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9. Volcanoes
The eruption of Mt. Vesuvius in 79 A.D. was the first volcanic eruption for which there is a detailed
written record. That eruption buried Pompeii and Herculaneum and more. There have been many
eruptions of Vesuvius since and geophysical analysis indicates that there were major eruptions
dating back 17,000 years, the ones in ca. 5960 B.C. and 3580 B.C. being amongst the largest ever in
Europe. The locals near Vesuvius took their Neopolitan word for a stream caused suddenly by rain
– lava – and used it also for the streams of molten rock pouring down the sides of Vesuvius. The
word became part of Italian and subsequently, about two hundred and fifty years ago, part of the
English language. The picture below shows the cone (gran cono = great cone) of Vesuvius in front
of part of the remains of the Somma Volcano from 17,000 years ago.
Worldwide, there is about one volcano eruption per week. At least 1300 volcanoes have erupted
during the past 10,000 years that can be documented (directly or indirectly). That doesn’t count
multiple eruptions per site. The count is problematic though. Seafloor estimates are much higher.
Some volcanic systems have hundreds of escape cones that could be counted separately or as due to
a single underlying feature. The eruption frequency has varied over earth’s history. There have
been periods of high activity and also of low activity.
Yellowstone Park is a huge plateau built up by three cycles of volcanic activity. These presumably
lasted only days to weeks, but occurred in the past two million years. Melded hot ash, pumice and
other material is as much as four hundred meters thick in places. That’s almost 25% higher than the
Eiffel Tower. The volume was 2450 cubic kilometers for the first eruption which covered an area of
over 15,000 square kilometers and took place 2.1 million years ago. The second eruption, dated at
1.3 million years, had a volume of 280 cubic kilometers and the most recent eruption, 0.64 million
years ago, produced another thousand cubic kilometers.
The map below shows the worldwide distribution of volcanoes. All but a few percent occur as
strings of eruptions. Most are caused by subduction of plates.
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Volcanic Gases (Krauskopf, Intro to Geochemistry)
In understanding the cumulative effects of volcanoes, the idea is to consider that the composition of
the gases from modern volcanoes is a reasonable model for primitive volcanoes. But we must keep
in mind that modern eruptions recycle constituents of sediments. It is realistic to think of that as
having occurred over eons except for the initial formation of atmosphere, prior to weathering to form
sediments. Throughout geological development, huge amounts of carbonate sediments would be
subducted by tectonic motion of plates. At high temperatures, they would react with abundant silica
(SiO2) and metamorphate into silicates, releasing carbon dioxide as a product. The carbon dioxide
would be trapped in the hot blend but would be released at sites of volcanoes and mid-ocean ridges.
Here, crustal plates separate. The mantle moves upward releasing pressure on the mantle below
causing the local contents to melt (in the absence of any applied heat). That is the typical behavior
of most substances that are near phase changing conditions to start with.
The violent and spectacular eruption of volcanoes is due in most part to the escape of gases
dissolved in the lava, the escape being enabled by the reduction in pressure as the molten material
approaches the surface. Roughly 5% of the weight of the magma represents dissolved gases. A
common scenario is one in which the dissolved gases collect at the top of the magma until the
pressure accumulation forces the gases through overlying rocks, carrying small bits of molten lava
with it. Fluid, molten lava follows the opened path of fissures. Gases continue to rise giving the
sometimes long succession of explosive displays. Gases can accumulate in a large bubble maybe ten
kilometers below the surface. Long term accumulation of gases seems to play an important role in
terms of buoyancy and viscosity reduction of the molten lava.
