Rocks Lab 2 - Student Handout
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Geology
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Dec 6, 2023
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GEOL 121
Tim Trostel
Lab Section L18
Rocks Lab 2 – Rock Classification and Identification
Learning Objectives:
1)
Describe the characteristics used to classify igneous, sedimentary, and metamorphic
rocks.
2)
Identify common igneous, sedimentary, and metamorphic rocks.
Part 1: Igneous Rocks
1) Igneous rocks form from the cooling and crystallization of magma. This cooling may happen
on Earth’s surface or deeper within the crust. Based on this understanding of how igneous rocks
form and what you learned in Rocks Lab 1, what characteristics of igneous rocks might be
useful for identifying and classifying igneous rocks?
Texture, composition, and color.
2) Examine igneous rocks A, B, and C in your sample set:
What are some ways these rocks are similar?
What are some ways these rocks are different?
They are similar because each has a mix of
color and minerals. They are also all igneous.
One is overall darker in color than the other
two. The others are lighter and one is mostly
white. Different size crystals.
3) Examine igneous rocks E, F, and G in your sample set:
What are some ways these rocks are similar?
What are some ways these rocks are different?
All of them have dark color, no crystals, same
poor cleavage.
G has small brown craters all over it. E and G
have a mix while F is all the same color.
Igneous rocks are classified according to their composition and texture. Composition
refers to the proportion of the different silicate minerals that are present in the rocks, and
Rocks Lab 2
GEOL 121
can be simplified to the major catagories: felsic, intermediate, mafic, and ultra-mafic.
Igneous textures generally reflect the method and rate at which the magma cooled.
4) For the unknown igneous rocks describe their composition and texture. Then use the
provided classification charts to identify the rock.
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GEOL 121
Sample
Composition
Texture
Rock ID
A
felsic
phaneritic
granite
B
intermediate
phaneritic
diorite
C
mafic
phaneritic
gabbro
D
felsic
aphanitic
rhyolite
E
mafic
porphyritic
porphyritic basalt
F
mafic
aphanitic
basalt
G
mafic
vesicular
scoria
5) How confident do you feel in your identifications?
Fairly confident
6) What is still unclear to you about igneous rock classification?
What and ultramafic rock looks like
Part 2: Sedimentary Rocks
3
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7) Sedimentary rocks form from the products of chemical and physical weathering. The
sediments and dissolved ions are transported and deposited by processes happening on
Earth’s surface. Based on this understanding of how sedimentary rocks form and what you
learned in Rocks Lab 1, what characteristics of sedimentary rocks might be useful for identifying
and classifying sedimentary rocks?
8) Examine sedimentary rocks K, O, and P:
What are some ways these rocks are similar?
What are some ways these rocks are different?
Fine grain, all of them break apart pretty easy.
The rock “O” reacts to the acid.
One of them is very dark and the other two are
very light in color. The dark one breaks apart
easier than the other two.
9) Examine sedimentary rocks J, L, and M:
What are some ways these rocks are similar?
What are some ways these rocks are different?
None of them react to the acid.
They are all different color and cleavage.
Sedimentary rocks are classified at the highest level according to the composition of the
material and the origins of the rock. Clastic/detrital rocks are the most common
sedimentary rocks; these rocks are further classified by their clast size.
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GEOL 121
10) For the unknown sedimentary rocks use the classification charts provided to identify the
rock and provide a justification.
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Rocks Lab 2
GEOL 121
Sample
Rock ID
Reasoning
J
Shale
Dark color, flat thin layers, brittle, no reaction to acid.
K
Sandstone
Soft layering, does not react to acid, light color
L
Sandstone
Soft layering, does not react to acid, dark/reddish color
M
Conglomerate
Clastic with other minerals and brown color.
N
Breccia
Has large other minerals in it and light brown color
O
Limestone
Reacts with acid and light in color, breaks apart easily.
P
Coal
Very dark color, breaks apart easily, lightweight.
11) How confident do you feel in your identifications?
Confident
12) What is still unclear to you about sedimentary rock classification?
How can sandstone be two different colors
Part 3: Metamorphic Rocks
6
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13) Metamorphic rocks form through the alteration of other rocks by high heat and/or pressure.
This can happen during burial to greater depths, tectonic processes, or the movement of heat
by magma. Based on this understanding of how metamorphic rocks form and what you learned
in Rocks Lab 1, what characteristics of metamorphic rocks might be useful for identifying and
classifying metamorphic rocks?
14) Examine metamorphic rocks R, W, and X:
What are some ways these rocks are similar?
What are some ways these rocks are different?
They are all fine grain and mixed with other
colors.
W reacted with acid. Two are foliated and one
is not.
15) Examine metamorphic rocks T, U, and V:
What are some ways these rocks are similar?
What are some ways these rocks are different?
Dark color, foliated.
Two have thin layers and are mostly the same
color. T is mixed with other colors.
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Rocks Lab 2
GEOL 121
Metamorphic rocks are classified primarily based on their texture, most importantly the
presence and degree of foliation. Mineral composition and grain size also play a role in
classification.
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Rocks Lab 2
GEOL 121
16) For the unknown metamorphic rocks use the classification charts provided to identify the
rock and provide a justification.
Sample
Rock ID
Reasoning
R
Gneiss
Foliated, white and black, no acid reaction.
S
Gneiss
Foliated and light color mixed with little black, no acid reaction.
T
Schist
Visible crystals and dark color
U
Phyllite
Fine grained, has thin layers, no reaction to acid.
V
Slate
Fine grain, flat layers, dark color, no acid reaction.
W
Marble
Reacts with acid, smooth and almost glassy.
X
Quartzite
Fine grain, looks crystally, bright luster.
17) How confident do you feel in your identifications?
Fairly confident
18) What is still unclear to you about metamorphic rock classification?
Why does marble react to acid?
9
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