Name __________________________ Date __________________ Grade 8 Science Group ____
D21. Explain how the internal energy of the Earth causes matter to cycle through the magma and the solid earth
IGNEOUS ROCKS
Read the information below on “Igneous Rocks”. Then answer the questions which follow by
writing the letter of the correct answer on the space to the left.
Igneous rocks (from the Greek word for fire) form from when hot, molten rock
(magma) crystallizes and solidifies. The melt originates deep within the Earth near active plate
boundaries or hot spots, then rises toward the surface. Igneous rocks are divided into two
groups, intrusive or extrusive, depending upon where the molten rock solidifies.
Intrusive (or plutonic) igneous rock forms when magma is trapped deep inside the Earth. Great
globs of molten rock rise toward the surface. Some of the magma may feed volcanoes on the
Earth's surface, but most remains trapped below, where it cools very slowly over many
thousands or millions of years until it solidifies. Slow cooling means the individual mineral
grains have a very long time to grow, so they grow to a relatively large size. Intrusive rocks have
a coarse grained texture.
Granite is an igneous rock that is composed of four minerals. These minerals are quartz,
feldspar, mica, and usually hornblende. Granite forms as magma cools far under the earth's
surface. Because it hardens deep underground it cools very slowly. This allows crystals of the
four minerals to grow large enough to be easily seen by the naked eye.
Extrusive (or volcanic) igneous rock is produced when magma exits and cools outside of, or
very near the Earth's surface. These are the rocks that form at erupting volcanoes and oozing
fissures. The magma, called lava when molten rock erupts on the surface, cools and solidifies
almost instantly when it is exposed to the relatively cool temperature of the atmosphere. Quick
cooling means that mineral crystals don't have much time to grow, so these rocks have a very
fine-grained or even glassy texture. Hot gas bubbles are often trapped in the quenched lava,
forming a bubbly, vesicular texture.
Obsidian is a very shiny natural volcanic glass. When obsidian breaks it fractures with a distinct
conchoidal fracture. Obsidian is produced when lava cools very quickly. The lava cools so
quickly that no crystals can form. When people make glass they melt silica rocks like sand and
quartz then cool it rapidly by placing it in water. Obsidian is produced in nature in a similar
way.
Pumice is a light, porous volcanic rock that forms during explosive eruptions. It resembles a
sponge because it consists of a network of gas bubbles frozen amidst fragile volcanic glass and
minerals.
_____1. The word, Igneous, means __(1)__ in Greek.
a. fire
b. rock
c. crust
d. pebble
_____ 2. Igneous rocks are grouped according to where the __(2)__ rock becomes solid.
a. minerals
b. igneous
c. trapped
d. melted
_____3. Another name for Intrusive rock is __(3)__ rock.
a. sedimentary
b. plutonic
c. volcanic
d. metamorphic
_____4. Intrusive rock results from __(4)__ cooling.
a. fast
b. speedy
c. slow
d. no
_____5. Due to this rate of cooling, intrusive rocks have __(5)__ sized crystals.
a. large
b. no
c. small
d. tiny
c. obsidian
d. pumice
_____6. An example of an intrusive igneous rock is
a. granite
b. basalt
____7. Another name for Extrusive rock is __(7)__ rock.
a. sedimentary
b. plutonic
c. volcanic
d. metamorphic
_____8. Extrusive rock results from __(8)__ cooling which produces very small crystals.
a. fast
b. slow
c. mediocre
d. exhaustive
_____9. An example of an extrusive igneous rock which has a glassy texture is
a. granite
b. basalt
c. obsidian
d. pumice
____10. __(10)__ becomes trapped in cooling lava to give pumice its sponge-like look.
a. gas
b. lava
c. grains
d. minerals