35. Massive Volcanic Debris Avalanche, Socompa Volcano, Chile
The catastrophic eruption of Mt. St Helens on May 18, 1980, focused attention on a previously little known type of volcanic eruption, in which collapse of a large segment of the volcano triggers a violent explosive blast. Many other examples of similar eruptions have now been recognized in the field and in geological literature. Examination of satellite images of the Andes has revealed many major examples. Illustrated is the 6050-meter-high (19,844 feet) Socompa volcano on the frontier between Chile and Argentina. At some time between about 10,000 and 500 years ago, the volcano experienced a massive collapse of a 70° sector of the original cone that triggered a debris avalanche that traveled up to 40 kilometers (25 miles) from the volcano, covering an area of 500 square kilometers (190 square miles). The summit of the volcano is at lower right; the debris avalanche deposit fills the center of the frame and is pale blue-gray.
At left center, the marginal levee of the deposit is conspicuous, illuminated by the low Sun angle. Less conspicuous is a low scarp running northeastward across the center of the deposit. This reveals that the avalanche was compound, with primary and secondary flows. The primary flow traveled northwestward, encountered sloping ground, came to rest transiently on it, and then slumped away northeastward in a secondary flow.
STS-41D, August-September 1984. Picture #14-39-034.