Sand Density

Background

Sedimentary rocks are formed from the accumulation of layers of sand, silt, clay, or the hard-shelled bodies of water-dwelling organisms. Geologists are able to closely inspect sedimentary rocks to explain what type of environment produced them.

While many sedimentary deposits are formed by water, others are deposited by wind—especially sand dunes. If a layer of rock is entirely composed of sand with cross-bedding features, geologists can safely assume that the rock was initially deposited by wind. Many thousands of years from now, sandstone may exist where the Great Sand Dunes stand today.

Understanding how water sorts materials of different density is also important to geologists who study sedimentary rocks. Many sedimentary materials are deposited in rivers, lakes, or oceans. Since materials of lighter density can be suspended in slow-moving water longer than denser sediments, lighter materials can travel farther in flowing water than materials of greater density. Gold miners know this well. When panning for gold, miners know that gold—with its high density—will tend to stay at the bottom of their pans, while the less-dense materials can be sloshed off the top.

At Great Sand Dunes, the process in which Medano Creek sorts materials of different density can be seen by following the creekbed upstream, whether or not the stream is in high flow. Larger and larger rocks can be found, the farther one travels upstream. If one was to travel downstream to the area where the terminus of the creek is during peak flow years, very fine sediments and silts may be found laying atop earth, some of which is blown back toward the dunes.

Procedure

Divide students into small groups or pairs for this activity. Pass out a jar, lid, piece of paper, and a test tube to each group. Each student should have a pencil and paper to write on.

Use the shovel to dig sand and fill the jars about one-third full. When taking the sand sample, find an area that has a variety of sand sizes. Consider using the location that contained the greatest variety of sand sizes from the All Sand Grains, Great and Small activity. Fill the jars almost to the top with water and firmly screw on the tops.

Have the students shake their jars thoroughly or have them stir up the sand in the jar until all the sand particles are suspended in the water. After the sand and water are mixed up, set the jars down and allow them to settle. Have the students watch how the sand settles out of the solution. The denser materials should hit the bottom first. Smaller materials will stay afloat longer, because they are more affected by friction with the water. Also, notice what happens to the dense, black, and small-grained magnetite. Older students should write their observations down on paper. Have them record any correlations they may find between which type of sand settles out first and which settles out last.

Students can inspect the contents with a hand lens and make observations about the material firsthand. After students have seen the larger material at the bottom, discuss why they think it is there. Afterwards, return the sand its source. If you use sand from the dunes, remember that the sand should not be taken from the park.

Critical Thinking

1. Generally, smaller particles are found at the top after the sand settles. While magnetite is one of the smaller particles found in sand samples from Great Sand Dunes, it also settles quickly to the bottom of the tube. Why does magnetite settle to the bottom quickly?
2. Is there a difference in the color of materials from top to bottom?
3. How does this activity compare with All Sand Grains, Great and Small? What is the difference between density and size?

Extension

Have your students compare sedimentary materials in Medano Creek at the Picnic Area with a location further upstream. Hike upstream to look for differences in creekbed materials. The Day-Use Learning Area Map will help you find other appropriate locations for comparison.

Adapted from Educator's Guide to Great Sand Dunes, by Lori Cooper, Friends of the Dunes.