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Of all of the different sizes of earth materials, perhaps the most fascinating is sand. Sand remains vital to our culture and economy as it has through time. Michael Welland’s fascinating book “Sand” (2009) devotes a chapter just to a brief review of the uses of various sands from aggregates, abrasives and construction material, to electronics, foundry sands, glass, jewelry, porcelain, and ores for some elements. Sand is the stuff of sandstones also important as reservoirs for water, petroleum and natural gas, as well as building material and industrial uses. Sand is an essential component of soils, important to soil properties such as porosity and permeability; think viniculture for example. Sand art goes back to ancient times, and sand in the hour glass was once the common measurer of time. What would an adventure movie be without quicksand? And there’s sand on Mars!
Here on earth, sand dominates specific landforms: think beaches, dunes, sand bars and stream beds. And every sand grain in those landforms has a history – an origin when its parent rock formed, weathering and release into a medium of transport (glaciers, rivers, wind, waves), a journey to its current resting place that may have had many episodes of re-erosion and reworking in several different environments through time. And the science of sand seeks to define the material, reconstruct its history, and in some cases to exploit the material as a commercial resource.
Sand then is common geologic material, easily collected, and brought into the class room for hands-on experiences for K-12 students and teaching integrated science. Sand is both a product and a reflection of earth materials and natural processes that typically take place at interfaces within and between Earth Systems. So if you can’t take your students on a field trip, bring the field to the students. Also encourage them to collect sand samples for their classroom collection, and continue to increase a diverse set of sand samples for future study. Sandy beaches are a good starting point as they form at the atmosphere/hydrosphere/lithosphere triple point, where wind driven waves act on shoreline sediments. And sand dunes form as wind energy is expended on land sediments. Sand and the properties of sand grains reflect these environments of deposition as well as those of the sands’ origin, weathering, transport and deposition.
Composition of the sand grains tells us of their ultimate origin and source (provenance). To study sand is to apply principles of geology, biology, chemistry and physics utilizing math, particularly statistics.
The need for definition and classification of materials is illustrated by sand. Even small children know sand from a sand-box, and they would know that a box of gravel or clay is not the same. If an adult ordered sand for landscaping and the delivery truck dumped a load of clay, they would know it was not the right sized material without looking up a formal definition of sand (gravel or clay wouldn’t do). And most students know these differences, but a formal definition illustrates the need for precision in communication, as well as for a size classification.
Specifically, sand is material having grains with diameters ranging from 0.0625 to 2.00 mm in size. This general sand-size category is subdivided into Very Fine (0.0625 to 0.125 mm), Fine (0.125 to 0.25 mm), Medium (0.25 to 0.50 mm), Coarse (0.50 to 1.00 mm), and Very Coarse (1.00 to 2.00 mm). Virtually all sands in natural environments are mixtures of these different sizes as well as with coarser (gravel sizes) and finer (silt and clay sizes) sediments. In part, particle size reflects the energy conditions of the depositional environment. The higher the energy, the coarser the sediment, so high wave-energy beaches usually have some gravel-sized material present (granules, pebbles, cobbles). In contrast, wind-blown sand dunes consist of well-sorted finer sand, free of both coarser gravel and finer clay (Fig. 1). As a result of the mixing of sizes, sands also are described by their textural characteristics which are expressed statistically, such as median and mean grain size, and degree of sorting (e.g., standard deviation, skewness and kurtosis). Textural characteristics also include grain shape (roundness/angularity, sphericity), and the nature of the individual grain surfaces (e.g., fracture, polish, staining).
Figure 1. Sand from Good Harbor Beach, Michigan. The sand on the left is beach sand, and the sand on the right is from the adjacent dune. You have to look close to see that the dune sand is dominated by slightly smaller (finer) grains, and under a magnifying glass or microscope it would be more apparent that the dune sand is better sorted, and more grains are angular. In both samples the majority of the grains are quartz, but their tan coloring is due to a slight iron-oxide staining on the grains. (Photo from Neal and Wilson, 2014)
Sands also are classified genetically on the basis of a combination of their mineral composition and original source of origin. Terrigenous sands are derived from the land; typically from crystalline igneous and metamorphic rocks, although the grains may have been recycled many times and included in sedimentary rocks (e.g., conglomerates and sandstones). Crystalline rocks such as granite and gneiss are composed of many different minerals, but feldspars are most common, followed by quartz. In the long history of a grain’s weathering and transport, many minerals break down into silt and clay. Feldspars, for example, are the most common rock forming minerals, but they break down into clays and silts due to both chemical and mechanical weathering. In contrast, quartz is very resistant to weathering and abrasion, so it persists as sand-sized material and often is the dominant mineral in beach and dune sands (Fig. 2).
