A. Igneous rocks

ROCKS

A rock is a natural aggregate (combination) of one or more minerals, mineraloids, glass, and/or organic material. There are three families of rocks distinguished from one another by the processes involved in their formation. The three rock families are:

A. Igneous—originating from a molten silicate melt.

B. Sedimentary—originating from the deposition of the by-products of weathering.

C. Metamorphic—develop via the change in form or chemical composition of pre-existing rocks and minerals by new conditions of temperature, pressure, and/or the addition of hot chemical fluids.

TABLE1. Classification of rocks

Igneous, sedimentary, and metamorphic rocks are described and identified on the basis of their composition and texture. Composition, in general, refers to the chemical makeup, the particular elements that are present in the rock. Texture, in general, refers to the size, arrangement, and shape (" morphology") of the constituent minerals or materials in the rock. There are different sets of textural terms for each rock family that often denote the same or closely similar conditions.

A. Igneous rocks

Igneous rocks are the solids produced by the cooling and crystallization of molten silicate material initially formed beneath the Earth's surface. Crystallization occurs when cooling allows for the growth of mineral crystal grains. The cooling rate and space available determine the size of the crystals that form. Large crystals form when magma, molten silicate material below ground, is insulated by the surrounding country rock (rock that has been intruded by the magma), and therefore cools very slowly. When magma solidifies underground, it forms intrusive (plutonic) igneous rocks. The shape and position of emplacement differentiate plutonic igneous rock bodies. A dike is a pluton that cuts across pre-existing rock (strata). A sill is a two-dimensional pluton that is placed parallel to and between layers (strata) of existing rock. Batholiths are very large, three-dimensional plutons, usually the result of multiple intrusions of magma, hundreds of miles in length and width, which cool and crystallize very slowly beneath the Earth's surface. A laccolith is a smaller three-dimensional pluton with a convex roof and a flat floor.

Volcanic (extrusive) igneous rocks form on or above the surface of the Earth by the cooling of lava (molten silicate flows on the surface), or by the deposition of violently ejected pyroclastic (pyro = fire, clast = fragment) material such as volcanic ash. Lava cools faster than magma because it is exposed to environments that allow for the rapid dissipation of heat and therefore prevent the formation of large crystals. In general, most extrusive igneous rocks develop crystals that are too small to be seen without the aid of a microscope. There are different types of basaltic lava. As is blocky, sharp-edged lava that moves very slowly and pahoehoe is ropy, " smooth" lava. Volcanic glass (obsidian) forms when lava is cooled too rapidly for crystals to develop.