SAND DUNES

Sand dunes are a natural wonder with beauty and majesty all their own. However, when you know more about the dunes, they take on even greater significance. Once you learn about the complex, dynamic dune ecosystem, you will appreciate dunes even more.  Before we describe sand dunes, we need to know what sand is. Sand is considered any loose, granular material having grains which are 0.05 to 2.0 millimeters in diameter. The designation "sand" does not refer to how the material was formed or what it is made of. Sand can, and does, come from all three major rock types: igneous, metamorphic, and sedimentary. Sand can be primarily one mineral or material, or it can be a mixture.
    Sand dunes are mounds of windblown sand which vary greatly in size, from less than one meter to tens of meters high. The size depends upon the supply of sand. There is even greater variation in the area covered by dunes. Many of the more recognizable dune forms are ridges or complexes of mounds or crescents. The shape of individual dunes is equally variable. Shape relates to the direction and strength of the wind forming it, as well as to the amount of sand available.  The impressive sand dunes along the eastern shore of Lake Michigan were created by the prevailing westerly winds blowing the sand deposited along the beaches into the dune formations.
    Michigan is home to the largest dune system in the world, associated with a freshwater lake.  Our sand dunes are found not only along Lake Michigan, however, as the maps below show.

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Dunes in Michigan


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Dunes in the UP of Michigan


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Formation of Dunes
Sand dunes form in many areas. However, some requirements must be met. Required elements are: 1) a source of dry sand, 2) a means to sort and transport the sand, and 3) a land area on which to deposit the sand. Studies indicate that a supply of dry sand is the most important factor. A means of transporting and sorting (i.e., wind. water) is not rare or out of the ordinary. Running water and normal winds can do this. Strong winds or water action, cannot initiate dune formation, however, without a supply of sand. As mentioned, the size and shape of a dune depend on the wind and amount of sand available. The amount of sand being equal, wind conditions have a strong influence on the shape of a dune field. Constant winds tend to form more regularly shaped dunes or even long ridges. Irregular or changing wind directions result in irregular-shaped dunes.
    The sand making up Michigan’s dunes is almost all one mineral-quartz. Because quartz grains can form from a number of different rocks, the original source of the quartz dune sand cannot be singled out. It should be noted that relatively few bedrock outcrops occur in Michigan. Instead, glacial drift almost completely covers the state. Therefore, one can safely assume that sand for the dunes comes initially from glacial deposits. Yet, much of the sand found in drift is mixed with a variety of other particles, from clay to cobble size. The drift must therefore be eroded, transported, sorted and redeposited by water---the running water of a river or the pounding surf of a beach–before it is sorted enough to be a uniform "sand". Water tends to concentrate sand-sized particles in particular areas.
    Continued erosion and transportation of sand replenishes the sand blown into the dune. As long as more sand is supplied, the dunes will continue to grow. When the supply diminishes, the dunes are likely to stabilize, or at least stop growing.
    The size range of sand grains that make up dunes is remarkably uniform because grains that are smaller than sand-size (silt or clay) are carried farther away by the action of wind and water. Grains or stones that are larger than sand-size (pebbles, cobbles, boulders, and the like) are left behind.


