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Identifying Rocks
The following was taken from the 1980 version of Holt's
Elementary Science (the information is still useful). I got the pictures
from various of the websites found in my Earth
Science Rocks and Minerals Links section.
Knowing what minerals are in a rock provides us with
just one clue to the rock's identity. When trying to identify most igneous,
sedimentary, and metamorphic rocks, other or additional clues are needed.
Let's find out what these clues are.
Many igneous rocks can be identified by the position
of their minerals. The minerals usually
fit together like pieces of a jigsaw puzzle. Specific igneous
rocks can usually be identified by the different minerals they contain.
Granite, for example often contains the minerals quartz, feldspar, and
mica.
See how the minerals in Granite have a definite
pattern?
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Rhyolite's minerals are harder to see but there
is a pattern to it all.
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Certain igneous rocks can also
be identified by the size of their crystals. Granite and rhyolite
are both igneous rocks made of the same minerals. Granite has large crystals
and rhyolite has small crystals.
Granite has larger crystals. |
Rhyolite's crystals are much smaller. |
You learned that sedimentary rocks are composed
of different-sized particles that have been cemented together.
These various-sized particles make many sedimentary rocks easy for us
to identify. Because of these differently sized particles, the
minerals in sedimentary rocks do NOT fit together like pieces of a jigsaw
puzzle. They are not arranged in any
order. Sedimentary
rocks also contain the remains of animals that lived long ago.
Here is a conglomerate, obviously sedimentary my dear Watson. |
This Atherfield rock has the fossil of a lobster, proof positive
that it's sedimentary. |
The position of the minerals in metamorphic rocks
is often the most important clue in identifying such rocks.
As you know, they are arranged in bands, or layers.
Another clue that helps identify metamorphic
rocks is the fact that they usually do not contain any remains or of past
life. Such evidence was probably destroyed by the heat and
pressure under which these rocks were formed.
Bands and layers very clear in this Gneiss. |
Check out these bands and layers. |
Main IDEAS:
Minerals play an important part in identifying rocks. Color, luster, texture,
and hardness are some properties of minerals. The minerals in igneous
rock usually fit together like pieces of a jigsaw puzzle. The minerals
in sedimentary rock are not arranged in any order. A metamorphic rock's
minerals are arranged in bands, or layers.
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