Section 3: Sedimentary Rocks Chapter 5: Rocks Section 3: Sedimentary Rocks

Objectives After completing the lesson, students will be able to . . . Describe how sedimentary rocks form; List and describe the three major types of sedimentary rocks.

Introduction Read the introduction on page 154.

From Sediment to Rock Sedimentary rocks form from particles deposited by water and wind. Sediment—Small, solid pieces of material that comes from rocks or organisms. Sediments include: Sand grains Pebbles Mud Water, wind, and ice can carry sediment and deposit it in layers.

Erosion Destructive forces are constantly breaking up and wearing away all the rocks on Earth’s surface. These forces include: Heat and cold Rain Waves Grinding Ice Erosion—The destructive process in which water or wind loosen and carry away fragments of rock.

Deposition Deposition—The process by which sediment settles out of the water or wind that is carrying it. After sediment has been deposited, the processes of compaction and cementation change the sediment into sedimentary rock. In addition to particles of rock, sediment may include shells, bones, leaves, stems, and other remains of living things.

Compaction At first, sediments fit together loosely, but over millions of years, thick layers of sediment build up. These layers are heavy and press down on the layers beneath it. Compaction—The process by which sediments are pressed together under their own weight. The weight of the new layers further compacts the sediments, squeezing them tightly together.

Cementation While compaction is taking place, the minerals in the rock slowly dissolve in the water. The dissolved minerals seep into the spaces between particles of sediment. Cementation—The process by which dissolved minerals crystallize and glue particles of sediment together into one mass. It often takes millions of years for compaction and cementation to transform loose sediment into solid sedimentary rock.

Types of Sedimentary Rocks Geologists classify sedimentary rocks according to the type of sediments that make up the rock. There are three major groups of sedimentary rocks: Clastic Rocks Organic Rocks Chemical Rocks

Clastic Rocks Clastic Rock— Sedimentary rock that forms when rock fragments are squeezed together under high pressure. Clastic rocks are grouped by the size of rock fragments, or particles, of which they are made.

Shale Shale forms from tiny particles of clay. For shale to form, water must deposit clay particles in very thin, flat layers, one on top of another. No cementation is needed to hold clay particles together. Shale feels smooth, and splits easily into flat pieces.

Sandstone Sandstone forms from the sand on beaches, on the ocean floor, in riverbeds, and in sand dunes. Sandstone is a clastic rock formed from the compaction and cementation of small particles of sand. Because the cementation process does not fill all the spaces between sand grains, sandstone contains many small holes and can easily absorb water through these holes.

Conglomerate and Breccia Some sedimentary rocks contain a mixture of rock fragments of different sizes. The fragments can range in size from sand and pebbles to boulders. If the fragments have rounded edges, they form a clastic rock called conglomerate. A rock made up of large fragments with sharp edges is called breccia.

Organic Rocks Not all sedimentary rocks are made from particles of other rock. Organic Rock— Sedimentary rock that forms where remains of organisms are deposited in thick layers. The term “organic” refers to substances that once were part of living things or were made by living things.

Coal Coal forms from the remains of swamp plants buried in water. As layer upon layer of plant remains build up, the weight of the layers squeezes the decaying plants. Over millions of years, they slowly change into coal.

Limestone Read this section on page 157.

Chemical Rocks Chemical Rock—Sedimentary rock that forms when minerals crystallize from a solution. Example: Limestone can form when calcite that is dissolved in lakes, seas, or underground water comes out of solution and forms crystals. Chemical rock can also form from mineral deposition left when seas and lakes evaporate. Rock salt Gypsum

Uses of Sedimentary Rock Read this section on page 158.