Chapter 7 Volcano Basics
1.0 Introduction
The second process we study that results from Earth‟s internal energy is volcanic
eruption.
Dormant or extinct volcanoes
look great (Fig.1; Mt. Fuji in
Japan)! There‟s also a great deal
of good that‟s associated with
volcanoes: many volcanic rocks
decompose very rapidly to form
wonderfully rich agricultural soil; a large proportion of the world‟s richest copper, gold
and silver deposits are associated with volcanoes; and the heat associated with the deep
magma and hot rocks supplies a significant proportion of geothermal energy to
consumers. Also, of course, the slopes of extinct volcanoes are used in many parts of the
world for recreational enjoyment such as down-hill skiing. But, of course, what we‟re in
this course to consider is not the good but the very bad!
Volcanoes are testament to Earth‟s huge internal energy, and the release of that energy
can be catastrophic. Imagine a volcanic eruption in which an area the size of Greater New
York City collapses, a region the size of Nova Scotia is suddenly buried under meters of
hot ash that snuffs out all life, and farmer‟s fields as far as 100 to 2000 km away
(depending upon winds) are covered in so much ash that all crops are killed. Imagine that so
much ash and gas is blown up into the atmosphere that Sun‟s light is dimmed to the point that there‟s no summer for two years. Unbelievable? It happened at least three times in
the western USA when a huge volcano erupted at Yellowstone National Park. We‟ll
consider those catastrophic events and others, but first we have to look at the source of
energy and material, and consider a few „normal‟ eruptions before we look at the deadly
ones.
Just as most ( but not a) earthquakes are tied to current plate tectonics of Earth, so most
(but not al) volcanic eruptions are clearly tied to plate tectonics – as we‟ll see later in this
chapter. In Figure 2 you see the strong correlation of plate boundaries, earthquakes and
volcanoes.
Want to see an eruption? Well, while almost
every day you can expect a new or continuing
eruption around the rim of the Pacific Ocean
(due to the strong subduction of the Pacific Plate and the
melting associated with), for sure there‟s going to
be eruptions at sea floor spreading centers (say,
the Mid-Atlantic Ridge of Fig. 2) – but flying
back and forth around the volcanic “Ring of
Fire” of the Pacific rim can be expensive, and
you can‟t see the eruptions under oceans. So
you‟d better head for an island volcano called Stromboli (Fig.3) off the coast of Sicily; it
erupts every 20 minutes to 1 hour every day!
2.0 Magma Characteristics
Magma is defined as molten rock, and may be a mix of actual hot liquid, gases and
scattered mineral crystals. When magma spills out on surface it produces hot lava (see lava
running down the side of the cone in Figu) When
magma blows out in a violent eruption, the stuff
thrown into the air is usually called pyroclastic
material (Fig.4; safe to say the driver of the jeep has
„the pedal-to-the-meta) and may be in the form of
blobs of hot magma ( sometimes called „bombs‟),very
large solid blocks, or very fine pulverized
material commonly referred to ( somewhat
misleadingly) as „ash‟. Initially, pyroclastic
material may be very, very hot because it is
being blown out by very hot gases. All of the
material erupted from a volcano forms volcanic
rock of various types ( we‟ll look at some of them bel) when cooled and solidified. Magma
can also solidify (i.e. cool and crystal) beneath surface ( in which case it can‟t be called
„volcanic), where it produces plutonic rocks. All types of rock produced from magma –
whether volcanic or plutonic - are classified as igneous rock.
