Igneous Rocks
Objectives- How are igneous rocks identified
What are igneous rocks used for
Two things are needed to identify igneous rocks: the texture (size and shape of the mineral) and
mineral composition.
Igneous rocks can form from magma or lava. Igneous rocks that form from magma are called
intrusive, ones that form from lava are called extrusive. Intrusive rocks, because they cool slowly, have
larger mineral grain (size). These are called coarse grained. Extrusive cool quickly, and have smaller
mineral grains, or fine grain. Sometimes, extrusive rocks cool so quickly, they have a glassy appearance
and have no crystal structure. Other times, the magma begins to cool slowly as expected, but then
undergoes a more rapid cooling. Consequently, the rock has both large and small crystals producing a
texture called porphyritic.
The color of the rock will be a clue to its mineral composition. If the rock is dark colored, it
contains low amounts of silica. Silica is found in many minerals. If the rock contains large
amounts of silica, the rock will be lighter in color.
Igneous rocks have many uses. Many people use granite for counter tops. Granite is also used
statues, grave stones, or building blocks. Pumice is abrasive (scratchy) and is used in cleaners. Obsidian
was used in arrow heads, knives and believe it or not, razors. Obsidian can be chipped to have an
incredibly sharp edge. Matter of fact, it can be many times sharper than surgical steel. Sharp enough to
be used for heart surgery in modern times.
Questions:
1. T/F Extrusive rock forms beneath Earth’s surface.
2. T/F Intrusive rocks have smaller crystals then extrusive rocks.
3. A rock with large crystals scattered on a background of much smaller crystals has a(n)
_____________________ texture.
4. What type of texture do extrusive rocks have?
Use the diagram to answer the rest of the questions.
Igneous Rock Classification
Granitic- light colored
Basaltic- dark colored
Intrusive
Granite
Gabbro
Extrusive
Rhyolite
Basalt
Vesicular
Pumice
Scoria
porphyritic
Porphyry- mineral has both large and small mineral crystals
glassy
Obsidian
5. Circle the letter of each sentence that is true about the silica composition of igneous rocks.
a. Igneous rocks low in silica are usually dark colored.
b. An example of an igneous rock low in silica is granite.
c. Igneous rocks high in silica are usually light colored.
d. An example of an igneous rock high in silica is basalt.
6. Describe what basalt would look like?
7. Vesicles are holes in the rocks. How would igneous rocks become vesicular?
In order to identify igneous rocks, knowing the texture of the rock and mineral content can help. The
terms intrusive, extrusive, porphyritic, glassy and vesicular are used to describe the texture of igneous
rocks. To review what these terms mean, complete the 4 questions below. The minerals in the rock
may be difficult to identify, but the color and density can help. If the igneous rock is light color and has a
low density, it is called granitic. If the igneous rock is dark colored and has a high density, it is called
basaltic.
8. What do intrusive igneous rocks look like?
9. What do extrusive igneous rocks look like?
10. What for porphyritic igneous rocks look like?
11. What do igneous rocks with vesicles look like?
Texture clues
Mineral
Clues
Rock # Intrusive or Other textures Granitic or
Extrusive
Basaltic
1.
I
E
P V G
Gr
B
2.
I
E
P V G
Gr
B
3.
I
E
P V G
Gr
B
4.
I
E
P V G
Gr
B
5.
I
E
P V G
Gr
B
6.
I
E
P V G
Gr
B
7.
I
E
P V G
Gr
B
8.
I
E
P V G
Gr
B
9.
I
E
P V G
Gr
B
10.
I
E
P V G
Gr
B
I- Intrusive; E- Extrusive;P- Porphyritic; V- Vesicles; G- Glassy;
Gr- Granitic; B- Basaltic
Rock #
1.
Rock Name
2.
3.
4.
5.
6.
7.
8.
9.
10.
Which rock(s) listed in the key are formed at or near the surface of the earth?
Which rock(s) listed in the key are formed below the surface of the earth?
Which rock(s) listed in the key form by slowing cooling?
Which rock(s) listed in the key are formed by rapid cooling?
Which rock(s) listed in the key are formed by rapid and slow cooling?
Which rock(s) listed in the key had gas(es) trapped in them when they formed?