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Lecture

Life’s a Beach—so What is a Beach?

Our first though when we hear “beach” is an area of broad tan-colored sand, as this is what we see on Gulf coast beaches, Atlantic coast beaches, and Los Angeles area beaches.  However, as the textbook states, a beach is simply an area where sediment is in transit - and that sediment can be of many compositions, colors and textures.  Later in this module we'll discover some beaches have no exposed sand at all, as the ocean meets the coast at a rocky cliff or mud flat. 

To keep in mind that all beaches are not created equally, it helps to remember:

Black Sand
A black sand beach at Kamoamoa, Hawaii.  Here, the beach sand is derived from erosion of the igneous rock basalt, which contains dark-colored minerals.  Basalt is the dominant rock-type in the Hawaii island chain.  The basalt comes from lava flows originating from the Hawaii volcanoes National Park—see the lava flow into the beach (steam is visible) west of this photo location in Google Maps link (U.S. Geological Survey).

When beaches do contain sand sized sediment, the sediment will move as outlined in section 10.2 of your textbook. 

The longshore current is a very important part of the beach environment. The longshore current is not very strong in the Gulf of Mexico, but on the Atlantic or Pacific Coast it is strong enough to drag anyone playing in the ocean.  If you are standing in the ocean directly in front of where you plopped down your beach umbrella, over the course of an hour or two the longshore current slowly pushes your body down the beach---several hundred feet—as each wave slowly nudges you along. 

While the backwash-swash affect pushes sediment onto or away from the beach, the longshore current moves the sediment parallel to the beach. This process of sediment transport is called longshore drift and moves in the same direction regionally (over a year). In the eastern U.S. the drift moves south as shown in Figure 10.15.

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