Plate Tectonics Flashcards
What is the lithosphere?
The solid top layer of crust in which plates are formed; include upper mantle and crust
What is the asthenosphere?
Soft, plastic like rock in the lower half of the mantle
How did the Earth become a heat source?
- colliding planetesimals that formed the earth from a cloud of gas and dust
- radioactive decay from natural material like uranium
- seismic data can give information about which rocks make up the earths, and whether these are molten or solid
How does the process of convection currents work?
1) due to radioactive decay, a heat source is created due to the kinetic energy of colliding particles
2) heat melts rock
3) rock rises due to low density
4) rose spreads out, carrying plates with it
5) cools and sinks
6) creates currents that are called convection currents
What is the plate tectonic theory of crustal evolution?
The hypothesis that Earth’s external shell is separated into a few plates that float over the mantle, the rough inward layer over the core
What are the two pressures exerted at plate margins?
- slab pull: at subduction zones gravity pull the ocean plate down into the mantle; destroying it
- ridge push: process of material pushing out from the ridge
What evidence can be shown to support plate tectonic theory?
- jigsaw fit
-> idea that continents fit together into one land mass called Pangea - paleomagnetism
-> as new material is constantly formed at mid ocean ridges, the magnetism at that time is taken with it
-> magnetism randomly flips now and again, meaning age of the spreading sea floor can be matched - geological fit
-> occurrence of matching geological features on different continents
-> the discovery that rock outcrops on eastern South America and Western Africa over 2000 million years old were continuous - tectonic evidence
-> occurrence of matching tectonic features on different continents
-> fragments of a fold mountain belt between 450 and 400 million years old were found on widely spread continents, found on England, Greenland, Canada and Scandinavia - fossil evidence
-> fossils found on separate continents were found nowhere else on earth, meaning they could have been once joined
-> this suggests continental drift, the most believable theory
What are features of the continental crust?
- thickness: 70 km thick
- age: up to 4 billion years old
- density: 2.7 g/cm³
- mineral content: oxygen, silicon, aluminium, iron
- rock type: granite
What are features of the oceanic crust?
- thickness: 7 km thick
- age: up to 200 million years old
- density: 3.0 g/cm³
- mineral content: iron, silica, magnesium, calcium
- rock type: basalt
What are the main plate boundary types?
- constructive
-> 2 plates moving apart, magma rises to fill the gap
-> volcanoes and earthquakes occur but don’t with force, rift valleys and mid ocean ridges
-> an example is the mid-Atlantic ridge - destructive
-> dense oceanic plate subducts below the less dense continental
-> as the plate descends, it melts
-> violent volcanoes (and earthquakes) occur as magma rises through fissures, ocean trenches
-> example includes the 11km deep Mariana Trench, where some use echolocation - collision (destructive)
-> when two plates of similar density collide
-> causes material to buckle into fold mountains
-> fold mountains
-> the Himalayas is an example - conservative
-> two plates of varying directions or speed slide alongside each other along a fault
-> earthquakes
-> such as the San Andreas Fault
How are rift valleys formed?
1) as plates move apart, mama rises through the magma chamber to fill the gap
2) this warps the crust, forming faults along the top
3) this allows gases and steam eruptions to escape
4) sections of crust descend into the mantle
5) causing sections to rise as magma pushes up
- very green due to fertile soils
- changes in altitude create relief rainfall
What are the three types of destructive plate margin?
- oceanic/oceanic
-> one is forced under the other as it is slightly more dense, or moving faster
-> earthquakes are frequent, with a shallow to deep focus
-> ocean trenches and island arcs, which curve - oceanic/continental
-> ocean trenches form
-> subduction and earthquakes with a shallow to deep focus
-> pressure on continental forms fold mountains, whereas some oceanic plate breaks off, adding to the fold mountains
-> molten material rises in magma plumes and burn their way through, causing violent earthquakes - continental/continental
-> prone to powerful earthquakes (Nepal 2015)
-> sediment from ocean floor/volcanoes from prev. subduction gets squeezed up
-> forms fold mountain chains with deep roots in the lithosphere
-> no volcanoes/ocean trenches
