Hundreds of hummocks extend from the base of Mount Shasta (top left) at least 45 kilometers NNE across the floor of Shasta Valley. This view is from the top of Gregory Mountain, located about 40 km from the summit of the volcano. The prominent cone on the right skyline is Black Butte, a collection of four overlapping lava domes that were erupted about 9,500 years ago. The hummockly landslide deposit was emplaced between 380,000 and 300,000 years ago.
The hummocks are part of an exceptionally large landslide that covers an area of about 675 km2 and has a volume of at least 45 km3. The hummocks consist of massive blocks of lava flows and layers of unconsolidated deposits of pyroclastic flows, lahars, tephra, and sediment which were transported from Mount Shasta intact within the landslide. The flat areas between the hummocks are underlain by thoroughly mixed and shattered volcanic rocks of the landslide.
Boulders of volcanic rock from Mount Shasta are scattered along the west side of Shasta Valley (out of view toward left) at heights of as much as 100 m above the adjacent surface of the landslide deposit. The boulders represent a lag that was formed after the main body of the landslide came to rest, when much of the still-fluid part (called the matrix facies) drained away and flowed out of the Shasta Valley. The boulders were left behind as the still-moving part of the landslide drained away downslope.
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