plate tectonics Flashcards
what are the layers of the earth
core, mantle, crust
what is the core
-made of iron and nickel
-4 times as dense as the crust
-inner core: 13000km thick, 5500 degrees celsius, hot due to pressure and radioactive decay, heat is responsible for the earths internal energy
-outer core: semi-molten, temps of 3000 degrees celsius, 1250km thick, creates the earths magnetic field
what is the mantle
-2900km thick
-mainly silicate rocks
-accounts for more than 80% of the volume of the earth
-asthenosphere: 100-300km deep, lower layer of mostly solid rock that flows, transfers heat from the core through convection currents
-lithosphere: upper layer of mantle and crust (-100km), broken up into tectonic plates, moves ontop of the asthenosphere
what is the crust
-thinnest layer (5-70km thick, max 100km under mountain ranges)
- comprised of silica rock
-oceanic crust: composed of basaltic rock aka simu (silica and magnesium)
->forms lower layer of crust
->denser than sial
-continental crust: granite rocks aka sial (silica and aluminium)
-> forms upper layer of crust
-> less dense than simu
->av 30 km thick
what processes create the earths internal heat energy
-conversion of gravitational energy
-radioactive decay of unstable isotopes
who created the plate tectonic theory
Alfred Wegner in 1912
what was the plate tectonic theory
the idea that 300 million years ago the continents were converged into one super continent called Pangaea which later split into two continents called Laurasia and Gondwanaland
how was this theory produced
he published his theory of continental drift, he claimed that evidence like south americas and africas coastlines aligning, and similar fossils being found in both of these coastlines
what is the actual evidence behind the theory
modern evidence for wegners theory would be the discovery of the mid-atlantic ridge, this shows the movement of the plates
what is ridge pull
it is the pushing force that plates experience as they slide down the raised asthenosphere underneath mid ocean ridges
what is slab push
it is the opposite of ridge push, as the plates move away from the diverging boundary, it ages and cools, therefore becoming denser, the cold dense lithosphere then sinks down into the asthenosphere and pulls the rest of the plate down with it
destructive plate margins
-subduction zones
-as the plates collide the oceanic plate is forced below the continental plate as it is more dense
-when the plate sinks into the mantle it melts to form magma, the pressure of the magma builds up and escapes through a composite volcano
-if two continental plates collide neither can sink meaning that the land buckles causing fold mountains, earthquakes can occur here
how are island arcs formed
when two oceanic plates converge the subduction causes the formation of an island arc.
-the descending plate encounters hotter surroundings, which toegther with frictional heat, causes the plate to melt
-as the subducted material is less dense that the surrounding asthenosphere it rises to the surface in plutons of magma
-on reaching the surface it causes explosive volcanoes,
-if the eruptions occur offshore then a line of volcanic islands form
how are ocean trenches formed
-these form at destructive plat margins between oceanic plates or oceanic and continental plates
-one subducts under the other
-example: Marina trench
constructive plate margins
-plates move apart
-magma rises in the gap
-shield volcanoes
-mid ocean ridges
-rift valleys