2.1 Viscosity and Volatiles Viscosity is defined as that property of materials that provides resistance to flow; the
opposite of viscous is, therefore, fluid. Viscosity/fluidity involves interaction of many
properties; this can be difficult to explain. As far as magma is concerned, one factor over-
rides all others: temperature; the higher the temperature of any material, the more fluid
(i.e. less viscous) it is. Reason: the higher the temperature, the greater the rate of atom
vibration – thus bonds between atoms (which would form structures in the liquid
inhibiting fluidity) tend not oo form very readily. In fact, based on nothing but
temperature, a liquid at 900 C would be about 5 orders of magnitude (i.e. 100,000 times)
more fluid than the same liquid at 600 C.o
Of course, there are other factors that influence viscosity, and next most important is
chemical composition, in particular the
content of two elements: silicon (Si) and
oxygen (O). Reason: Si and O atoms have a
very strong affinity for each other and tend to
form little tetrahedral structures where each
silicon atom ( small central atom of Fi) is
surrounded by 4 oxygen atoms ( large
surrounding atoms of Fig.); the more of
these little structures in a liquid, the more
viscous the liquid. But it‟s actually worse
than that: the more of these SiO (4 1 Si and
4 Os called silica tetrahe) in a liquid, the
more they will tend to bond together to
form long strings, sheets and frameworks
of Si and O atoms by sharing corner
atoms of those tetrahedral (Fig.6). This
can really
quickly affect
viscosity. Of
course, the
tendency to
form these
silica
tetrahedra is
offset by
temperature:
few at high
temperature,
rapidly
increasing
number at
lower
temperatures.
[For all you
Science people: please understand that I’m using the word ‘atom’ for simplicity; if I wanted to use proper terminology, I’d have to
consider ‘ions’.]
If increasing content of silica structures increases viscosity, magma with increasing
content of crystals would also become increasingly viscous. Certainly as magma cools,
more and more crystals form – eventually turning the material into a solid rock. Take a
look at Figure 7; start with a magma we‟ll call basalt (defined later) that‟s really hot and
contains no crystals, and now slowly cool it. The first crystals that form will be those that
can form stable bonds between atoms at high temperatures (olivine and pyroxene); at
ever lower temperatures other minerals become stable, until finally, at the lowest
temperature of the magma (just before the whole thing becomes solid) quartz (SiO ) 2
forms.
There‟s one final complication to the matter of
viscosity, and that‟s the volatile content of the
magma – particularly water since it‟s by far the most
common volatile compound. The water molecule (1
atom of oxygen and 2 of hydrogen) is unique. The
water molecule is not symmetrical; the two hydrogen
atoms are tacked on to the oxygen atom in such a way
that one end of the molecule (the hydrogen end) is
relatively positively charged while the oxygen end is slightly negatively charged. In other
words, we have a dipolar molecule (Fig.8).
Why is that significant? At magma temperatures of some moderate value, where silica
tetrahedral would normally start forming and making structures (where the oxygen of one
tetrahedron shares as the oxygen of another tetrahedron and on and on), the slightly
positive ends of water molecules attach lightly to the oxygen instead – thus breaking up
the chain effect. The result is to keep the magma more fluid than it would be if it
contained no water. When water is attached to other molecular structures in this way, it is
called dissolved water. But there has to be a limit. You can readily understand that if the
magma happens to contain more water than there are opportunities for attachment to
silica tetrahedral structures, that extra water will simply be left as contained bubbles of
water – that is, exsolved water. It so happens that as the temperature of the magma
lowers, water is less able to attach to structures, and the amount of exsolved water
increases (we’ll see how that leads to explosions ).ter
2.2 Types of Magma
We can get a good idea of how the properties
of magma pretty much control the type of
volcanic activity we see by restricting our
discussion to those magmas that lead to three
types of volcanic rock: basalt, andesite and
rhyolite. Geologists go to great lengths to
determine the source of a melt or magma that
produced particular sequences of rock. They have determined that as far as volcanic rocks are concerned, nearly all magma has begun
with a partial melt of Earth‟s asthenosphere; sometimes that partial melt is spewed out of
a volcano with virtually no modifications to the initial composition; that‟s going to be
called basalt. Most basalt flows from a volcano as very hot and very fluid lava. There are
two basic textural types of basalt lava: aa (which is a rubbly-looking rock) and pahoehoe
(which is a smooth-surfaced, ropey-looking rock (Fig 9). Both are well-exposed in Hawaii, thus
the Hawaiian names. The most obvious (and only significant) chemical difference in them is
that aa contains much less water than does pahoehoe.
Andesite is volcanic rock produced from a modified partial melt. In other words, the
primary melt that would ordinarily yield basalt has been changed in composition
somehow, the result being a magma that‟s more Si-rich and more volatile-rich than the
original. Rhyolite is a volcanic rock even more modified (still higher Si and even higher
volatile content) from the original partial melt that yields basalt. Table 1 shows the
essential differences in resultant rock appearances and mineral content, in both chemical
and physical properties of the magma, and finally in the style of eruption expected from
volvanoes fed by the different magmas. I strongly recommend you review this table
carefully and thoroughly; not only will you be better able to understand the following
discussions, but the content is also appropriate for exam questions!
