Earth Science Laboratory


    Igneous rocks all come from magma - molten rock material, often rich in gases, originating deep below the earth's surface. They are classified on the basis of texture (shapes and sizes of crystals), mineralogy (types of minerals found together), and their mode of origin. Magma cools very slowly deep below the earth's surface, which allows large and coarse crystals to form; these igneous rocks are called intrusive (plutonic). Magmas which rises to the surface (lava) cools quickly in the much lower temperatures at the earth's surface, so that only very tiny crystals of minerals form in the rock; these igneous rocks are called extrusive (volcanic). A special kind of rock texture commonly found in volcanic rocks is termed porphyritic, which is a mixture of conspicuously coarse crystals dispersed within much finer grains of other minerals. These coarse grains are called phenocrysts, and the finer grains are collectively called the groundmass. Porphyritic texture results from a magma body which initially started to initially cool deep underground (where coarse crystals formed); then the magma migrated upwards toward the earth's cooler surface (where the finer crystals formed).

    The great majority of igneous rocks are made up of silicate minerals, and of these silicates, those listed in the Bowen's Reaction Series are most important. Generally, igneous rocks that are composed mainly of potassium feldspars and quartz are known as felsic ("fel" from "feldspar," and "sic" from "silica"). Felsics are usually fairly light in overall color, usually from white, to shades of pink and red, to medium gray. Igneous rocks composed of plagioclase feldspars and silicate minerals high in iron and magnesium, such as olivine and pyroxenes (augite), are known as mafic or ferromagnesian. Mafic rocks tend to be rather dark in overall color (black, dark gray, or dark green). Igneous rocks composed of mostly plagioclase feldspars and amphibole (hornblende) are neither felsic nor mafic, but are called intermediate. Igneous rocks composed almost entirely of olivine and/or pyroxenes are known as ultramafic (generally called peridotite); these are characteristic of the rock deep in the earth's mantle.

    Unlike minerals, rocks do not have to be completely crystalline; for example, a rock formed from lava which cooled very quickly (i.e. under seawater) may have no crystalline structure at all - it is composed entirely of volcanic glass (obsidian). Other quickly-cooled extrusive rocks such as scoria or pumice may contain cavities produced by escaping volcanic gases. In looking at the hand specimens in lab, note that the minerals in the coarse-grained intrusive (plutonic) rocks are still smaller than the silicate mineral specimens that you had been working with previously. Because mineral crystals in igneous rocks are intergrown into a mosaic, it is usually not possible to isolate the individual grains to look for cleavage planes, perform hardness tests or other physical tests. An example of such a mosaic is granite, a coarse-grained, intrusive rock containing potassium feldspar, quartz, plagioclase feldspar, and small amounts of biotite and hornblende.

    Minerals in extrusive (volcanic) rocks are often microscopic and cannot be seen at all with the naked eye or even with a powerful magnifying glass. The minerals in a rock such as basalt may, however, be seen under a microscope designed especially for viewing thin, transparent slices of rocks. Basalt contains mostly pyroxenes and plagioclase feldspars. To identify a hand specimen of basalt, it is necessary to judge its overall appearance rather than try to identify its component minerals.

    On your lab worksheet today, indicate the color, grain size, type of chief feldspar (K-feldspar or plagioclase), other silicate minerals, and other textural features of the assigned rock samples. Tip: First try to group together the felsic, mafic, and intermediate rocks - identify the coarse-grained rocks first. Use your "Rocks & Minerals" Golden Guide to help obtain this information for the fine-grained extrusive rocks. Please do not hesitate to ask questions of your lab instructor.

Copyright  © 1983 by William K. Tong