Question

why does plastic flow only occur below 50 meters of ice?
a. it is cold enough below that much ice to cause plastic flow.
b. it takes the weight of that much ice to cause the plastic flow.
c. fifty meters of ice can block enough sun to cause plastic flow.
d. basal slip only occurs at depths below 50 meters.

Explanation:

Plastic flow in ice is related to the pressure exerted by the weight of the ice. When there is enough ice (below 50 meters), the weight of the overlying ice creates sufficient pressure to cause the ice to deform and flow plastically. Option A is incorrect because cold temperature alone (without sufficient pressure) does not cause plastic flow. Option C is wrong as blocking the sun is not the mechanism for plastic flow. Option D is incorrect because basal slip is a different glacial movement mechanism and not the reason for plastic flow starting below 50 meters.

Answer:

B. It takes the weight of that much ice to cause the plastic flow.