Evidence of Mt. Lassen Glaciation

January 16, 2009

011609

Provided and Copyright by: Thomas McGuire, Textbook Author/Educator
Summary Author: Thomas McGuire

As the Pacific and Juan de Fuca lithospheric plates dive beneath Northern California, melting of near-surface rocks has produced dozens of volcanoes in the Coast Ranges. As Mount St. Helens has shown us, none of these cones can be dismissed as inactive. In past epochs, glaciers have covered many of the higher peaks of the Coast Ranges, leaving behind abundant evidence of glaciers' erosive action. This particular exposure of glacial polish and striations is on Mt. Lassen in northern California. Similar surface features can be observed on bedrock surfaces throughout the northern part of the U.S. and most all of Canada. Such features can be used to determine the direction of movement of the flowing ice. Ice, which is technically a mineral, has a hardness of only 1.5 on Mohs scale. So how can glaciers scratch and polish these rock surfaces? The answer is: glaciers often transport great quantities of rocks and sediment that act like sandpaper under the weight of the ice.

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