Figure 2. Sand from Cocoa Beach, Florida. Clear, colorless quartz is the dominant grain type with a few dark grains of heavy minerals and sand-sized rock fragments. Note the larger shell fragment (clam) near the center of the photo. The dominance of the clear, unstained quartz accounts for the light color of SE U. S. beaches, and reflects the long abrasion history of these sands in order for the resistant quartz to become concentrated. (Photo by Mark Luttenton)
In addition, other minerals that occur in small amounts in the source rocks also are resistant (e.g., zircon, rutile, tourmaline) and may be found in very low percentages in sands. These latter minerals have high specific gravities, and are grouped together as heavy minerals. Sometimes these heavy minerals are preferentially sorted from the more abundant light minerals, and concentrated as placer deposits in steams, and especially by storm waves on the back of terrigenous sand beaches. These placers are the black or reddish brown patches of sand seen on some Great Lakes beaches. The dominant black mineral is usually magnetite (iron oxide) and can be separated from the other minerals with a magnet (Fig. 3), while the reddish-brown sands have concentrations of garnet. There are over 30 different minerals and mineral varieties in the heavy minerals found on Great Lake’s beaches including magnetite, various garnets, amphiboles, pyroxenes, epidotes, zircon, rutile, and tourmaline. Although small in total percent of beach and dune sand (usually less than 1 %), these heavy minerals are important in determining the kind of inland source rocks that gave rise to the sands. In contrast, quartz is so common in various rocks that it usually is not a good indicator of specific source rocks.
Figure 3. Sample from a concentrated area of heavy minerals in a Lake Michigan beach sand. Although there is an abundance of clear to slightly stained quartz grains, there is a high concentration of black grains (magnetite) and lots of salmon pink grains (garnet). Other heavy minerals are present, but in trace amounts, and a microscope or magnifying lens would be needed to identify them. Note the bar scale which is 0.5 mm. (Photo by Mark Luttenton)
Some terrigenous grains are sand-sized rock fragments. One needs a good magnifier or microscope to determine such small rock fragments, but often they are dark in color, derived from fine-grained igneous or metamorphic rocks such as basalt or slate.
Figure 4. Coarse beach sand from eastern Lake Superior’s Batchawana Bay, Ontario, provides a good example of sand-sized rock fragments. Look closely at individual grains of sand and you will see that many are composed of still finer particles. Most of these rock fragments are from igneous and metamorphic rocks, and are angular, suggesting they have not had a long abrasion history.
A second common category is carbonate sand, composed of the calcium carbonate minerals, aragonite or calcite. Generally these grains are more common in ocean beaches and shoals, and originate from the breakdown of the skeletal remains of marine organisms, but they can also originate from chemical precipitation (e.g., ooids, Fig. 5). Specific grain types are clues as to the offshore sources of the sediment, such as reefs, or shoals. Shells of clams and snails and grains derived from their breakdown do occur in fresh-water sand deposits as in Great Lakes beaches and river sand bars. Since the introduction of zebra and quagga mussels into the Great Lakes such carbonate grains are more common.
Figure 5. Carbonate sand from the Bahama Banks composed almost entirely ooids, all of which are very well rounded, and have a polished surface. Many of them are nearly spherical in shape. Note the bar scale which is 1 mm., indicating that there is a range of sand sizes in the sample. (Photo by Mark Luttenton)
A third type of sand is volcanic, derived directly from volcanic activity. Our inland lakes and rivers lack such sands, however, some of the islands of the Caribbean and Mediterranean, and other active volcanic areas, like Hawaii, have volcanic sand beaches. Sometimes gravel-sized pieces of floating pumice are known to have been carried for long distances by ocean currents, and can arrive on non-volcanic beaches. Volcanic sands are usually very dark in color, typically sand-sized rock fragments of basalt, or red if that is the color of the volcanic pyroclastic material. However, the famous Hawaii green sand is composed of grains of the mineral olivine derived from basalts (Fig. 6).
