BACKGROUND:
Students have learned that there are three different types of rocks, each
of which possesses different characteristics. However, rocks are difficult
to identify because there are great variations in their appearance and
composition. For example, the layers of sedimentary rocks are easily confused
with the squished bands of minerals in metamorphic rocks. Likewise,
fine-grained (volcanic) igneous rocks often resemble well-cemented
sedimentary rocks.
Some features, however, are consistently useful for rock
identification.
For example, rocks that have a gritty feel (like sand paper) are usually
sedimentary. Many igneous rocks show randomly oriented large minerals,
especially those that have been cooled very slowly. Rocks that look
"squished" are usually metamorphic.
The ambiguities in rock identification make the process fun. It is like a
mystery. If students bring you rocks, which you cannot identify, have them
ask several people until someone can identify it. The fun is in the
research!
PROCEDURE:
- Read the worksheet with the students so they understand the meaning of
each sentence. Have them then use the hints to try and match the rocks
to the questions. If available, have students use a hand lens to observe
the specimens. Encourage them to use other descriptive terms to help
identify their samples. The key objective is for students to develop
observational techniques that force them to think logically about
selecting a rock that fits each description.
- After students have worked through the lab sheet, go over each
specimen and make sure the students understand the correct answers.
- In conclusion, reinforce that there are only 3 major types of rocks
(igneous, sedimentary, and metamorphic), but that there are hundreds of
specific rock names. Each rock has its own characteristics. The students
will learn to recognize various rock types by going through the process
of formal identification of rock samples. Review how the different types
of rocks are formed.
IGNEOUS
OBSIDIAN - Also known as volcanic glass. 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.
PUMICE - Students will immediately notice that pumice is
spongy or "full of holes." 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 visible minerals.
SCORIA - Scoria is composed of volcanic glass and preexisting
rock fragments that became incorporated into the magma as it erupted.
The volcanic glass looks similar to pumice, but is reddish in color,
because it contains more iron than pumice. Scoria lacks large visible
minerals; small ones may be 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.
SEDIMENTARY
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.
MUDSTONE WITH FOSSIL SHELLS - Mudstone is a variety of shale
that is more massive. The samples in the kit contain marine fossils,
indicating that these rocks formed in the ocean.
METAMORPHIC
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.
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.
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.
- Please note that rocks with the same name can vary in appearance.
Geologists use other information besides appearance in order to identify
rocks. For example, mineral compositions are key in determining the
names of many rocks.
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