plate boundaries Flashcards
what is a destructive plate boundary?
plates move towards each other (convergant)
destructive oceanic-continental?
denser oceanic plate subducts under continental plate when they collide and melts into the mantle. friction in the subduction (benioff) zone causes intermediate and deep earthquakes.
forms deep ocean trenches as water fills up gap where subduction occurs.
forms fold mountains as continental plate is pushed upwards and folded- it is also continuously growing.
volcanic eruptions are generated as magma created by melting oceanic plate pushes up through faults in continental crust to reach the surface.
destructive oceanic-oceanic?
denser/faster plate subjects beneath the other and melts in the mantle.
forms deep ocean trenches eg the Marianas Trench (1100m deep!)
forms undersea volcanoes caused by rising magma from the benioff zone. over millions of years layers of lava and debris pile up until volcano rises above sea level and forms chais of volcanoes known as island arcs.
causes shallow- deep earthquakes.
destructive continental-continental?
collision margin occurs! neither plate is subducted when they meet head on as both are lighter than lithosphere and similar have density. instead they collide and crust crumples and is pushed upwards.
form high fold mountains eg Himalayas when India collided with Asia.
causes shallow earthquakes and no volcanic activity.
what is a constructive plate margin
plates move away from one another (divergant)
constructive in oceans?
formation of mid ocean ridges.
transform faults occur at right angles cutting across ridges as they speak at different rates.
volcanic eruptions occur due to rising magma from magma which pushes up through plates forming underwater volcanoes which can grow to above sea level eg Iceland on Mid-Atlantic Ridge.
causes shallow focus earthquakes- low risk as occur in middle of ocean underwater.
constructive on land?
formation of rift valleys.
caused by heating of brittle continental crust which leads to fracturing and rifts. sides of rift move apart and central sections drop down forming valleys.
overtime this will break apart land masses and any surrounding water will fill valleys and eventually become a sea eg E African Rift Valley, 50km wide and 600m deep.
what is a conservative plate boundary?
plate slide past each other resulting in major break in crust called a fault. on large scale called a transform fault.
plates often stick as they move past each other causing stress to build up which is suddenly released as a strong shallow focus earthquake. no volcanoes occur here.
example is San Andreas Fault Line in California. 1300km long.
