2.10 Convergent plate boundaries - subduction (oceanic)

Question 1

What is meant by the term convergent boundary?

Answer 1

a place where two lithospheric plates are moving towards each other

Question 2

What is meant by the term subduction?

Answer 2

the movement of an oceanic plate into the mantle as a result of convergence

Question 3

What are the main landforms created by subduction at oceanic-oceanic convergent boundaries?

Answer 3

ocean trench
volcanic island arc

Question 4

Why is volcanic activity common at oceanic-oceanic convergent boundaries?

Answer 4

partial melting of the basaltic oceanic crust (the subducting plate) occurs as lower temperature minerals melt
melted minerals separate and, being less dense that the surrounding oceanic crust, rise up through the crust

Question 5

What is the composition of the mantle?

Answer 5

ultramafic (<45% silica)
peridotite

Question 6

What is flux melting and why does it occur at oceanic-oceanic convergent boundaries?

Answer 6

water released into mantle rock from the subducting oceanic lithosphere lowers the melting point of minerals in the mantle rock, leading to partial melting

Question 7

What is decompression melting and why does it occur at oceanic-oceanic convergent boundaries?

Answer 7

Lower pressure in the mantle is created as the subducting plate drags on the surrounding mantle.
Lowering the pressure reduces the melting point of minerals in the mantle leading to partial melting.

Question 8

What is the composition of magma produced by partial melting of mantle material?

Answer 8

mafic (45-52% silica)
basaltic

Question 9

What is the nature of the magma at oceanic-oceanic convergent boundaries?

Answer 9

intermediate (52-66% silica)
this is due to the partial melting of oceanic crust combining with the basaltic magma created by flux and decompression melting of mantle

Question 10

Why does the chain of volcanoes curve (an island arc)?

Answer 10

it reflects the curving shape of the convergent plate boundary beneath

Question 11

What is an ocean trench?

Answer 11

a long, narrow, linear part of the Earth’s surface producing the deepest parts of the oceans

Question 12

Why are ocean trenches found at oceanic-oceanic and oceanic-continental convergent boundaries?

Answer 12

they occur when the subducting plate is curved as it is forced under the other plate

Question 13

What are the main landforms associated with oceanic-continental convergent boundaries?

Answer 13

ocean trenches
fold mountains
batholiths
limited volcanic activity

Question 14

What is the composition of lava generated at oceanic-continental convergent boundaries?

Answer 14

silicic (>66% silica)
rhyolite
and
intermediate (52-66% silica)
andesite

Question 15

Why is lava produced at oceanic-continental convergent boundaries so high in silica?

Answer 15

Mafic/intermediate magma rising from the subducting oceanic plate is hot enough to cause partial melting of the continental crust that it passes through, incorporating more silica into the magma.

Question 16

What is a batholith?

Answer 16

a large igneous intrusion

Question 17

What are fold mountains and why are they found at oceanic-continental convergent boundaries?

Answer 17

Fold mountains are areas of thicker continental crust created by the intense folding and faulting of rocks.
Fold mountains occur at oceanic-convergent boundaries because the continental crust is compressed and sediments are scraped off the subducting plate and folded.

Question 18

What is the Benioff (or Benioff-Wadati) zone?

Answer 18

a sloping area of increasingly deep-focus earthquakes generated along the subducting plate