All Sand Grains, Great and Small

Background

Sand is defined by geologists to be any rock which is less than 2 mm in diameter and greater than 1/16 mm diameter. In a very small sense, sand grains are both great and small.

When the wind becomes strong enough and the vegetation becomes sparse enough, sand can be moved from one location to another through a process called saltation —the faster the wind, the greater size of sand grain can be moved. Saltation comes from the Latin word saltare, to jump. Through this process, sand bounces off of other particles and small ridges develop in the sand, called ripples.

It is thought by geologists that sand originally arrived at Great Sand Dunes from the drying up of ancient lakes and streams that once were found in the upper half of the San Luis Valley. Sand was carried to these ancient lakes by creeks and streams which originated from the metamorphic rocks of the Sangre de Cristo Mountains, as well as from volcanic rocks of the San Juan Mountains.

Explore the Great Sand Dunes’ web pages for an animation of the Great Sand Dunes’ formation, pages on geology and hydrology, and links to the latest research.

Not all the sand is the same color. Examine the sand by picking up a handful and viewing it through a hand lens. How many colors do you see? When viewed from a great distance, dark patches and streaks can be seen in the dunefield. These dark patches are fine-grained, iron-bearing sands known as magnetite. Magnetite mostly comes from the Sangre de Cristo Mountains. The mineral magnetite is strongly attracted to a magnet and has its own magnetic charge. Lightning on the surface of the earth causes the most highly charged magnetite. Magnetite is very dense, or heavy, because of its iron content. Use a magnet to sort the fine magnetite from a handful of sand.

Both wind and water naturally sort sand at Great Sand Dunes. Wind sorts the sand as it blows over dune crests. Heavy particles, like magnetite, remain close to the dune crest. The wind carries lighter particles farther away from the crest. Also, the mean grain size is larger on the upwind side of the dunes because the larger grains are not blown upslope and over the crest as easily as smaller grains. Since the dominant wind direction is from the southwest, on the large scale, sorting increases significantly from the southwest to northwest, (i.e., sand grains are more uniform in size with increasing distance downwind.) See the Sand Density activity to learn how water sorts sand grains.

Procedure

Divide students into groups of four types: base (#1), crest (#2), upper slipface (#3), lower slipface (#4). The students are to go to their assigned area of a dune and take sand samples to compare. Use the collection cups and have the students decide how the samples will be taken to make the best comparison between locations. One cup of sand for each group is adequate for this activity. Afterwards, return the sand samples to the dunes.

Follow the dune profile below for locations to take samples on a dune. To find sand with the best examples of sorting, choose a dune which is near the creekbed.

After the samples have been collected, provide one Sand Comparison Worksheet for each group on which to record their data. Have each group weigh their sample. Bring a scale that can accommodate for the weight of the sample size you intend to take. A 16oz/500g kitchen scale will work well for one cup samples.

Each group should now sort their sample using a sand sifter. Remind students to shake the sifter well in order to sort the grain sizes. Weigh the portion of the sample collected in each chamber of the sand sifter. Use the worksheet to compare the results. On the worksheet fill in the appropriate size range categories that correspond to the sand sifter that your students are using. Typical sand sifters contain mesh sizes that increase in size by one order of magnitude, from smallest at the bottom to largest at the top.

Finally, students should graph the results in the lower half of the worksheet.

Have each group present the shape of their bar chart to the class. Compare samples by collection location.

Critical Thinking

1. Which location had the most the large-grained sand?
2. Which location had the most the small-grained sand?
3. Where were the grain sizes most uniform?
4. Where did you find sand with the most varied sizes?

Extension

Compare the results from this activity with the results from the Sand Density activity.

The sorted sand from this activity may be used to create a sand filtration system. The Sand Filters activity helps students understand how capillary action filters water naturally.

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