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Some analyses from 1994 of volcanic gases taken by Symonds et al. in volume percent after
correction for what's already present in air are
Gas
Carbon dioxide
Carbon monoxide
Hydrogen
Sulfur dioxide
Hydrogen sulfide
Hydrogen chloride
Water vapor
Kilauea
Hawaii
49 %
1.5
0.5
12
0.04
0.08
37
Momotombo
Nicaragua
1.4 %
0.01
0.7
0.5
0.2
2.9
97
Erta' Ale
Ethiopia
11 %
0.4
1.4
8.3
0.7
0.4
77
Among other species also present are sulfur trioxide, nitrogen, argon, and volatile salts. Sulfur
trioxide, SO3, may be from the oxidation of sulfur dioxide, SO2. Only traces of ammonia, NH3, are
emitted implying ammonia was never an appreciable atmospheric constituent.
However, although the chemicals present in volcanic gases are roughly the same set amongst all
studied volcanoes, the abundances vary very markedly from site to site, and even at the same site at
different times. Much of the variation is attributed to chemical reactions that occur whose results
depend on temperature, pressure, and air and water encountered. The generalization is that water
vapor dominates, usually amounting to more than 90% of the emissions. The carbon-containing
gases are next, with carbon dioxide being the most prevalent among them. Volcanism is the greatest
source of natural carbon dioxide supplied to the air. Approximately 100-200 million tons of carbon
dioxide are released into the atmosphere each year by volcanoes. This happens to be less than 2% of
that emitted as the result of current human activities and less than 0.01% of the total in the
atmosphere.
But well before the Vesuvius eruption that buried Pompeii, the Carthaginian admiral Hanno sailed
around west Africa around 500 BC and recorded sighting the eruption of what is now Mt.
Cameroon. His route is shown below. This was the only known volcanic eruption on the continent
of Africa for the next two thousand years.
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There are three lakes in the world where carbon dioxide has accumulated at the bottom in huge
amounts. These are all in Africa: Lake Nyos and Lake Monoun both in Cameroon and Lake Kivu
on the border between Congo and Rwanda. The carbon dioxide allegedly has seeped into the lake
bottoms from magma pools fifty miles below in the crust. The carbon dioxide is trapped there in a
highly unstable situation. It dissolves in the first water in contacts, at the bottom. Continual feeding
with no mixing of water layers leads to saturation of the bottom of the lake with carbon dioxide.
Elsewhere, as is more frequent, the carbon dioxide that seeps up goes either directly into the
atmosphere or bubbles into springs that release the gas to the atmosphere rather than feed a lake
bottom. Lake Nyos is a crater lake inside a dormant volcano up in the mountains of northwest
Cameroon. Its waters don't rise and fall, but are quite still so that the bottom-dwelling gas is not
circulated. Escape doesn’t happen. The gas-loaded water is kept at the bottom by the pressure of
the uncarbonated water above.
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Like a soda can being opened, releasing the pressure within, in August of 1986, some undetermined
happening caused the carbon dioxide to erupt more than 200 feet high. A huge cloud of water and
gas moving at a reported forty-five miles per hour spilled downhill, the heavy and suffocating gas
racing into the valleys and villages below, killing 1700 people and hundreds of animals. It is
estimated that a cubic kilometer of carbon dioxide gas was released.
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Two years earlier and about sixty miles away, Lake Monoun’s carbon dioxide storage erupted
killing 37 people. These phenomena could have been triggered by events as simple as a landslide,
earthquake, or perhaps even a change in weather or gusty winds. As of the year 2000, Lake Monoun
was 83% saturated at the greatest depths and Lake Nyos was 60% saturated with carbon dioxide at
its bottom. Tests are being run to see if large pipes might controllably release the pressure directly
to the atmosphere.
Among explanations of mass distinctions is the possibility that the Lake Nyos scenario took place on
a much vaster scale involving deep anoxic ocean water suddenly turning over and releasing carbon
dioxide.
Flood Volcanism
In the state of Washington lies one of the youngest of a kind of layered structure known as a flood
basalt. It has a volume of about 0.25 million cubic kilometers. In the picture below you can discern
the separate layers, each one to two meters thick, that were deposited over a period of about a
million years at roughly sixteen million years in the past. The responsible eruptions eject vast
amounts of high-temperature magma (liquid rock) from the earth’s interior. The process is called
“flood volcanism”.
