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Igneous Rocks
Answer Key
6. Answers may vary. A typical answer might
1. No, the drops in the cold water hardened
faster than the drops in the hot water.
2. While the drops of melted wax that hardened
in the hot water spread out and formed thin
disks, the drops that hardened in the cold
water retained their droplet shape.
mention granite used as a building material,
obsidian used to make cutting tools, and
pumice used as an abrasive in polishing.
7. Coarse-grained
8. Fine-grained
10. extrusive
9. Glassy
11. intrusive
3. The drops that hardened in the cup of water
at room temperature would have flattened
and spread out more than the spherical drops
in the cold water, but they would not have
spread out as far as the drops in the hot
water.
4. Rock formed from lava at or near Earth’s
surface will have a smaller grain than rock
formed from magma deep below its surface.
1. Granite, diorite, and gabbro are intrusive;
rhyolite, andesite, and basalt are extrusive.
2. They are similar in mineral composition and
color. They are different in texture; granite is
coarse-grained and rhyolite is fine-grained.
3. Both rocks contain amphibole, feldspar, and
pyroxene.
4. Gabbro is coarse-grained, while basalt is
Geologists classify igneous rocks by their origin,
texture, and mineral composition. A geologist
would know that granite is formed by hot magma
cooling far below Earth’s surface, so it is an
intrusive rock. Because magma cools slowly, a
granite sample has large crystals that give it a
coarse grain. Granite can be made of a variety
of combinations of minerals. By looking at the
different colors of the grains in a granite sample,
a geologist could make an educated guess about
some of the minerals it contains.
1. Igneous rocks are classified by their origin,
texture, and mineral composition.
2. basalt
3. granite
4. Lava that is low in silica usually forms darkcolored rocks; magma that is high in silica
usually forms light-colored rocks.
5. Igneous rocks are hard, dense, and durable.
fine-grained. Gabbro forms underground
from magma, while basalt forms on the
surface from lava.
5. Grantit is like gabbro in texture.
6. Granite has more silica than basalt.
7. A rock with more silica is likely to be lighter.