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Regents Earth Science Lab – An Introduction to Igneous Rocks
An introduction to igneous rocks requires exploration of three core ideas. Each of these is
introduced below:.
Formation, Classification, and Identification of Igneous Rocks.
Igneous rock evolution
Plate tectonics and igneous rock distribution
FORMATION, CLASSIFICTION AND IDENTIFICATION
The inside of the earth is very hot - hot enough to melt rocks. And the deeper you go the hotter it
gets. Below the surface the molten rock is called magma; at the earth's surface it becomes lava,
although nothing has changed except the name.
The fresh magma is white hot, brilliant enough that you would have trouble looking at it. But as it
cools it turns yellow, and then various shades of red. Eventually, it cools enough to solidify completely
and form an igneous rock, such as the granite and basalt below. Granite and basalt are the two most
abundant igneous rocks at the earth's surface.
Granite
Basalt
Magma/lava is a mixture of elements such as silica, iron, sodium, potassium, etc. As the
magma/lava cools these elements chemically combine, or crystallize, in geometric patterns to form the
eight rock forming minerals. For example, in the granite above the pink is orthoclase, the black biotite,
and clear to gray mostly quartz.
These eight minerals form the bulk of igneous rocks. They are arranged in Bowen's Reaction Series
(BRS) by temperature of formation, high temperature ones at the top and low temperature ones at the
bottom. Although it is useful to know these minerals they are not essential for a basic understanding of
igneous rocks.
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Please note: Temperatures decrease toward the bottom of the chart. The minerals at the bottom would be the last to
form as the magma cools.
Cooling is progressive in a magma/lava. Some minerals become solid at high temperatures (top of
BRS) and others at lower temperature (bottom BRS), so that part way through the cooling the
magma/lava is a mixture of minerals and still molten rock.
Magma/lava also contains lots of gases such as water, sulfur dioxide, carbon dioxide, etc., and these
are driven off into the atmosphere during cooling.
If cooling is "slow" (thousands to millions of years, below ground,) the
minerals grow large enough to see with the eye, as with the granite to
the left.
These are "coarse grained" (or phaneritic). Any rock in which the
grains can be seen by eye are coarse grained.
If cooling is "quick" (days to weeks) as at the earth's surface, the
minerals do not have enough time to grow, and so are microscopic in
size. These are fine grained (or aphanitic). For example, the rhyolite to
the left.
If cooling is "very quick" (hours to days) the elements and compounds are frozen in place, no
minerals form, and the result is a glass. For example, the scoria (above left,) and the obsidian (above
right,) are glassy-textured igneous rocks.
Basalt and granite are two of the most common igneous rocks found at the earth's surface. They
illustrate the diversity of properties that igneous rocks have.
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Granite: a felsic rock
Basalt: a mafic rock
1. Dark color
2. High specific gravity
3. Olivine/pyroxene/Calcium plagioclase -rich
4. Fine grained - crystals only seen under high power
1. Light color
2. Low specific gravity
3. Quartz and orthoclase and sodium plagioclase -rich
4. Coarse grained - crystals large enough to see by eye
5. Forms on the continents deep underground
6. Forms low in Bowen's Reaction Series
5. Forms at the surface, principally in the ocean basins, but
also in isolated "hot spots" on the continents.
6. Forms high in Bowen's Reaction Series
Igneous rocks are classified in several different ways, but all rock classifications are a combination of
texture and color/composition of the rock. The variety of igneous textures is in the table below.
Texture
Cooling History
Examples
Glassy
Very fast cooling; non-crystalline.
Obsidian
Vesicular (cellular)
Very fast cooling with rapid gases
escaping, forming bubbles in the
non-crystalline rock.
Slow cooling; microscopic crystal growth.
Pumice, scoria
Phaneritic
(coarsegrained)
Very slow cooling; crystals grow to visible size.
Granite, diorite, gabbro
Porphyritic (two grain
sizes)
Two stage cooling; one slow, underground creating
visible phenocrysts, the second fast at the earth's
surface, producing a fine grained groundmass.
Any aphanitic rock with
the adjective porphyry
Aphanitic (finegrained)
Rhyolite, andesite, basalt
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The color/composition of the rock is at its simplest divided into dark colored rocks (mafic),
intermediate colored rocks (intermediate), and light colored rocks (felsic). If we combine
texture/cooling history and color/composition in a grid we get the classification in the table below.
OBSERVE: There are a few aphanitic and glassy/cellular rocks for which color will just not work for classification, and if
you try to use color the identification will be wrong. There is no way to know this ahead of time. You just have to
memorize these specimens.
They include:
Obsidian: Glassy, and black or red. It belongs in the light-colored felsic category because its chemistry is like those rocks.
Obsidian is dark because it is a glass with many impurities which absorb the light and make it dark.
Scoria: Ranges from dark red to black. Composition ranges from intermediate to mafic.
OBSERVE: Although mafic, intermediate and felsic are the main categories, many other kinds of igneous rocks exist. One
additional category that does not easily fit onto the above classification table is ULTRAMAFIC. These, like mafic rocks are
olivine or pyroxene rich, but lack plagioclase feldspar. Examples are dunite (mostly olivine) and peridotite (olivine and
pyroxene). That is, in Bowen's Reaction Series the composition of these rocks is high on the left.
Different igneous rocks are found in different places on Earth. These differences are related to the
density of the rocks and to plate tectonic processes that cause movement of the continents and the
ocean basins over time. Continents tend to be made up of the less-dense felsic igneous rocks, such as
granite. These less-dense rocks have “floated up” and rest on the denser rocks below. Ocean basins
are made of the denser mafic igneous rocks such as basalt and gabbro, and volcanic arcs of
intermediate rocks such as diorite and andesite.
This information is from the website http://csmres.jmu.edu/geollab/Fichter/IgnRx/Introigrx.html
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Problem: What are the properties of igneous rocks that may be used to identify igneous rocks?
Materials: Set of igneous rocks, hand lens
Procedures and Observations:
1. Review the background information section of this investigation.
2. Look at the set of igneous rocks that you have been given. Each specimen is numbered. Enter the number of
each specimen that you have been told to examine in the Data Chart on the next page of this lab. Use your
observations and the Scheme for Igneous Rock Identification from the Earth Science Reference Tables, page 76
(also shown here for your convenience,) to help you to identify each of the specimens.
3. Complete the Data Table. Be sure to use the hand lens for a better look at the texture and the minerals
present in the samples.
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Data Table: Igneous Rocks
Specimen Texture
Number
(glassy, finegrained,
coarse grained
Overall Dominant
Color
Minerals Seen
(if any)
Cooling History: Fast/Slow
(Evidence?)
Name of
Rock
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Analysis and Conclusions:
1. Which of the rocks that you observed are probably volcanic? What characteristics indicate this?
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2. Obsidian was used by many Native American tribes as the material from which arrowheads and spearheads
were made. What property or properties of obsidian made them a good choice for such uses?
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3. Both granite and gabbro are coarse-grained igneous rocks, yet they are very different from each other. Using
information from the Scheme for Igneous Rock Identification, describe three ways in which granite and gabbro
differ from each other.
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4. Pumice is the only rock that floats when placed in water. Why does pumice have a density less than that of
water?
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