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Rocks
Earth’s crust is made of rock. Rocks are mixtures of minerals and other materials, although
some rocks may contain only a single mineral. Geologists collect and study samples of rock in
order to classify them.
When studying a rock sample, geologists observe the rock’s color and texture and determine
its mineral composition. Texture is the look and feel of a rock’s surface. Some rocks are smooth
and glassy, while others are rough or chalky. Most rocks are made of particles of minerals or other
rocks, which geologists call grains. A rock’s grains give the rock its texture. To determine a rock’s
texture, geologists use a number of terms based on the size, shape and pattern of the rock’s grains.
Course-grained rocks have large and easy to see grains. Fine-grained rocks have grains that are so
small they can only be seen with a microscope. Geologists may use mineral properties and their
tests to determine the mineral composition of the rock.
There are 3 major groups of rocks: igneous rock, sedimentary rock and metamorphic rock.
These terms refer to how the rocks in each group formed. Igneous rock forms from the cooling
of molten rock. Sedimentary rock forms when particles of other rocks or the remains of plants and
animals are pressed and cemented together. Metamorphic rock forms when an existing rock is
changed by heat, pressure, or chemical reactions.
Igneous Rocks
Igneous rock is any rock that forms from magma or lava. Magma is melted rock. If magma
reaches the surface of the crust it is called lava. Most igneous rocks are made of mineral crystals.
Geologists classify igneous rocks according to where they formed. Extrusive rock is igneous rock
formed from lava that erupted onto Earth’s surface. Basalt is the most common extrusive rock.
Igneous rock that formed when magma hardened beneath Earth’s surface is called intrusive rock.
Granite is the most abundant intrusive rock.
The texture of an igneous rock depends on the size and shape of its mineral crystals.
Rapid cooling lava forms fine-grained igneous rocks with small crystals. Slow cooling magma
forms course-grained rock with large crystals. Intrusive rocks have larger crystals than extrusive
rocks.
Many igneous rocks are hard, dense and durable. As a result, people have long used igneous
rock for tools and building materials. Granite is widely used in statues and construction. Basalt,
pumice, and obsidian also have important uses.
Name _________________________________________________________ Class ____________
Rocks and Igneous Rocks
Understanding Main Ideas
Fill in the blanks in the table below.
Origin of
Igneous Rock
Igneous Rock
Classification
Resulting Texture
Example
1. __________________
2. ____________________
3. ________________
4. __________________
5. ____________________
6. ________________
Slow cooling of
magma far
beneath Earth’s
surface
Rapid cooling of
lava in which
tiny crystals
form
Answer the following questions in complete sentences. If you need additional space, use a piece of
loose-leaf and staple it behind this sheet.
7. What do geologists look for when observing a rock sample? _____________________________
________________________________________________________________________________
8. Name the 3 major groups of rocks and describe how each forms.__________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
9. What qualities of igneous rocks have long made them useful for tools and building materials?
________________________________________________________________________________
________________________________________________________________________________
Building Vocabulary
10. The look and feel of a rock’s surface is its _____________________________.
11. The particles of minerals or other rocks that make up a rock are called ___________________.
12. Igneous rock formed from lava on Earth’s surface is called ____________________rock.
13. Igneous rock formed from magma below Earth’s surface is called _________________ rock.
Intrusive and Extrusive Igneous Rocks
Instruction
Have individuals respond “agree” or “disagree” to each statement. For each “agree” statement
have students provide supporting evidence or an example and for each “disagree” statement have
them reword the statement to make it valid.
Activity
For each statement below, choose either Agree or Disagree. If the statement is true, give an
example or a connection to your life. If you disagree, reword the statement to make it true.
Statement
Agree or
Disagree?
If agree: Example or connection
If disagree: Reworded to make it true
14. Igneous rocks only form on
the surface of the Earth.
15. The slower the magma cools,
the bigger the crystals formed.
16. Crystal size has nothing to do
with texture in rocks.
Write the term intrusive or extrusive next to the sentence or term.
17. Formed from lava that erupted onto Earth’s surface _____________
18. Formed when magma hardened beneath Earth’s surface. ___________________
19. Basalt is a common example. _______________________
21. Granite is the most abundant. ______________________
22. Forms the core of many mountains. __________________________
23. Lava forms fine-grained rocks with small crystals. ______________________
24. Magma forms coarse-grained rock with large crystals. _____________________________
25. Scientist who study rocks are called _____________________.