Gravel, sand, silt, and clay, collectively known as sediment, are produced by the mechanical and chemical breakdown of rocks. Once disaggregated from the original source rock, this material is then eroded and transported by either wind, water, or ice, often ending up at the deposits of rivers or lakes, or ultimately as sediment in the sea.
In STEM Club last week we had learned how to identify rocks and minerals. This week, we took it a step further and discussed how scientists use the characteristics of composition and texture to help hypothesize where a sand sample may have been collected.
The composition of sediment is largely dependent on the source material. For example, the sand around volcanic islands is often composed of volcanic rock fragments, volcanic glass (obsidian), and mineral (such as olivine) associated with volcanic rocks. Sediment found on beaches of southern California are largely composed of quartz and other minerals associated with igneous rocks, which form the bulk of our local mountain ranges.
In areas where there is no good source of sedimentary material from nearby mountains or volcanoes, the sand is often entirely composed of organic material (i.e. shell fragments, coral, etc). These sediments are called carbonates, because they are nearly entirely composed of calcium carbonate (CaCO3).
The texture of sediment is largely determined by the transportation process. The three important characteristics used to assess the texture of sediment are rounding, size, and sorting.
As material is transported, it is subject to abrasion and impact with other particles which tends to “round-off” the sharp edges or corners. A well rounded sand grain has probably traveled a great distance from its original source area, while angular grain has probably only been transported locally. However, large particles tend to round much faster than smaller particles.
The terms gravel, sand, silt, and clay carry with them a size connotation. Gravel is any material greater than 2 millimeters in its largest dimension. This includes boulders, cobbles, pebbles, and granules (in decreasing size order). Sand is any material between 2 mm and 0.06 mm in size.
We usually sub-divide this category into very coarse, coarse, medium, fine, and very fine. In practical terms, very fine sand is about the smallest grain size you still see with your naked eye. Silt is material which is finer than sand, but still feels gritty when rubbed on your teeth. Clay is the finest material of all, and pure clay will feel smooth on your teeth, and will form a sticky ball when wet.
As a general rule, material gets smaller the more it has been transported. Therefore very coarse material usually indicates a short distance of transport and vice versa.
The sorting of a sediment is simply how well the sedimentary material is separated out by size. For example, if all the grains in a sediment sample are very nearly the same size then we say the sample is “well- sorted”. If a sedimentary sample were to contain pieces gravel, as well as sand and silt, it would be a “poorly- sorted” sample.
Sorting is primarily effected by the agent of transport. Water is an excellent medium for sorting of particles by size and density. Wind is probably the best sorting mechanism of all, but of course, only operates on the finer grain sizes (not much gravel is moved by wind transport). Ice is the poorest sorting mechanism, transporting and depositing all sizes of sediment with equal ease.
Some time ago, I found a wonderful Sands PowerPoint but I have sadly been unable to locate it again. Please – if you are familiar with the original source, I would so much like to point my readers directly to the source and give proper credit.
The following image is a slide excerpted from the presentation. Three sand samples have been photographed under magnification from which the roundness of each sample can easily be identified. The graph below each sample illustrates how well the sedimentary material or sand sample was separated by size.
Sample 1 – This is a medium sized sand with a few sea shell fragments. The sorting of the sand is good. There is not a huge variation in the size of the sand. The graph shows a bell curve with a relatively equal distribution of small, medium, and larger size grains. This is a good example of a BEACH SAND.
Sample 2 – This is a classic example of a DUNE SAND. It is very well sorted and fine grained. Well sorted means that the size of the grains is not varied. Most of the grains of this dune sample are in one size group (96.7% are in the size .009 to .005).
Sample 3 – This is a very poorly sorted sand. There are coarse particles and fine grained particles. This, along with the fact that the sand particles are very angular are the indications that it is a RIVER SAND. In other words, it has not traveled far from its original source.
Hands-on Lab Activity
- At least 4 different sand samples collected from different locations; the greater the variety the better
- Metric ruler
- Microscope slides
- Sand gauge (optional)
- Sediment or sand sieve kit (optional)
In their science notebooks, I instructed the students to create a data table with the following 8 column headings:
- Sample Name or Number and Location
- % Dark Minerals
- Grain Size
- Organic Material
- Other (probable origin, sketches of grain shape, etc.)
I then encouraged them to examine at least 4 different sand samples from the sand collection provided. To guide their observations, students were asked to fill out the table as they went along. A microscope was also available so they could observe their samples under magnification.
Using the data on their charts and their microscope observations, students should have been able to identify the environment of deposition (beach, dune, or river) of the various sand samples.