Igneous_rocks_lab_FA23
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Geology
Date
Dec 6, 2023
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1
PHYSICAL
GEOLOGY
GEOSC 001
LAB MANUAL
FALL 202
3
modified from original by Prof. Roger Cuffey
NW
SE
64
Name
GEOSC 001
FALL
202
3
LAB 9:
IGNEOUS ROCKS & PROCESSES
Section
Date
In lecture and in your textbook, the processes that form igneous rocks are considered in detail. A quick
word of nomenclature: Igneous rocks formed by magma that cools
beneath the surface
of the earth are
called intrusive. Igneous rocks formed by the cooling of magma
after erupting
to the earth’s surface (lava)
are called extrusive. The magma itself forms by melting of rocks in the mantle or in the crust. This melting
takes place under well-defined conditions and the melting products are readily identified. In this lab, you
will be looking at the end products of those processes, i.e., the rocks they leave behind.
The exercise
today will help you identify different igneous rocks and understand a bit more about the conditions under
which they were produced.
You may need to refer to your textbook for further information.
Part 1: Igneous Texture and Cooling History
The size of the minerals (crystals) in igneous rocks reflects the rate of cooling of the magmas. In volcanic
eruptions, magma is forcibly ejected from the earth in the form of lava. In this case, magma cools very
rapidly, leaving no time for large crystals to form. In the case of magma that cools very slowly beneath
the surface (because it failed to erupt, or was left behind), there is ample time for large crystals to grow.
This is an important idea: in order to form large crystals in igneous rocks, long cooling times are necessary.
Igneous rocks that have large crystals form intrusively, because of the slow cooling of the magma.
Igneous rocks that have small crystals form extrusively, leaving little or no time for large crystals to grow.
Obsidian
Q1. Could you see any individual minerals in the obsidian?
Q2. Describe the fractures you see on the obsidian sample.
Granite
Q3. Granite has large mineral grains (phenocrysts). What might explain the difference between the
obsidian and granite?
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Basalt
If you were to take away all of the water on Earth, it would no longer be known as the blue planet. It
would become the black planet, because the crust beneath the oceans, which cover 70% of the Earth’s
surface, is basalt, the most common igneous rock. Examine the basalt (the dark rock) you have been given.
Q3. Describe the minerals t
hat compose basalt. We don’t expect you to identify any individual minerals
and sometimes the individual grains may be difficult to see, but please describe what you observe.
Q4. Now look back at the granite. Do you think that any of the minerals are found in both samples?
Q5. Which of these rocks (basalt and granite) is intrusive and which is extrusive?
Q6. If a rock had large crystals surrounded by very fine crystals, what can you say about its cooling
history?
This texture is known as porphyritic, and you can see it in some samples in lab. This texture shows us that
the molten magma cooled slowly over a small temperature range, allowing large crystals to grow. It then
erupted quickly, causing the remaining liquid to cool quickly into a solid, and grow a very fine suite of
crystals known as groundmass.
Part 2: Composition of Igneous Rocks
Igneous rocks are primarily composed of silicate minerals, a class of common minerals including quartz,
orthoclase and plagioclase feldspars, mica, amphibole, pyroxene, and olivine
–
all of which we will see
next week in lab. Igneous rocks are further classified by their composition and overall color. Felsic rocks
contain large amounts of
fel
dspar and
si
lica (quartz). Mafic rocks have minerals with large amounts of
ma
gnesium and iron (
fe
rrum). These groups can typically be distinguished based on their color: mafic
rocks tend to be dark (basalt, gabbro) and felsic rocks tend to be lighter colored (rhyolite, granite).
Mafic and felsic (and intermediate) lavas are often found to have erupted from a single volcano. This
observation suggests a genetic relationship between these various rock types. Here we explore a simple
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process that can explain this relationship: fractional crystallization. To be fair, the process does not seem
simple when you first think about it, but it is a good test of your ability to think in geological terms.
We start by considering a large mafic (basalt) magma body newly arrived in a storage chamber beneath a
volcano at the earth’s surface. This magma formed by melting the mantle, so it is rich in Mg, Fe, and Ca.
It is liquid at a temperature just over 1200° C but it soon begins to cool; crystals form as it starts to solidify.
Let’s also assume that the volca
no fed by our new magma erupts periodically and allows us to observe
changes within the chamber over tens of thousands to millions of years.
