Rocks: Materials of the Lithosphere opens with a discussion of the rock cycle as part of the Earth system. The origins and processes involved in forming the three major rock groups—igneous rock, sedimentary rock, and metamorphic rock—are also reviewed. A discussion of the crystallization of magma precedes an examination of the classification, textures, and compositions of igneous rocks. Following an investigation of the origin of sediment, the classification of sedimentary rocks, as well as some of their common features, is discussed. The chapter also examines the agents of metamorphism, the textural and mineralogical changes that take place during metamorphism, and some common metamorphic rocks. In conclusion, resources from rocks and minerals are explained.
Learning Objectives
After reading, studying, and discussing this chapter, you should be able to:
•Diagram and discuss the rock cycle.
•List the geologic processes involved in the formation of each rock group.
•Briefly explain crystallization of magma.
•List the criteria used to classify igneous rocks.
•List the names, textures, and environments of formation for the most common igneous rocks.
•Discuss the origin of materials that accumulate as sediment.
•List the criteria used to classify sedimentary rocks.
•Explain the difference between detrital and chemical sedimentary rocks.
•List the names, textures, and environments of formation for the most common sedimentary rocks.
•List the common features of sedimentary rocks.
•Describe the agents of metamorphism.
•List the criteria used to classify metamorphic rocks.
•List the names, textures, and environments of formation for the most common metamorphic rocks.
•Discuss metallic and nonmetallic mineral resources.
Chapter Summary
•Igneous rock forms from magma that cools and solidifies in a process called crystallization. Sedimentary rock forms from the lithification of sediment. Metamorphic rock forms from rock that has been subjected to great pressure and heat in a process called metamorphism.
•The rate of cooling of magma greatly influences the size of mineral crystals in igneous rock—the faster the rate of cooling, the smaller the crystals. The four basic igneous rock textures are 1) fine-grained, 2) coarse-grained, 3) porphyritic, and 4) glassy.
•The mineral makeup of an igneous rock is ultimately determined by the chemical composition of the magma from which it crystallized. N.L. Bowen showed that as magma cools, minerals crystallize in an orderly fashion. Crystal settling can change the composition of magma and cause more than one rock type to form from a common parent magma.
•Igneous rocks are classified by their texture and mineral composition.
•The process of weathering produces sediment—unconsolidated particles of rock created by the weathering and erosion of rock, by chemical precipitation from solution in water, or from the secretions of organisms. Sediment is transported and deposited by water, wind, or glaciers. Compaction and cementation transform sediment into sedimentary rock.
•Detrital sediments are materials that originate and are transported as solid particles derived from weathering. Chemical sediments are soluble materials produced largely by chemical weathering that are precipitated by either inorganic or organic processes. Detrital sedimentary rocks, which are classified by particle size, contain a variety of mineral and rock fragments, with clay minerals and quartz the chief constituents. Chemical sedimentary rocks often contain the products of biological processes such as shells or mineral crystals that form as water evaporates and minerals precipitate. Lithification refers to the processes by which sediments are transformed into solid sedimentary rocks.
•Common detrital sedimentary rocks include shale (the most common sedimentary rock), sandstone, and conglomerate. The most abundant chemical sedimentary rock is limestone, composed chiefly of the mineral calcite. Rock gypsum and rock salt are chemical rocks that form as water evaporates and triggers the deposition of chemical precipitates.
•Some of the features of sedimentary rocks that are often used in the interpretation of Earth history and past environments include strata, or beds (the single most characteristic feature), bedding planes, and fossils.
•Two types of metamorphism are 1) regional metamorphism and 2) contact metamorphism. The agents of metamorphism include heat, pressure, and chemically active fluids. Heat is perhaps the most important because it provides the energy to drive the reactions that result in the recrystallization of minerals. Metamorphic processes cause many changes in rocks, including increased density, growth of larger mineral crystals, reorientation of the mineral grains into a layered or banded appearance known as foliation, and the formation of new minerals.
•Some common metamorphic rocks with a foliated texture include slate, schist, and gneiss. Metamorphic rocks with a nonfoliated texture include marble and quartzite.
•Some of the most important accumulations of metallic mineral resources are produced by igneous and metamorphic processes. Vein deposits (deposits in fractures or bedding planes) and disseminated deposits (deposited distributed throughout the entire rock mass) are produced from hydrothermal solutions——hot, metal-rich fluids associated with cooling magma bodies.
•Nonmetallic mineral resources are mined for the nonmetallic elements they contain or for the physical and chemical properties they possess. The two groups of nonmetallic mineral resources are 1) building materials (e.g., limestone and gypsum) and 2) industrial minerals (e.g., fluorite and corundum).