SCIENCES-IGNEOUS ROCKS LAB
PROBLEM: How can you
tell the difference between plutonic and volcanic rocks?
HYPOTHESIS:
MATERIALS: Earth
Science - Igneous Rocks , Swift
GH microscope or Hand
Lens
PROCEDURE: General
Questions - Answer the following questions.
A. What type of igneous rocks take the longest to
cool? Why?
B. Volcanic rocks cool more quickly than plutonic
rocks. Name some environments where volcanic rocks are likely to form.
(Hint: can volcanoes form underwater?)
C. If plutonic rocks are formed deep inside the
crust of the Earth, will we ever see them on the surface of the Earth?
Explain your answer.
D. Examine each of the igneous rock samples.
Complete the chart below. It is more important to visualize the
environment where each rock formed than to memorize the names of each
rock.
Answer the following questions
using specimens provided by your instructor. Be sure to examine each
specimen, and use the classification charts to help you.
1. PUMICE
Pumice is a type of pyroclastic
material. It has a vesicular texture (it is full of holes). These formed
because the magma cooled when it was full of gas bubbles. The gas
escaped, leaving the holes behind. Pumice is composed of volcanic glass
rich in silica (SiO2).
A. Why is pumice very light?
B. How did pumice form? Was it
cooled quickly or slowly?
How can you tell?
C. Does pumice float on water?
Why?
2. SCORIA
This volcanic rock may look
similar to pumice. In fact, it formed the same way as pumice. It is a
volcanic rock that solidified when it was full of gas bubbles. The main
difference between the two rocks is composition. Scoria contains much
more iron (Fe) and magnesium (Mg) and much less silica (SiO2)
than pumice.
A. Describe scoria.
B. What is the difference between
pumice and scoria?
3. OBSIDIAN
Obsidian is volcanic glass. It is
magma that became solid so quickly that there was no time for minerals
to form. Obsidian is easy to break and shape. For this reason, native
peoples often used obsidian to make arrowheads, knives, and other
cutting tools. Be careful, your obsidian specimens might be sharp!
A. What common material does
obsidian resemble?
B. Can you see any crystals in
obsidian?
C. This obsidian was used by the
Clear Lake Indians of California. What do you think they used it for?
(Hint: is obsidian sharp?)
D. Clear Lake was a
__________________ area at one time.
4. BASALT
Basalt is the most abundant
igneous rock in the Earth’s crust. Most of the ocean floor is
underlain by basalt. It is a volcanic igneous rock, and is typically
dark in color because it contains large amounts of iron (Fe) and
magnesium (Mg) compared to other igneous rocks. Basalt is also very
common on islands like Hawaii.
A. Describe this specimen of
basalt.
B. Can you see any minerals in the
sample?
C. Why is basalt always a dark
color? (Hint: Has something to do with minerals.)
5. ANDESITE
Andesite is a volcanic igneous
rock. It is named after the Andes Mountains of South America, which
contain many active volcanoes. Most volcanoes on land are composed of
andesite, in the form of either lava or pyroclastic material.
A. Is andesite lighter in color
than basalt?
B. Basalt contains a lot of
"dark" elements like iron and magnesium. Notice that andesite
is not as dark as basalt. Does andesite thus have more or less
"dark" minerals?
C. Are any minerals visible in
this andesite?
6. RHYOLITE TUFF
Tuff is a name for a pyroclastic
rock where the magma fragments are small. A rock with big magma
fragments is called a breccia. Tuffs sometimes contain mineral crystals
that were blown out of the volcano along with the magma. Tuff is very
common in the Western United States, especially in Washington and
Oregon, where there are many volcanoes.
A. Describe this rock.
B. Why is rhyolite light in color?
7. GRANITE (South
Carolina)
This granite is a plutonic igneous
rock. It formed from a magma that cooled slowly underground. Granites
are rich in silica (SiO2), which makes them light in color.
A. Are the minerals that make up
this granite visible to the naked eye?
B. What is the general percentage
of light minerals compared to dark minerals? (Hint: greater or less than
50%)
C. What is the difference between
basalt and granite?
D. What can you attribute this to?
8. GRANITE (Texas)
This is another granite, from a
different part of the United States. It looks different from the
specimen in Question 7, because it formed from a slightly different
magma. Both rocks are called granite because they have the same general
texture and composition. It is important to remember that most rock
names are not very specific.
A. What is the general percentage
of light minerals compared to dark minerals in your sample?
B. Can you see distinct minerals
in this specimen?
C. Does it look the same as #7?
Why are they both called "granite?"
9. DIORITE
Diorite is another phaneritic
igneous rock. Like granite, it formed by slow cooling inside the crust
of the Earth. If the magma which cooled to make a diorite was instead
erupted on the Earth’s surface, it would make andesite. This means
that diorite and andesite have the same composition, but have very
different textures.
A. What is the percentage of dark
minerals in this specimen?
What percentage are lighter
minerals?
B. Describe the dark minerals
C. What is the difference between
diorite and the two granites in questions 7 and 8?
10. GRANODIORITE (Sierra
Nevada)
Granodiorite is not on your identification chart.
It is an igneous rock which is between granite and diorite in
composition. Granodiorites can form directly by melting inside the
Earth, or by the mixing of other kinds of magma. The formation of magma
inside the Earth can be very complex.
A. Estimate the percentages of dark and light
minerals in this specimen.
B. Describe the size of the minerals. Why do you
think the minerals are smaller than the granites or diorite?
11. GABBRO
Gabbro is another phaneritic igneous rock. Its
dark color indicates that it contains lots of iron (Fe) and magnesium
(Mg). Gabbro forms from the same kind of magma that creates basalt and
scoria. However, to make gabbro, the magma cooled slowly inside the
Earth.
A. Estimate the percentages of dark and light
minerals in this specimen.
B. Describe the size of the minerals.
C. What is the difference between gabbro and
granodiorite, diorite, and the granites?
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