Activity 4: Sedimentary Rocks
LIMESTONE, SANDSTONE AND SHALE, OH MY!
Sedimentary rock is one of the three main rock types. It is formed by weathering, erosion and lithification. Students will learn to differentiate between limestone, sandstone, and shale according to the physical characteristics of the rock.
Introduce students to sedimentary rocks, how they form, the different types and how to identify them.
Students will be able to:
30 minute setup
Sedimentary rocks are classified into three major groups: clastic, biologic and chemical. When sediments consolidate into a cohesive mass, they become sedimentary rock. The composition, texture (how the rock looks) and other features of a sedimentary rock can tell us a story about its origin.
Clastic Sedimentary Rocks
Clastic sedimentary rocks may have particles ranging in size from microscopic silt and clay to huge boulders! The chart to the right shows different particle sizes.
Shale is the name given to rock that has mostly clay-sized particles in it. Shale also has the unique property of breaking along flat planes, because the flat clay minerals in the shale separate easily along their flat surfaces.
Rock made from silt is called siltstone, and rock made from sand is called sandstone.
Conglomerate is rock made from varying sized particles that are cemented together by a matrix or mineral cement.
Biologic Sedimentary Rocks
Biologic sedimentary rocks form when a large number of living things die, pile up, compact, and are cemented to form rock. Accumulated carbon-rich plant material eventually forms coal.
Deposits made mostly of small animal shells form kinds of limestone. A common mineral found in ocean water is calcium carbonate. This is the dominate mineral in limestone. Shells rich in calcium carbonate form as animals draw abundant minerals from the ocean water. Dolostone is a limestone that is slightly altered by the addition of magnesium. Dolomite is a calcium-magnesium carbonate.
Biologic sedimentary rocks are named based on composition rather than on sediment size. Any rock that contains calcium carbonate reacts with a weak acid. Calcium joins to the acid and carbon dioxide is released, causing the acid to fizz. This reaction is one way to tell a limestone from other rocks. Dolostones react, too, but you need to scratch the surface of the rock, forming a powder, to expose a fresh spot before it reacts with acid.
Chemical Sedimentary Rocks
How do Sediments Become Sedimentary Rocks?
Cementation is the word given to the sticking together of sediments by additional material between grains. Water flowing between grains contain silica, carbonate or other dissolved materials that precipitate out of solution and attach to grain surfaces. Eventually, enough mineral precipitates out that it cements grains into a whole.
Sedimentary Rocks are Clues to Past Environments
Why do Geologists Need to Identify Sedimentary Rock?
What do geologists look for in sedimentary rock in order to identify them? What does grain size tell them? What does the acid test tell them? What are the two steps that sediments undergo to become sedimentary rock? Why do not all sediments become sedimentary rock? What are some important natural resources that we find in sedimentary rocks?
For more advanced students, you may wish to present them with information about depositional environments. For background information and an activity, go to Extension: Depositional Environments.