What geological feature is described as a giant crack in the Earth's crust?

A fault line is indeed described as a giant crack in the Earth's crust. It is a fracture or zone of fractures between two blocks of rock. Fault lines are critical in geology because they are the locations where seismic activity occurs, and they can indicate the movement of tectonic plates. The movement along these faults can lead to earthquakes, which are often caused by stress accumulating in the Earth's crust until it is released through this cracking action.

In contrast, volcanoes are geological features created by the eruption of magma from beneath the Earth's crust and are not simply cracks. Mountain ranges are formed by the process of tectonic forces pushing up the earth's crust rather than forming cracks. Plateaus are elevated flat areas and do not represent cracks in the Earth’s structure. Thus, the characteristics of a fault line specifically highlight it as the correct answer in the context of geological features described as cracks in the Earth's crust.