Rock Characteristics:
Sedimentary Rocks:
As sedimentary rocks form in layers, they can be distinguished from igneous and metamorphic rocks in the field. A hand specimen in the field usually breaks along layered surfaces. Of the three major rock types, sedimentary rocks contain the most fossils. Fossils are never found in igneous rocks and rarely in metamorphic rocks. The origin of the particles that make up sedimentary rocks determine their appearance and give clues to their identity. Sedimentary rocks easily erode down to particle size.
Origin:
Sedimentary rocks form at or very near the Earth's surface. At the surface, rock particles are usually transported by wind, water, and ice and then later deposited on dry land, on the beds of rivers and lakes, and in marine environments, such as beaches, deltas, and the sea. The three major types of depositional environments include continental, transitional, and marine. Of course, transitional environments represent coastal regions where both continental and marine deposition occurs. Note: Left click to enlarge photo.
Fossil Content:
As mentioned above, fossils occur mainly in sedimentary rocks. They are the remains of animals and plants preserved in layers of sediment. The type of fossils that are found in a rock give an indication of the rock's origin. For example, a marine fossil suggests that the rock formed from sediments deposited in the sea or near shore (coastal). Rocks that are especially rich in fossils include limestone. See tabs titled Fossils & More and Fossil Preservation.
Grain Size:
The classification of grain size in sedimentary rocks include such terms as coarse-, medium-, and fine-grained are usually used. Grains may range in size from boulders to very small particles of clay. Coarse-grained rocks composed of fragments easily seen with the naked eye include conglomerate, breccia, and some sandstones. Such coarse-grained rocks reveal how far they were transported before being deposited. Medium-grained rocks, the grains of which can be seen with a hand lens, include other sandstones. Fine-grained rock includes shale, clay, and mudstone.
Grain Shape:
The way the grains that make up sedimentary rocks are transported influences their shape. For example, wind erosion creates round sand particles, while water-based erosion gives rise to angular, sand-sized particles, but smooth, round pebbles. Note that when classifying an assortment of rocks, such as represented in the left photo, geologists simply name them as gravel. Conglomerates contain a matrix with surrounded by pebbles with rounded edges, while breccias contain a matrix surrounding angular rock fragments. Both generally represent transport by water; however, breccias have the shorter distance for travel.
Classification:
Classification explains the source of the rock's grains. Clastic rocks contain particles from pre-existing rocks. The term bioclastic indicates that the rock is made of shells or other fossil fragments, and the term chemical indicates that the mineral were produced by chemical precipitation. Biochemical indicates that the rock fragments were made by both chemical precipitation and fossils. When classifying clastic rocks, no grain (particle) size or particle name will be observed and/or noted for labs and/or fieldwork assignments.
Igneous Rocks:
Igneous rocks crystallize from molten magma or lava. The starting composition of the magma, the manner in which it travels towards the Earth's surface, and the rate at which it cools all help to determine its composition and resultant characteristics. These characteristics include grain size, crystal shape, mineral content, and overall color. Origin indicates whether the rock is intrusive (magma crystallized beneath the Earth's surface) or extrusive (lava crystallized at the Earth surface). When identifying igneous rocks, geologists also consider such things as occurrence, texture, and composition.
Occurrence:
Occurrence describes the form of the molten mass when it cooled. For instance, a pluton is a very large, deep intrusion that can measure many miles across. A dike is a narrow discordant sheet of rock and a sill is a concordant sheet. Dikes cut across (strata) layered rocks while sills follow along the bedding plane of strata. Note: left-click on image to enlarge.
Composition:
Feldspars, micas, quartz, and ferro-magnesians make up the bulk of igneous rocks. Igneous rocks are arranged into groups according to chemical composition: felsic rocks, with over 65 percent total silica content; intermediate rocks, with 55-65 percent silica content; mafic rocks, and with 45-55 percent total silica content. Ultrabasic rocks have less than 45 percent total silica content. By examining the graph to the left, one can determine what type of minerals will occur in each of the groups. For example, a felsic igneous rock may contain orthoclase, quartz, plagioclase, biotite, and amphibole. Note: left-click on image to enlarge.
