Black
sand can be seen as a layer on top of silica sand in regions with
high wave energy, on the flanks of volcanoes, and in areas where
most of the source rock is mafic, or dark-colored and poor in silica.
It can be composed of a number of different dark minerals - most
are iron-rich and heavier than silica sand. This weight enables
it to remain when high-energy waves wash the lighter sand grains
out into the surf zone. |
 Volcanoes
such as Hawai'i's Mauna Loa produce lava flows that come into contact
with the cold ocean water, as shown at right. The extreme difference
between the ocean temperature and the temperature of the flowing
lava cause the lava to fracture into tiny shards of black glass.
This glass is collected by waves into beaches. Lava flows also harden
and form layers like the cliffs seen in the image above. These are
eroded by wave action and are broken into grains and clasts which
are included in the black sand.
 Mafic
rocks such as basalts (like those produced on Mauna Loa), gabbro
(an intrusive igneous
or plutonic rock), and other dark-colored low-silica rocks will
erode down to components that are also dark. The serpentinized basalt
at left is a metamorphic rock - basalt that was pressurized and
subjected to high heat. The west coast has a great deal of this
type of rock, and much of the black sand found on beaches in that
region come from rocks like this one.
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Some dark-colored minerals, like magnetite,
are magnetic - take a magnet with you to the beach and run it through
the sand, magnetite will stick to the ends. You might want to cover
it with plastic first, the magnet will still work, and it's easier
to get clean. The dark minerals in beach sand at right, from Fort
Funston, (Golden Gate National Recreation Area, California) are primarily
magnetite and amphiboles, which are non-magnetic black minerals.
Both of these mineral types tend to fracture into very small grains
that collect on the surface of the sand, by virtue of being smaller
and, therefore, lighter. This is an ideal locale type to try out
a magnet. The other grain types in this particular image are primarily
multi-colored chert, a type of silica.
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