as lightweight metallic alloys. Examples of freshwater evaporate deposits can be found in tectonic valleys in the western United States. As little as 13,000 years ago at the end of the last ice many of these valleys, which formed due to extension of the crust (Chapter 4), contained deep lakes whose water was relatively fresh. Since then most of the lakes have completely evaporated as the climate has become more arid, leaving evaporate deposits on the now exposed lake beds. One of the few remaining lakes is the now highly saline Great Salt Lake in Utah, shown in Figure 12.16. This lake lies in a topographic depression that receives water from streams, but the water can only escape through evaporation. Because the overall evaporation rates are relatively constant, water levels today in the Great Salt Lake rise and fall primarily in response to yearly changes in stream runoff.
Phosphorites
Similar to the element nitrogen, phosphorus (P) is one of the essential plant nutrients found in soils (Chapter 10). Consequently, economical mineral deposits containing phosphorus are important in the production of modern fertilizers. Phosphorous-bearing minerals, however, are normally widely dispersed within igneous rocks, and are therefore not economical to mine. When rocks undergo chemical weathering, however, the phosphorus is released in the form of the phosphate ion (PO43-), which is then transported to water bodies where it accumulates. Once in aqueous form, animal life is able to extract both phosphate and calcium ions to form teeth and bones composed mainly of the mineral apatite (CaPO4). As a result, most limestone rocks that contain the skeletal remains of marine organisms are relatively rich in phosphorus compared to other rock types. In situations where unusually large amounts of skeletal matter