Extrusive Igneous Rock
Igneous rock that forms from magma that erupts,
or extrudes, onto the Earth’s surface is called
extrusive igneous rock. Extrusive rock is common
around volcanoes. It cools quickly on the surface
and contains very small crystals or no crystals.
When lava erupts from a volcano, a lava flow
forms. Figure 5 shows an active lava flow. Lava does
not always flow from volcanoes. Sometimes lava
erupts and flows from long cracks in the Earth’s
crust called fissures. Lava flows from fissures on
the ocean floor at places where tension is causing
the ocean floor to be pulled apart. This lava cools
to form new ocean floor. When a large amount of
lava flows out of fissures onto land, the lava can
cover a large area and form a plain called a lava
plateau. Pre-existing landforms are often buried by
these lava flows.
How does new ocean floor form?
Figure 5 An active lava flow is shown in
this photo. When exposed to Earth’s surface
conditions, lava quickly cools and solidifies
to form a fine-grained igneous rock.
extrusive igneous rock rock that forms as a
result of volcanic activity at or near the Earth’s
surface
S6E5.e
Review
Summary
rock forms
• Igneous
when magma cools and
hardens.
texture of igneous
• The
rock is determined by
•
•
the rate at which the
rock cools.
Igneous rock that solidifies at Earth’s surface is
extrusive. Igneous rock
that solidifies within
Earth’s surface is
intrusive.
Shapes of common
igneous intrusive bodies include batholiths,
stocks, sills, and dikes.
Using Key Terms
1. In your own words, write a definition for each of the following
terms: intrusive igneous rock and
extrusive igneous rock. S6CS10.c
Understanding Key Ideas
2.
is an example of a coarsegrained, felsic, igneous rock.
a. Basalt
b. Gabbro
c. Granite
d. Rhyolite
Critical Thinking
6. Making Comparisons Dikes and
sills are both types of igneous
intrusive bodies. What is the
difference between a dike and
a sill?
7. Predicting Consequences An
igneous rock forms from slowcooling magma deep beneath
the surface of the Earth. What
type of texture is this rock most
likely to have? Explain.
3. Explain three ways in which
magma can form.
4. What determines the texture
of igneous rocks?
Math Skills
5. The summit of a granite batholith has an elevation of 1,825 ft.
What is the height of the batholith in meters? S6CS3.b,
M6M1
Developed and maintained by the
National Science Teachers Association
For a variety of links related to this
chapter, go to www.scilinks.org
Topic: Igneous Rock
SciLinks code: HSM0783
101