Clastic rocks

Clastic rocks are stones composed of particles from other rocks and minerals. Such rocks are composed of any combination of the three basic rock types—igneous, other clastic (sedimentary), and metamorphic. Clastic rocks are identified by the size of their particles and by their composition. The largest of these particles is gravel, followed by sand and mud. Particle size is important because it helps scientists understand the amount of energy used to form and deposit clastic rocks. Additionally, clastic rocks are identified by their minerals: quartz, feldspar, fragmented rock (lithic fragments), and matrix. Clastic rocks are deposited in currents, such as in waves, rivers, glaciers, and wind.

Basic Principles

Also known as sedimentary rocks, clastic rocks are composed of particles and larger pieces of the other two basic types of rocks. The first of these types of rock are igneous rocks, which are cooled pieces of magma, some of which have been discharged through volcanic eruption. Igneous rocks are the predominant substance on Earth's surface and include such minerals as granite and basalt. The second of these basic types is metamorphic rock, composed of minerals that, when moved into a new, high-pressure or high-temperature environment, change in composition.

Clastic rocks are identified by scientists in two ways. First, geologists and mineralogists assess the size of the particles that constitute them. If the particles are more than two millimeters in diameter, they are considered gravel. Sand particles are between 0.0625 and 2 millimeters, while mud, clay, or silt are smaller than 0.0625 millimeters. Second, geologists and mineralogists identify the composition of the rock using three basic minerals—quartz, feldspar, and lithic fragments of other minerals (QFL). Geologists and mineralogists also use the term matrix to describe the silt and clay particles that are not visually identifiable.

Background and History

The study of geology and mineralogy dates to ancient Greece, when Aristotle speculated about the role of minerals in living organisms. The iconic philosopher and scientist commented that just as a sculptor forges rock into a piece of art, so, too, does a seed create a living organism. Though humanity has developed an appreciation for stone and minerals (particularly precious stones and metals), few studied the composition of the rocks and minerals found on Earth's surface.

In the sixteenth century, however, Saxon scientist Georgius Agricola left his medical training and instead began to study mining and geology. Agricola wrote the seminal book On the Nature of Metals (1556), which reviewed the types of known metals and minerals and the methods scientists used to locate them. Agricola's work is considered the first scientific study of the clastic rocks found on and under Earth's crust, earning him recognition as the founder of geology as a scientific discipline.

Two centuries later, Scottish scientist James Hutton, the founder of modern geology, observed the flooding that occurred in rivers near his home. He noticed the amount of sand, pebbles, and other rocks that appeared when flood waters receded and concluded that the rocks and soil that appeared on Earth's crust were deposited over millions of years of flooding, mountain erosion, and other natural processes. Despite prevailing views that the flood of the Bible was responsible for the global landscape, Hutton argued that the sediment that covered Earth was placed there during a long-term, ongoing process of recycling and erosion, a concept dubbed uniformitarianism.

Clastic Rock Formation

Igneous Rock Origins. Earth's surface is covered with rocks of varying size, origin, and composition. Some of this sediment (clast) originates under Earth's crust, pushed outward in superheated form (magma) until it cools or is ejected violently through volcanic activity. Other forms of clastic rock are formed after a mountain or other geological body erodes. Still others change in composition after being introduced to a new environment.

The first type of rock from which sediment is derived in clastic form is igneous rock. This rock is the product of magma flow from Earth's mantle to its outer crust. As the molten rock flows outward, it can either reach areas beneath the crust and cool or reach the surface and be ejected from active volcanoes. In time, the igneous rock is weathered by heat, pressure, and other conditions. This weathering may cause the rock to break into smaller particles, such as clay or sand, and to ultimately fuse with other sediments to produce clastic rocks. Scientists believe that the appearance of clasts formed from igneous rocks may yield clues about tectonic plates (massive plates floating beneath Earth's outer crust) and other geodynamic processes. The rocks also can help scientists understand how Earth's geology has evolved.

Metamorphic Rock Origins. Metamorphic rocks, the second type of basic rock that becomes reconstituted as clastic rock, undergo a chemical reconfiguration when moved from one set of elements into another. A metamorphic rock may originally have been, for example, igneous. However, if the sediment that sank downward under the ocean floor was exposed to intense heat from magma found under the crust, the minerals that constitute it would become unstable under those new conditions. To reestablish equilibrium (a state of balance), the minerals would change in shape and chemical composition. Metamorphic rock sometimes changes into a flatter shape, such as slate. In others, the minerals they contain become dehydrated, causing the chemicals in the rock to break down.

Weathering

Clastic rocks are the products of two general types of weathering: mechanical and chemical.

Mechanical Weathering. This type of weathering occurs when the rock is broken down by a physical element but is not altered chemically. Mechanical weathering includes dramatic changes in temperature, whereby heat and extreme cold cause igneous and metamorphic rocks to expand, contract, and fall apart into smaller particles (gravel, sand, or silt and clay). Examples of weathering include the expansive effects of frozen water on rocks, the growth of tree and plant roots within those rocks, and the activities of burrowing animals.

