Every rock has specific mineralogy and chemical composition based on which they are characterized and separated from other rock types. Then there is texture such as grain-size which help researchers to distinguish them visually.
In general, rocks are classified into three main groups; Igneous, Sedimentary and Metamorphic. Each of them has a separate method of formation. They are, however, associated with rock cycles, a fundamental theory in the geology that explains the transitions of rocks types on a geologic time scale. Below, we have briefly described 15 different types of rocks that you must know.
Igneous rocks form through cooling and solidification of magma that originates from deep within the Earth’s core. The magma, while it carries various compounds, is largely composed of silica. Igneous along with metamorphic rocks accounts for about 95% of the Earth’s crust.
Igneous rocks, based on their mode of occurrence, can be divided into two sub-categories; intrusive and extrusive igneous rocks. Intrusive igneous rocks are those which form within the Earth’s crust through solidification of magma. Such rocks are rough or have a coarse texture.
Extrusive igneous rocks, on the other hand, form on the Earth’s surface. But unlike intrusive rocks, extrusive igneous rocks are smooth and fine-grained. This is because they cool and solidify at a much faster rate.
Basalt is perhaps one of the most commonly known igneous rock formed from rapidly cooling lava which is rich in magnesium and iron but has low silica content.
Minerals which are present in substantial amounts are pyroxene, plagioclase, feldspar and sometimes olivine.
Basalt, usually, is between grey and black in color but rapid weathering or rust can change its color to brown.
Mars and Venus along with few planetary satellites like Io (Jupiter’s) and the moon are known to have an active eruption of basalt due to widespread volcanism.
Andesite is an extrusive igneous rock that has higher silica content than basalt but lower than that of Rhyolite. Plagioclase feldspar along with pyroxene (sometimes hornblende) are two dominant minerals found in the rock.
Research conducted in 2011 discovered that andesite makes most of the Martian crust along with basalts. It is generally believed that most of the Earth’s continental crust is andesitic. Andesite acquired its name from the Andes mountain range in South America.
A Carbonatite rock with calcite, magnetite and olivine Image Courtesy: Eurico Zimbres
Igneous rocks, irrespective of their mode of occurrence, whose mineral composition is dominated by carbonate minerals (more than 50%) are called carbonatites. Due to the similarity in their texture, carbonatites can easily be confused with marble, a metamorphic rock.
Carbonatites are extremely rare and are actually formed by unusual processes. The Ol Doinyo Lengai volcano in Tanzania is the only active carbonatite volcano in the world known to have erupted in the recorded past. Its eruption in 1960 helped geologists to confirm that carbonatites are actually derived from magma.
Chemical Composition: Silica, Alumina
A common type of igneous rock, granite is intrusive in nature and have a granular texture. Granites usually have high amounts of quartz and feldspar (alkali feldspar), mica and certain other amphibole minerals are present in small qualities.
One of the oldest standing structures in the world, the Menkaure’s Pyramid (smallest of the three pyramids of Giza) is partially made of granite. The Great Pyramid of Giza itself contains a sarcophagus made of granite. An 11th-century Indian temple in Tanjore, South India is probably the first ever building entirely made of granite.
Today, it’s mostly used in sculptures, flooring of certain buildings and for engineering purposes.
Image Courtesy: NASA
Gabbro is an intrusive igneous rock which is characterized by its dark, sometimes greenish color and rough texture. Relatively fine-grained gabbro is known as dolerite or diabase. It forms when magma, rich in magnesium (Komatiite) and iron, gradually cools and crystallize beneath the Earth’s surface.
Unlike granite, gabbro is mainly composed of calcium-rich plagioclase feldspar and pyroxenes while trace amounts of olivine can also be found. Gabbro makes up a large part of the oceanic crust and is a major component in large ophiolite zones (a section of sea crust which is exposed). It usually carries valuable elements like silver, gold, platinum, and chromium.
Sedimentary rocks form mostly from organic deposits that accumulate near the Earth’s surface. These deposits can be either pre-existing rocks or remains of a once-living organism. The sedimentary rocks occur in layers of sediments, which leads to a structure called bedding. Each bed or layer has different characteristics than the subsequent layer.
They cover about 73% of the Earth’s total land area but only 8% of its volume. Researches have also found evidence of sedimentary rocks on Mars which might have formed underwater on that planet.
Image Courtesy: James St. John/Flickr
Arkose, a sandstone rock by nature, is generally characterized by a substantial amount (at least 25%) of feldspar present in the rock along with quartz and sometimes mica. One other significant characteristic of arkose rocks is that it carries slight amounts of calcite which reacts with hydrochloric acid and produce a fizz.
Such rock is usually formed from weathering of granitic rocks, although other feldspar-rich igneous rocks will do the job. They are mostly reddish or grey in color and have coarse sand grains.
Bituminous coal Image Courtesy: USGS
Coal has been used as a primary source of energy for centuries and even with all hydroelectric and nuclear power it still plays an important role in the world’s economy. While it’s mostly made of carbon, elements such as hydrogen, oxygen, and sulfur are also present in certain quantities.
The process of coal formation is long-lasting. When a fauna or plant dies it gradually turns into a heap of organic matter known as peat. After few million years or so, the accumulated organic matter provided with heat and temperature turns into coal.
Based on their quality, coal can be divided into three grades; lignite (lowest quality), bituminous and anthracite (high-grade coal). While lignite and bituminous are both sedimentary rocks, anthracite is actually classified as metamorphic rock.
The People’s Republic of China is the leader in both the consumption and production of coal in the world, followed by India, the United States and Australia (by total production).
