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Chapter 6: Sedimentary and Metamorphic Rocks NOTES Chapter 6: Sedimentary and Metamorphic Rocks NOTES (Can be accessed on notes section of website) 6.1 Formation of Sedimentary Rocks Main Idea: Sediments produced by weathering and erosion form sedimentary rocks through the process of lithification. Weathering (1) Sediments: small pieces or fragments of rock that created by weathering. There are two types of weathering: physical and chemical During physical weathering, minerals remain chemically unchanged. Rock fragments break off of the solid rock along fractures or grain boundaries. Plant growth, frost wedging, and animal activity can expedite this process. Chemical weathering is when minerals in a rock are dissolved or otherwise chemically changed (moving water or acid rain). When exposed to both chemical and physical weathering, granite eventually breaks apart and might look like this decomposed granite. Erosion (2) The removal and transport of sediment is called erosion. The four main agents of erosion are wind, moving water, gravity, and glaciers. After rock fragments and sediments have been weathered out of the rock, they often are transported to new locations through erosion. Eroded material is almost always carried downhill. Deposition (3) Deposition occurs when transported sediments are deposited on land or sink to the bottom of a body of water, forming layers with the largest, most dense grains at the bottom. As moving water slows down, some sediment deposits are sorted into layers of different-sized particles. The largest particles settle out first, then the next largest, and so on. Wind only moves small grains, so deposits made by wind are usually fine and well-sorted. Some sediment deposits contain particles of all sizes because they are dumped in unsorted piles when, for example, a glacier melts or there is a landslide. Lithification (4 and 5) As more sediment is deposited in an area, the bottom layers are subjected to increasing pressure and temperature. These conditions cause lithification, the physical and chemical processes that transform sediments into sedimentary rocks. Lithification begins with compaction. The weight of overlying sediments (see right) forces the sediment grains closer together, causing physical changes. Cementation occurs when dissolved minerals precipitate out of groundwater and their growth glues sediment grains together into solid rock. Sedimentary Features The primary feature of sedimentary rock is horizontal layering called bedding, which results from the way sediment settles out of water or wind. When particle sizes become progressively heavier and coarser toward the bottom layers it is called graded bedding. Cross-bedding is formed as inclined layers of sediment are deposited across a horizontal surface. When sediment is moved into small ridges by wind or wave action or by a river current, ripple marks form. As sediment is transported, pieces that began with an angular shape knock into each other and become rounded as their edges are broken off or furthered weathered. Fossils are the preserved remains, impressions, or any other evidence of once-living organisms. During lithification, parts of an organism can be replaced by minerals and turned into rock, such as shells that have been turned into stone. 6.2 Types of Sedimentary Rocks Main Idea: Sedimentary rocks are classified by their mode of formation and there are three types. Review - Saturated: the maximum possible content of dissolved minerals in solution. 1) Clastic Sedimentary Rocks The most common sedimentary rocks, clastic sedimentary rocks, are formed from the abundant deposits of loose sediments that accumulate on Earth’s surface. Clastic refers to rock and mineral fragments produced by weathering and erosion. These rocks are further classified according particle size. Conglomerates have rounded, gravel-sized particles, while Breccias are composed of angular, gravel-sized particles. Sedimentary rocks containing sand-sized rock and mineral fragments are medium-grained clastic rocks. Porosity is the percentage of open spaces between grains in a material, such as rock. When open spaces between grains in a rock are connected to one another, fluids can move through porous rock such as sandstone. Sandstone layers can be valuable as underground reservoirs of oil, natural gas, and groundwater. Sedimentary rocks consisting of silt- and clay-sized particles are called fine-grained rocks (low porosity). 2) Chemical Sedimentary Rocks (see figure above) When the concentration of dissolved minerals in a body of water reaches saturation, crystal grains precipitate out of solution and settle to the bottom. The resulting layers of chemical sedimentary rocks are called evaporites. The constant evaporation from a body of salt water results in precipitation of large amounts of salts. 3) Biochemical Sedimentary Rocks Biochemical sedimentary rocks are formed from the remains of once-living organisms. The most abundant of this type of rock is limestone, which is composed primarily of calcite. After the death of organisms that used calcium carbonate to make their shells, the shells settle to the bottom of the ocean and can form thick layers of carbonate sediment. During burial and lithification of the carbonate sediment formed from the shells of once-living organisms, calcium carbonate precipitates out of the water, crystallizes between the grains of carbonate sediment, and forms limestone. 6.3 Metamorphic Rocks Main Idea: Metamorphic rocks form when existing rocks are exposed to increases in temperature, pressure and/or hydrothermal solutions. Review - Intrusive: rocks that form from magma that cooled and crystallized slowly beneath Earth’s surface. Recognizing Metamorphic Rocks When high temperature and pressure combine and change the texture, mineral composition, or chemical composition of a rock without melting it, a metamorphic rock forms. The high temperatures required for metamorphism are ultimately derived from Earth’s heat, either through deep burial or from nearby igneous intrusions. The high pressures required for metamorphism come from deep burial or from compression during mountain building. During metamorphism, the minerals in a rock undergo solid-state alterations, which means that they change into new minerals that are stable under the new temperature and pressure conditions. Metamorphic Textures Foliated metamorphic rocks are characterized by layers and bands of minerals caused by high pressure during metamorphism. Foliation develops when pressure is applied from opposite directions. The foliation develops perpendicular to the pressure direction. Nonfoliated metamorphic rocks are composed mainly of minerals that form blocky crystal shapes. Increasing grain size parallels changes in composition and development of foliation. Grain size is not a factor in nonfoliated rocks. Different combinations of temperature and pressure result in different grades of metamorphism. Each of the grades—low, intermediate, and high—is associated with a different suite of minerals and textures. Types of Metamorphism When high temperature and pressure affect large regions of Earth’s crust, they produce large belts of regional metamorphism, ranging from low grade to high grade. Regional metamorphism results in changes in minerals and rock types, plus folding and deforming of the rock layers that make up the area. When molten material, such as that in an igneous intrusion, comes in contact with solid rock, a local effect called contact metamorphism occurs. High temperatures and moderate-to-low pressure form mineral assemblages that are characteristic of contact metamorphism. When very hot water reacts with rock and alters its chemical and mineral composition, hydrothermal metamorphism occurs. Chemical changes are common during contact metamorphism near igneous intrusions and active volcanoes. Many of the economic mineral resources that make the modern way of life possible are produced by metamorphic processes. Although deposits of pure metals are occasionally found, many metallic deposits are precipitated from hydrothermal solutions. Many nonmetallic resources are also produced by metamorphism, including talc, asbestos, and graphite. The Rock Cycle - Interactive Video The three types of rock are grouped according to how they form. Igneous rocks crystallize from magma; sedimentary rocks form from cemented or precipitated sediments; and metamorphic rocks form from changes in temperature and pressure. Any rock can be changed into any other type of rock. This continuous changing and remaking of rocks is called the rock cycle. .
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