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USGS Geology in the Parks

Green Sand Beach, Hawai'i

Steep Slope and Colored Sand

Steep sand slope

When you've gotten down to the beach you might notice that parts of it aren't all green. In fact, the higher up the beach you are, the more the sand appears grey, rather than green.

The slope of the beach gives a clue as to why this is the case: further down the beach slope the sand has been more 'worked' by waves. It is closer to sea-level, and is washed by wave-action for a larger proportion of the tidal cycle. Therefore, the lower sand has has more of the black grains washed away. It has a lower black sand component and a higher proportion of olivines the further down the beach you go.

Another image of steep sand slopeTo the right is another image demonstrating the slope of the beach, seen here at the at the base of the cinder cone cliff. The green backpack is for scale and color reference. The bump in the sand slope is called a berm, and represents a previous tide height, either normal or a remnant from a storm (click here for more information on beach profiles). It is difficult to see from this view, but there are layers of the cinder cone which provide the majority of the olivines. These layers have a distinctly green tinge to them. Also visible are larger cinders that protrude from the ash layers - these, upon erosion from the wall, join the sand as clasts which are broken into sand grains by the waves.

 

Green sand up close


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