CHAPTER 3 Resource Integration Guide Chapter Outline Chapter Summary Lecture Suggestions Key Terms Web Links Virtual Field Trip Suggested Responses Chapter Outline 3.1 and the Rock Cycle 3.1a The Rock Cycle and Systems Interactions 3.2 Igneous 3.2a Magma: The Source of Igneous 3.2b Types of Igneous 3.2c Naming and Identifying Igneous 3.2d Common Igneous 3.2e Granite and Rhyolite 3.2f Basalt and Gabbro 3.2g Andesite and Diorite 3.2h Peridotite and Komatiite 3.3 Sedimentary 3.3a Clastic Sedimentary 3.3b Organic Sedimentary 3.3c Chemical Sedimentary 3.3d Bioclastic Sedimentary 3.3e Carbonate and Global Climate 3.3f Physical Sedimentary Structures 3.4 Metamorphic 3.4a Metamorphic Grade 3.4b Metamorphic Changes 3.4c Textural Changes 3.4d Mineralogical Changes 3.4e Types of Metamorphism and Metamorphic
Chapter Summary Geologists divide rocks into three groups igneous rocks, sedimentary rocks, and metamorphic rocks depending upon how the rocks formed. Although rocks may seem permanent over a human lifetime, they are continuously recycled in the outer layers of over geologic time. The processes forming new rocks from old ones are summarized in the rock cycle. can change along many different paths. Rock cycle processes exchange energy and materials with the atmosphere, hydrosphere, and biosphere. s temperature rises about 30 C for every kilometer of depth in the crust and at a lower rate in the mantle. Between depths of 100 and 350 km, temperatures of 600 C to 1,400 C melt rock into magma, which expands by about 10 percent and, because of its lower density, rises toward s surface. Igneous rocks solidify from the cooling magma. Two types of igneous rocks are distinguished: extrusive (volcanic) rocks and intrusive (plutonic) rocks. Extrusive or volcanic igneous rocks are fine-grained rocks that solidify from magma erupted onto s surface. Fluid magma that flows onto s surface is called lava; the term also refers to the rock that forms when lava cools. Since magma cools rapidly on s surface, crystals do not have much time to grow and remain small, resulting in a fine-grained texture. Intrusive or plutonic igneous rocks, on the other hand, solidify slowly within s crust and are, therefore, of medium- to coarse-grained texture. Geologists use both mineralogic composition as well as texture to name igneous rocks. Most common igneous rocks are classified in pairs, each member of a pair containing the same minerals but having a different texture. Granite (an intrusive igneous rock) is the most abundant rock in continental crust, whereas basalt (an extrusive igneous rock) is the most common rock in oceanic crust. Sedimentary rocks are divided into four categories: clastic, organic, chemical, and bioclastic sedimentary rock. Sedimentary rocks make up only about 5 percent of s crust, but are widely spread in a thin surface layer and hence cover about 75 percent of the continents. Water and air attack rocks at s surface; this process is called weathering, leading to the decomposition and disintegration of rocks and minerals by chemical and physical processes. The resulting smaller particles, such as gravel, sand, silt, or clay, are called clastic sediment. They are transported by streams, glaciers, wind, and gravity, and deposited in unconsolidated sediment layers at lower elevations. The pore space between sediment grains is reduced through compaction, and precipitation of mostly calcite, quartz, or iron oxides from circulating water leads to the cementation of the particles. Through this process of lithification, clastic sedimentary rock is formed. The most common clastic sedimentary rocks are shale, siltstone, and sandstone. Lithified gravel is called conglomerate. Organic sedimentary rock forms by lithification of plant and animal remains. Examples for organic sedimentary rock are chert (composed of silica from tiny marine organisms) and peat (formed from partially decayed plant remains), which converts to coal through compaction. When salts concentrate in water through evaporation and eventually precipitate, the resulting chemical sedimentary rocks are called evaporites. Marine organisms such as clams, oysters, and corals extract dissolved calcium from seawater and combine it with carbon dioxide to form their shells and hard
parts. After the organisms die, the remains of shells and corals accumulate to form carbonate rocks, such as limestone, the most common type of bioclastic sedimentary rock. Interactions between limestone and carbon dioxide (in the atmosphere and ocean) are important determinants of global climate. Sedimentary structures are features that develop during or shortly after deposition of the sediment and contain information about the circumstances of transportation and deposition as well as environmental conditions. Examples are bedding (sedimentary layering, also called stratification), cross-bedding (small beds at an angle to the main sedimentary layering), ripple marks (small, parallel ridges and troughs), mud cracks (fractures developing when mud dries), and fossils (remains or traces of