Abstract
Volcanic eruptions influence land surfaces in various ways and to varying degrees, both direclly and indirectly. Pyroclastic fall and flow deposits fill or covers land surfaces, resulting in damage to vegetation via rapid accumulation of tephra and through thermal effects.Large-scale pyroclastic flows and debris avalanches may cause a variety of topographical changes of extraordinary scale. The former often cover undulating topography with thick flow accumulations, creating flat surfaces that are preserved as pyroclastic flow uplands. The latter may create buried valleys, eroding valley bottoms and walls, and damaging vegetation and soils.The basic properties and characteristics of blast phenomena were first clarified and illustrated in detail by the 1980Mt. St. Helens eruption. When a large scale debris avalanche enters the sea or a lake, it generates a tsunami which affects the coastal areas over wide areas.After rapid deposition of tephra, the surface tephra materials are easily moved by running water and wind action. Sand dunes composed of secondary tephras are common, especially in Hokkaido.Debris flows and mud flows commonly occur in volcanoes such as Usu and Sakurajima which exhibit continuous small scale tephra-producing activity. These flows contribute to the growth of volcanic fans.Strong rainfalls sometimes trigger mud and debris flows, which contribute to developing volcanic fans, on both active volcanoes and older highly dissected volcanoes. Such volcanoes have extensive volcanic fans surrounding their cones.
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