3.1.5.2 Plate Tectonics Flashcards
What’s the crust
Earth’s outer shell that we live on
2 types of crust
Oceanic
Continental
What’s the collective name for the crust + upper mantle where tectonic plates are formed
Lithosphere
What’s the widest section of the earth called (2900km thick)
Mantle
What’s the rock in the upper mantle like
Solid
Sit on top of asthenosphere
What’s the asthenosphere
What can it do
Layer of softer, plastic-like rock
Can move very slowly, carrying the lithosphere on top
What’s the lower mantle like
Most dense part of mantle
What’s the core
The centre + hottest part of the earth - reach 5000*c
What’s the core mostly made up of
Iron + nickel
Extremely dense
Describe 2 parts of the core
Outer core - semi-liquid and mainly iron
Inner core - solid + made of an iron-nickel alloy
What’s the earth’s main internal energy sources
The core - heat producing convection currents
Radioactive decay
What are the 2 theory’s for plate tectonics
Wegner’s theory of plate tectonics + continental drift
Hess’s theory of sea-floor spreading
Describe wegners theory
A single continent existed about 300 million years ago that he named Pangea + it then later split into 2 continents of Laurasia + Gondwanaland
Further splitting of these land masses formed today’s continents
This is the continental drift theory
What did Werner name the single continent
What 2 continents did it split into
Pangea
Laurasia
Gondwanaland
2 pieces biological evidence that supported wegners continental drift theory
- fossil brachiopods found in Indian limestone are comparable with similar fossils in Australia
- fossil remains of the mesosaurus reptile are found in South America + South Africa
- fossiled remains of a plant that existed when coal was being formed have even located only in India + Antarctica
2 other pieces of evidence that support wegners theory
- bulge of South America fitting into indent below west Africa
- evidence of late-Carboniferous glaciation, deposits from this are found in South America, Antarctica + India
- striations on rocks in Brazil + west Africa
- rock sequences in northern Scotland agree closely with those found in eastern Canada
What did Hess discover + study to suggest that sea-floor spreading was occuring
The mid-Atlantic ridge
What’s Hess’ evidence sea-floor spreading was occuring
The alternating polarity of the rocks that form the oceanic crust
- iron particles in lava on ocean floor are aligned with earth’s magnetic field
- as lavas solidify, these particles give a permanent record for earth’s polarity at time of eruption
- but at regular intervals, earth’s polarity reverses, resulting in a series of magnetic ‘stripes’ with rocks aligned alternatively towards north + south poles
What pattern that Hess saw, suggests the oceanic crust is slowly spreading away from this boundary
The striped pattern, mirrored exactly on each side of the mid-oceanic ridge
With distance from the mid-oceanic ridge, what happens to oceanic crust
It gets older
What does sea-floor spreading imply
But what must actually be happening
Earth must be getting bigger
But actually, plates must be being destroyed somewhere to accommodate their increase in size at mid-oceanic ridges
What are the 7 major plates
African plate Antarctic plate Eurasian plate Pacific plate Indo-Australian plate North American plate South American plate
What are the 8 minor plates
Arabian plate Caribbean plate Cocos plate Nazca plate Juan de Fuca plate Philippine sea plate Scotia plate Okhotsk plate
3 plates where there are destructive margins
Pacific plate
South American plate
Caribbean plate
2 plates that are diverging (constructive)
Pacific plate + Nazca plate
Where’s the main collision margin
Arabian plate
3 factors about the oceanic crust
- thinner
- young
- heavier + more dense
3 factors about the continental crust
- thicker
- older
- lighter + less dense
What’s the oceanic crust made up of
Basalt - silicon, magnesium + oxygen (SIMA)
What’s the continental crust made up of
Granite - silicon, aluminium, oxygen (SIAL)
What was the generally accepted view of plate movement
Explain
It’s due to convection currents
Convection currents Move apart -> plates move apart
Convection currents converge -> plates converge
What’s ridge push
Forces pushing from the ridge, dragging plates down at trenches ‘ acting along sides of plate at conservative margins
What produces the ocean ridge at constructive margins (ridge push)
Upwelling of hot magma at ocean ridges generates a buoyancy effect that produces ocean ridges, 2-3km above ocean floor
What’s gravitational sliding
Due to gravity, oceanic plates experience a force that acts away from the ridge -> sliding away
What’s slab pull
Cool,thick oceanic lithosphere is gravitationally unstable as it’s denser than the asthenosphere
As the plate descends under the continental plate, a gravity generated force pulls the whole oceanic plate down due to the plate’s negative buoyancy
3 forces plates are controlled by
Convection currents
Ridge push + gravitational sliding
Slab pull