plants or animals preserved in rock). Metamorphic rocks form when any rock is altered by high temperature, pressure, or an influx of hot water. Metamorphism occurs in solid rock, changing its texture and/or mineral content. Metamorphic layering resulting from mineral orientation perpendicular to the direction of forces squeezing a rock is called foliation. It produces slaty cleavage, a parallel fracture pattern often cutting across original sedimentary bedding. Metamorphic grade refers to the intensity of metamorphism a rock has undergone; it increases with depth below s surface. Four different geologic processes create changes that lead to metamorphism, described in the following. Contact metamorphism affects rocks heated by a nearby igneous intrusion. Burial metamorphism alters rocks as they are covered deeply with massive amounts of sediments. In regions where tectonic plates converge, high temperature and deformation from rising magma and plate movement all combine to cause regional dynamothermal metamorphism. Hydrothermal metamorphism is caused by hot solutions soaking through rocks and is often associated with the emplacement of ore deposits. Slate, schist, gneiss, and marble are common metamorphic rocks. The older igneous and metamorphic rock that lies beneath the thin sedimentary rock layer is called basement rock. The term bedrock refers to the solid rock of any type underlying soil and unconsolidated sediments. Rock unchanged by weathering or metamorphism is called parent rock, and older rock present in an area and then cut into by intrusions or mineral deposits is termed country rock. Lecture Suggestions Show igneous rock pairs of granite and rhyolite and/or basalt and gabbro to illustrate their similarities and differences (in color, mineral content, and texture). Bring a conglomerate sample, and have students discuss the nature or possible origin of its components. Demonstrate mineral regulation due to squeezing during metamorphism by randomly sticking pennies into a lump of playdough and pressing down on it. Show samples of limestone and marble, granite and gneiss, or other pairs of parent rock and metamorphic rock. Show pictures from famous national parks (such as Grand Canyon, Half Dome in Yosemite Park, Devil s Tower, and so on) or from outcrops in your area, and let students assign the correct rock type (igneous, sedimentary, metamorphic).
Key Terms magma igneous rock weathering sediment sedimentary rock metamorphic rock texture rock cycle extrusive igneous rock lava intrusive igneous rock bedrock parent rock country rock basement rock lithification precipitation pore space compaction conglomerate shale peat carbonate rocks coquina chalk sedimentary structures bedding cross-bedding ripple marks mud cracks fossils biogenic structures metamorphism metamorphic grade foliation slaty cleavage metamorphic halo Web Links http://geomaps.wr.usgs.gov/parks/index.html A link to information on rocks and minerals from the United States Geological Survey s website on the geology of the National Parks. http://geology.about.com/library/bl/images/blrockindex.htm Rock list and picture gallery. http://www.rocks-rock.com/rock-types.html Information on different rock types, with further links to rock descriptions. Virtual Field Trip Suggested Responses to Follow-Up Questions Answers will vary, but should include the following information. 1. The principle of superposition will be violated when tectonic forces turn existing layers upside down, tilt them, or shove an older layer over a younger one. 2. We would find marine fossils and biogenic traces such as burrows from worms. Other structures found in a beach environment would be cross-bedding, which are nearhorizontal layers that are inclined internally. Cross-bedding forms when wind or water currents organize the sand into semiparallel ridges called dunes. Continued flow of water and wind cause sand grains to be eroded off the more gently-sloping side of the dune that faces upstream, and these sand grains travel in the current to the brink of the dune. There, the sediment avalanches down the steep side of the dune that faces downstream. The inclined layer of sediment that results from the avalanche forms a cross-bed. Multiple such avalanches form sets of cross-beds. Depending on the type of beach, we might also find ripple marks.
3. Geologists will mostly rely on lithologic correlations to correlate sediments over large distances, such as Utah and Arizona. Lithologic correlations include: 1) mapping of formations, such as the Tapetas Sandstone and Moenkopi Formation, which are formallydefined, mappable rock units that occur only in a specific region; 2) mapping of index fossils and fossil assemblages, noting that the best index fossils are abundantly preserved in rocks, geographically widespread, in existence as a species or genus for only a relatively short time, and easily identified in the field; and 3) mapping of key beds, which are thin, widespread, easily recognized sedimentary layers deposited rapidly and simultaneously over a wide area.