Module 6 Assignment Igneous Rock Identification Isaias Espiritusanto
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University of Texas, Permian Basin *
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Course
1301
Subject
Geology
Date
Apr 3, 2024
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docx
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5
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Module Assignment – 6: Igneous Rocks and Volcanism
(50 pts)
Part One: Identification of Igneous Rocks (36 pts)
Classification of igneous rocks is based on their texture (i.e. grain size) and mineralogical make up. The size of the mineral grains, or phenocrysts, in igneous rocks are directly related to their cooling history. Fine-grained igneous rocks cooled quickly, whereas large grained igneous rocks cooled slowly. The fine-grained textures are what we call aphanitic and are often associated with quick cooling of lava on the surface of the Earth. The large grained igneous textures are called phaneritic and are associated with slow cooling of magma within the Earth. Color can also be useful as it is related to the mineralogical composition of the rock.
In this laboratory exercise you will collect data from the selected igneous rocks to fill in the table below. Then using some of the specimens you will evaluate the magma composition, cooling history, elemental makeup, and plate tectonic setting for the formation of various igneous rocks. All of these attributes will help us to illustrate the life of an igneous rock
. You will need the following to complete this assignment:
The laboratory resources in the Kendall Hunt Platform
o
Igneous Rock Texture Flow Chart
o
Igneous Rock Texture Photo Guide
The following from your Physical Geology Rock and Mineral Kit
o
9 rocks (#16-#24)
o
Copper nail, iron nail, glass plate to test hardness, white streak plate to collect streak color and a hand lens (10X)
Instructions and Observations
Step 1: Retrieve the igneous rock samples (16 to 24) from your kit. Remember the best
practice is to set them on a white sheet of paper. Read through the Igneous Rock Texture Flow Chart
and Photo Guide
then use what you learned to identify each of the igneous rocks.
Step 2: Describe, in your own words, the texture and color of each rock in the kit. Can you identify any individual mineral grains? After describing the phenocrysts, identify the igneous rock texture based on phenocryst size (i.e. is the rock aphanitic, phaneritic, porphyritic, or pegmatitic), then write in the composition term (mafic, intermediate, or felsic) based on the rocks color.
Step 3: Name each sample using the distinguishing characteristics and write the name in the following table.
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Igneous Rock Identification Table (1 pt for each blank space)
#
Texture and
Color
Texture
Composition
Rock Name
16
Grey and white.
Mixed crystal
sizes mostly
small to medium
Mixed-
phaneritic
Sodium-rich
plagioclase
feldspar and
hompiper
diorite
17
White/Black and
tan specks
Mixed-
phaneritic
Orthoclase
Feldspar and
quartz
Granite
18
Grey
Felsic
aphanitic
texture
Feldspar & quartz
Rhyolite
19
Black
Glassy texture
Volcanic glass
Obsidian
20
Light orange
Volcanic tuff
Volcanic ash
Tuff
21
Grey/silver
Intermediate Aphanitic Texture
‘Plagioclase and mafic minerals
Andesite
22
Red/coral
Mafic versicolor
texture
Basalt
Scoria
23
Grey
Intermediate Aphanitic Texture
Plagioclase and mafic minerals (basalt/granite)
Andesite
24
Light Grey/ Black Specks
Por phyntic texture
porphyrite
rhyolite
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