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There are a number of fairly well characterized such basalt fields that have appeared throughout the
world during the past quarter of a billion years as indicated on the map below. They seem to have
occurred on the average of 23 million years apart, although there is no significance to that figure.
The oldest and by far most striking of these is in Siberia.
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Basalt “province”
Columbia River (US)
Ethiopia
North Atlantic
Deccan Traps (India)
Madagascar
Rajmahal (India)
Serra Geral (Brazil)
Antartica
Karoo (Namibia)
Newark (US)
Siberian Traps
Location
80˚ E, 10˚ N
100˚ E, 60˚ N
Age (My)
16
31
57
66
88
116
132
176
183
201
249
Those mostly Russian buildups are mostly buried under almost two kilometers of sediment
deposited subsequently. Recent explorations and dating of these so-called Siberian Traps have
shown the fields to be incredibly extensive, covering an area of about two and a half million square
kilometers, larger than that of Europe. (“Trap” comes from the Sanskrit word for “step”.) They are
up to two kilometers in thickness and comprise a volume estimated to be between two and three
million cubic kilometers. Obviously, the transport of this material to the surface resulted in the
emission to the atmosphere of large quantities of volcanic gases, of which carbon dioxide is a
significant component. The age of the Traps, 249 My, is provocatively close to that of the severe
extinction at the end of the Permian period in which three-quarters or more of all animal species
disappeared. The rise of the dinosaurs followed this extinction.
The largest such basalt province though is under water. The Ontong Java Plateau is one of several
ocean basin flood basalts. It is northeast of Australia at approximately 160˚ E and the equator. Its
area is about that of Alaska’s, five times that of the Deccan Traps and a third larger than the Siberian
Traps. The depth of the deposit averages 30 km and its volume is a mind boggling 50M km3. It
dates back to about 120 My ago with indications that there is also a younger component 90 My old.
Other effects
Mt. Pinatubo’s eruption, its first in 500 years, likely contributed to the reduced atmospheric growth
rate of carbon dioxide in the early 1990’s. Surface air temperature dropped 2˚C in the summer and
over 3˚C in the winter the next two seasons in the northern hemisphere. Ozone levels seem also to
have been affected, decreasing 5% in mid latitudes.
Earthquakes
In California is a large remnant volcanic caldera known as the Long Valley Caldera. In its
southwest corner is Mammoth Mountain, which is volcanically active and has been for
approximately four million years. Except it hasn’t erupted in nearly two centuries. However,
underneath the mountain there is a lot of minor earthquake activity, perhaps a couple of hundred
events per month at times. In the early 1990’s forest rangers noted that there were areas of dead
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(and dying) trees at the foot of the mountain. Studies eliminated both drought and insect damage as
causes. It was then found that the soil air contained between 20 and 95 % carbon dioxide compared
to 1% in regions away from the dead tree affliction. (See “Carbonates and Water”). That’s enough
to kill the trees, essentially by asphyxiating them at the roots where oxygen is needed.
Approximately 100 acres have been destroyed by releases of perhaps 300 tons of carbon dioxide per
day. The carbon dioxide very likely comes from magma intrusions into limestone-rich sedimentary
rocks underlying the region.
Measuring the rate of carbon dioxide emissions in calderas and on the sides of volcanoes is difficult
and can be dangerous. But there are now portable carbon dioxide detectors that facilitate these
studies. Air samples are quickly passed through a tube crossed by a beam of infrared light, filtered
so that a wavelength of 4.26 micrometers is all that is used. This corresponds to a wave number in
the infrared absorbtion spectrum of 2347 cm-1 discussed earlier that is pretty much specific to carbon
dioxide. By using a reference sample containing 372 ppm carbon dioxide, small changes in CO2
levels can be obtained quickly, automatically, accurately, and precisely: to better than 0.1 ppm.
“Civilization exists by geologic consent, subject to change without notice.”
WILL DURANT, Historian
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