Island Arcs and ocean plate subduction

Question 1

Mariana Trench details

Answer 1

• 10,920m deep
• 2,250km long
• 69km wide
• subduction of pacific plate beneath philippine plate

Question 2

Exploration of the Mariana Trench

Answer 2

• challenger expedition during 1870s (recorded depth, 9,636m)
• 1951 used echo sounding maximum depth now 10,900m (known as Challenger Deep)
• Donald Walsh and Jacques Piccard in the bathyscape Trieste in 1960

Question 3

Mariana activtiy

Answer 3

• the trench has a series of earthquakes, behind that there are a chain of volcanoes
• shallow and intermediate earthquakes, brittle activity, very solid slab of crust, old cold dense slab of crust
• the weight of the volcanic arc is holding down the bending plate = negative anomaly

Question 4

Trenches and gravity anomalies

Answer 4

• variations in the strength of gravity occur depending on the density distribution of the rocks beneath the surface

Question 5

The fore arc basin and sediment accretionary prism

Answer 5

• sediments are scraped off the ocean plate to form the accretionary wedge
• seamounts on the subducting plate can deform the sediments of the accretionary wedge

Question 6

volcanic arcs

Answer 6

• occur over cool down-going lithosphere

- melting of the overlying mantle by adding water and then buoyant rise

Question 7

3 ways to melt rocks

Answer 7

1) heat them a lot
2) take the pressure off (move them up towards the earth surface)
3) add water

Question 8

What causes the lithosphere to sink into the asthenospheric mantle?

Answer 8

• it is colder and heavier than surrounding mantle asthenosphere so it sinks under gravity
• the ridge push is also a gravity force as the oceanic plate slides down hill away from the mid-ocean ridge
• it is not clear if there if always a relationship between mantle convection flow and direction of plate movement

Question 9

The back arc basin

Answer 9

Topographic feature extending behind the volcanic arcs

Question 10

Summary of main forces affecting oceanic plate motion

Answer 10

• roll-back of the subducting lithosphere may cause extension of the back-arc region
• trench suction
• slap pull and ridge pull
• overriding plate resistance and bending resistance
• mantle drag

Question 11

Positive anomalies

Answer 11

occur when the underlying rocks are more dense than predicted (pre-cambrain basement and basalt rocks)

Question 12

Negative anomalies

Answer 12

occur when the underlying rocks are less dense than predicted by modelling (sedimentary basin and large granite intrusions)