Page 396 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

360 part III The Earth–Atmosphere Interface
▲Figure 12.9 Chemical sedimentary rock. Travertine, a chemical limestone composed of calcium carbonate, at the natural hot springs of Pamukkale in southwestern Turkey. [funkyfood London–Paul Williams/ Alamy.]
Limestone is also formed from a chemical process in which CaCO3 in solution is chemically precipitated out of groundwater that has seeped to the surface. This process forms travertine, a mineral deposit that commonly forms terraces or mounds near springs (Figure 12.9). The pre- cipitation of carbonates from the water of these natural springs is driven in part by “degassing”: Carbon dioxide bubbles out of solution at the surface, making the remain- ing solutes more likely to precipitate. Cave features such as speleothems, discussed in Chapter 11, are another type of travertine deposit.
Hydrothermal deposits, consisting of metallic min- erals accumulated by chemical precipitation from hot water, often are found near vents in the ocean floor— often along mid-ocean ridges created by spreading of the seafloor (discussed later in the chapter). As water seeps into the magma below the crust, it becomes superheated and then gushes out of the ocean floor at high speed. Such hydrothermal vents, called “black smokers,” belch dark clouds of hydrogen sulfides, minerals, and metals that the hot water (in excess of 380°C) leached from the basalt (Figure 12.10). These materials may build up to form towers around the vents that support life forms uniquely suited to the chemical conditions of the vent fluids. The deposits are caused by mineral precipitation but are closely associated with igneous activity within newly forming crust.
Salt deposits that precipitate when water evaporates can build up to form another type of chemical sedimen- tary rock. Examples of these evaporites are found in Utah on the Bonneville Salt Flats, created when an an- cient salt lake evaporated, and across the dry landscapes of the American Southwest. The pair of photographs in Figure 12.11 dramatically demonstrates this process; the first photo was taken in Death Valley National Park the day after a record 2.57-cm rainfall, and the second photo was taken a month later at the exact same spot, after the water had evaporated and the valley was covered in evaporites.
▲Figure 12.10 Chemical sediments at a hydrothermal vent. Black smokers and associated mineral deposits along the Mid-Atlantic ridge. [Science Source.]
Both clastic and chemical sedimentary rocks are deposited in layered strata that form an important re- cord of past ages. Using the principle of superposition discussed earlier, scientists use the stratigraphy (the ordering of layers), thickness, and spatial distribution of strata to determine the relative age and origin of the rocks. Different strata, such as those at Green Point in Newfoundland (Figure 12.8a) may form cliffs or slopes depending on the resistance of the rock to exo- genic forces such as weathering and erosion. The strata also correspond to the region’s climatic history, since each layer was formed under different environmental conditions.
Metamorphic Processes
Any igneous or sedimentary rock may be transformed into a metamorphic rock by going through profound physical or chemical changes under pressure and in- creased temperature. (The name metamorphic comes from a Greek word meaning “to change form.”) Metamor- phic rocks generally are more compact than the origi- nal rock and therefore are harder and more resistant to weathering and erosion (Figure 12.12).
The four processes that can cause metamorphism are heating, pressure, heating and pressure together, and compression and shear. When heat is applied to rock, the atoms within the minerals may break their chemical bonds, move, and form new bonds, leading to new min- eral assemblages that develop into solid rock. When pres- sure is applied to rock, mineral structure may change as atoms become packed more closely. When rock is subject to both heat and pressure at depth, the original mineral assemblage becomes unstable and changes. Finally, rocks may be compressed by overlying weight and subject to shear when one part of the mass moves sideways rela- tive to another part. These processes change the shape of the rock, leading to changes in the mineral alignments within.