Skills Practice Lab
Classification of Rocks
There are many different types of igneous, sedimentary, and metamorphic rocks. Therefore, it is
important to know distinguishing features of the rocks to identify the rocks. The classification of rocks is
generally based on the way in which they formed their mineral composition and the size and
arrangement (or texture) of their minerals.
Igneous rocks differ in the minerals they contain and the sizes of their crystals. Metamorphic rocks
often look similar to igneous rocks, but they may have bands of minerals. Most sedimentary rocks are
made of fragments of other rocks that are compressed and cemented together. In this lab, you will use
these features along with your class notes and a rock identification key to identify various rock samples.
OBJECTIVES
Compare and contrast the features of igneous, sedimentary, and metamorphic rocks.
Identify igneous, sedimentary, and metamorphic rocks.
Determine the formation conditions of various rocks based on their characteristics and identities.
PROCEDURE
1. Examine the rocks carefully. You can use a hand lens to study the fine details of the rock samples.
Look for characteristics such as the shape, size, and arrangement of the mineral grains. USE THE
KEY starting on the next page to determine the name of each rock sample. Record the
characteristics you chose at each step along the way in the column labeled “description of properties”
to provide a detailed description of the rock. Perform various tests as described below ONLY when
required by the key.
a) Acid Test: Certain rocks react with acid, which indicates that they are composed of calcite (which
is the mineral found in the shells of marine organisms). If a rock contains calcite, the rock will
bubble and release carbon dioxide. Go over to the Acid test station & using the dropper in your kit
and 10% dilute hydrochloric acid, test various samples for their reactions. CAUTION: Wear
goggles & only apply one drop of acid to your rock. Blot with paper towel afterward to prevent
excess acid from burning the next person to touch your rock. Wash your hands thoroughly after
testing the rock with acid. Record in your data table which samples reacted with acid, and what their
reaction was.
b) Water Test: Go to the water test station & gently drop the rocks you are testing into the water one
at a time and record if each sinks or floats.
2. Complete your table by identifying the class of rock—igneous, sedimentary, or metamorphic. Use
your book & class notes to determine which class of rock each belongs to.
Rock Identification Key
1.
a. The rock contains crystals (has sparkles in it when light hits it) …………………………… Go to 2
b. The rock does not contain crystals (no sparkles) …………………………………………..…Go to 3
2.
a. The rock has fine-grained crystals (smaller than salt/pepper) .……………………………….Go to 4
b. The rock has coarse-grained crystals (salt/pepper sized or larger)..………………….………Go to 5
3.
a. The rock is made of rock/mineral fragments (visible pieces of rock/shells or
feels gritty & sediment rubs off when scratched with your fingernail) ................................…Go to 6
b. The rock is not made of rock/mineral fragments ……………………..……………………..…Go to 7
4.
a. The rock has a vesicular texture & floats in water (perform water test to determine this)….Pumice
b. The rock does not have a vesicular texture or float in water .……………………..…………..Go to 8
5.
a. The rock is foliated (banded with layers of multiple materials or of the same material) ……….Gneiss
b. The rock is not foliated……………………………..………………………………………..…..Go to 9
6.
a. The rock is made of coarse sediment (visible rock fragments or shell fragments) …………….Go to 10
b. The rock is made of fine sediment (not visible fragments, but detectable by touch)…………..Go to 11
7.
a. The rock has a glassy texture……………………………..……………………………………Obsidian
b. The rock does not have a glassy texture……………………………………………..…………Go to 12
8.
a. The rock is banded with layers of sediment…………………………………………………Sandstone
b. The rock is not banded with layers of sediment..………………………….……………..…..Go to 13
9.
a. The rock contains calcite from the shells of living organisms, so it reacts with acid
(perform acid test to determine this)…………………………….…………………………….Marble
b. The rock does not react with acid………………………………………………………..……..Go to 12
10.
a. The rock contains calcite from the shells of living organisms, so it reacts with acid
(perform acid test to determine this)…………………………….………………………...Limestone
b. The rock does not react with acid…………………………………………………..…….Conglomerate
11.
a. The rock is light in color (white or crème) …………………………………………………….…Chalk
b. The rock is not light in color (rust, tan, grey, green)………………………………..…………….Shale
12.
a. The rock is made of compressed layers of fine muddy sediment ……………………………...… Slate
b. The rock is made of layers of organic material (organic material is usually black) ..…Anthracite coal
13.
a. The rock is Mafic (dark in color)………………………………………………………………...Basalt
b. The rock is not Mafic …………………………………………………………………………….Schist
14.
a. The rock is Felsic (made of light colored minerals like whites, pinks or clear crystals)…………Halite
b. The rock is not Felsic……………………...………………………………………………..…..Go to 15
15.
a. The rock is Intermediate (a mix of dark & light minerals)……………………………………...Granite
b. The rock is not Intermediate……………………………………………………………………..Gabbro
ANALYSIS AND CONCLUSIONS
1. What properties were most useful and least useful in identifying each rock sample? Why?
2. Why were some samples hard to identify?
3. Describe any characteristics common to all of the rock samples.
4. How can you distinguish between a sedimentary rock and a foliated metamorphic rock if both have
observable layering?
5. Why do limestone and marble react with acid?
6. What processes formed rock samples A, D, G, & I?
7. What processes formed rock samples B, E, F, H, & J?
8. What processes formed rock samples C, K, & L?
9. What is the difference between the way rock sample C & rock sample L formed? How can you tell?
10. What type of environment existed at the time rock sample A was formed? How can you tell?
11. How were the rock fragments in rock sample D transported? How can you tell?
12. What is the difference between the way that rock sample B and rock sample H were formed?
13. What was the environment like when rock sample L was formed? How can you tell?
14. Circle either “I” for intrusive or “E” for extrusive to show where each of the following rocks
formed. Then, explain how you can tell for each.
Rock
Sample
Intrusive
or
Extrusive?
B
I / E
E
I / E
F
I / E
H
I / E
J
I / E
How can you tell?
Bonus problem: Complete the following chart to show sets of rocks that change from one type of
rock to another due to Metamorphism (Hint: see 6.4 for most of these & page 160 for the coals):
Sedimentary Rocks
Metamorphic Rocks
Sandstone 

Bituminous coal 

Slate 
Limestone 

Gneiss
DATA TABLE:
Sample
A
B
C
D
E
F
G
H
I
J
K
L
Descriptions of Properties
Rock Class
(S, I, or M)
Rock Name