2.11 Divergent plate boundaries

Question 1

What is a rift valley?

Answer 1

an area of lower lying land that occurs when a linear strip of crust has slipped down along normal faults

Question 2

Why do rift valleys form at divergent boundaries?

Answer 2

normal faults form due to extension of the crust, which is either pulled apart by tension or gently arched upwards by rising magma

Question 3

What is the nature of earthquake activity at divergent boundaries?

Answer 3

shallow-focus, low-magnitude earthquakes occur along the fault lines

Question 4

What is a mid-ocean ridge?

Answer 4

where divergent boundaries occur under oceans (most divergent boundaries) an area of raised ocean floor can be observed with a central rift valley

Question 5

How do mid-ocean ridges form?

Answer 5

• diverging plates cause (or are caused by - this is debated) upwelling of hot mantle material leading pushing the crust up
• newly created oceanic lithosphere is hotter and thinner and therefore ‘floats’ at a higher level on the asthenosphere than older, colder and thicker oceanic lithosphere at the edges of oceans (isostasy)

Question 6

What type of melting generates the magma at divergent boundaries?

Answer 6

decompression melting

Question 7

What is the nature of the magma produced at divergent boundaries?

Answer 7

mafic (45-52% silica)
basaltic (dolerite or gabbro at depth)