OBSIDIAN - Also known as volcanic glass. Most children recognize
obsidian as the rock that many Indians used to make arrowheads. The
Indians chose obsidian for the same reasons that a geologist can recognize
it. It is very hard, but more importantly it breaks into sharp edges that
easily cut through many materials. Note that broken obsidian looks like
broken glass. Obsidian occurs in almost any color, depending on what trace
elements are present in it. Black and brown obsidian are most common.
Obsidian is an amorphous solid; that, it is a solid rock composed of
silicon dioxide, but this material lacks crystalline structures. It is one
of very few exceptions to the rule that rocks are made of minerals.
The obsidian that is in your kit comes from volcanoes near Clear Lake,
California. Obsidian is formed when lava is cooled very quickly; it
freezes before crystals can form. Have your students try to determine
which part of a lava flow will cool quickly enough to form obsidian
(answer - the outer surface or "skin" of the flow).
PUMICE - Students will immediately notice that pumice is spongy
or "full of holes" or vesciular. This characteristic makes pumice extremely
lightweight; it even floats in water (you may wish to show this to your
students). It is commonly light gray to blackish-gray in color. It is
easily broken and has sharp edges. Like obsidian, pumice is volcanic
glass; it thus looks glassy (especially with a magnifying glass) and lacks
Pumice forms during eruptions of magma containing large quantities of
gasses, such as water vapor, sulfur dioxide, and carbon dioxide. The gas
"froths" the magma as it erupts, forming bubbles. This is
physically analogous to opening a soda can; carbon dioxide bubbles form in
the drink as the can is opened. Like obsidian, the magma then cools
quickly, preserving the bubble shapes. The gas often escapes, leaving
numerous holes in the pumice. Pumice is used as an ornamental building
stone. "Pumice rock" is also sold in beauty stores for cleaning
dead skin cells from areas like feet or elbows.
SCORIA - Scoria is composed of pieces of volcanic glass and preexisting
rock fragments that became incorporated into the magma as it erupted.
Many scoria have fragments of basalt included hence if can look like a red
vesicular basalt. The
volcanic glass looks similar to pumice but denser. Scoria is reddish in color, because
contains more iron than pumice. Scoria lacks large visible minerals; small
ones may be it visible with a magnifying glass. Scoria is often sold as
"lava rock" for use as a landscaping material.
GRANITE - Granite is composed of visible minerals, most commonly
quartz, mica and feldspar. Quartz looks clear and glassy, mica is black
and flaky, and the feldspars (commonly two or more different types are
present) are either pale pink/orange or white in color. The relatively
large size of the minerals indicates that the magma that formed the
granite cooled slowly. This took place deep inside the earth, not on the
surface, like pumice or scoria; it is a plutonic rock. Ask your students
if they think granite is made of the same minerals as basalt (no, they
cooled differently and came from a different "mother" magma). It
may help to have them imagine that the minerals in the granite were tiny;
would this make them dark?, (No, they would still be light colored). This
indicates that rocks composed of different minerals likely have different
magma "mothers." Try using the analogy that rocks are like
people, no two are the same! Granite is used as ornamental and building
CONGLOMERATE - Conglomerate consists of pebbles, gravel, sand,
and boulders that have been cemented together to make a solid rock. These
materials were mixed naturally in rivers or in some parts of oceans and
lakes. Any type of preexisting rock can become part of a conglomerate.
To explain cementation, try telling students that Mother Nature has a
cement that she sometimes pours onto the beaches of lakes, oceans, and
rivers. When it hardens, it becomes conglomerate, if the pieces are big,
or sandstone, if they are small. In reality, the two most common cementing
substances are natural solutions of calcium carbonate and silica dioxide.
Crystals of calcite and quartz, respectively, precipitate from these
solutions in the spaces between grains, cementing the rock together.
SANDSTONE - The gritty feel of the surface of sandstone hints
that this rock was once sand that has been cemented together. Sandstones
have quite varied compositions; some are composed entirely of quartz, and
others are mixtures of rocks, crystals and fossils. Almost any combination
is possible. Sandstones thus come in a wide array of colors. By
definition, the grains in a sandstone are "sand-sized"; most
students will recognize this if you demonstrate "sand size" by
showing them a bag of sand.
SHALE - Shale is composed of very small particles of mud, which
have been compacted and cemented together. Individual mud grains are very
small; they will rarely be visible. Shales are quite variable in color.
CHERT - Chert can range in color from white
to red to brown in color and is largely composed
of very small quartz crystals. The red color comes from trace amounts of
iron, and brownish tinges can be caused by the presence of organic matter. Chert is very hard. Chert was also used by Indians (the variety called
flint) for making tools. Chert forms from the skeletons of microscopic
one-celled protozoa called radiolarians. These are sometimes preserved in
the rock, but can only be seen with a microscope. Chert forms on the ocean
floor, where the skeletons of these organisms are deposited after they
COAL - Coal is a sedimentary rock composed of the
accumulation of vegetable matter that has been consolidated between other rock strata to form coal
seams. Coal has several forms depending on the effects of microbial action, pressure and heat over a considerable time period.
Anthracite is the most compressed and hard form; bituminous is not as hard;
and lignite still has vegetable matter present.
MARBLE - marble is composed exclusively of large commonly visible
crystals of calcite. The gray/white bands in some of the samples are due
to impurities within the calcite. Marble actually comes in a variety of
colors, including black, gray, white, and pink. Marble, like all rocks
that have calcite in them, fizz if you put a weak acid on it (usually 10%
solution of hydrochloric acid). Marble forms when a rock containing
calcite in it (such as limestone) was put under high temperature and
pressure conditions. Marble has been used throughout history because it is
easy to break and to carve. Some marble (especially in Italy) is noted for
its smooth, small crystals that make it excellent for statues. Many of the
statues of Michelangelo were made from marble. Marble is also used as an
ornamental building stone. If you live near or in a city, have your
students try to find buildings made of marble. If you are in an old
school, some of the bathroom stalls or floors may be made of marble.
SERPENTINITE - Serpentinite has a smooth, soapy feel, a green
mottled color, and a somewhat flaky texture. It is composed mainly of the
mineral serpentine. Serpentinite is so named because of its mottled color,
which resembles the back of a sea-serpent. The geologic origin of
serpentinite is still debated, but many scientists agree that it formed from
a rock like basalt that was put under high temperature and pressure.
Serpentinite is the state rock of California. Serpentinite is used for
carving and as an ornamental building stone.
SCHIST - Schist is composed of visible minerals, mostly micas.
Schists form under moderately high pressure conditions; this causes the
naturally platy mica crystals to line up, giving the rock a platy look. This
is a good example for illustrating the characteristic "squished"
look of metamorphic rocks to your students. Have them imagine that a heavy
Mother Nature sat on some rocks - look at what she did!