Geology and National Parks
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Green Sand Beach, Hawai'i
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.
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.