A layer of chert on top of mudstone and siltstone Image Courtesy: Minghong
The term “chert” denotes to any rock which is made of either microcrystalline, cryptocrystalline or microfibrous quartz. Simply put, chert is a fine-grained sedimentary rock comprised of quartz (the mineral form of silicon dioxide). In oceans, chert is formed by a gradual accumulation of petrified silicon-rich algae and protozoa (siliceous ooze).
It is one of the many cryptocrystalline materials from which stone tools were created in prehistoric times. Today, chert is mostly used in the construction industry (roads) despite its challenging physical properties. Flint, Jasper, Radiolarite, Mozarkite, and Onyx are some the most common varieties of chert.
Dolostone is a name given to a type of sedimentary rock which is primarily composed of dolomite, a carbonate mineral. It was initially named after the mineral but was changed to dolostone to avoid any confusion. Dolomite is a relatively common rock type with low solubility (in groundwater) and high resistance to erosion.
Due to its similar appearance, dolomite was erroneously acknowledged as limestone for quite some time and was only distinguished in 1778 by a French war surgeon named Belsazar Hacquet.
10. Oil Shale
Ojamaa mine (VKG Ojamaa kaevandus), Estonia Image Courtesy: Kaupo Kikkas
Oil shale is a sedimentary rock that carries kerogen, a solid organic matter from which hydrocarbons can be retrieved. It is not to be confused with shale-hosted oil that contains tight oil also known as petroleum.
These rocks neither have a precise geological definition nor a definite chemical formula and vary significantly in their age, type of kerogen, chemical and mineral composition.
Because of their availability, Oil shales are one of the most important energy alternatives. But at the same time, it’s more expensive than the conventional fuel. Many countries including Germany, Estonia, Brazil, Russia, China, and Turkey are actively using oil shale as a substitute for crude oil.
Metamorphic rocks are rocks that started out either as igneous or sedimentary but have gone through significant chemical and physical changes. Metamorphism occurs when a protolith (original rock) endure high temperature (sometimes close to 300 °C) and pressure (near 1000 bar). Rocks which are already in their metamorphic form can go through metamorphism.
Under extreme temperature and pressure conditions, these rocks undergo extreme physical and chemical changes. More specifically, metamorphism enables specific chemical reactions to take place within the rock allowing minerals to rearrange and in some cases form a new one.
Foliation (repeated layering) is a prevalent physical feature in many metamorphic rocks which is caused by unaligned pressure. The process, however, doesn’t make rocks hot enough to melt. In such a scenario, they would become igneous rocks.
Some of the common metamorphic rocks are schist, marble, and gneiss.
A chunk of anthracite Image Courtesy: Wikimedia Commons/Jakec
Anthracite is basically a hard variety of coal which possesses the highest amount of carbon content and energy density among all coal types. Based on the level of carbon content, anthracite can be categorized into three types; standard grade (mostly used for power generation), high grade and ultra-high grade.
Historically, anthracite has been used in blast furnaces for smelting iron. However, in order to cope with the increasing demand, the industry shifted toward coke due to its relatively higher structural strength.
Anthracite is actively mined in China, the United States, Russia, Ukraine, South Africa, and Australia along with few other nations. It accounts only for a slight percentage of the international coal reserves.
Orthogneiss (igneous) rock in the botanic garden, Prague, Czech Republic
Gneiss is a class of metamorphic rocks which are characterized by gneissic banding; alternating light and dark colored strips. Such rocks are formed when a sedimentary or igneous rock is exposed to tremendously high temperature (more than 300°C) and pressure.
Gneisses which are obtained or derived from igneous rocks, such as granite, are collectively called orthogneiss. On the other hand, gneisses obtained from sedimentary rocks are called paragneiss. Some gneiss rocks are even named after a particular component or mineral such as albite gneiss, garnet gneiss or biotite gneiss.
They are generally medium grained and are highly recrystallized (due to high temperature and pressure).
Image Courtesy: Aleksander Kaasik Marble Wall in Ruskeala, Russia
Marble is what we get when certain carbonate minerals such as dolomite or limestone go through metamorphism and recrystallization. The pure white marble, which is used in some of the most famous and oldest sculptures in the world, is obtained from metamorphism of pure, low silicate dolomite.
Distinctive patterns and layers in various colored marble are usually due to impurities present in protolith, or original dolomite. For example, the green coloration in serpentinite marbles is caused by high amounts of magnesium along with silica impurities present in the dolomite.
On a different note, dust released from marble has been linked to respiratory diseases after long exposure. According to the National Institute for Occupational Safety and Health, the recommended respiratory exposure limit (8 hour work day) is 5 mg/m3.
Maurienne Valley Quartzite, French Alps Image Courtesy: Wikimedia Commons
Quartzite is a relatively hard metamorphic rock made out of pure quartz sandstone. Much like marble, pure quartzite is either grey or white. It may, however, occur in colorful shades due to certain mineral impurities mostly iron oxide.
It is resistant to chemical weathering. That’s the reason why ridges and hilltops made of quartzite usually have thin or no layer of soil and vegetation. Due to its hardness and chemical resistant properties, quartzite is used in railways as track ballast which allow tracks to operate smoothly for a lengthy period of time.
High-grade quartzite is used to produce silicon carbide, silica sand, and ferrosilicon for industrial purposes. It is also used for decorative purposes.
Image Courtesy: Michael C. Rygel/Wikimedia Commons
Schist is a foliated (repetitively layered), intermediate-grade metamorphic rock which contains large amounts of platy minerals such as chlorite, talc, graphite, and micas often mixed with quartz.
The word schist is taken from the Greek word ‘schízein‘ meaning “to split” which appropriately describe the ease with which schists can break along the flat planes.
The specific names of different schists are usually based on their mineral components. For example, the most common schists type the mica schists, are primarily composed of minerals within the mica family such as biotite and muscovite.