Earth Science Unit 1 Lesson 7 Sedimentary Rocks Sedimentary rocks are like mystery novels. They are full of clues about Earth's past. Fossils, chemical composition, and layer thickness can tell us what life was like

hundreds, thousands, even millions of years ago. Curves in the sedimentary rock layers of mountains tell us the land was once flat. Rock salts, like halite and gypsum, tell us an area once had salt water. All these clues

help us solve the mystery of Earth's past. In this lesson, you will learn how sedimentary rocks form and how they are classified.

Lesson Outline

Sedimentary rocks form from rock fragments, sediment, and minerals. The rocks build in layers.

Clastic sedimentary rocks contain rock fragments and sediment cemented together. The clastic sedimentary rocks include shale (from clay), mudstone, siltstone, sandstone, and conglomerates.

Chemical sedimentary rocks are made of mineral crystals. Gypsum is a chemical sedimentary rock, as are stalactites and stalagmites in caves.

Organic sedimentary rocks develop from the remains of living organisms. Limestone and coal are organic sedimentary rocks.

OBJECTIVES

Describe how sedimentary rocks form. Compare clastic, chemical, and organic sedimentary rock. Identify common characteristics of sedimentary rock.VOCABULARY

clast an individual particle or grain in sedimentary rock

degradation breaking-down process that changes the minerals in rock

evaporite chemical sedimentary rock formed when minerals are left behind by evaporated water

sediment weathered rock, bone fragments, soil, and other particles carried by wind, water, and ice

stalactite chemical sedimentary rock hanging from the ceiling of a cave

stalagmite chemical sedimentary rock growing from the floor of a cave

Formation

Mosaic is an art form in which small pieces of colored glass, stone, or other material are pieced together to form a whole.

Formation of Sedimentary Rocks

Sedimentary rocks are similar to a mosaic art piece. They form from rock fragments, minerals, and pieces of dead organisms. Over time, these particles, called sediment, consolidate into new rock.

Sedimentary rock forms after the Earth's surface is weathered and eroded.

Weathering is the breakdown of minerals and rocks by wind, water, ice, and/or

gravity. Weathering causes degradation in rocks and minerals. Almost all minerals can

degrade, except quartz. Quartz maintains its chemical composition during weathering. Other minerals,

like orthoclase, will degrade into new minerals. Chemical weathering changes orthoclase into clay. Another example is the calcium in plagioclase dissolves in water and forms

calcium carbonate (CaCO3).

After rocks are weathered, water, wind, ice, and/or gravity transports and sorts particles of the rocks through a process called erosion.

Different-sized particles move at different speeds and in different ways.

[Graphic showing types of rocks under water: beach has sandstone, near beach has shale and far from beach has limestone.] In water, large particles roll and bounce along the bottom; on the other hand, clay particles are small enough to become suspended in the water.

Since calcium carbonate dissolves, it simply moves with the water. Another way rocks can be eroded is by exposure to wind. Wind can also help

erode weathered rocks, especially very small particles of rocks. Over time, wind can cause large particles of sediments to be broken down into smaller pieces.

Eventually, particles of rock fall out of water and wind through the process of deposition. By the time they deposit, the particles have sorted themselves by size. Large particles fall out first, followed by smaller and smaller particles. This is why many sedimentary rocks are made entirely of same-sized particles.

There are three main types of sedimentary rocks: clastic (made from rock fragments), chemical (made from dissolved mineral), and organic (made from animal or plant remains).

Clastic Sedimentary Rocks

Have you ever eaten a layer cake? Clastic sedimentary rock is similar to a cake, because it usually forms horizontal layers.

Weathering breaks down rocks on Earth's surface. Water and wind carry the rock fragments away, and the pieces mix with other

pieces of stone, shells, and even bones. When the sediment settles, it lays down in layers and new sediment deposits over old sediment. Over time, pressure from higher layers compacts lower layers into rock. The rock forms when dissolved minerals and pressure cement particles together.

The particles in clastic sedimentary rock range in size from microscopic clay particles to large rocks. Geologists base the name of a clastic sedimentary rock on the size of its clasts. Clasts are broken rock fragments. Rocks with the smallest clasts are clays.

Shale is an example of a clay sedimentary rock. Rocks with clasts larger than

clay are silts and sands.

Siltstone is made of silt-sized clasts, while sandstone is made of larger, sand-

sized clasts.

Another example of clastic sedimentary rocks is mudstone. Mudstone is a mixture of silt and clay.

Finally, rocks with particles larger than two millimeters are called conglomerates.

Sand or mud usually cements together the clasts in conglomerates.

Remember that clastic sedimentary rocks are commonly known and arranged from smallest clasts to largest. Shale is made of clay particles that are the smallest, and then there is mudstone, which is a mixture of clay and silt. Siltstone and sandstone are larger clasts, in comparison to mudstone. The largest clastic sedimentary rocks are the conglomerates.

Sandstone: Shale:

Conglomerates:

The following list summarizes the clastic sedimentary rocks, arranged from smallest clasts to largest.

shale (from clay) mudstone (mix of clay and silt) siltstone sandstone

conglomerates (particles larger than 2 mm)

Chemical Sedimentary Rocks

Have you ever been to Luray Caverns in Virginia? The caverns display one of the most amazing collections of chemical sedimentary rocks in the country.

Chemical sedimentary rocks form by chemical precipitation. Water dissolves minerals in rocks. When water moves through a rock, it carries

the dissolved minerals away. Eventually, the water deposits the minerals in a new location. This process is chemical precipitation. As the minerals harden, they form chemical sedimentary rocks.

Sometimes, water carrying dissolved minerals evaporates, leaving the minerals behind.

Rocks formed from evaporating water are evaporites. You can usually find evaporites near inland lakes in dry climates and in deserts

that at one time had water. The Bonneville Salt Flats in Utah is made of almost 160 square miles of chemical

sedimentary rock. When the ancient Lake Bonneville evaporated, it left behind a thick layer of salt. Salt flats, also called playas, occur all over the world. They are usually made of halite and gypsum.

Not all chemical sedimentary rocks are evaporites. Sometimes, water simply

can't hold any more minerals and must deposit the excess. In caves, stalactites form when dripping water from the ceiling leaves behind small mineral deposits. When the water hits the ground, it leaves behind another mineral deposit. Over time, this deposit forms stalagmites. Sometimes, stalactites and stalagmites meet and form large columns.

Organic Sedimentary Rock

Have you ever drawn outside using sidewalk chalk? It may not seem like it, but chalk is a type of sedimentary rock.

Chalk is an organic sedimentary rock called limestone. Organic sedimentary rocks contain the remains of living organisms.

Limestone is made of shell, coral, and the mineral calcite (calcium carbonate). Most limestone forms under ocean water. Marine organisms, like plankton and coral, use dissolved calcite to build their shells. When the organisms die, their shells fall to the ocean floor. Over time, the layers of organic material form rock.

Sometimes, organic sedimentary rock forms from a build-up of dead plants and animals. A few layers of dead organic material will compact into peat. Over millions of years, the peat transforms into coal. Since once-living organisms form coal, it often contains fossils. That is why we call coal a fossil fuel.