Chapter 4 Section 3 Sedimentary Rocks

What You Will Learn Describe the origin of sedimentary rock. Describe the three main categories of sedimentary rock. Describe three types of sedimentary structures.

Where does Sand come from? Sand is a product of weathering, which breaks rock into pieces. Over time, sand grains may be compacted, or compressed, and then cemented together to form a rock called sandstone. Sandstone is just one of many types of sedimentary rock.

Origins of Sedimentary Rock Wind, water, ice, sunlight, and gravity all cause rock to physically weather into fragments. Through the process of erosion, these rock and mineral fragments, called sediment, are moved from one place to another. Eventually, the sediment is deposited in layers. As new layers of sediment are deposited, they cover older layers. Older layers become compacted.

Origins of Sedimentary Rock Sedimentary rock forms at or near the Earth’s surface. It forms without the heat and pressure that are involved in the formation of igneous and metamorphic rocks.

The most noticeable feature of sedimentary rock is its layers, or strata. A single, horizontal layer of rock is sometimes visible for many miles. Road cuts are good places to observe strata. Figure 1 shows the spectacular views that sedimentary rock formations carved by erosion can provide.

Composition of Sedimentary Rock Sedimentary rock is classified by the way it forms. Clastic sedimentary rock forms when rock or mineral fragments, called clasts, are cemented together. Chemical sedimentary rock forms when minerals crystallize out of a solution, such as sea water, to become rock. Organic sedimentary rock forms from the remains of once-living plants and animals.

Clastic Sedimentary Rock Clastic sedimentary rock is made of fragments of rocks cemented together by a mineral such as calcite or quartz. Figure 2 shows how clastic sedimentary rock is classified according to the size of the fragments from which the rock is made. Clastic sedimentary rocks can have coarse- grained, medium-grained, or fine-grained textures.

Chemical Sedimentary Rock Chemical sedimentary rock forms from solutions of dissolved minerals and water. As rainwater slowly makes its way to the ocean, it dissolves some of the rock material it passes through. Some of this dissolved material eventually crystallizes and forms the minerals that make up chemical sedimentary rock.

Organic Sedimentary Rock Most limestone forms from the remains, or fossils, of animals that once lived in the ocean. For example, some limestone is made of the skeletons of tiny organisms called coral.

Coal Another type of organic sedimentary rock is coal. Coal forms underground when partially decomposed plant material is buried beneath sediment and is changed into coal by increasing heat and pressure. This process occurs over millions of years.

Sedimentary Rock Structures Many features can tell you about the way sedimentary rock formed. The most important feature of sedimentary rock is stratification. Stratification is the process in which sedimentary rocks are arranged in layers. – Strata differ from one another depending on the kind, size, and color of their sediment.

Sedimentary rocks sometimes record the motion of wind and water waves on lakes, oceans, rivers, and sand dunes in features called ripple marks, as shown in Figure 5.