558 SEDIMENTARY ENVIRONMENTS/Lake Processes and Deposits Hydrothermal Processes Figure Satellite image of Lake Michigan taken on 20 August 1999 The surface waters over much of the southern half of the lake have a milky appearance (‘whiting’) due to the extensive development of phytoplankton blooms and the accompanying precipitation of calcite (Part of NASA Visible Earth image #ev4338_S1999254181550.png) Which minerals are precipitated is determined by the chemistry of the inflowing water This is in marked contrast to the formation of marine evaporites, where the uniform composition of seawater ensures that only a limited suite of minerals can occur Because the precipitation of salts requires the evaporation of large volumes of lake water, their formation and accumulation tend to occur in lakes that are hydrologically closed (i.e they have no outflow) and in regions with a dry or arid climate In these settings, water loss due to evaporation can exceed inflow Such lakes tend to alternate between brief periods of flooding, following rainfall, and longer periods of saline or even completely desiccated conditions Because of their large variations in volume, many saline lakes are surrounded by extensive salt flats, where salt precipitation may continue owing to groundwater seepage and loss through capillary effects (Figure 10) Lake Baikal and many lakes in the East African rift system, the western USA, and the Andes receive a portion of their inflow from the subsurface These springs may enter the lake from above or below the lake surface, and their temperature may be anywhere between ambient and boiling The water may be rich in dissolved ions, which can have a measurable influence on the chemistry of the lake water and, in saline lakes, the nature of any evaporite minerals that may precipitate In addition, effects induced by changes in physicochemical conditions at the point where a spring vents into a lake may trigger mineral precipitation and the accumulation of hydrothermal deposits Silica and carbonates are the commonest precipitates; the latter may form spectacular deposits (Figure 11) Sulphide mineralization has also been recorded in some of the East African rift lakes The spring deposits typically take the form of irregular masses of travertine and sinter, which in many cases are rather porous and show complex growth patterns One reason for these features is that many springs, even those that emerge at close to boiling point, have dense growths of filamentous and encrusting microbial communities associated with them These colonies clearly influence the manner in which the minerals accrete The larger voids represent conduits through which fluids passed; some of the sublacustrine springs in Lake Tanganyika are marked by chimney-like structures of aragonite, many of them carrying fronds of milky white filamentous bacteria, swaying in the stream of hot water exiting the conduit Tectonic Processes Although lake basins have a variety of origins, outside areas of glaciation most large long-lived lakes occupy basins formed by tectonic processes The morphology of such a basin is to a large degree determined by its tectonic setting The Lake Baikal and East African rifts, and many of the basins in the western USA, for example, are the result of crustal extension, which typically produces half-graben structures characterized by marked topographical asymmetry (Figure 12) (see Tectonics: Rift Valleys) The form of the basin determines the position of the lake, which will occur at the topographically lowest point of the basin; typically, this means that the deepest water is close to a border fault Sediment accumulation is also greatest here, where maximum subsidence rates produce the space needed to accommodate new deposits Continuous Miocene–Holocene subsidence has resulted in the accumulation of several kilometres of lacustrine sediment beneath Lake Baikal and in parts of the East