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A turbidity current is a current of rapidly moving, sediment-laden water moving down a slope through water, or another fluid. The current moves because it has a higher density than the fluid through which it flows—the driving force of a turbidity current derives from its sediment, which renders the turbid water denser than the clear water above. The deposit of a turbidity current is called a turbidite. Turbidity currents are an example of density or gravity currents, which include: oceanic fronts, avalanches, lahars, pyroclastic flows and lava flows. Seafloor turbidity currents are commonly used to describe underwater currents in lakes and oceans, which are usually triggered by earthquakes, slumping and sediment-laden rivers. They are characterized by a well-defined front, also known as head, followed by a layer known as the body of the current. Turbidity currents are characteristic of areas where there is seismic instability and an underwater slope, especially submarine trench slopes of convergent plate margins, continental slopes and submarine canyons of passive margins. With an increasing continental shelf slope, current velocity increases, as the velocity of the flow increases, turbulence increases, and the current draws up more sediment. The increase in sediment also adds to the density of the current, and thus its velocity even further. Turbidity currents are traditionally defined as those sediment gravity flows in which sediment is suspended by fluid turbulence.〔Sanders, J.E. 1965 Primary sedimentary structures formed by turbidity currents and related resedimentation mechanisms. In: Primary Sedimentary Structures and Their Hydro-Dynamic Interpretation – a Symposium Middleton, G. V.), SEPM Spec. Publishers , 12, 192–219.〕〔Meiburg, E. & Kneller, B. 2010, "Turbidity currents and their deposits", Annual Review of Fluid Mechanics, vol. 42, pp. 135-156.〕 However, the term 'turbidity current' was adopted to describe a natural phenomenon whose exact nature is often unclear. The turbulence within a turbidity current is not always the support mechanism that keeps the sediment in suspension; however it is probable that turbulence is the primary or sole grain support mechanism in dilute currents (<3%).〔Kneller, B. & Buckee, C. 2000, "The structure and fluid mechanics of turbidity currents: A review of some recent studies and their geological implications", Sedimentology, vol. 47, no. SUPPL. 1, pp. 62-94.〕 Definitions are further complicated by an incomplete understanding of the turbulence structure within turbidity currents, and the confusion between the terms turbulent (i.e. disturbed by eddies) and turbid (i.e. opaque with sediment).〔McCave, I.N. & Jones, K.P.N. 1988 Deposition of ungraded muds from high-density non-turbulent turbidity currents. Nature, 333, 250–252.〕 Kneller & Buckee, 2000 define a suspension current as 'flow induced by the action of gravity upon a turbid mixture of fluid and (suspended) sediment, by virtue of the density difference between the mixture and the ambient fluid'. A turbidity current is a suspension current in which the interstitial fluid is a liquid (generally water); a pyroclastic current is one in which the interstitial fluid is gas.〔 ==Triggers== 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「turbidity current」の詳細全文を読む スポンサード リンク
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