Name: ___________________________________ Date: ______________
8th Grade Science
Earth Surface
Mr. Vorstadt
Igneous Rock
Surface
Volcanic rocks are composed of very small
No Crystals
crystals because the molten rock has cooled
very quickly at or near the Earth's surface.
The rapid cooling does not give the crystals
Small Crystals
enough time to grow large. Volcanic rock is
sometimes called extrusive rock because
the molten rock (magma) is extruded at the
surface, where it is known as lava. For this
reason, all volcanic rocks have a fine grained Large Crystals
texture.
Fast cooling
Slow cooling
Very slow
cooling
Increasing Depth
All igneous rocks have formed by the
solidification of molten lava or magma
Plutonic rocks contain large crystals
because
Crystal Size and Cooling Rate
they have cooled and solidified slowly, deep
underground. The slower the magma cools,
the larger the crystals can grow Plutonic rock is also known as intrusive igneous rock
because it forms by slow cooling within the Earth.
Igneous rocks are classified on the basis of their crystal size and their mineral
composition. The felsic rocks are light in color and relatively low in density. They are
rich in feldspar, a light color family of aluminum silicate minerals. Mafic rocks contain
more iron and magnesium. Therefore, mafic rocks are usually more dense and darker in
color than felsic rocks.
Granite is a light colored, coarse grained plutonic rock. The large grains (crystal) make
individual mineral crystals readily visible without magnification. Granite is usually pink or
gray in overall color because of the abundance of feldspar minerals.
Rhyolite is light colored and it has a mineral composition like granite, but rhyolite is fine
grained due to rapid cooling and solidification. Magnification is required to see the
individual minerals crystals.
Obsidian is volcanic glass. It has cooled so rapidly that crystals could not form. Due to
the dispersal of the dark colored mineral, both mafic and felsic varieties of Obsidian are
usually dark in color.
Pumice is a volcanic rock so low in density that some samples may float on water. As
magma rises to the surface, gases trapped in the molten rock expand to form tiny air
pockets. .Although it doesn't look much like obsidian, pumice is actually a frothy form of
volcanic glass.
Scoria is a rock full of larger air pockets, formed as gases expand within the cooling
lava. Scoria is more dense than pumice, and the bubbles are clearly visible.
Basalt is a fine grained, dark colored volcanic rock. Because it forms near the surface,
basalt is composed of very fine crystals. It is rich in iron and magnesium minerals, so it
is dark in color and relatively dense,
Gabbro is a dark (mafic) plutonic rock. It is coarse grained, like granite, but because it
is rich in iron and magnesium, it is dark in color. Gabbro is the coarse grained
equivalent of basalt.
Use the chart on the other page and the descriptions on the igneous
rock to answer the following questions.
1. What name is applied to a coarse grained, felsic igneous rock? ________________
2. If granite rock was made of smaller crystals it would be called? ________________
3. What name is applied to a coarse grained, dark colored igneous rock? __________
4. What are the five minerals found in granite? _______________________________
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5. Name the four minerals found in basalt and give the approximate percentages of
each mineral? _________________________
_____%
_________________________
_____%
_________________________
_____%
_________________________
_____%
6. Why do plutonic rocks have larger crystals than volcanic rock? ________________
__________________________________________________________________
7. According to the chart what is the texture of Pumice? ________________________
8. What is the fine grained rock similar in composition to rhyolite? ________________
9. What is the difference between rhyolite and granite? ________________________
10.
In order for igneous rock to formed the _________________________ and
_________________________.
11.
How large are the crystals of granite? __________
12.
What five minerals are commonly found in basalt? ________________________
________________________________________________________________
13.
How do extrusive rocks differ from intrusive rocks? ________________________
14.
What is the color of rhyolite? _________________________________________
15.
What is the approximate percentage of plagioclase in diorte? _______________
16.
What is the word (s) used to describe the grain size in pumice? ______________
17.
What two common minerals are found in granite, but not in basalt?
__________________________________________________________________
18. What igneous rock contains exceptionally large crystals?
__________________________________________________________________