- (B) The asthenosphere is the region below the lithosphere, estimated as
being from 50 to several hundred miles (85 to several hundred km) thick.
FREE-RESPONSE ANSWER
10 Total Points Possible
(a) Maximum 6 points total: 2 points each for identifying, describing, and
providing an example of geological features A, B, and C.
(A) (i) Identify AND describe the geological action that is occurring at A in
the diagram. (1 point maximum) Feature A is a subduction zone. One
lithospheric plate is subducting (sinking) below another, largely due
to differences in density (the denser plate sinks), and is an example of
a convergent plate boundary. As the subducted plate sinks to greater
depths, the temperature increases and the plate begins to point where
it begins to melt. This molten magma is less dense than the solid rock
around it, so it rises and forms a chain of volcanic mountains parallel
to the plate boundary.(ii) Give an example of geological feature A occurring on Earth. Only 1
example is required. (1 point maximum) Possible answers include:
■ The Cascade Mountains in Washington State (volcanic arc) ■ The
Alaskan Peninsula and Aleutian Range (volcanic arc) ■ The Andes
volcanic belt (volcanic arc) ■ Aleutian Islands (island arc)
■ Japan (island arc)
■ Philippines (island arc)(B) (i) Identify AND describe the geological action that is occurring at B in
the diagram. (1 point maximum) Feature B is a divergent plate
boundary. Lithospheric plates are moving apart. The space created
between them is filled by hot molten magma coming up from the
asthenosphere that then cools and adds to the crust. In oceanic crust,
they are known as a mid-oceanic ridge.(ii) Give an example of geological feature B occurring on Earth. Only 1
example is required. (1 point maximum) Possible answers include:
■ Mid-Atlantic Ridge (mid-oceanic ridge) ■ East Pacific Rise (mid-
oceanic ridge) ■ African Rift Valley (rift valley)