Comment: 11:39 - 12:48 (01:09)
Source: Annenberg/CPB Resources - Earth Revealed 23. Glaciers
Keywords: "James Sadd", moraine, deposition, glacier, "glacial ice", "glacial melt", "glacial flow", "glacial advance", "glacial retreat", geologist, "Ice Age", "geologic time", climate, "sea level", evolution, life, "glacial cycle", "overlapping moraine"
Our transcription: Understanding moraine deposition is of great importance to geologists because it can be used to reconstruct the history of glacial movement.
For example, overlapping moraines not only show the position of a glacier at various times, but also indicate that glaciers typically advance and retreat over and over.
It was evidence like this that helped geologists recognize that the Earth has experienced repeated cycles of glaciation, or "ice ages," over the last two million years.
An ice age consists of a gradual cooling of the climate and growth of glaciers worldwide terminated by a warm inter-glacial climate during which glaciers melt back and retreat.
By reconstructing the glacial history of the Earth, we now understand that changes in sea level and the evolution of life are also linked with glacial cycles.
What's less clear, however, is the cause of ice ages, and a number of theories attempt to explain the processes responsible for glacial cycles.
There have been about ten ice age cycles over the last one million years.
Any theory that is advanced to explain these cycles must take into account the regular repetition of glacial activity and the link between glacial cycles and global climate.
Geology School Keywords