Earth Materials

Chapter 11 Earth Materials

This chapter deals with the systems and cycles of the solid Earth. This chapter discusses the basic materials of the solid Earth – rock and minerals – and some principals of their formation. These are linked to the cycle of rock change which describes how different rock types develop as Earth materials are cycled and recycled through geologic time.  The elements oxygen and silicon account for about seventy-five percent of the Earth’s crust, while the metallic elements iron, aluminum, and the base elements account for most of the rest.  The elements of the crust are combined in inorganic chemical compounds called minerals.  These minerals are mixed together in various proportions to form many kinds of rock.  The rocks of the Earth’s crust are grouped into three major classes: igneous, sedimentary, and metamorphic rocks.  Igneous rocks form when molten material from the Earth’s interior cools and solidifies in the crust.  Magma cooled slowly below the surface forms coarse-textured intrusive igneous rocks. Lava cooled rapidly at the surface forms fine-textured extrusive igneous rocks.  Igneous rocks consist mainly of silicate minerals containing silicon, oxygen, and metallic elements. The kind of metallic elements present determines the mineral density.  Less dense felsic minerals dominate the igneous rocks of the upper crust, while more dense mafic and ultramafic minerals dominate those of the lower crust.  Silicate minerals undergo chemical changes called mineral alteration when exposed to air and water at the Earth’s surface.  Most clay minerals are produced by mineral alteration.  Weathering, or the breakdown of rocks into smaller particles known collectively as sediment, occurs through both mineral alteration and physical disintegration.  Layers of mineral sediment and organic matter accumulate in oceans and low-lying land areas to be compacted and hardened into sedimentary rocks. Different types of sediment produce different kinds of sedimentary rock.  Igneous and sedimentary rocks can be altered by heat and pressure to form metamorphic rocks.  The cycle of rock change describes the circulation of rock material between the Earth’s interior and the crust. This is a very slow process powered by the heat of radioactive decay deep within the Earth.