GEO1115_2021_Activity8.4S
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University of Ottawa *
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Geology
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Dec 6, 2023
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THEME 8 – IGNEOUS ROCKS [ 70/73 : A+ ]
A
CTIVITY
8.4S – V
ISUAL
IDENTIFICATION
AND
ASSOCIATED
QUESTIONS
( /73)
Reference material:
-
Lecture of associated theme (Brightspace)
-
Glossary (
https://mysite.science.uottawa.ca/dumas/
)
-
Reference document “Section of the lab manual” in Brightspace
Activity:
Answer the questions associated to each of the following pictures. N.B. The rocks shown in the pictures below can be any of the rocks identified with a red “
X
” in Table 1 of the “Section of the lab manual” (reference document on igneous rocks). Also, the same rock (rock name, texture) can be tested more than once with different pictures.
Igneous rock 1 ( /4)
a)
Name this rock.
Obsidian
b)
Give the word that describes its texture.
Vitreous
c)
What was the rate of cooling of the magma that generated this rock (slow or rapid)?
Slow
d)
What is the name given to the type of fracture shown?
Conchoidal fracture
Igneous rock 2 ( /7)
a)
Give the word that describes the texture of this rock.
Phaneritic
b)
Name the dominant mineral in that rock (
hint: green colour
), and give its chemical formula. Explain how the chemical formula of this mineral relate to the composition of this rock (i.e. felsic, mafic, or ultramafic).
Olivine - (Mg,Fe)
2
SiO
4
It is ultramafic due to the very high Mg and Fe content in the chemical composition in the rock
c)
Qualify the viscosity (low or high) of the magma that generated this rock, and justify your answer.
Low as it is mainly composed of olivine which is one of the first minerals to crystallize, then the magma is still extremely hot
d)
Name this rock.
Peridotite
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Igneous rock 3 ( /5)
a)
Give the word that describes the texture of this rock, and explain why you say so.
Porphyritic because there is multiple distinct sizes of crystals, larger ones interspersed within ones which are smaller and invisible to the naked eye.
b)
What is the specific textural name given to the large crystals in that rock?
Phenocrysts
c)
Briefly explain what this texture implies in terms of rates and depths of cooling.
This occurs as rising magma cools in two stages, first lower within the Earth’s crust it cools slowly and
forms the larger crystal pieces. Then as it rises and reaches a much shallower depth it cools much more quickly and crystallizes into extremely fine crystals that are invisible or nearly invisible to the naked eye.
Igneous rock 4 ( /8)
a)
Give two different words that describe the textures present in this rock.
Aphanitic and Vesicular
b)
Give the word that describes its composition, and briefly comment on what this implies regarding the relative abundance of Si, Fe, and Mg in the rock (low or high).
It is mafic, this means high in Fe and Mg and low in Si
c)
Name this rock.
Vesicular Basalt
d)
Name two minerals that form this rock and give one diagnostic physical property for each of these two minerals.
-
Olivine
o
Very dark, generally dark green, mineral
-
Amphibole
o
A more intermediate shade
e)
What crystalline
rock would be of equivalent composition but of different texture?
Gabbro
Igneous rock 5 ( /7)
a)
Give the word that describes the dominant texture of this rock.
Aphanitic
b)
What was the rate of cooling of the magma that generated this rock (slow or rapid)?
Rapid c)
Give the word that describes its composition, and briefly comment on what this implies regarding the relative abundance of Si, Fe, and Mg in the rock (low or high).
Felsic, therefore high in Si and low in Fe and Mg
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d)
Name this rock.
Rhyolite
e)
Name two minerals that form this rock and give one diagnostic physical property for each of these two minerals.
-
Quartz
o
Very light coloured
-
Plagioclase Feldspar
o
A more intermediate/medium coloured mineral
Igneous rock 6 ( /8)
a)
Give the word that describes the texture of this rock.
Phaneritic
b)
Give the word that describes where this rock was formed with respect to the Earth’s surface, and explain why you say so.
Intrusively as the crystals are large and visible therefore it must have cooled slowly underground.
c)
What was the rate of cooling of this rock?
Slow
d)
Give the word that describes its composition. Hint: the white crystals are calcium-rich plagioclase feldspars.
Mafic
e)
Qualify the viscosity (low or high) of the magma that generated this rock, and justify your answer.
Low as mafic magma has low viscosity
f)
Name this rock.
Gabbro
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Igneous rock 7 ( /8)
a)
Give the word that describes the texture of this rock.
Phaneritic
b)
Give the word that describes its composition.
Felsic
c)
Qualify the viscosity (low or high) of the magma that generated this rock, and justify your answer.
High as felsic magma has high viscosity
d)
Name this rock.
Granite
e)
Name two main minerals that form this rock, and give one diagnostic physical property for each of these two minerals.
-
Quartz
o
Very light coloured
-
Plagioclase Feldspar
o
A more intermediate/medium coloured mineral
f)
What crystalline
rock would be of equivalent composition but of different texture?
Rhyolite
Igneous rock 8 ( /6)
a)
Give the word that describes the texture of this rock, and briefly define this texture.
Pegmatitic, meaning very large and course crystals.
b)
What was the rate of cooling of the magma that generated this rock (slow or rapid)?
Slow
c)
Based on Table
1 in the “Section of the lab manual” (reference document on igneous rocks), what rock name would you give to this sample?
Pegmatite
d)
Give the name of the two very common minerals identified by the two red arrows (1 and 2) on the picture. Hint: the two minerals are listed in Table
1 in the “Section of the lab manual” (reference document on igneous rocks).
-
1. Quartz
-
2. Potassium Feldspar
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Igneous rock 9 ( /7)
a)
Give the word that describes the dominant texture of this rock, and briefly explain what causes it.
Vesicular, meaning there are vesicles, or cavities, throughout the rock
b)
Name this rock.
Pumice
c)
Qualify the viscosity (low or high) of the magma that generated this rock, and justify your answer.
High since pumice is felsic and felsic magma has a high viscosity
d)
Give the word that describes where this rock formed with respect to the Earth’s surface, and explain why you say so.
Extrusive, since the crystals are small and not visible
Igneous rock 10 ( /8)
a)
Give the word that describes the texture of this rock as a whole.
Porphyritic
b)
What is the specific textural name given to the large white crystals in that rock?
Phenocrysts
c)
Have those large white crystals cooled slowly, or quickly?
Slowly
d)
Give the word that describes the texture of just the darker (~black) portion of that rock, and state whether it represents rapid or slow cooling.
Aphanitic, represents rapid cooling
e)
What is the specific textural name given to the darker material surrounding the large white crystals in that rock?
Aphanitic
f)
Assuming this rock has an intermediate composition, what would you call this rock?
Porphyritic Diorite
g)
Assuming again that this rock has an intermediate composition, and based on Table
1 in the “Section of the lab manual” (reference document on igneous rocks), give the most likely mineral name for the large white crystals in that rock.
Plagioclase Feldspar
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Igneous rock 11 ( /5)
a)
Name this rock.
Obsidian
b)
Give the word that describes where this rock formed with respect to the Earth’s surface, and explain why you say so.
Extrusive as there are no large or visible crystals so it must have cooled quickly
c)
Give the word that describes its composition, and qualify the viscosity (low or high) of the magma that generated this rock.
Felsic, High