What Is a Sedimentary Rock?

Ripples in beach sand , such as those in the upper photograph (A) may someday become a rock like the sandstone in the lower photograph (B). This sandstone was part of a beach over 200 million years ago in the Triassic period. Click on image for full size (143 Kb) Both images courtesy of Martin Miller, University of Oregon

Sedimentary rocks make up about 75% of the rocks that are at the Earth’s surface. They preserve a chronicle of the environments and events that happened at the surface throughout Earth history, as well as the creatures that lived there.

Millions of little fragments of rock called sediment are the building blocks of a type of sedimentary rock known as clastic rocks. Sediment is found almost everywhere. It is the sand you find at the beach, the mud in a lake bottom, the pebbles in a river, and even the dust on furniture. The sediment particles are carried in streams, other waterways, glaciers, or the wind. They eventually settle out of the air or water and form a rock if they are cemented together.

But not all sedimentary rocks are made in this way. Chemical sedimentary rocks are not made of particles that come from somewhere else. They are made of mineral crystals formed by chemical processes. Organic rocks are made from the remains of living things such as clamshells, plankton skeletons, dinosaur bones, and plants.

Ripples in beach sand , such as those in the upper photograph (A) may someday become a rock like the sandstone in the lower photograph (B). This sandstone was part of a beach over 200 million years ago in the Triassic period. Click on image for full size (143 Kb) Both images courtesy of Martin Miller, University of Oregon

Have you ever been to the beach and nestled your toes in the sand? Over thousands of years that sand might become part of a sedimentary rock!

Sedimentary rocks make up about three-quarters of the rocks at the Earth’s surface. They form at the surface in environments such as beaches, rivers, the ocean, and anywhere that sand, mud, and other types of sediment collect. Sedimentary rocks preserve a record of the environments that existed when they formed. By looking at sedimentary rocks of different ages, scientists can figure out how climate and environments have changed through Earth’s history. Fossils of ancient living things are preserved in sedimentary rocks too.

Many sedimentary rocks are made from the broken bits of other rocks. These are called clastic sedimentary rocks. The broken bits of rocks are called sediment. Sediment is the sand you find at the beach, the mud in a lake bottom, the pebbles in a river, and even the dust on furniture. The sediment may, in time, form a rock if the little pieces become cemented together.

There are other types of sedimentary rocks whose particles do not come from broken rock fragments. Chemical sedimentary rocks are made of mineral crystals such as halite and gypsum formed by chemical processes. The sediment particles of organic sedimentary rocks are the remains of living things such as clamshells, plankton skeletons, dinosaur bones, and plants.

Ripples in beach sand (A) may someday become rock like in the lower picture (B). Click on image for full size (143 Kb) Both images courtesy of Martin Miller, University of Oregon

Someday, the sand that you see at the beach might become a sedimentary rock!

The sand at the beach is made of little pieces of rock that broke off larger rocks. These little broken pieces are called sediment. There are many types of sediment including sand, mud, pebbles, and even dust. It takes a very long time, thousands and thousands of years, but sediments can form a rock if they become stuck together.

Sedimentary rocks record what environments were like a long time ago. Fossils are sometimes preserved in sedimentary rocks too. They record what animals and plants were like thousands or millions of years ago.

Some types of sedimentary rocks are made of special sediment. The particles do not come from other rocks. They can be crystals that form from chemicals in seawater, like in chemical rocks, fossils of ancient plants and animals, like in organic rocks.