Download Chapter 10: Volcanism and Extrusive Rocks

Chapter 10: Volcanism and Extrusive Rocks
This chapter continues our discussion of igneous rocks, specifically extrusive (volcanic) rocks.
You can just skim the first few pages, which cover interesting volcanic eruptions at Mt. St.
Helens, Mt. Vesuvius, and Mt. Mazama (which created Crater Lake). The “meat” of this chapter
(or should we say the “stone”?) begins with the section entitled, EXTRUSIVE ROCKS AND
GASES.
Volcanic materials include lava (molten rock material at Earth’s surface), various gases, and
explosively erupted rock fragments called pyroclasts. We briefly discussed the relationship
between lava and magma in Chapter 11.
Speaking of volcanic gases, I once heard a geology professor from the University of Southern
California say that the gases that accompany volcanic eruptions include some of the same ones
that come from the tailpipe of your car--gases such as carbon monoxide, carbon dioxide, water
vapor, and various nitrogen oxides.
Other volcanic gases include sulfur dioxide, hydrogen sulfide, and hydrochloric acid; however,
the single most abundant gas to accompany volcanic eruptions is water vapor. In fact, the
earliest atmosphere here on Earth consisted in part of volcanic gases from Earth’s interior.
Pyroclasts consist of rock debris of explosive origin and range in size from dust and ash up to
blocks and bombs.
We’ve already been introduced to a number of important volcanic (extrusive) rocks in the last
chapter. Let’s get some more practice with classification of the volcanic rocks we’re already
familiar with. See if you can fill in the following table:
<<<<<<<<<<<<<<<<<<Composition>>>>>>>>>>>>>>>>>>
Felsic
Intermediate
Mafic
Volcanic (Extrusive)
Rock Name
In order from felsic to intermediate to mafic, the correct answers are rhyolite, andesite, and
basalt.
Remember, for the volcanic rocks listed above, the rock is classified on the basis of origin
(intrusive vs. extrusive) and composition (felsic, intermediate, or mafic).
Another important characteristic of igneous rocks in general and volcanic rocks in particular is
texture. In this context, texture doesn’t mean how the rock feels (rough vs. smooth, for example)
but rather the sizes, shapes, and arrangements of its constituents. See Table 10.1 for a
summary of volcanic rock textures.
Rock Photos
Here are some photos of common volcanic rocks, including rhyolite, andesite, basalt, obsidian,
pumice, tuff, and volcanic breccia.
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Rhyolite (courtesy Wikipedia: http://en.wikipedia.org/wiki/Rhyolite).
Rhyolite is typically light colored due to an abundance of non-ferromagnesian (iron/magnesiumpoor) minerals like quartz and feldspar.
Andesite (courtesy of USGS; http://volcanoes.usgs.gov/images/pglossary/andesite.php).
Andesite is typically a bit darker than rhyolite due to higher percentages of ferromagnesian
minerals, including pyroxene and amphibole.
Basalt (photo courtesy of Wikipedia; http:/en.wikipedia.org/wiki/Basalt).
Basalt is typically darker colored than rhyolite and andesite because it contains high
percentages of ferromagnesian minerals.
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Obsidian (photo courtesy of Wikipedia; http://en.wikipedia.org/wiki/Obsidian).
A quick note about obsidian (volcanic glass). Under some circumstances, volcanic rock cools
too quickly for the atoms in the rock to link up into ordered crystals. In this case, the rock still
solidifies but it doesn’t crystallize. Instead, the solidified material retains its fluid properties (i.e.,
randomly arranged atoms that slowly flow with time) and a volcanic rock called obsidian is
created.
Although you wouldn’t think so, most obsidian is felsic in composition. It’s dark colored due to
trace impurities, including iron and magnesium.
In other situations, obsidian can form even when a felsic lava cools somewhat more slowly. The
reason for this is that felsic lava tends to be very thick, somewhat like toothpaste (for reasons
we’ll discuss below); so thick, in fact, that the atoms have trouble migrating through the thick
fluid to link up into crystals. Many people, including myself, are fascinated by obsidian, perhaps
because it’s…well…shiny and pretty! Here’s a link to a website devoted to obsidian if you’re so
inclined: http://en.wikipedia.org/wiki/Obsidian.
Pumice (courtesy Wikipedia commons; http://commons.wikimedia.org/wiki/File:Pumice_santorini.jpg).
Pumice is just “frothed up” obsidian, basically volcanic glass with lots of gas escape bubbles
called vesicles. In fact, pumice has such a low density that it floats in water.
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Penny for scale
Tuff (courtesy USGS; http://geomaps.wr.usgs.gov/parks/rxmin/gtuff.html).
Tuff (hardened volcanic ash) consists of a mixture of small volcanic rock and mineral fragments
(pyroclasts) and is indicative of explosive volcanic eruptions.
Volcanic breccia (courtesy USGS; http://pubs.usgs.gov/of/1999/of99-168/of99-168-pamphlet.html).
Volcanic breccia (photo above) is a pyroclastic igneous rock, like tuff. The main difference
between these two rocks is that volcanic breccia is composed of larger fragments (pyroclasts).
Eruptive Style Controls
At this point I’d like to discuss a topic your book avoids, or at least minimizes. What is it that
controls the eruptive style of a volcano? Why do some volcanoes erupt non-violently, whereas
others produce devastating, violent eruptions?
Simply put, three factors control eruptive style, including: (1) silica content, (2) magma
temperature, and (3) dissolved gas content.
Fluid thickness (resistance to flow) is a property known as viscosity. The extremely high