Color:
Color is generally an accurate indicator of chemistry, reflecting a mineral's content. Light colored indicates a felsic rock, with over 65 percent silica. Mafic rocks are dark colored, with low silica, and a high proportion of dark dense ferro-magnesian minerals such as augite. For example, many granite rocks have a mineral content consisting of potassium (K)-feldspar, quartz, and amphibole. K-feldspar minerals display a hue of pink and quartz minerals generally exhibit off-white, while amphibole minerals appear black. Note: left-click on image to enlarge.
Grain Size:
Grain size indicates whether a rock is plutonic (formed below ground) or extrusive (formed above ground). Those that form below ground are coarse-grained and those that form above ground (fine-grained). Course-grained igneous rocks such as granite have crystals over 5 mm in diameter. Medium-grained rocks such as diorite have crystals 0.5 - 5 mm in size, and fine-grained rocks such as basalt have crystals that are less than 0.5 mm in size. The image to the left is a close-up shot of the medium-grained igneous rock known as diorite or informally known as salt-and-pepper granite.
Crystal Shape and Texture:
Crystal shape: Slow cooling gives the minerals time to develop well-formed (euhedral) crystals; whereas, fast cooling allows time only for poorly-formed (anhedral) crystals to grow. Grains that show no recognizable crystal form are said to be anhedral (A). Grains that show imperfect but recognizable crystal form are said to be subhedral (B). Grains that show sharp and clear crystal form are said to be euhedral (C).
Texture: Texture refers to the way the grains or crystals are arranged and their size relative to one another. To learn more about texture, see the tab titled Rock Textures.
Texture: Texture refers to the way the grains or crystals are arranged and their size relative to one another. To learn more about texture, see the tab titled Rock Textures.
Metamorphic Rocks:
Metamorphic rocks exhibit certain typical features. The minerals of which they are made usually occur as crystals. Crystal orientation is determined by whether the rock formed as a result of both heat and pressure, or heat alone. Their size reflects the degree of heat and pressure to which they were subjected. Thus, examination of the crystals in a metamorphic rock can help to establish its origin and its identity. When identifying metamorphic rocks, geologists examine and consider the following rock characteristics: structure, grain size, pressure and temperature, and mineral content.
Structure:
Structure indicates the way minerals are oriented in a rock. Contact metamorphic rocks have a crystalline structure, though the minerals are usually randomly arranged. Regional metamorphic rocks, however, are foliated: the pressure forces certain minerals to become aligned. The metamorphic rock in the above photo represents random orientation; therefore, it was more than likely formed by contact metamorphism. The metamorphic rock in the left photo exhibits preferred orientation and is classified as foliated; therefore, this rock known as mylonite was formed by regional metamorphism.
Grain Size:
Grain size indicates the temperature and pressure conditions to which the rock was subjected; generally, the higher the pressure and temperature, the coarser (larger) the grain size. Therefore, slate which forms under low pressure, is fine-grained (smaller). The "parent rock" for slate is shale. This means that shale (sedimentary rock) was at one time buried and later transformed into slate (metamorphic rock). Schist, formed by moderate temperature and pressure, is medium-grained, and gneiss, formed at high temperature and pressure, is coarse-grained. Gneiss is the highest-grade of metamorphic rock formed from either the "parent rock" shale or granite. Note: left photo represents schist.
Pressure and Temperature:
As rock is buried deeper in the crust, the temperature and pressure increase. Higher temperatures and pressures lead to metamorphism. Medium- to high-grade metamorphism occurs at a minimum temperature of approximately 480 degrees Fahrenheit. Temperatures in some metamorphic rocks can be much lower -- and a maximum temperature of 1,472 degrees Fahrenheit. Anything above this temperature melts to become magma or lava. Intensity of pressure ranges from 2,000 to 10,000 kilobars. An enlargement of the graph may be obtained by left-clicking on it.
Mineral Content:
The presence of certain minerals in metamorphic rocks can help the identification process. Garnet and kyanite occur in gneiss and schist rocks, while crystals of pyrite grow on the cleavage surfaces of slate rocks. Minerals such as brucite can occur in marble rocks. The left photo shows a schist rock contain three large garnet crystals. Also note that schist rocks are composed a matrix of micas. In the case of the garnet schist to the left, the dominant mica variation is muscovite. Biotite or chlorite may also be present in schist rocks.