Chemical Weathering. This involves the introduction of a substance that, when applied to an igneous or metamorphic rock, causes the rock to break down chemically. One example of this process simply involves rain. As it falls, rain collects carbon dioxide in the atmosphere. When the soil absorbs the rain, it takes on more carbon dioxide trapped beneath the surface. The water, as a result, becomes acidic. When this acidic substance comes into contact with an igneous or metamorphic rock, the rock is eroded, and the chemically altered sediment is drawn from the stone. The mineral apatite, for example, is easily weathered through this chemical process, and the calcium dissolved from the apatite enriches noncarbonated soils.

When weathering occurs, the sediment particles are carried away in streams to rivers, lakes, and oceans, where they are deposited on the floors of those bodies of water and cement with other particles to form different types of clastic rocks. Sandstone, for example, contains a large amount of weathered quartz sand grains. In another example, rounded rock fragments may be bound together with clay and silt grains to form conglomerates. Clay and silt grains may also be cemented to form shale or siltstone.

Computer Modeling

Much study has focused on the sedimentation process (the conditions and forces that meld particles of original rocks into clastic rocks). Scientists search for common characteristics and trends. Geologists and mineralogists frequently use computer models to analyze trends in clastic rock formation in certain environmental settings. For example, scientists collated statistical data from several clastic rock formations in shallow-water environments. Using these data, they created a computer model that simulated clastic rock formation and deformation according to high and low sea-level conditions. Because of the complexity of studying clastic rock formation and composition, computer models are proving to be an increasingly reliable tool for scientists.

Stratigraphy

When clastic rocks are carried to their ultimate destinations, they frequently become cemented together in basins to form large, solid slabs that, through time, become layered. Studying these layers—a scientific approach known as stratigraphy—helps scientists find clues about myriad geological changes that have occurred in and since Earth's prehistory. For example, stratigraphic analysis of different clastic basins can yield information about the geodynamic activity during a particular era or epoch. A 2009 stratigraphic study of two basins in New Zealand revealed movement in the tectonic plate (massive slabs of rock moving constantly beneath Earth's outer crust) during the late Permian and early Cretaceous periods (144 million to 65 million years ago). This development allowed for the generation of oil and coal deposits in these areas. Scientists continued to achieve similar clastic rock discoveries across the globe, including in South India's Cauvery Basin in 2015, China's Songliao Basin in 2017, and the Guyana-Suriname Basin off of the northern coast of South America in the 2020s.

Relevant Organizations and Institutions

Clastic rocks are studied by people interested in Earth's past, people concerned with climate change, and those seeking prime locations for exploratory drilling and mining. Many groups and institutions are especially interested in the study of clastic rocks.

The United States government plays a significant role in studying sedimentary rocks. For example, the US Geological Survey (USGS) maintains a Mineral Resources Program designed to collect data on all three types of basic rock from deposits throughout the United States. The program provides maps and downloadable data about clastic rocks, information on scholarly studies, and research results. Meanwhile, the National Science Foundation provides funding for scientists working in sedimentary geology.

Universities and their faculties and research staff are key to studying clastic rocks and sedimentary geology. Stanford University, Pennsylvania State University, and the Massachusetts Institute of Technology are among the institutions whose geology and Earth science departments offer courses in this field.

Energy companies always seek new oil, coal, and gas deposits. Such findings are usually unearthed in or near clastic rock basins. For this reason, the petroleum industry often employs full-time geophysicists, mineralogists, and geodynamics consultants to help find such deposits. This interest has created an entire industry of businesses whose staff explores such basins for evidence of oil and coal deposits.

Implications and Future Prospects

Clastic rocks provide a number of clues to geodynamic processes, including plate tectonics and volcanism. Those scientists who seek to learn about climate change may also look to the strata of clastic rock to help them understand the conditions that existed during prehistoric periods, which featured significant climate shifts. Furthermore, clastic rocks can help energy companies, including coal mining and exploratory oil drilling businesses, locate coal and oil deposits.

Twenty-first-century technologies have greatly aided the study of clastic rocks. Computers capable of collating large volumes of data and creating two- and three-dimensional models based on these data are helping scientists better understand how clastic rocks are formed under a wide range of environmental conditions. The Internet allows scientists worldwide to quickly share data on rock samples found in various basins and analyze such data collectively.

Principal Terms

clast: pieces or particles of other rocks that together form clastic rocks

clay: a finer grain of mud particle that comprises clastic rocks

conglomerate: rounded form of gravel-sized clast

igneous rock: crystalline solid formed by the cooling of magma

magma: molten rock formed beneath Earth's crust

metamorphic rock: type of rock that changes composition when introduced into a different environment

QFL: the three main types of minerals found in sedimentary/clastic rocks—quartz, feldspar, and lithic fragments

sandstone: clastic rock composed of sand-sized particles

sedimentary rock: type of rock formed from the debris that is broken off other, older rocks; also known as clastic rock

silt: a more coarse grain of mud particle that comprises clastic rocks

Bibliography

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