Location

Mount Shasta is located in northern California about 65 kilometers (40 miles) from the border with Oregon. To the north, the Shasta River flows through a long broad valley leading to the Klamath River and Oregon. The Klamath Mountains of the Coast Range flank the valley on the west, with the Cascade Range on the east. Tertiary age volcanic rocks border the valley in the east and north. To the south the Cascades are replaced by the Sierra Nevada Mountains, which diverge from the Coast Range to form the Great Valley of California. USGS: Mount Shasta No large cities are in the vicinty of Mount Shasta, but there are three small towns within 20 kilometers (12 miles): Weed, Mount Shasta City, and McCloud, the largest. In a 1985 study, Osterkamp and others concluded that none of these towns were even at low risk from debris avalanche during the next 100 years, (Magnitude and Frequency of Debris Flows and Areas of Hazard on Mount Shasta, Northern California, USGS Professional Paper 1396-A, 1986.) However, a 1998 study modelled mudflows (lahars) for 27 volcanoes, concluding that the risk from lahars is 20 times that from debris flows, (Richard M. Iverson et al Objective delineation of lahar-inundation hazard zones. GSA Bulletin, August 1998; v. 110; no. 8).

The physical difference between debris avalanches and lahars lies in their composition: debris avalanches may contain large quantities of rock that are carried long distances by a wet mixture of sand, silt, and clay. Lahars either lack rocks, or contain relatively smaller quantities of smaller rocks. Since both materials are made mobile by water, the more fluid-like lahars travel a greater distance and cover a greater area with the same volume of material. One debris avalanche, dated 300,000 to 380,000 ago, contained "mega-blocks" as large as one kilometer wide to 2.5 kilometers long that were transported intact, (Dwight R. Crandell, 1989. Gigantic Debris Avalanche of Pleistocene Age from Ancestral Mount Shasta Volcano, California. USGS Bulletin 1861, online version.; Downloadable PDF version.) Close inspection of the Landsat image reveals that the outline coincides with some major features, such as the Shasta Gorge, indicating that georeferencing was reasonably accurate. But the outline does not appear to coincide with other features in the valley as depicted in another Landsat composite (bands 4-5-7, not shown). As explained in Crandell's text, a basalt lava flow overlies part of the avalanche, thus what is seen in the Landsat image are the overlying deposits. For notes on making this image: Go to GIS Methods

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