3.0 Volcano Settings Over 90% of volcanism is associated with edges of tectonic plates (look back at Fig. 2).
Of the remaining 10%, most is erupted from hot spots marking the tops of mantle plumes.
Figure 10 is a hypothetical cross section through Earth‟s lithosphere and asthenosphere
illustrating volcanic activity at a spreading center (right side) and at a subduction zone
(left side). We‟ll look at those settings very briefly; we‟re going to delay our discussion
of volcanic eruptions associated with mantle plumes until the next chapter.
3.1 Spreading Centers
Most active plate spreading takes place underwater. From the point of view of volcanic
eruptions, the mid-ocean ridges constitute a comparatively simple environment (Fig.10).
As we learned in Chapter 3 (Plate Tectonics), at oceanic spreading centers the
asthenosphere - at temperatures usually between 1200 C and 1300 C - is as close as it
ever gets to the surface, and the cool lithosphere can be as thin as 3 km. As the plates
diverge, the hot asthenosphere rises
toward the crest of the mid-ocean ridges.
So how does a melt form from the hot
mantle/asthenosphere rock? The answer
comes from Figure 11.
Figure 11 ( Pressure decreases upward on the
vertical axiwas constructed to discuss a
different tectonic environment, but the
principle is the same at a spreading ridge
in the middle of an ocean. The
asthenosphere is so hot it‟s just at the point
of melting; it is commonly described as
„hot plastic‟ rather than either solid or
liquid. Let‟s say that we are considering
asthenosphere at the position A of Figure
11. The point A indicated rock that is stable just inside the „solid‟ field. If we increased the temperature of the rock at point A
(i.e. moved it horizontally to the right), obviously it would begin to melt because it‟s now
in the „melt‟ field of stability. On the other hand, if we decreased the pressure on point A
rock (i.e. moved A vertically upward), it would also begin to melt as it crossed over from
solid to melt stability fields. As you can see from Figure 10, at an ocean spreading center
the asthenosphere is very close to the ocean floor, thus here we are looking at the
situation of reducing the pressure on the asthenosphere rock rather than increasing the
temperature on it.
OK, we have a valid
mechanism to make a melt
from asthenosphere rock.
So how much melts? All
of it? Nope! Not even
close! At most, 30-40% of
the asthenosphere rock
will melt, and that melt
will be basalt in
composition. Figure 12 is
a simple-minded
illustration of the melting
of selective minerals,
while the ones with higher
melting temperatures resist
breakdown. The magma
we produce at spreading
centers under the oceans is
called MORB for Mid-
Ocean Ridge Basalt.
Whatever the
nomenclature, note that
the rock produced is
basalt. At ocean spreading
centers, the asthenosphere rock that‟s labeled „solid‟ in Figure 11 ( the rock that’s going to
partially me) is called peridotite; it has a density of about 3.3 g/ci.e. grams per cubic
centimete). The partial melt (that will go to form basalt) that develops from the peridotite
has a density of about 2.9 g/cc; obviously, the melt is relatively buoyant. Assuming any
fractures or vents exist (which they certainly will at spreading centers) up the partial melt
magma goes! And 80% of all volcanic rock is formed just like that. As you can readily
see, there‟s nothing catastrophic about eruptions in this environment.
3.2 Subduction Zones
Let‟s change our focus to the left end of Figure 10 – the subduction zone. Here, the
production of volcanic magma and its eruption is not at all simple, and the resulting
activity can most definitely be catastrophic. A quick look back at Figure 2 is in order here – note the many, many volcanoes located at the subduction ends of plates ( labeled with a
saw-tooth pattern and called „convergent zones‟ in the). For example, look at the grouping
around the Pacific Ocean: up the coast of South America, all along the west coast of
North America, across the Aleutian Islands to Asia, then all the way back down into the
South Pacific again. There are so many active volcanoes in this belt it is commonly called
“The Ring of Fire”.
Just so you don‟t have to keep flipping back all the time, I‟ve reproduced the left end of
Figure 10 and called it Figure 13.
