Rocks: Materials of the Lithosphere
The following statements summarize the primary objectives presented in the chapter.
- 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 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.
- Weathering is the response of surface materials to a changing environment. Mechanical weathering, the physical disintegration of material into smaller fragments, is accomplished by frost wedging, expansion resulting from unloading, and biological activity. Chemical weathering involves processes by which the internal structures of minerals are altered by the removal and/or addition of elements. It occurs when materials are oxidized or react with acid, such as carbonic acid.
- 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.