The mighty rivers like the Colorado wear away the
rocks as water passes on it's way to the sea. Tiny bits of
decomposed rock are carried along in the water. When water reaches
a lake, it slows down and some of the sand particles are deposited to
make beaches. We start with this image of sand from Lake Powell to
illustrate some of the basic things you should look for when
investigating sand. First, the size of the grains is always
important. Put a metric ruler under your microscope to get an idea
of their size. The shape of the material is also important.
Is it smooth or rough? Third, can you identify the material that
makes up the sand? Is it made from minerals like quartz, feldspar
or mica or is it made from tiny bits of broken shells?
Some samples are best observed on a white
background and others on a black background. Try both to see which
Here we see the same sample, magnified more on a
black background. See how different it looks. Sand is a
byproduct of weathering. The particles in sand are usually very
small bits of something very large upstream, like a huge mountain.
But not all sand is deposited from rivers. On some beaches, wave
action on coral underwater deposits tiny bits of coral on the
beach. From a distance, the color of the sand is related to the
composition of the individual particles. Lots of quartz will
produce a shiny white beach. Lots of feldspar will make a more
orange colored beach. Common black minerals in sand are mica and
One winter day I found this expanse of black sand
on a beach north of San Diego. The waves had been big for a few
days and most of the lighter sand with quartz and feldspar had washed
out to sea leaving this heavy material behind. I took a sample
back to my lab for analysis. It was very pretty with many
interesting minerals. It was also quite heavy as compared to
regular sand. I wondered why it was so dense so I grabbed a magnet
off my refrigerator door and ran it through the sample.
black sand after magnet
Many of the black particles stuck to the
magnet. Those metallic particles are called magnetite. I
looked at what was left. There was still some black
minerals. My guess is that they are hornblende. The green
crystals could be olivine.
If you ever drive the road to Hana on the island
of Maui (Hawaii) be sure to stop at Waianapanapa Beach (3 miles before
Hana). It is a
gorgeous black sand beach. Unlike the sand above, you can see that
these particles are dull in color and likely made from something other
than magnetite or hornblende. The sand here is composed of basalt
(lava) which is weathered by waves and wind at the oceans edge.
At Waianapanapa you will also see many photogenic volcanic arches and sea stacks
oceans edge. This is a good clue as to why the sand on the beach
Some beaches aren't made of minerals at all but
rather tiny bits of broken shells. Notice the size scale
here. These particles are quite large. When you walk on a
beach like this you can feel the coarseness of the sand under your
feet. Because there is very little rain in Baja, there is very
little transport of material from the land. As one might guess,
the sand must be made from some material other than decomposed
rock. After time, wave action will smooth out the sharp edges of
the shells. In this picture you can see some smooth and some sharp
Another example of a beach made of shell
fragments. If you spend some time sifting through the sample you
should find some less eroded material. You could eventually start
to identify and classify each particle and make a hypothesis as to what
type of mollusk that particle originally came from.
Oahu, HI Sand
Like the two samples above, Hawaiian beach sand is primarily made up of the shells of micromolluscs (tiny
snails) and broken and worn pieces of larger shells and corals. On
beaches with intense wave action (like this North Shore sample), the
shell pieces are rounded by the energy of the waves. On more
protected Hawaiian beaches the sand is coarser and the particles are
more defined (compare to the Cabo sample above).
In French Polynesia there are no big rivers that
flow to the sea nor is there freezing of rock to break it up. Some
of the minerals (mostly volcanic) find there way to the beach but the
beaches in Tahiti are predominantly very white and made up of bits
of coral. The islands are mostly protected by barrier reefs so
there is little wave action. The sand particles are generally not
smooth like those of the North Shore above and from this sample you can
see why the beaches in Tahiti are so very white and soft.
You don't have to live near the beach to collect
sand. Take a trip to your local lumber company and go to the area
where bricks and concrete are sold. Likely you will find the big
bags of sand used for sandblasting or to mix with cement. Some is very fine and white,
almost pure quartz. Look around the floor. There will surely
be a spill from one of the bags. Take a sample. Hmmmm,
I don't think it would be shoplifting but never cut open a bag for a
sample! From our scale, notice how small these particles are.
Yes, it really is green sand. It is strange
to walk on a green beach and there are very few green sand beaches in
the world. One is on the island of Guam and another is found on
the big island of Hawaii (2 hour hike from South Point). The beach formed by the erosion and concentration of olivine crystals derived from
a surrounding lava cone. Olivine is a mineral commonly found in
basalt lava. The waves removed the lighter grains of sand leaving the denser olivine crystals behind to form the beach.
This is not sand because the particle size is too
small. It is called ash and is
presented here as a reference. Like dust, the particles are
extremely small but on close inspection (40X) we see very tiny bits of
quartz (Silica) and other minerals, all with very rough edges. It
was ejected from the Mt. St. Helens volcano on May 18, 1980. Can
you explain why the grains aren't smooth?
|If you are considering
purchasing a microscope to study sand, we recommend the zoom model 420
(10X-40X) or the model 446TBL-10 from
MicroscopeWorld.com (see "suppliers" at the top). The 446TBL-10 is a stereo microscope with two powers
10X and 30X. You might also choose a 446TBL-15 with 15X and
45X. Stereo is best as it gives a 3-D effect. If you are really getting excited
about sand then jump back on
the Micro-Bus and head over to the sand collectors information
page your resource to sand other sand collection websites.