Figure 6. The famous Hawaii green sand beach is dominated by Olivine grains although white carbonate shell fragments (reflectant grains) indicate some of the sediment is derived from offshore. (Photo by Mark Luttenton)
In most cases the type of sand and its composition is the determinant of the color of the beach (Griggs, 2015). In addition, high-energy environments are characterized by gravel deposits, and, like sand grains, pebbles and cobbles can reveal much about the composition of the source rocks. In addition, pebble shape may reveal the processes of abrasion to which the rock has been subjected such as long term rolling in the surf or on the beach. Great Lakes gravel beaches provide good collecting to build demonstration sets of pebbles that show the range of shapes and different compositions. Sand, however, is easier to sample in terms of less bulk and the convenience of small samples needed for classroom use. Usually you can find significant variations in sands at a single beach locality (Fig. 7).
Figure 7. Good Harbor Bay Beach, Michigan. This shoreline view shows a good locality for taking a set of sand samples for contrasting sand compositions and size characteristics (site of Fig. 1 samples). In the far background is a higher, older sand dune, while the area of green dune grass is an active area of wind-transported sand accumulation. The inner dune formed during an earlier low-stand, and the last rise in lake level eroded that dune which was fronted by a beach. When lake level again went down, the beach widened and the lower dune formed. Typically in the summer months the inner, near-shore sand bar welds onto the beach as seen here, forming the trough and widening the beach. This beach front will likely be eroded by high-energy storm waves in the fall through winter and the inner bar will reform. Note the heavy-mineral black sand placers (evidence of higher energy waves at different levels on the beach), and the concentrations of light-colored shell material in the foreground (selective sorting due to the different specific gravity of the grains by size and composition0. (Photo from Neal and Wilson, 2014)
There are a multitude of websites with recommendations for using sand for instructional purposes, and aimed at various will age groups (e.g., Clary and Wandersee, 2011), including college-level students (e.g., Videtich and Neal, 2012). We recommend exploring such sites, and then design your own exercises around a single bulk sand sample and/or a set of varied samples (e.g., a range of samples from different beaches and or environments [beaches, dunes, streams, soil]) to describe, compare and contrast. Samples can be “knowns” where you give the students location/environment information or “unknowns” where the students try to determine where the sand came from in terms of depositional environment or source. You can get by with little equipment (e.g., a standard binocular microscope or good magnifying glass; magnet; wide-mouth, capped, clear bottles; graph paper).
Begin with description:
Pass a magnet over the sample. Are there magnetic minerals present?
Compare and Contrast:
Interpreting the results from the sample descriptions is the fun part of sand studies, whether it’s a single sample, or a set of samples for comparison.
Other activities might include demonstrations of size sorting based on settling rates (e.g., using a water-filled clear bottle with a mix of grain sizes from granules to clay as well as some black heavy minerals, suspend the mixture and observe the settling sequence by size and composition of grains). Or, use black magnetite sands for demonstrations of magnetism.
Build a Sand Sample Collection:
As you develop your own set of class-room activities using sand, encourage your students to bring in sand samples from their home region, and travels. Emphasize that they use a scientific approach by labeling the sample location and recording information such as the environment sampled, and conditions in that environment (e.g., beach face with wave energy; protected back side of sand dune; sand bar in creek), and photographing the sample area. Sand-collector organizations can be found on the internet, and sample exchanges are a good way to build a collection that includes places you will never visit.
Clary, Renee, and Wandersee, James, 2011, A Scientific World in a Grain of Sand: Investigating Local Sand Samples on a Shoestring Budget: The Science Teacher, p. 29-33. www.scienceofsand.info/sand/pdfs/NSTA_sandarticle.pdf
Griggs, Gary, 2015, The Colors of Beach Sand: Coastal Care Beach-of-the-Month, Feb. 2015. http://coastalcare.org/2015/02/the-colors-of-beach-sand-by-gary-griggs/
Neal, W.J., and Wilson, G.C., Beaches of Sleeping Bear Dunes National Lakeshore, Michigan: Coastal Care Beach-of-the-Month, Oct. 2014. http://coastalcare.org/2014/10/beaches-of-sleeping-bear-dunes-national-lakeshore-michigan-by-william-j-neal-gregory-c-wilson/
Welland, Michael, 2009, SAND The Never Ending Story: Univ. of California Press, Berkeley, CA, 343p.
Videtich, P.E., and Neal, W.J., 2012, Using Sieving and Unknown Sand Samples for a Sedimentation-Stratigraphy Class Project with Linkage to Introductory Courses: Journal of Geoscience Education, v. 60, no. 4, p. 311-324. http://nagt-jge.org/doi/pdf/10.5408/11-279.1