A geologist is looking at deposits along a river below a large volcano. She finds large boulders mixed with sand and mud, and all of the material is comprised of volcanic rocks, particularly the mud which is volcanic ash. She quickly realizes these deposits represent an ancient ______.
A) rock avalanche
B) lahar
C) earthflow
D) slump

Question

Quizplus · Accepted Answer

The presence of large boulders mixed with volcanic sand and mud, particularly volcanic ash, suggest that the deposits were formed from a volcanic event. In this case, the most likely explanation is a lahar, a type of volcanic mudflow that occurs when volcanic materials mix with water to create a fast-moving slurry. The large boulders are likely the result of debris picked up and transported by the lahar. The other options, such as a rock avalanche, earthflow or slump, do not fit the criteria of the presence of volcanic material in the debris.

A Geologist Is Looking at Deposits Along a River Below