Here are some key questions to ask yourself about this magma body. Our goal is to think through the
processes associated with both crystallization and eruption.
Will this mafic magma body crystallize into solid gabbro (the intrusive equivalent of basalt)?
No, because we specified that there will be periodic eruptions from the volcano. During the slow cooling
process, the first crystals the form will sink to the bottom of the magma chamber. The remaining liquid
has a slightly different composition, and these differences are exacerbated over time. There will be some
gabbro at the bottom of the chamber when it is cooled entirely, but there will be other rocks as well.
How do we know that melt composition in the magma body can change during crystallization?
We can use the composition of volcanic rocks to track this process. When individual volcanoes erupt a
range of lava compositions, we know that the melt composition has changed during crystallization.
How does this change occur?
The first minerals to grow at high temperatures (typically olivine and pyroxene) consume large amounts
of Mg and Ca. These minerals are denser than the magma and settle to the floor of the chamber. The
remaining magma is now cooler and somewhat depleted in Mg and Ca, so (1) it is no longer a basalt, and
(2) the minerals that need those conditions can no longer grow. Instead, over time we see a gradual change
in the composition of the magma and the formation of minerals that are appropriate to the new thermal
and chemical conditions. This process is shown schematically below. It is termed fractional crystallization
because during each step (really a continuum) a
fraction
of the melt undergoes
crystallization
. Notice
that the composition of the magma changes as cooling proceeds. Successive volcanic eruptions will tap
this evolving liquid, giving us a range of lava types at the earth’s surface.
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Rocks are made of minerals, and for igneous rocks we can use the minerals we observe to determine the
temperature at which a rock crystallized. We can also infer a great deal about the composition of the lava,
and the way in which it erupts.
Rock Type
Basalt
Andesite
Rhyolite
SiO
2
content
45-55 wt.%
55-65 wt.%
65-75 wt.%
Magma temperature
1000-1250
o
C
800-1000
o
C
600-800
o
C
Viscosity
Low
Increasing
High
Gas escape from magma
Easy
Increasing
Difficult
Eruptive style
Peaceful
Increasing
Explosive
Q7. Consider our large magma body that was initially basaltic in composition. Assuming that geologic
processes operated ideally (which they do not, but today we can pretend), what would be the sequence
of lavas erupted from the volcano as it cooled?
Q8. How would the dangers associated with eruptions change over the life of this imaginary volcano?
Q9. Pumice and obsidian (sample 4) are formed from similar magma types and often during the same
eruption. What is different about the rocks? What would cause this difference? Think about what,
besides lava, gets ejected in volcanic eruptions.
68
Q10.
Sample 5 has an unusual feature among the other rocks you’ve been provided.
It doesn’t have a
purely extrusive texture.
The splintery dark minerals (the only visible individual crystals) are the
amphibole hornblende.
Develop a crystallization history for the sample.
Describe the scenario below.
Part 3: Identifying Igneous Rocks (use the table on the next page)
Igneous rocks can be described in terms of their cooling rate (texture) and composition (color and
mineralogy).
Q11.
Fill in the table on the following page.
You should identify the 9 samples based on texture and
composition.
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69
GEOSCIENCES 001 LAB: IGNEOUS ROCKS
SAMPLE
MAFIC, INTERMEDIATE OR FELSIC
GRAIN SIZE /
COOLING RATE
TEXTURE
ROCK NAME
1
2
3
4
5
6
7
8
9
70
IDENTIFYING IGNEOUS ROCKS
TEXTURE
MINERAL COMPOSITION
FELSIC
(light
–
pink/white)
INTERMEDIATE
(white/gray)
MAFIC
(dark /
black)
ULTRAMAFIC
(very dark / green)
INTRUSIVE
Phaneritic (coarse
grained)
GRANITE
DIORITE
GABBRO
PERIDOTITE
EXTRUSIVE
Aphanitic (fine
grained)
RHYOLITE
ANDESITE
BASALT
Porphyritic (larger
crystals in fine
groundmass)
ADD PORHYRITIC TO ROCK NAME
Vesicular (vesicles /
open holes)
PUMICE
SCORIA
Glassy (vitreous)
OBSIDIAN
(black)
Pyroclastic (contain
other fragments)
TUFF