viscosity of felsic lava prevents the atoms from moving through the cooling liquid to link up into
crystals as the rock cools.
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In a cooling, silica-enriched magma, the silicon and oxygen atoms link up to form complex
tetrahedral structures—this, in turn, tends to increase the viscosity (fluid thickness) of the
magma…imagine a magma developing millions of tiny silicate strands, sheets, and 3dimensional frameworks, all tangled up like knotted spaghetti.
In essence, gas-charged, silica-enriched magmas of intermediate to felsic composition generally
erupt violently because such magmas have a high viscosity. They’re very thick, which means
that the gases in the magma can’t easily escape as the magma rises to the surface. This
causes gas pressures to rise until…boom!--they finally escape in an explosive manner! A good
analogy from cooking is the way that homemade spaghetti sauce (which is thick and rich)
behaves when brought to a boil on your stove. Droplets of sauce start flying! Think of spaghetti
sauce as highly viscous magma.
Magma temperature is also important because it, too, affects viscosity. Most fluids become
more viscous as the temperature drops. Using another food analogy, think of how thick cold
pancake syrup becomes when you put it in the refrigerator. On the other hand, if you’re like me
and like your syrup warm, you’ve undoubtedly noticed that warm pancake syrup is thin and
runny—that is, it has a low viscosity. Temperature measurements collected from erupting
volcanic materials indicate that intermediate and especially felsic magmas tend to exhibit lower
temperatures than hotter, mafic magmas. Lower temperatures, in turn, mean higher magma
viscosity and more explosive volcanic eruptions.
Finally, the dissolved gas content of a magma also has a direct bearing on how a magma will
erupt. The more dissolved gas, the more explosive the eruption. As a magma rises toward the
Earth’s surface, it experiences lower confining pressure, which causes more and more of the
dissolved gas in the magma to come out of solution. The same thing happens when a bottle of
soda is opened…gas bubbles appear. As more and more dissolved gas exits the magma,
magma pressure increases until it exceeds the confining pressure and a violent eruption is
produced. There’s also a direct relationship between the composition of a magma and its
dissolved gas content. In general, silica-enriched magmas of intermediate and felsic
composition tend to contain a higher percentage of dissolved gases than low-silica, mafic
magmas.
To summarize, high silica content, low magma temperatures, and high dissolved gas content
are all associated with more explosive volcanic eruptions. The bottom line is this—highly
explosive volcanic eruptions are powered mostly by expanding gas, not heat energy.
For a dramatic demonstration of the violent effects of expanding gas, here’s a link to a famous
experiment most of us have seen involving Mentos mints and diet Coke: Mentos Diet Coke
Experiment. Check it out. You’ll be amazed!
The quick explanation is that this experiment works because the rough, dimpled mint surfaces
together with chemicals associated with the dissolving mints and the sweetener in diet Coke all
work together to cause dissolved carbon dioxide (CO2) in the Coke to quickly exolve (rapidly
leave the diet Coke solution), forming millions of tiny bubbles of CO 2 gas on the mint surface.
So many bubbles form so quickly that they can’t all escape at once, so gas pressure builds
until…well, you’ll see!
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Watch this video, then imagine scaling things up in your mind to a diet Coke bottle millions of
times larger…and voila! You’ve got yourself an explosive volcano! In the case of violent
volcanic eruptions, the expanding gas is primarily water vapor, and lowered confining pressures
associated with the rising magma have the same effect as adding the Mentos mints to the diet
Coke. The take-home message is that it’s expanding gas, not heat, that drives explosive
volcanic eruptions.
Relating all this to plate tectonics, Earth’s most violent volcanic eruptions are typically
associated with subduction zones, where water- and silica-enriched magmas tend to form. As
the subducting oceanic plate sinks into Earth’s mantle, it “sweats” out its water, which causes
the surrounding mantle rock directly above the subducting plate to undergo partial hydration
melting, with produces mafic magma. As the resulting water-rich, mafic magma rises into the
Earth’s crust, it assimilates and mixes with silica-enriched material typically found in the crust,
resulting in violent, water-vapor-charged andesitic eruptions typical of most subduction zones
(particularly ocean-continent subduction zones). Here’s a diagram (below) to help you visualize
things. Read the text in this diagram from the bottom up…
Volcanic Arc
Trench
Sea Surface
Assimilation of silica-rich
crustal rocks by mafic magma
produces explosive,
intermediate (andesitic)
volcanic arc magma.
Hydration melting of
ultramafic mantle rock
produces mafic magma…
Rising
H2O
Dehydration of
subducting plate
produces water…
Magma generation at an ocean-continent
subduction zone.
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One last topic to mention in this chapter has to do with the common types of volcanoes, nicely
summarized in Table 10.2 of your text. Spend some time studying this diagram! You should
become very familiar with the differences between shield volcanoes, composite volcanoes, and
cinder cones in terms of relative size, shape, composition, and eruptive style. To summarize:

Shield volcanoes are large, broad volcanic structures associated with low-viscosity, relatively nonviolent eruptions of mafic magma.

Composite volcanoes are smaller than shield volcanoes (but still pretty large) and have steeper
sides; they consist of alternating layers of lava and pyroclastic material. they’re steep-sided in part
because the viscosity of their associated lava is higher than that of shield volcanoes, which means
their lavas don’t flow as far and tend to pile up around the volcanic vent.

Cinder cones are quite small in comparison with shield volcanoes and composite volcanoes. They
consist almost entirely of pyroclastic fragments, most commonly of basaltic (mafic) composition.
Cinder cones are explosive, but less so than composite volcanoes, and tend to be steep-sided due to
the accumulation of volcanic cinders right around the volcanic vent. They’re more explosive than
shield volcanoes due to gas-enrichment of their magma.

Although calderas aren’t really volcanoes, they’re an important volcanic landform that originates from
highly explosive, gas-charged eruptions of typically felsic composition. In a caldera eruption, a
magma chamber partially blows its top, causing what’s left of the roof of the chamber to cave in,
creating a large depression. Caldera eruptions are the most explosive of all and can be absolutely
massive. Crater Lake, in Oregon, formed during a caldera eruption several thousand years ago.
Volcano photos
Check out this image of Mauna Loa, Earth’s largest volcano, a shield volcano on the big Island
of Hawaii:
N
Lava flows
central rift zone
14 mi approx
Mauna Loa shield Volcano (courtesy of Google Earth).
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Not only is Mauna Loa massive in map view, it’s tall as well. From the base of the volcano on
the sea floor to the summit elevation of 13,679 feet, Maun Loa is over 30,000 feet tall, making it
taller than Mt. Everest.
The volcano is so massive that it’s literally cracking apart under its own weight, as evidenced by
the central rift zone that runs along most of the volcano’s length.
Most of the lava flows that pour out of Mauna Loa erupt from this rift zone rather than the central
crater. Although Mauna Loa is massive and tall, it’s not very steep-sided, as shown below:
Mauna Loa, looking southwest
(photo by J.D. Griggs, courtesy of USGS; http://hvo.wr.usgs.gov/maunaloa/).
The gentle side slopes reflect the low viscocity of the mafic magma feeding the volcano.
Here’s a photo of Mt. Mayon in the Philippines, a classic composite volcano. Mt. Mayon’s steep
sides imply that it was created by fairly viscous lava:
Mt. Mayon.
(photo courtesy of Wikipedia http://upload.wikimedia.org/wikipedia/en/8/81/Mt.Mayon_tam3rd.jpg).
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This image of Sunset Crater near Flagstaff, Arizona, is a classic cinder cone:
Sunset Crater
(photo courtesy Wikipedia; http://en.wikipedia.org/wiki/Sunset_Crater).
Note that Sunset Crater isn’t very large, as evidenced by the trees for scale.
Crater Lake.
(photo courtesy UGSS Cascades Volcano Observatory; http://vulcan.wr.usgs.gov/)
The Crater Lake caldera formed approximately 7,000 years ago during the eruption of a large
stratovolcano known as Mt. Mazama. During the eruption, Mt. Mazama was destroyed as the
underlying magma chamber blew its top, leaving a large depression that later filled with water to
become Crater Lake.
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Here’s a great website created by Dr. Vic Camp, the volcanologist at San Diego State
University, that summarizes the differences between shield volcanoes, composite volcanoes,
and cinder cones: How Volcanoes Work
From the homepage, click on the “Volcano Types” link under the heading, “Volcanic Landforms”
on the left side of the page. Make sure you can compare and contrast the important
characteristics of cinder (scoria) cones, stratovolcanoes, and shield volcanoes.
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