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A depression in geology is a landform sunken or depressed below the surrounding area. Depressions may be formed by various mechanisms. Erosion-related: * Blowout: a depression created by wind erosion typically in either a partially vegetated sand dune ecosystem or dry soils (such as a post-glacial loess environment).〔 * Glacial valley: a depression carved by erosion by a glacier. * River valley: a depression carved by fluvial erosion by a river. * Area of subsidence caused by the collapse of an underlying structure such as sinkholes in karst terrain. * Sink: an endorheic depression generally containing a persistent or intermittent (seasonal) lake, a salt flat (playa) or dry lake, or an ephemeral lake. Collapse-related: *Sinkhole: a depression formed as result of the collapse of rocks lying above a hollow. This is common in karst region. * Kettle: a shallow, sediment-filled body of water formed by melting glacial remnants in terminal moraine.〔(【引用サイトリンク】 Dictionary of Geologic Terms – K )〕 *Thermokarst hollow: caused by volume loss of the ground as the result of permafrost thawing. Impact-related: * Impact crater: a depression created by an impact such as a meteorite crater. Sedimentary-related: * Sedimentary basin: In sedimentology, an area thickly filled with sediment〔(【引用サイトリンク】 Dictionary of Geologic Terms – B )〕 in which the weight of the sediment further depresses the floor of the basin. Structural or tectonic-related: * Structural basin: A syncline-like depression; a region of tectonic downwarping as a result of isostasy (the Hawaiian Trough is an example) or subduction (such as the Chilean Central Valley). * Graben or rift valley: down dropped and typically linear depressions or basin created by rifting in a region under tensional tectonic forces. * Pull-apart basin caused by offset in a strike slip or transform fault (example: the Dead Sea area). * Oceanic trench: a deep linear depression located in the ocean floor. Oceanic trenches are caused by the subduction (when one tectonic plate is pushed underneath another) of oceanic crust beneath either other oceanic crust or continental crust. * A basin formed by ice sheet: an area depressed by the weight of the ice sheet resulting in post-glacial rebound after the ice melts (the area adjacent to the ice sheet may be pulled down to create a peripheral depression.)〔 Cites American Geological Institute's Glossary of Geology (3rd edition, revised in 1987).〕 Volcanism-related: * Caldera: a volcanic depression resulting from collapse following a volcanic eruption.〔(【引用サイトリンク】 Dictionary of Geologic Terms – C )〕 * Pit crater: a volcanic depression smaller than a caldera formed by a sinking, or caving in, of the ground surface lying over a void. * Maar: a depression resulting from phreatomagmatic eruption or diatreme explosion. ==See also== * Cryptodepression * List of places on land with elevations below sea level 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Depression (geology)」の詳細全文を読む スポンサード リンク
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