Activity 4: Sedimentary Rocks

LIMESTONE, SANDSTONE AND SHALE, OH MY!

Summary:

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.

Instructional Method:

Experiment

Goal:

Introduce students to sedimentary rocks, how they form, the different types and how to identify them.

Objectives:

Students will be able to:

• Define clastic and carbonate sedimentary rocks in their own words.
• Separate random sedimentary rocks into their correct rock type.
• List the tests used to identify each rock type

Time:

30 minute setup
30 minute activity
10 minute cleanup

Materials Needed:

• Pieces of rock: Conglomerate, sandstone, shale, limestone
• Paper
• Weak acid (vinegar)
• Rubber gloves
• Goggles
• Magnifying glass
• Printable rock cycle diagram

Vocabulary:

chert	erosion	sandstone	stalactite
clastic	limestone	sediment	stalagmite
compaction	lithified	sedimentary rock	texture
conglomerate	matrix	shale	weathering
dolostone	precipitate	siltstone

 

Background:

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 sediments are composed of broken fragments of preexisting rocks that are the products of weathering and erosion. Clastic sediments vary widely in size, shape and composition. A single clastic sedimentary rock may be composed of one type of rock and mineral or many types of rocks and minerals. The types of sediment found in a clastic sedimentary rock can tell geologists much about past environments. Clastic sediments and the rocks formed from them are named on the basis of particle size rather than mineral composition.

 

Sediment Size Chart

USGS

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.

 

Sandstone with bedding layers

USGS

 

Conglomerate

USGS

 

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.

 

Limestone with large fossils

USGS

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
Water traveling through rock dissolves minerals and transports them away. Chemical sedimentary rocks form from these minerals. Eventually dissolved minerals are redeposited, or reprecipitate, when the water evaporates or when the solution becomes over-saturated with minerals or when the water enters an area of low pressure, like a cavity in the rock. Some chemically precipitated rocks include stalactites and stalagmites you see in caves (also known as travertine). Chert is a precipitated rock from silica-saturated waters. Rock salt, known as table salt, is the result of water evaporating from salt-rich water.

How do Sediments Become Sedimentary Rocks?
To become a sedimentary rock, sediments must be lithified, or cemented into stone. As sediments pile atop other sediments, the earliest deposits become deeply buried. Weight of overlying sediment places pressure on those below. Sediment under pressure compacts. The tighter the sediment is squeezed, the less room there is for water or air between grains and grains to stick together. Something else has to happen to turn these sediments into rock.

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
Sedimentary rocks can tell a geologist much about past environments. These rocks are keys to reading Earth's past. Connecting to the following activity will further develop this idea, Extension: Depositional Environments.

Why do Geologists Need to Identify Sedimentary Rock?
Sedimentary rock is where you find important things like oil, natural gas, coal, salt, potash, iron ore, and other important resources. Geologists also find fossils in sedimentary rock. In order for geologists to know where to find these things, they need to know what types of sedimentary rock to find them in.

Instructional Procedures:

1. Explain to students what sedimentary rocks are and how they form. Be sure to introduce them to different types of sedimentary rocks, especially sandstone, limestone and shale.
2. Divide students into groups of three and give each group a piece of limestone, sandstone, shale or conglomerate.
3. Have students examine grain size for each rock. It may be helpful to use a magnifying glass. Have students test the rock to see whether it is a clastic sedimentary rock or a biologic (carbonate) sedimentary rock by putting a drop of weak acid on the rock and seeing whether or not it will fizz. While students are performing each experiment they need to be taking notes describing each outcome.
4. Have students discuss and compare what type of rock they think they have based on their tests.
5. Troubleshooting: If there is a sandstone, shale or conglomerate that fizzes, it is probably due to a calcium carbonate cement. This does not mean that the rock is a limestone, it just means that the cementing agent is made of calcium carbonate. You may want to test all of your rocks before giving them to students to avoid confusion.

Discussion:

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?

Extension:

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.

 

Included National Parks and other sites:

Bryce Canyon National Park
Dinosaur National Park
Zion National Park

Photos:

Bryce Amphitheater
Zion Overlook

Utah Science Core:

2nd Grade Standard 6 Objective 1,2,3
4th Grade Standard 3 Objective 1,2

 

(top of page)

---

GEODETECTIVE Home - Rocks & Minerals Home - Rocks & Minerals Discovery Chest
Rocks & Minerals Kid's Page

Contact our Education Outreach Specialist here.