding crust forming a slope. New crust cools and becomes denser, gravity causes denser rock to move downslope putting pressure on the plate causing it to move apart
what’s a different name for ridge push?
Gravitational sliding
Theory of continental drift: Alfred Wegner
Suggests that all continents were one joined together as a super continent called Pangea
Evidence for theory of continental drift
•Edges of continents fit together
•Identical fossils found in South America and Africa
•Tropical coal deposits found in Antarctica, North America and the UK
•Similar glacial deposits in Antarctica, South America and India
Theory of paleomagnetism: Alfred Holmes
Suggested the movement of plates was caused by thermal convections in the mantle
Evidence for theory of paleomagnetism
•Magma erupts from mid-ocean ridges, the magnetic material aligns itself with earth’s magnetic field
•Some material has reversed polarity
•Alternating magnetic strips can be found along sea floor, showing the older crust is away from the ridge
What is sea floor spreading?
Tectonics plates diverge and magma rises up to fill the gap, then cools forming new crust. Over time the new crust is dragged apart allowing even more new crust to form. When this happens at a plate margin under the sea the sea floor gets wider called sea floor spreading, creating structures called mid-ocean ridges -> ridges of higher terrain on either side of the margin
Process of constructive margins
1) Two plates pull away from each other
2) A gap appears and molten magma rises to fill the gap
3) Magma solidifies to create a new rock build up on sea bed
4) Overtime, the layers increase and break through the surface of the ocean
How do volcanoes form at constructive margins?
Two plates are moving apart. The mantle is under pressure from plates above, the pressure is released at the margin causing the mantle to melt, producing magma . The magma is less dense than the plate above, so it rises and can erupt to form a volcano
How do earthquakes form at constructive margins?
Two plates moving apart (diverging) but don’t move at the same speeds, one is faster than the other causing a pressure build up. When there’s too much pressure the plate cracks, making a fault line and causing an earthquake. Further earthquakes can occur once it’s created
Formation of ocean ridges at constructive margins + examples
1) Diverging plates are underwater, an ocean ridge forms e.g. the Mid-Atlantic ridge is where Eurasian and North American plate move apart
2) Underwater volcanoes erupt along the mid-ocean ridge and build up to be above sea level e.g. Iceland has been formed by build up of underwater volcanoes along the ridge
Formation of rift valleys at constructive margins + examples
1) Where the plates diverge beneath land rising magma causes the continental crust to bulge and fracture, forming fault lines
2) As the plates keep moving apart, the crust between parallel faults drop down to form a rift valley e.g. the East African Rift System -> a series of rift valleys that stretch from Mozambique to the Red sea about 4000Km formed between Nubia and Somalian plates
3) Volcanoes are found around rift valleys e.g. Mt Kilimanjaro and Mt Kenya, apart of the East African Rift System
Process at destructive margins: Oceanic-continental
1) Two plates converge and the heavier oceanic plate gets subduct beneath the lighter continental crust
2) Oceanic plate sticks and locks as it tries to slide under the plate
3) Heat from friction and the mantle start to melt the subducted plate
What is formed at oceanic-continental destructive margins and examples?
1• A deep sea trench e.g. Peru-Chile trench in pacific ocean
2• Fold mountains -> Made up of sediments that have accumulated on the continental crust, which are folded upwards along with the edge of the continental crust
How are volcanoes at oceanic-continental destructive margins formed?
The oceanic crust is heated by friction with the upper mantle, which melts into magma. The magma is less dense than the continental crust above and will rise back to the surface to form volcanoes
How are earthquakes at oceanic-continental destructive margins formed?
As one plate moved under the other they can get stuck, causing a pressure build up. Once it becomes too much the plates jerk past each other causing an earthquake
Processes at oceanic-oceanic destructive margins
1) Most of the same processes occur, since the denser oceanic plate subducts forming a deep sea trench, triggering earthquakes and volcanic eruptions
2) Volcanic eruptions that take place underwater create Island Arcs -> Clusters of island that sit in a curved line where magma has pushed up through the trench and solidifies creating islands e.g. Mariana Islands
Processes at continental-continental margins
1) Two plates of continental crust move towards each other, neither is subducted so there aren’t any volcanoes but the pressure build up can cause earthquakes
2) Fold mountains -> e.g. Himalayas
Also known as a collision boundary
Process at conservative margins + example
1) Two plates are sliding in different direction or speeds past each other
2) The two plates get locked together in places and pressure builds up
3) They eventually break free causing them to jerk past each other or crack forming fault lines (Two plates moving parallel to each other) releasing a sudden surge of energy forward -> Earthquakes
e.g. Pacific plate moving past North American plate causing many earthquakes along the margin and fault lines like San Andreas in California
