Crustal rock can be categorized as one of three general types: sedimentary, igneous, and metamorphic. Sedimentary rock is formed underwater (usually in the ocean, but also in lakes and riverbeds) when accumulated sand and mud are compacted over long periods of time. Igneous rock is cooled, hardened magma, and can be divided into volcanic rock, which is formed on the surface or at relatively shallow depths, and plutonic rock, which forms deep underground. Metamorphic rock is created when high temperature and/or pressure alters the mineral assemblages of sedimentary or igneous rock. Since the surfaces of rocks tend to be weathered and dirty, it is sometimes hard to see what they are. To identify rocks, it is recommended that you spilt them open with a hammer to expose the fresh surface inside.
Make sure that no one is standing too close before you use a hammer.
The head on a rock hammer typically has a flat and a pointed side. Normally, use the flat side to split rock samples.
Be careful, since tiny fragments may spray out when the rock breaks. It is particularly dangerous to get these fragments in your eyes, so wear glasses when splitting rocks (if available, safety goggles would be even better). You may also want to protect your carotid artery by wrapping a scarf or towel around your neck.
It is easier to hammer a rock once you have it stabilized with your free hand or foot. Be very careful not to hammer on yourself instead! To minimize damage in case you do, wear gloves, and steel-toed shoes (safety shoes).
There is a technique to splitting rocks. Place the rock on a flat surface, and aim a sharp whack to hit the center of the rock with the edge of the hammer’s flat head. The rock should break quite nicely.
| Rocks that form the crust | Sedimentary | Sandstone, Mudstone, Conglomerate, Chert, Limestone, Tuff | |
|---|---|---|---|
| Igneous | Volcanic | Rhyolite, Andesite, Basalt | |
| Plutonic | Granite, Diorite, Gabbro | ||
| Metamorphic | Schist, Gneiss, Amphibolite, Greenstone....etc. | ||
| Rock that forms the mantle | Peridotite | ||
Next, we will introduce the common types of rock. Please use the Common Minerals chapter to learn more about the minerals mentioned in the text.
Rock formed of compacted accumulated sand. Its color depends on the grains of sand that make up the sandstone. High concentrations of quartz and feldspar produce a whitish color, while inclusions of volcanic rock or other rock fragments will produce shades of gray.
Rock formed of compacted accumulated mud or silt. (Strictly, a rock made of silt is called siltstone. However, differentiation of these rocks are sometimes difficult in the field). Commonly blackish in color, with a finer grain than sandstone.
Formed of granule (particles with a diameter > 2mm) or larger size particles (pebble, cobble). Fine mud and calcareous materials, filling the spaces between the granule particles, act as glue. The granule particles that form conglomerates are usually smoothed and rounded, but some conglomerate rocks, called breccia, are made up of angular fragments.
Sedimentary rock formed by the precipitation and accumulation of fine silica particles (silicon dioxide: SiO2) in the deep ocean. Generally consists of the accumulated siliceous skeletons of plankton. Characterized by extreme hardness, chert is often used for decorative gravel. Silica itself is colorless, but tiny amounts of impurities included with the silica produce cherts in a wide range of colors, including red, green and gray.
Sedimentary rock formed by the accumulation of calcium carbonate (CaCO3) on shallow seabeds. Generally consists of the calcium carbonate skeletons of plankton. Coral limestone is formed from coral reefs. Limestones composed of pure calcium carbonate are white or ivory (sometimes with a touch of pink), while those containing impurities such as sand are gray. Limestone is less hard than steel and can be marked by a knife, nail, or other sharp bit of steel. Limestone can be readily distinguished from other rock because putting hydrochloric acid (HCl) or citric acid (lemon juice) on it causes it to release carbon dioxide (CO2), bubbling or fizzing, and to dissolve.
Sedimentary rock formed of compressed volcanic ash. It thus could be said to possess the characteristics of both sedimentary and igneous rocks. Tuff is basically whitish, but its color can vary, depending on the type of volcanic ash it contains and any alterations it has undergone. It may be covered with numerous holes formed by air or gasses escaping from in between the compressed particles of ash as the rock formed. Tuff is characterized by a rough surface and low density.
Igneous rocks are categorized as either volcanic or plutonic rock, depending on their structure. Because volcanic rock is formed when magma cools quickly, it is characterized by a porphyritic structure, in which larger mineral crystals (phenocryst) that form relatively early in the magma’s cooling process are suspended in a fine-grained matrix. Depending on the rock, however, phenocrysts may not be visible at all. Plutonic rock, on the other hand, cools and hardens slowly, in the deep underground. In that slower process, all the minerals develop large grain sizes; the result is an equigranular texture, with grain sizes visible to the naked eye.
