hazards key info Flashcards
slap pull
at destructive plate margins the denser crust (oceanic) is forced under the less dense crust.
ridge push
at constructive plate margins, magma rises to the surface and forms new crust, it heats the surrounding rock causing it to expand and rise above the surface, forming a slope.
the new crust cools and becomes denser, gravity causes the denser rock to move downslope pulling the plates apart even more.
sea floor spreading
as plates diverge (move apart), magma rises up to fill the gap created, then cools to form new crust. over time the new crust is dragged apart and the process continues. when this happens under the sea, the sea floor gets wider.
it Cretes mid ocean ridges as magma rises either side (volcano).
constructive margins
plates moving away from one another
constructive margins volcanoes
when the plates are diverging, the mantle is under pressure from the plates above. when they move apart, the pressure is released at the margin.
the release of pressure causes the mantle to melt, producing magma. the magma is less dense than the crust above forming a volcano.
Rift Valley
when plates diverge beneath land, rising magma cause the continental crust to bulge and fractured forming fault lines. as the plates keep moving apart, the crust parallel drops down to form a Rift Valley.
example: East African rift system around 4000km. Nubian and Somalian plates diverging.
constructive margins earthquake
the plates don’t diverge in a regular way, some plates move faster then other. this causes pressure which can eventually lead to fault and cracks in margin and earthquakes.
destructive margins
where the plates converge (move together)
destructive (oceanic-continental)
the oceanic plate is sub-ducted under the continental as it is more dense, forming a deep sea trench and fold mountains as sediment accumulated on the continental crust is buckled up.
the oceanic crust is heated by friction and contact with the mantle to form magma. the magma is less dense than the continental crust above and will rise to form volcanoes.
earthquakes can also form if the plates get stuck when being subducted and pressure builds up.
destructive (oceanic-oceanic)
most of the same processes will occur as o-c. the denser plate will be subducted forming a deep sea trench, fold mountains, earthquakes and eruptions.
eruptions that take place under water form island arcs.
conservative plate margins
only form earthquakes where to plates are moving beside one another, the two plates get locked together and pressure builds up and causes an earthquake.
magma plumes
magma plumes are vertical columns of extra-hot magma that rises up from the mantle. the magma plume remains stationary but the crust moves above it forming volcanoes away from margins.
Hawaii’s chain of islands were formed by magma plumes.
volcanic hazards at constructive margins
-basaltic lava is formed here. very hot and has low viscosity. the eruptions are non violent and take place over a long time.
volcanic hazards at destructive margins
-andesitic and rhylotic lavas are formed here- they are cooler and more viscous so flow less easier.
when plates are subducted as the lava is viscous it causes blockages in volcanic vents which lead to more explosive eruptions.
pyroclastic flows or nueè ardentes
a mixture of super-heated gas, ash and volcanic rock that flows down the sides of volcanoes. often travels at 80km/h
-travel fast with no warning so very dangerous.