The initial magma begins the same
way as for spreading centers – as a
partial melt of asthenosphere. But
the reason for generating that melt
is not because of either rising
temperature or decreasing pressure
– it‟s because we‟re adding water
to system. In Figure 11 there‟s a
broad solid line between “solid”
and “melt”; if we added water to
that system, it would have the
immediate effect of moving that
solid line toward lower
temperatures. In other words, if we
add water to a relatively dry rock
(at high temperatures, of course),
the rock will begin to melt at
much lower temperature than in a dry system.
Where does the water come from? The slab of material being subducted (in Fig.13) is
ocean floor lithosphere – and the top layer is completely saturated with water. As that
slab drops further and further into the hot asthenosphere (and you’ll recall that it drops
because cold ocean lithosphere is more dense than hot asthenosphere), the increased heat
it‟s subjected to drives water off – and straight up into that wedge of hot asthenosphere
between the top of the ocean lithosphere slab and the base of the continental crust
(Fig.13). And once again, just as at oceanic spreading centers, a partial melt of basalt-
equivalent is produced, and up it goes.
At the base of the continental crust the basaltic melt comes in contact with some rocks of
dramatically different composition. The „roots‟ of continental crust blocks are composed
of very old and very evolved rocks that are granitic in content. That means they contain
an abundance of those minerals we saw at the low temperature end of the mineral
sequence shown in Figure 7 ( please go back and lo): potassium-rich feldspars, some micas
and quartz. The result when a high-temperature basaltic magma encounters a low
temperature granitic rock is melting of the granitic rock and, thus, modification of the
magma. Depending upon how much continental crust it has to move through to reach
surface, the magma will be increasingly modified. It‟s important to learn what those modifications are because they change a volcano from a peaceful eruption to a
dramatically explosive eruption.
The schematic of Figure 13 shows blobs of
partial melt magma gradually moving from
the point of origin (just above the
subducting slab) upward toward surface.
These blobs are meant to represent „magma
chambers‟ where the magma makes room for itself in the continental crust rocks as it
moves in „spurts and stops‟ toward surface. Figure 14 ( ignore the terminology right now, just
look at the proc) shows some of the detail right at the
walls of a hypothetical magma chamber. The hot
magma works it way into fractures in the surrounding
solid rock, loosens blocks of that rock which gradually
fall into the magma, and melts some/all of those blocks
(depending on their composition). This is a process
called assimilation. By this process, the originally
basaltic magma changes composition to some
intermediate composition between the original and the
composition of the rock it melts. In fact, there are a few
really important processes like this simultaneously
changing the magma composition.
Another is illustrated in Figure 15; as magma gradually
moves toward surface, it cools. At cooler temperatures
crystals begin to form in the melt; if they are crystals of
minerals that are relatively heavier (i.e. higher specific
gravit) than the melt, they‟ll fall through the magma
chambers toward the base, where they may form a rock
consisting almost entirely of these minerals (plus
whatever magma is trapped interstitially). This is a
process called fractional crystallization (i.e. a fraction
of the melt crystallizes and thus removes itself from the
bulk of the magma that sooner or later will move further up toward surface).
OK, let‟s look at one more process that modifies magma composition. Figure 16 shows
one bleb of supposedly basaltic magma joining and mixing with a larger bleb of magma
that‟s been sitting in a magma chamber for some time, thus already quite modified in
composition. This is a process called magma mixing. The result is simply to produce
magma of some composition intermediate to that of the two „pots‟ that mix.
The result of these modification processes has been to produce magma dramatically
different in all properties to those of the original partial melt. Certainly, the magma that
finally makes it to surface will be cooler, more silica-rich, more volatile-rich and thus
considerably more viscous and explosive than a basaltic magma. The rocks produced
from such magma are commonly andesite or rhyolite; take another look at the basic
properties of those rock types in Table 1.
4.0 Volcanic Eruption Hazards
We‟re not going to waste time discussion
potential hazards around spreading center
volcanoes; they‟re non-existent or
insignificant. We‟ll move straight to the
dangerous category: subduction zone
volcanoes, where we‟ll look at three major
volcanic products: lava, pyroclastic
products and gas. There also are some
secondary products which we‟ll mention as
we proceed; see Figure 17 for a very
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