Background Essay: Coastal Geological Materials

Coastal erosion occurs in areas of high energy — where wave action wears down rock and other hard material into smaller and smaller pieces. Coastal deposition, however, happens in environments where most of the energy from waves has been dissipated before it reaches the shore. This allows sand or other rock and mineral fragments suspended in the water to settle and contribute to the development of the coastline. Perhaps the most familiar depositional features of coastal environments are beaches.

Beaches are sloping bands of accumulated sediment that form at the boundary where land and sea meet. Beaches contain particles of different colors and sizes, characteristics that are largely determined by the nature of the parent, or source, material. For instance, sand — in technical terms, sediment with particle grain size of 0.0625 to 2 mm (0.0025 to 0.0787 in.) — is created by the weathering of a variety of rocks and minerals. Quartz sand may be white, pink, or yellow, while sand originating from basalt lava is gray or black.

The process by which sediments are transported by wind, waves, and currents and distributed according to grain size is called sorting. Beaches with well-sorted sediments contain particles of uniform size and shape. By contrast, beaches with poorly sorted sediments — often originally formed by glacial melt — contain particles of varying sizes and shapes.

Although many kinds of beaches exist, five beach types are most common:

• White sand beaches — White sand is the most common type of beach material. It is composed primarily of quartz, a very hard mineral derived from igneous and sedimentary rocks.
• Black sand beaches — When basalt lava cools in seawater, it is quickly weathered. Waves then transport and deposit the dark mineral grains on nearby shores.
• Shell and coral beaches — Shell fragments and eroded coral make up the white and pink beaches often found in tropical climates. Fish bones and shark teeth may also be among the calcium-rich sediments.
• Mud flats and marshes — Composed of silt- and clay-sized rock particles, muddy beaches are rarer than sandy beaches largely because low-energy conditions are required for these extremely fine-grained sediments to settle.
• Gravel beaches — Gravel beaches are found in high-energy coastal environments. Only strong ocean waves are capable of moving these coarse-grained rock fragments.

To learn more about erosion and other processes that shape coastlines, check out Coastal Geological Processes.

To learn more about coastal environments characterized by beaches, check out Sandy Coasts.

To learn more about erosion and weathering in other environments, check out Erosion and Weathering.