Earth and the crust Flashcards
define lithosphere
the solid layer of crust in which plates are formed
define asthenosphere
soft, plastic like rock in the upper mantle, just below the lithosphere
what is the thickness of continental crust?
25-70km
what is the thickness of oceanic crust?
7-10km
what age is continental crust?
2 billion years +
what age is oceanic crust?
180-200 million years
what is the density of continental crust?
2.7g cm3
what is the density of oceanic crust?
2.9g cm3
what rock type is continental crust?
granite
what rock type is oceanic crust?
basalt
what is the mineral content of continental crust?
aluminium, silicon, oxygen
what is the mineral content of oceanic crust?
silicon, magnesium and oxygen
what is the theory of ‘Paleomagnetism’?
- new material is constantly being produced
- earths magnetism flips back and forth everynow and again which results in a striped rock pattern
- all possible due to ocean floor spreading (mid ocean ridge)
- it is symmetrical on both sides of the ridge
- process called ridge push
what is the process in subduction zones called?
- gravity pulls the oceanic plate down in the mantle
- this destroys crust material and keeps the Earth in shape
- this process is called slab pull
for a stable earth, what must be equal to what?
ridge push = slab pull
what is earth thought to have arisen from?
‘planetesimals’ out of ash cloud, stuck together which produces heat from decay of uranium in core
what is the process of movement in the mantle?
- radioactive decay in the core heats material
- heated rock rises as it is less dense
- at the top semi molten rock spreads out, moving plates above with it
- cools and sinks back down to be reheated
- process repeats
what is jigsaw fit?
similarities in coastlines (east coast of S. American and west coast of Africa) seem to fit like a jigsaw, suggesting they were once connected.
any gaps in the fit are likely due to erosion
what is tectonic evidence?
fragments of old fold mountain belts (400-450 million years old) found spread
pieces of Calodien belt found Greenland, Canada, England, Ireland, Scotland and Scandinavia
what is geological fit?
geology of S. America and W. Africa reveal ancient rock outcrop over 200 mill years both consistent on both continents
how was the east African ridge produced?
(1) elevated heat flow creates bulges in Kenya and Ethiopia
(2) the brittle crust cracks into faults
(3) convection currents in asthenosphere causes the extension of crust
(3) rift continues to extend, crust becomes thinner and erupts
(4) eventually African plate splits in 2 : Nubian and Somalian
what is the benioff zone?
- the further the rock descends, the hotter the surroundings get
- this, along with frictions heat, melts the plate into magma
what are the types of destructive boundaries?
oceanic - continental
oceanic - oceanic
continental - continental
how do fold mountains?
- oceanic plate is subducted as it is more dense
- material is scraped off the subducting plates and folds up into fold mountains