Scientists are people who try to understand the world. When you want to understand a thing, the first step is often to describe it in a way that shows its differences from other things that seem like it. A scientist who studies life, who we call a biologist, divides living things into plants and animals. A scientist who studies stars, who we call an astronomer, divides the stars based on their size and the color of their light. Scientists who study the earth and its rocks, who we call geologists, are the same: to describe the rocks they see, geologists had to define rock types. The simplest way to divide up the rocks is based on the way the rock was formed, so that’s what they did. So now, all rocks fall into one of three types:
Each of those three basic rock types is then divided into special rock names based on composition, or the kinds of minerals present in the rock; and texture, which refers to the sizes and shapes of the grains that make up the rock, and the way that they touch each other. The first thing a geologist does when looking at a rock sample is to decide whether it is igneous, metamorphic, or sedimentary. How do they decide? Here are some hints:
Igneous rocks: The word igneous looks like the word ignite because both words refer to fire. Igneous rocks are rocks that form from fire, or at least from great enough heat to melt older rocks. These rocks form when the molten rock, called magma, cools. As it cools, different minerals form crystals at slightly different times. The last mineral grains to form have to fill in gaps between older grains, which gives igneous rocks a characteristic texture: the mineral grains lock together like jigsaw puzzle pieces, except instead of being flat the puzzle is three-dimensional.
Geologists divide igneous rocks further based on the size of the grains and the minerals that are present. Rocks with grains too small to see with the naked eye occur when the molten rock cools quickly, like it does when lava runs down the side of a volcano. This kind of rock includes basalt, the black rock that covers most of the Hawaiian Islands. Rocks with larger grains happen when the magma cools slowly, deep underground. This kind of igneous rock includes granite, a gray or pinkish rock often used for building stone and countertops. There are hundreds of igneous rocks, many of which cannot be told apart without careful study.
Sedimentary Rocks: These are the rocks that form at the Earth’s surface, at temperatures that support life. They are called sedimentary because the first ones identified were made of tiny bits of older rocks that had been broken and shaped by the actions of wind, water, and ice. This sediment had been dropped from the water in rivers and oceans to form layers of sand. With time, burial, and circulation of underground water; the sediments became compressed and cemented together. That process is called lithification (a big word that means making rock). The sedimentary rocks that are made up of bits of older rocks are called clastic sedimentary rocks for clasts, or grains of sediment. Clastic sedimentary rock can be recognized because the grains touch each other but do not interlock. They include sandstone and shale. Most are deposited by water in rivers, lakes, and seas; but some were deposited by water and some by ice.
Clastic sedimentary rocks differ from two other kinds of rocks that also form at the surface. The first is chemical sedimentary rocks, which form when minerals precipitate (crystallize) from water as it evaporates. The most common chemical sedimentary rocks are halite, or rock salt; and gypsum. If you dissolve some table salt in a cup of water and allow it to evaporate, you can form your own tiny deposit of halite! The mineral grains in chemical rocks, like igneous rocks, have an interlocking texture. The key to telling them apart is identification of the minerals. Fortunately, halite and gypsum are very easy to identify.
The other kind of sedimentary rocks are organic sedimentary rocks, which are made of the skeletons of dead plants and animals. Coal is an organic sedimentary rock that is made up mostly of plant remains, while limestone is mostly made of the shells of water-dwelling animals like clams and coral. Organic sedimentary rocks are easy to identify because most contain fossils.
Metamorphic rocks: Metamorphic rocks are rocks that have been changed. You might have seen the word metamorphism used to describe how a caterpillar changes into a butterfly. Metamorphism happens when an existing rock finds itself under great heat and pressure but don’t quite melt. The minerals are usually the same after this happens, but the texture changes. Some metamorphic rocks are made entirely of minerals that are all lined up like the sheets of paper in a book. This is called foliation (a folio is a book with big pages). Other metamorphic rocks do not have foliation. Most of these are rocks that are almost all one mineral, either quartz (the most common mineral in beach sand) or calcite, which is the mineral that sea shells are made of. These two rocks are called quartzite and marble. They result from the metamorphism of sandstone and limestone, in that order. Like igneous rocks the grains have interlocking textures, but careful study can often identify the shaped of sedimentary grains or fossil remains.
The rock cycle: The rocks that make up the earth are always moving through a giant cycle of formation, erosion, uplift, and transportation. The processes happen too slowly for us to see, however! Any rock can get melted and turned into an igneous rock; or any rock of any kind can be metamorphosed. They are long, slow processes that take millions and even billions of years
Geologists like to dream about the rock cycle and draw fun pictures of how rocks might move through the cycle over time. Here are a few:
In summary, there are three kinds of rocks: igneous, sedimentary, and metamorphic. The division is based on where and how the rocks formed, not on their content. The key to telling the three types apart is to study how the mineral grains are arranged. After a geologist has determined which of the three types a rock falls into, the next step would be to define a more specific type.