Many of our state parks, including Warren Dunes and Lakeport State Parks, are associated with sand dunes.  And of course, Sleeping Bear National Lakeshore is a major attraction, with its fantastic beaches and perched dunes.  All along the east coast of Lake Michigan dunes make up a strikingly large part of the landscape.   This image, from near Holland, shows the beautiful dunes and the beaches that adjoin them. 
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WHERE DO WE FIND SAND DUNES?
In Michigan, sand dunes are found along much of the shoreline of the Great Lakes. The best known and most studied Michigan sand dune areas are along Lake Michigan. Here, the supply of dry sorted sand and a prevailing wind direction toward land lead to dune development. However, close inspection shows that some of the dunes or dune-like structures are not close to today’s lake shoreline. These inland dunes reflect previous dune-forming conditions. These dunes are a result of massive ice sheets which covered Michigan, and much of North America, during the Pleistocene Epoch. Glaciers transported sand and other materials and deposited them as glacial drift. Large masses of glacial ice gouged out the basins that now confine the Great Lakes.
    However, the glaciers did not simply advance and retreat. Instead, a complex sequence of advances and retreats occurred. These fluctuations had great impact on the landforms. At times drift was deposited, forming barriers which ponded meltwater in large lakes. When the barriers were eroded or otherwise breached, lake levels changed. When the water level dropped, sand deposits could dry out and then be blown into dunes. Therefore, one can find dunes along the ancient lake shores where sand accumulated. Yet, the supply of sand, water and wind varied greatly from area to area. The distribution of dune landforms usually implies the existence of an old beach. However, the absence of dunes does not necessarily mean a beach did not exist.
    In some places in Michigan, dunes formed on top of the glacial drift. These dunes are called perched dunes. As the water level changed and migrated, so did the dunes. If the shoreline retreated, the dunes were more likely to stabilize. And, if conditions were proper, new dunes formed along the new shoreline. The older dunes, formed at a higher lake level, remained on the glacial drift. If the shoreline later advanced with a rising lake level, the older stable beach lines and bluffs were eroded.
    When you think of sand dunes, you commonly associate them with the desert. If Michigan is not a desert, then why do we find dunes here? The answer lies in the Great Lakes, primarily Lakes Michigan, Superior and Huron. All along the shores of these beautiful lakes, wave action sorts the sediments in the near-shore area. As the waves pound the beach, much of the finer-than-sand-sized materials are carried out into deeper water while the sand-sized grains and larger particles and pebbles are moved nearer to the beach. During storms, large quantities of sand are moved past the beach. Eventually these piles dry out. Until the sand dries, water between the grains holds the sand together. The adhesive quality of the water makes the sand temporarily immobile. The adhesion does not remain after the sand dries. The dry sand can then be transported and winnowed by the wind. The wind carries the sand inland, where it is deposited as a dune.
    In Michigan the supply of sand is not constant. So, in time, the dunes become covered with grasses and other forms of vegetation-even trees. When vegetative cover prevents the wind from moving the sand, the dunes are stabilized (as below).
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    However, if the protective vegetation is removed, or if there are exceptionally high winds or the groundwater level drops, the sand is exposed to wind erosion, and movement or migration begins again. The area where migration begins is called a blowout. Renewed dune movement can bury anything in its path, even the forests which once may have stabilized the dune.

There are three major types of dunes in Michigan: inland dunes, coastal dunes, and perched coastal dunes.  Many people think of dunes as a kind of desert, and dunes do form in desert areas. However, Michigan’s Great Lakes shoreline is not even remotely related to a desert environment. In fact, the shoreline receives large amounts of rainfall. The rainfall and relatively high water table help to stabilize the dunes. Some plants common in dunes may be rare or absent elsewhere, particularly in dryer upland areas. Furthermore, there is a great diversity of animal life in dune landscapes.

Fulgurites
One interesting feature of sand dunes is fulgerites--tubes of fused sand, formed when lightning hits the sand. The tremendous heat and energy released when lightning strikes the sand welds the grains together as glass. (Temperatures exceed 1700C). Fulgurites are rather fragile and hard to find. They are easily confused with tubes formed around the roots of plants growing on the dunes. An easy test will distinguish a fulgerite from a root cast. Place the specimen in a container of vinegar. If it fizzes and falls apart, it was a root cast. The cementing material was dissolved by the slightly acid vinegar. However, if the specimen is a fulgurite, it will not be affected to any great extent by the vinegar.

Uses of dune sand
Dune sand has a number of uses. The size, uniformity, chemical purity and nature of dune sand make it a singular resource. The demand for this resource is increasing, because of an increase in the number of products using dune sand and because other new uses have been developed.  The major use of dune sand is in foundries. The sand is used to make molds and cores. The molds are used to form metal into a variety of shapes. Core sand fills the spaces where metal is not needed or wanted. Silica (quartz) sand is also used in glass manufacture. Quartz comprises 50-65% of the mix used to make glass. Railroads use large amounts of sand to improve traction on wet or slippery rails. Sand is used in sandblasting. The floors of some large open hearth furnaces are lined with silica sand. Sand is an excellent filter for removing sediment and bacteria from water.  The petroleum industry also uses sand. Under pressure, sand is injected into an oil well. The sand goes into the cracks and crevices where the oil is trapped. When the pressure is released, the sand helps hold the cracks open. When this process works, oil production can be increased greatly. The process is called sand-fracturing.  Finally, one area of increased use of sand is in fiberglass manufacturing.

For information on sand dune mining, go here.

This material has been compiled for educational use only, and may not be reproduced without permission.  One copy may be printed for personal use.  Please contact Randall Schaetzl (soils@msu.edu) for more information or permissions.