Earth Beneath My Feet:

Layers of Sedimentary Rocks in the Earth's Crust

Sedimentary Rocks

INTRODUCTION

In this experiment, volunteers will help students build a model of the Earth’s crust using food. There are many layers of the earth’s crust, which are in turn made up of different types of rock. By describing what each layer of the sandwich represents, students should learn how the earth’s crust is developed and what materials have combined to form it.

MATERIALS

WHAT TO DO

  1. Have a class discussion about the structure of the earth, including the crust, mantle, and core of the earth. In the discussion of the crust, make a special point of mentioning that the crust of the earth is made up of several layers of different types of rock.
  2. Now turn to the sandwich-making to explain the development of the earth’s crust. The first layer to put down is the stack of plates. This represents granite, which is the igneous rock that was laid down billions of years ago. We use a stack of plates because granite is so much thicker than everything else – we can not drill through the granite base.
  3. Next is the white bread. This represents sandstone, which would have been formed by a river. When a river flows over bedrock, over time it erodes the bedrock, creating small particles of sand. Eventually, with the mineral deposits from the water the sand becomes cemented together and forms sandstone on the river bottom.
  4. Now add the apple butter with raisins OR crunchy peanut butter. A long time ago, a great flood occurred, which caused mud and rocks to be deposited over the sandstone. With time, this combination of mud and rocks eventually became a sedimentary rock called conglomerate.
  5. Now the light brown bread is added to represent shale. As time continued, the river slowed down and carried small pieces of rock called silt. The silt accumulated and eventually became shale.
  6. Next add the cream cheese and M & M’s OR icing to represent fossilized limestone. At this point in Earth’s history, an ice age ended. This means that the glaciers melted, the oceans rose, and water covered what was previously land. Creatures living in the ocean died and left their skeletons and shells on the ocean floor. The crust of the earth, the shells, and the skeletons cemented together over time, creating what is called fossilized limestone.
  7. Last, add a piece of dark brown bread. This represents topsoil, which covers the other rock layers many long years after the ocean and river have disappeared and the rock layers rose up. There are several processes which can cause the layers to rise. Ask students to name a few. Some answers might include earthquakes, tectonic plates shifting, and so forth.
  8. Now, sketch the sandwich and ask students to tell you what each layer represents. Label the sketch as they answer correctly.
  9. Use the film canister to take a "core sample" of the sandwich.
  10. Deform the sandwich to show students what might happen during such occurrences like earthquakes. Cut the sandwich in different directions to reveal different patterns made by the rock layers.
  11. Show students the pictures of rock layers and compare with the sandwich.

QUESTIONS

  1. What does each part of the sandwich represent? Do the materials match the character (look) of the actual sediments or sedimentary rock? In what environments do they form? (See "What To Do" for answers.)
  2. What does the core taken with the film canister show? (By coring through the different layers, the composition of the layers can be determined as well as the thickness of the layers.)
  3. How deep can we drill? (Oil companies dig as deep as 8 kilometers and this is in sedimentary rock. Igneous and Metamorphic rock has only been drilled to depths of 2 or 3 kilometers.)
  4. What are the rock units in Ohio?
    (Eastern Ohio - Limestone, Sandstone, Siltstone, and Coal
    Central Ohio - Shale at surface
    Western Ohio - Limestone
  5. What patterns occur when the rocks are folded? When they are cut at angles? (See definition section on anticlines and synclines. Cutting the rock shows shifting of layers, which is similar to what would happen in an earthquake.)
  6. How deep do these sedimentary rock layers go in Ohio? (700 meters on the west of Ohio and 4000 meters on the southeastern side.)
  7. Do they contain resources? (Some gas and oil.)

SUMMARY

The earth is made up of three different major sections: the crust, the mantle, and the core. The crust beneath the continents averages at about 30 km, and beneath the ocean at about 5 km. The mantle is about 2900 km thick. The outer core, made up of liquid, is approximately 2200 km thick. The inner core, which is made up of very hot, very dense solid metal, is about 1250 km thick. Under large mountain ranges like the Alps and the Sierra Nevada, the crust of the earth may be up to 100 km thick.

Rocks are composed of tiny bits of minerals. They are formed naturally over millions of years. Sometimes there are natural occurrences such as a river flowing over rock, which cause the rock to break down. This creates sediment, or small pieces of rock. The sediment may then be cemented together with minerals to become sedimentary rock. The different types of sedimentary rock include shale, sandstone, conglomerate, coal, and chemical. Shale is made up of cemented silt and clay. Sandstone is made of sand and mineral deposits found in the water. Conglomerate rock is made of cemented round gravel. Coal is from plant material. Limestone is made of chemicals like calcium carbonate, often precipitated from warm shallow seawater.

Generally, newer sedimentary rock is found above older sedimentary rock. However, sometimes this is not the case due to folds in rock. These are bends or wavelike features found in rocks, which may be caused by the ground rising, especially during earthquakes.

EXTENSION

  1. Deformation of layers: Folds and faults (or leave for later)
  2. Measurement of Porosity of Sand
    1. Take a graduated cylinder and measure a known volume of dry sand (50mL).
    2. Place this dry sand in a cup.
    3. Measure a known volume of water (100 mL). Slowly add the water to the sand. Where is it going?
    4. Continue to add the water to the sand until the sand is completely saturated with the water. Do not overfill the sand with water. How much water was used to fill the air spaces between the sand particles? This describes the pore volume of the sand. Groundwater is found in sedimentary deposits like what was just made. As much as 40-50% of the total volume may be filled with water. The porosity is calculated by placing the pore volume over the total volume.

VOCABULARY

SOURCES

Presentation, "Sandwich Geology" Trisler et al., 2001, SECO Cincinnati.
"Science Is . . . A Resource Book for Fascinating Facts, Projects, and Activities." S. Bosak, Scholastic Canada, 1998, p.229.
"Magic School Bus: Inside the Earth." Joanne Cole, Teacher Created Materials, Inc., p.12.
"Physical Geology." Plummer, McGeary and Carlson, McGraw-Hill, 1999.
GEOEDGROUP- The Ohio State University
USGS website, http://pubs.usgs.gov/publications/text/inside.html

© S. Olesik, WOW Project, Ohio State University, 2000.

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