Photoglossary

Hummocky Terrain   Located near Mt. Shasta in northern California are hundreds of small hills and mounds (hummocks) whose origin had been a puzzle to geologists for several decades. Some scientists thought they were produced from glacial deposits, while others believed they were created by minor eruptions in which volcanic material broke through vents in the Earth's crust. It was not until the eruption of Mt. St. Helens in Washington State on May 18, 1980 that the origin of this hummocky terrain was finally determined.         When Mt. St. Helens erupted, it produced a sudden, high-velocity movement of massive amounts of rock and soil in a landslide called a debris avalanche. Debris avalanches form when a portion of the face of a volcano catastrophically fails and moves rapidly downslope. Debris avalanches are composed of wet or dry mixtures of soil, rock, and other volcanic materials. They are the most hazardous of all volcanic events because they destroy everything in their path. Debris avalanches contain both large, coherent pieces of the source volcano, called blocks, as well as finer-grained mixtures of volcanic fragments known as the matrix. The most characteristic topographic feature produced from a debris avalanche are irregular-shaped hummocks which form from the accumulation of this material. Much of the resulting hummocky terrain is located near its volcanic source, but it can extend outward for hundreds of kilometers (miles).         When the hummocky terrain located in the blast zone of the Mt. St. Helens eruption was compared to the deposits surrounding Mt. Shasta, it was discovered that they were similar both in composition and structure. The knowledge derived from the study of the Mt. St. Helens debris avalanche has helped scientists identify the origin of similar features in such diverse locations as the Bandai Volcano in Japan, the Gulunggung Volcano in Indonesia, and on Molokai Island in Hawaii. A kind of hummocky terrain has also been observed near the Prometheus volcano on Io. Scientists are investigating the possibility that these hummocks may have formed as a result of processes similar to those that occur on Earth.             Photo Credits: Harry Glicken, USGS (top) Austin Post, USGS, (middle) Lyn Topinka, USGS (bottom)      Io Home Page Photoglossary Home Page