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Why is there sand on beaches?

14 August 2013
Pigeon_Point_beach (2)

The beautiful beach at Pigeon Point, Tobago. This would make a great wallpaper too!
Courtesy: Wikimedia Commons

Our earth is a beautiful place. One of the things which have always bugged me is the presence of sand on beaches. So to start my article, I will firstly explain how beaches are formed.

Beach formation

Beaches are simply accumulations of unconsolidated sand or gravel that extend from mean low tide to the uppermost extent of wave impact. In simple words, the beach “material” accumulates on the beach during long periods of accretion or moves away from the beach during periods of erosion. Beaches are never stationary; they keep moving. They can be hundreds of kilometres long or very short. Trivial fact: The term “coast” is a general term referring to the junction of land and water, but a beach is made of specific materials which I will explain below.

Why only sand and not something else?

When the sea erodes the cliffs, large rocks fall away from the cliffs and into the sea. These rocks are tossed about by the sea and the edges become eroded, creating smaller, smoother rocks. They are then knocked around about and eroded further until large pebbles are formed. These turn into smaller pebbles which are eventually ground together by the sea to create sand.
An important natural process in the formation of the shore is longshore drift. Not all waves approach parallel to the beach but instead break obliquely. These waves carry sand and pebbles obliquely up the beach, with the backwash runs directly down the beach. This results in the sand and pebbles being gradually moved sideways along the shore.

What is in beach sand?Scala_dei_Turchi_Sand_062313 (1)

Most beach materials are the product of weathering and erosion. Most sand is made of quartz because it’s very stabled – other minerals weather into clays, dust, or dissolve completely. Quartz is both highly resistant to weathering and hard enough to resist being ground into dust. (It still does eventually, known as silt).

Most of the world’s beaches are made of quartz-rich sand. The land masses and mountains are composed of rocks that are in turn themselves composed of many common materials, such as quartz, feldspar, pyroxenes amphiboles and mica. During the weathering process ground waters and dissolved carbon dioxide react with the rocks to break them down.

Difference between beach sand and desert sand

Frankly, it all depends on the location. Beach sand is likely to be better sorted and more rounded grains than some desert sand. However, there are many exceptions. Beach sand, as I wrote above, may have a higher percentage of quartz than some desert sand. Other beach sand may actually be carbonate “sand” or volcanic ash (like the “black sands” of Hawaii on the Punaluu Black Sand Beach) and not typical siliciclastic sand.

Most of the beaches have their sand “supplied” by rivers and longshore drift. This sand is more “mature” that is the weathering processes have removed feldspars, clays, and other types of rock fragments leaving most quartz grains (since it is quite stable and resists weathering conditions).

Whereas, deserts sand is closer to its provenance, that is the rock it weathered from. That makes it more likely to have rough grains, and still have feldspars and rock fragments in it besides quartz. Since there isn’t any water washing up on shore in the desert, the sand usually arrives by the wind, instead. However, an exception like The Sahara, whose sand comes from older sandstones that were deposited by beaches and rivers.

Another type of wide-spread exception, if you will, known as aeolian sands are actually better sorted, and more rounded than most beach sands. These wind-blown sands actually are the most well-rounded examples of sands found, in many cases. They tend to be very well sorted with all the grains being the same size and made of pure quartz sand as they have accumulated by wind moving the sand over long distances and over many years.

Sources of beach sand

There are many sources of sand on beaches including the continental shelf. Longshore transport is just one of several sources of beach sand, bringing sand from updrift beaches, sand bars, or tidal deltas. Low bluffs or cutbanks can form the shores of some estuaries and also provide sand to their narrow beaches. Dunes along barrier islands supply a lot of sand to the beaches. This occurs when strong winds blow sand in an offshore direction or when storms erode and cut back the dunes. Rivers can also furnish sediment to beaches and are the major source of beach material in California. Dams on rivers have a huge impact on California beaches and lead to increased erosion. The shells on a beach also supply sediment as they are broken up by the waves or by scavenging organisms such as skates or rays (or beach buggies). Coral reefs are also a major source of sand.

References & Resources

  1. Beach Basics, Coastal Care, Santa Aguila Foundation. Retrieved from:
    http://coastalcare.org/educate/beach-basics/
  2. Coastlines – beach formation (CLIP 8433), BBC Learning Zone Class Clips. Retrieved from:
    http://www.bbc.co.uk/learningzone/clips/coastlines-beach-formation/8433.html
  3. Beach Formation, Penzance. Retrieved from:
    http://www.pznow.co.uk/marine/beachformation.html
  4. beach – National Geographic Education. Retrieved from:
    http://education.nationalgeographic.co.uk/education/encyclopedia/beach/?ar_a=1
  5. Glenn, C. R., Beach Sand, Ask-An-Earth-Scientist, Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawai’i. Retrieved from:
    http://www.soest.hawaii.edu/GG/ASK/beach_sand.html
  6. Yahoo! Answers
  7. Morelock, J., Beach System, Department of Geology, Universidad de Puerto Rico. Retrieved from:
    http://geology.uprm.edu/Morelock/beachsys.htm
  8. All images via Wikimedia Commons.

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Atul Anand Sinha - post author

I love understanding and writing about the universe, especially about its physical nature and its relation to mathematics. I also am a recreational programmer.

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