Plate tectonics Flashcards
What evidence is there for plate tectonics?
-past and present distribution of earthquakes
-changes in the earths magnetic field
-the ‘fit’ of the continents: Francis Bacon 1620
-glacial deposits in Brazil match those in West Africa
-fossil remains in India match those of Australia
-geological sequence of sedimentary and igneous rocks in parts of Scotland match those in Newfoundland
What occurs at divergent (constructive) plate boundaries?
-new crust formed
-mostly in the middle of oceans
-two plates move apart causing magma from the asthenosphere to rise
-rift valleys occur
What occurs at convergent (destructive) plate boundaries?
-two plates move together and denser oceanic plate subducts below continental plate
-oceanic trench left behind and built up pressure from melting plate causes explosive volcanoes bursting through continental plate
-oceanic and oceanic causes island arcs to form as lava cools and creates new land
What occurs at conservative plate boundary?
-parallel plates move in different speeds or directions
-no plates destroyed so no landforms created
What is sea floor spreading?
-when lava cools on the sea floor, magnetic grains in the rock acquire the direction on the earths magnetic field - paleomagnetism
-ridges are elevated above the ocean floor as they consist of rock that is hotter and less dense than the older, colder plate
-hot mantle material wells up beneath the ridges to fill the gap, material rising decompresses and undergoes partial melting
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What are the causes of the difference in sea floor spreading rates?
-slow ridges are fed by small and discontinuous magma chambers allowing for the eruption of a comparatively wide range of basalt types, eg Mid Atlantic Ridge
-fast spreading ridges have large, continuous magma chambers that generate similar magmas, because of higher rates of magma discharge, sheet lavas are more common. eg East Pacific Rise
What are deep sea trenches?
-long, narrow depressions in the ocean floor and are found adjacent to land areas and associated with island arcs
-trench usually asymmetric, with steep side towards the land mass
-numerous in Pacific Ocean
What is the Benioff zone?
-plane that dips on average at an angle of 45° away from underthrusting oceanic plate where large number of events take place
What is food mountain building?
- ocean and continental plate meet, lighter and less dense continental plate may fold and buckle into fold mountains eg Andes
-two continental plate meets both may fold and buckle eg Himalayas
What are ocean ridges?
-giant submarine mountain chains with a total length of more than 60000km
What are island arcs?
-formed when oceanic lithosphere is subducted beneath oceanic lithosphere
-typical of margins of shrinking oceans like Pacific