Volcanic and plutonic rock can be further broken down into three categories, depending on their chemical composition (the amount of silica they contain). In descending order by silica content, they are felsic, intermediate, and mafic rocks. (Mafic rocks are rich in iron-magnesium minerals.) Another way to categorize them is by color. The more silica an igneous rock contains, the whiter it will be; the less silica it contains, the blacker it will be. This range of color can be stated numerically with the color index. The color index indicates the ratio of colored (=iron-magnesium mineral) to colorless (= quartz, feldspar) minerals in igneous rock: the blacker a rock is, the higher the color index number; the whiter the rock, the lower the color index number will be.
| Felsic | Intermediate | Mafic | |
| Silica Content (wt%) | >66 | 66-52 | <52 |
| Color Index | <10 | 10-35 | >35 |
| Volcanic Rock | Rhyolite | Andesite | Basalt |
| Plutonic Rock | Granite | Diorite | Gabbro |
Cross-polarizing microscope photographs of typical structures of volcanic rock (left) and plutonic rock (right)
Volcanic rock containing large amounts of silica and typically white to pale gray in color. The rock-forming minerals are mostly quartz and plagioclase, commonly dotted with colored minerals like amphibole. The surface is often rough, and marked with small holes formed by gases escaping the cooling magma.
Gray volcanic rock containing a middling amount of silica. Andesite is the most common volcanic rock found in subduction zones such as Japan. The rock-forming minerals include quartz, plagioclase, pyroxene, amphibole, biotite, and, rarely, olivine.
Volcanic rock containing very little silica. Fresh specimens are nearly black, but weathering and chemical alteration can cause a green or brown color. Contains no quartz, but is mainly made up of pyroxene (either clinopyroxene or a combination of clinopyroxene and orthopyroxene) and plagioclase. May contain olivine.
Plutonic pock made mostly of quartz and feldspar (plagioclase or potassium feldspar or a combination). Commonly contains mica (biotite and/or muscovite), amphibole, magnetite, or ilmenite. Often used as a building material or for gravestones.
Plutonic rock with chemical composition in between that of granite and gabbro. Mostly consists of plagioclase, amphibole, and pyroxene (clinopyroxene, orthopyroxene), biotite, and a small amount of quartz. May be differentiated from gabbro because it contains more amphibole than pyroxene. Samples that contain a relatively large amount of quartz are called quartz diorite.
Plutonic rock consisting mainly of pyroxene (clinopyroxene or a combination of clinopyroxene and orthopyroxene) and plagioclase. Contains no quartz; any white minerals in its make up are plagioclase. May also contain amphibole or biotite.
Every mineral has a range of temperatures and pressures under which it remains stable. Familiar examples are diamond and graphite. Both are stable forms of carbon, but carbon under high pressure yields diamond and under low pressure, graphite. When rock is subjected to extreme temperature and/or pressure change after its formation, the mineral assemblage that make it up will change to become stable under those conditions. This process is known as metamorphism, and the resulting rocks are called metamorphic rock.
Rocks that undergo metamorphism deep underground and under great differential stress , causing a flaky scaled (foliated) structure, are called crystalline schist (or, simply ‘schist’). Crystalline schist is further defined by its original rock type (protolith). For example, sandstone-origin schist is called psammitic schist, while schist that originated as mudstone is pelitic schist. One exception is schist formed from basalt. In this case, the resulting rock is called greenschist. Schist is flakey because it contains large amounts of lamellar minerals like mica and chlorite, which line up like scales.
Like schist, gneiss has a strongly banded appearance, but because its costituent minerals are coarser, it is less flaky than schist. Formed at higher temperatures than schist (>600℃), it has coarser-grained mineral crystals. Mica and garnet can often be detected in it with the naked eye
Metamorphic rock originating as basalt or gabbro, almost entirely consists of amphibole and plagioclase. Hard and dark green, its massive structure is distinctly unlike that of schist or gneiss.
Similar to greenschist, but without its laminar structure. The yellow-green of epidote and the dark green mottled pattern of chlorite are highly distinctive.
All of the rocks discussed so far are found in the earth’s crust. Peridotite, however, is the main component of the earth’s mantle. Therefore, it is seen above ground only infrequently, and only in specific regions. It mainly consists of a combination of olivine and pyroxene, but plagioclase and spinel may also be present. The color is typically the bright green. Because its texture is coarse-grained, it is not uncommon for each mineral to be visible to the naked eye. Olivine is quite unstable on the surface conditions, and is easily metamorphosed into serpentine. Rocks formed almost entirely of serpentine are known as serpentinite.
