Experiment

This activity created in partnership with AGI.

Throughout the Earth’s history, the cycle of erosion and deposition by streams, rivers, oceans, and wind has added layer upon layer of rock, soil, and organic material to the surface. Over time, these layers of sediment were covered with more layers, and then even more. As layers got buried, through increased temperature and pressure the sediments hardened and became what we know today as sedimentary rock. As you might imagine, the layers at the bottom were formed before the layers at the top.

In photographs of sedimentary rock formations, you can see these layers clearly. Patterns of color and texture are visible indicators of differing conditions at the time the sediment was deposited. Although there are many places on the surface of the Earth where sedimentary layers are exposed—for example, the Grand Canyon in the United States—most sedimentary rock is beneath the surface and thus not visible.

Geoscientists study the geological history of the Earth by looking at these rock layers. The layers can provide information about the climate and life forms of millions of years ago. They can also show remnants of events such as earthquakes and the presence of resources such as petroleum. But how do geoscientists get all of this information from the rocks that lie below the surface?

Video ©American Geosciences Institute, 2012. The video Oil Formation over Time shows how oil develops in rock layers.

Sedimentary rock layers can be observed at a few places on the Earth’s surface.There are natural exposures, where erosion has worn away the soil on the surface. There are human-caused exposures, where hills are cut into for roads or buildings. However, these account for only a small portion of the sedimentary rock in the Earth’s crust. In order to learn about the sedimentary rock layers that we cannot see, geoscientists must drill into the surface and bring up core samples of the rock layers. Core drilling is done on land, through ice, or into the ocean bottom all over the world. Photographs like the one seen here of sedimentary rock core samples clearly show the layers.

Information from core samples, combined with that from other imaging techniques, allows geoscientists to make maps that show the depth and thickness of the sedimentary rock layers lying below the surface of the Earth. These subsurface geologic maps can be useful for many purposes, including the exploration for petroleum or natural-gas deposits.

Our Experiment

In this activity, you will prepare a three-layer model of a sedimentary rock formation and then figure out how to map the layers. Students should work in groups of at least two or three to design and make a model. Then the groups will switch models to see if they can map the layers accurately. It is important that all groups create their rock layers in secret to make the mapping a challenge to the other groups.

Tools and materials

• Transparent plastic box approximately 32 cm by 19 cm by 11 cm (12.5 in by 7.5 in by 4 in)
• Approximately 6 kg (13 lb) of dark sand and 3 kg (6.5 lb) of a lighter-colored sand, or approximately 3 kg each of three differently colored sands, in separate bowls.
• Two or three large paper or plastic cups
• Two or three wooden rods or sticks
• Marker
• At least 12 transparent plastic drinking straws
• Graph paper
• Ruler
• Water
• Paper towels
• Several pages of blank paper or lightweight cardboard
• Tape

What to do

With the marker, label the four sides of the box as  “north,” “east,” “south,” and “west.”
Add a few milliliters of water to each of the differently colored sands. Stir with wooden rods or sticks until all of the sand is moistened. Add more water only as necessary, to moisten all sand particles
Place a 2- to 3-cm-thick (0.75- to 1.2-in) layer of moistened dark sand in the transparent box. Then make the layer uneven by working its surface so that it is slanting, wavy—anything but perfectly flat. Press down on the layer gently to remove all pockets of air. Do not let the other groups see what your group is doing. Still out of view of the other groups, place a second and then a third uneven layer of moistened sand on top of the first one in the transparent box, alternating the sand colors. (If you have sand of three different colors, you can place the layers in any order.)
Use your paper or lightweight cardboard to make a tracing of each of the four sides of the transparent box so that the pattern of the layers is clearly shown. You can also take photographs of each side of the box. Do not show this information to the other groups.
Cover the sides of the transparent box with blank paper or lightweight cardboard, and tape, so that the pattern of the layers is not visible.
Exchange boxes with another group.   Now it’s up to your group to figure out the layer pattern of the box you received. Although this is only a model of the layers of the Earth, what do you think a geologist would do? You can use any of the materials furnished for this, but do not remove the material covering the sides of the box.
On a blank piece of paper, draw what you think the pattern of layers would look like for each of the four sides of the box.   When everyone is finished, compare your drawings with the drawings or photographs created by the current owner of your group’s box. How close were you?   Remove the material covering the sides of your box. Are the layer patterns consistent with those of the drawings or photographs?

	Digging Deeper Find out more about sedimentary rock layers and core samples