Hazards: Physical Flashcards

Question

Outline the process of slab pull at ocean trenches:

Answer 1

At destructive plate boundaries the subducting oceanic plate (usually composed of basalt) is denser than surrounding material.Downward gravitational force acts upon the cold dense descending plate as it sinks into the mantle.This pulls the rest of the plate behind it.

Answer 2

plates move towards each other

Answer 3

plates move alongside each other in opposite directions or the same direction at different speed

Answer 4

Plates diverge in oceanic areas (due to convection currents and gravitational sliding) - sea floor spreading.As plates move apart this leaves cracks and fissures, lines of weakness that allows magma from the mantle to escape from the highly pressurised interior of the planet. This magma fills the gap and eventually erupts onto the surface and cools as new land. Creating ridges of undersea mountains and volcanoes.

Answer 5

When plates diverge beneath land, rising magma causes the continental crust to bulge and fracture, forming fault lines,As the plates keep moving apart the crust between the parallel faults drop down to form a rift valley.The crust here is thinner due to tension in the crust causing the plate to thin as it starts to split. Through this thinning crust, magma forces its way to the surface to from volcanoes

Answer 6

Denser oceanic plate subducts under the less dense continental plate > downwarping forms an ocean trench. As the oceanic plate descends friction with the plate above and increasing pressure and heat in the asthenosphere, melts the subducting ocean plate at the benioff zone. Some of this molten material works its way up through the continental plate above through fissures, as molten material is less dense than the asthenosphere.Magma eventually re-emerges at the surface to create explosive composite volcanoes (explosive as they have silica-rich andesitic lava). Sediments on the edge of the continental crust cumple upwards as the oceanic plate subducts forming fold mountains. As one plate is subducted, plates can become stuck > pressure builds up > suddenly jerk past each other > earthquakes.

Answer 7

Denser oceanic plate subducts under the less dense oceanic plate > downwarping forms an ocean trench. As the oceanic plate descends friction with the plate above and increasing pressure and heat in the asthenosphere, melts the subducting ocean plate at the benioff zone. Some of this molten material works its way up through the less dense oceanic crust through fissures. Magma eventually re-emerges at the surface to create an island arc of explosive composite volcanoes.

Answer 8

Neither plate subducts into the mantle because of their similar density. Instead, the two plates crumple into one another and fold upwards into fold mountains.As neither is subducted there are no volcanoes but the pressure build up can cause earthquakes.

Answer 9

Two plates move alongside each other (in opposite directions or the same direction at different speeds). Plates get stuck (‘lock’) due to friction and pressure builds up.Plates suddenly jerk past each other or crack forming fault lines, this releases the pressure sending shockwaves through the earth’s crust as earthquakes

Answer 10

Stationary area of crust High heat flow (rising magma plume due to concentration of radioactive elements below the crust). Thinner crust (low pressure and high heat allow magma plume to melt and thin the lithosphere)Located away from plate boundaries

Answer 11

Hawaiin hotspotReunion hotspot (deccan traps)

Answer 12

Intense radioactivity (due to concentration of radioactive elements) below the crust creates a magma plume (column of extra-hot upwelling lava).Plume from the asthenosphere rises upwards and high heat and low pressure allows molten rock to melt the plate above (so plate is thinner than average) and push through the crust above.Lava breaks through the surface forming active volcanoes above the plume.The magma plume remains stationary but the tectonic plate moves. Upwelling lava creates a steady succession of new volcanoes.Volcano above the hotspot is active and the remainder form a chain of islands of extinct volcanoes. The oldest volcanoes put pressure on the crust causing subsidence and marine erosion results in old volcanoes becoming seamounts.