Destructive Plate Boundaries

Question 1

Plate types

Answer 1

. Oceanic-Continental
. Oceanic-Oceanic
. Continental-Continental

Question 2

Oceanic-Continental

Answer 2

. The oceanic crust which is thinner and more dense than the continental crust, sinks below the continental crust, into the upper mantle (subduction)

. Destructive margins are some of the most seismically active zones in the world, with shallow- to deep- focus earthquakes

. Deep ocean trenches are found along the seaward edge of destructive margins. They mark where the oceanic plate begins to descend beneath the continental plate

Question 3

Oceanic-Continental - The Trenches

Answer 3

. As the dense oceanic Nazca plate is subducted beneath the continental South American plate a deep ocean trench is formed

. The trench extends some 8km down and in places is 64km wide

. These trenches are largely unexplored

Question 4

Oceanic-Continental - Fold Mountains

Answer 4

. Pressure put onto the continental crust can cause if to buckle upwards and form fold mountains

. Material on the descending oceanic plate is scraped off and folded to form the mountains

. E.g. Andes in South America which are further added to by volcanoes formed due to the subduction

Question 5

Oceanic-Continental - The Volcanoes

Answer 5

. As it is less dense than the surrounding asthenosphere, the molten material begins to rise in great plumes up through fissures/faults in the buckled continental plate and by burning their way through overlying rock. Eventually these reach the surface to form explosive volcanoes.

. The andesitic lava has a viscous nature (flows less easily). This creates complex , composite, explosive volcanoes. If the eruptions take place off shore, a line of volcanic islands known as an island arc can appear

. Benioff zone – the further the rock descends, the hotter the surroundings become. This, together with the heat generated from friction, begins to melt oceanic plate into the magma.

Question 6

Oceanic-Oceanic

Answer 6

Two oceanic plates moving towards each other.

. One is forced under the other and the processes involved with subduction begin.

. The crust that is subducted may be marginally the denser of the two plates or is the one which is moving more quickly.

. Subduction occurs in the same way as at Oceanic-Continental but it is less clear which plate should descend.

Question 7

Oceanic-Oceanic - Trenches and Arcs

Answer 7

. Earthquakes are frequent – subduction produces shallow to deep focus earthquakes, some of which can be very powerful. Sub-sea earthquakes may result in tsunami formation

. Ocean trenches and island arcs are the main features, as the interaction takes place well offshore.

. Marianas trench reaches a depth of over 11km

. Beyond the trench are the Mariana islands

Question 8

Continental-Continental - Collision Boundaries

Answer 8

. Continental plates are of lower density than the asthenosphere beneath them. This means that subduction does not occur.

. Instead they collide with each other

. These boundaries are prone to very powerful Earthquakes (e.g. Nepal 2015) as the collision occurs

. Sediments scraped off vanishing ocean floors between the plates or volcanoes associated with earlier subduction are compressed to form young fold mountain chains with deep roots in the lithosphere