Learning from Sand (background reading)

by Linda Maston-McMurray, GL teacher, San Antonio, Texas

What can you learn from your sand?

- In studying sand, the first thing one can do is decide whether the sand is made of fragments of once-living things (biogenic) or made of nonliving material (abiogenic).

- Next, one can determine whether the sand came from an active continental edge or a passive continental margin by the sand's composition. Sands from active volcanic regions will have a higher percentage of dark minerals, giving them a salt-and-pepper type appearance. Sands from passive regions may include some dark minerals, but the percentage will be low. They are essentially light-colored. You may find sands from areas that were active in the past. In such cases, metamorphic rocks have eroded down and formed sands that appear to be from an active edge, but in fact are not. Indeed, the margin ceased activity many millions of years ago! See if you can determine why "passive" sand can look "active."

- Continents are basically composed of granite, and regardless of where the sand was formed, it will reflect these granite origins, unless it is biogenic. Granite is light colored and diverse ranging from grays to pinks.

- Oceanic crust is dark and basaltic. Therefore, sand formed in places like Iceland, Hawaii, and Malaysia consists primarily of dark, heavy minerals. Those sands are often high in basalt, olivine, or magnetite.

- Sands can be told apart by the degree to which they have been sorted. Sands formed along beaches tend to be well-sorted (with uniform grain size) and well-rounded (no rough edges). Sands formed in deserts, lakes, and rivers may at first glance appear the same, but they are usually poorly sorted. Shake a small sample of such sand in a clear container and look underneath it. You will see that it separates by grain size, and that the granules have many rough edges. "Roundness" does not mean that the grain is spherical. It means that, regardless of shape, the grain has no (or very few) rough edges

Faster running water generally can carry larger pieces of rock, so there is less likelihood of finding only very fine grains in a swift river. The same principle applies to beaches. Winter storms provide more energy to the water, so beach sand is coarser in winter than in summer.

You can also determine the approximate slope of a beach by looking at the sand. Beaches made of coarser sand have steeper slopes than beaches made of fine grain sands. You can get a sense of this by dumping a bucket of dry sand onto a flat surface and measuring the angle of the slope that it forms. This also changes seasonally; winter beaches will be just a bit steeper than summer beaches.

This piece was contributed by Global Lab teacher Linda Maston-McMurry, formerly in San Antonio, Texas.