Lecture Notes for G&G 120b: Global Change
Living on the Volcano
1. Introduction
There is a common image of volcanoes, permanently stamped on our
culture since the release of the movie King Kong in 1933. Explorers
from civilization approach a mysterious tropical island, where a
primitive village emerges from the jungle at the base of a great cone-
shaped mountain. Amid ominous tremors that threaten to topple stone
images of the local gods, the villagers anoint a sacrifice to the
volcano's wrath, typically a beautiful maiden. Hoping to avert the
avalanche of red-hot lava that the special-effects crew is preparing
just offstage, the villagers treat the volcano as though it had the
whims of a person or a god, in near-complete ignorance of the physical,
chemical and geological forces that determine its behavior.
Nowadays, our society is more likely to sacrifice politicians than
beautiful maidens. Luckily for them, our understanding of volcano
processes has much improved. A growing understanding of volcanic
hazards has led to notable successes in predicting the dangers that
volcanic eruptions pose to people and property. In striking contrast
to our sketchy knowledge of the earthquake process, both the timing and
the destructive power of many volcanic events can be estimated with
crude accuracy, at least in the short-term. Intensive efforts in
monitoring earth tremors, magmatic properties, gas emissions and the
changing shape of the volcano have led to the remarkably low loss of
life in the spectacular eruptions of Mt. St. Helens (1980) in the
Pacific Northwest and Mt. Pinatubo (1991) in the Philippines.
Nevertheless, the variety of hazards associated with volcanoes
continues to surprise scientists and public officials, and not all
hazardous regions can be monitored adequately. In addition, some
volcanic eruptions are sufficiently powerful to affect the mankind's
well-being globally as well as locally. By spewing ash and sulfur
dioxide gas high into the atmosphere, volcanoes cool the climate and
thin the ozone layer. Though fierce in eruption, volcanoes often sit
atop valuble geothermal energy resources. On a geologic time scale,
volcanoes are the most visible part of a natural underground refining
system that uses heat and fluids to concentrate useful metals into
minable ores.
We first examine the factors that influence the production of molten
rock, or magma, within the Earth, and how the composition, temperature
and eruptive environment influence the violence of its emergence at the
surface. Variations in these properties are governed largely by the
role that a volcano plays in the system of plate tectonics. Not
restricted to exotic tropical localities, active volcanism can be found
at most plate boundaries where plates either converge or move apart,
including the glaciated terrains of Iceland and the Kamchatka Peninsula
in eastern Russia. We illustrate the range of volcanic hazards with
type examples ranging from Hawaii in the US to Vesuvius in ancient Rome
to Pinatubo in the Far East. Finally, we examine the techniques with
which volcanoes can be monitored by scientists and public officials,
and their hazards mitigated to the extent possible.
2. The Factors Behind Volcanism
A. Physical Properties of Magma
Volcanoes form from successive eruptions of magma, molten rock, which
wells up to the surface as lava from the Earth's interior. A ready
supply of magma for volcanism is not available everywhere. The
temperature of the Earth's interior increases steadily as one descends
beneath the surface, but so does the pressure of overlying rock.
Rising temperature can melt rock, but rising pressure inhibits
melting. As a result, in an average slice through the Earth, there is
no molten rock between the core-mantle boundary and the surface, save
perhaps in the asthenosphere at the base of the tectonic plates
(100-200 km), where a partial melt of a few percent may exist. As a
result, unusual conditions must prevail in regions that produce magma
for surface volcanism (Figure 1 - diagrams wet&dry solidus - defined in
caption).
In one scenario, a rising blob or column of hot rock rises within the
mantle as a plume or hot spot. Though more ductile than surrounding
rock, the plume remains solid until it reaches the outermost layers of
the mantle, where the decreasing overburden triggers pressure-release
melting. Only part of the rock is melted, and this ascends beyond its
parent rock to erupt at the surface. In volcanism associated with a
mantle hot spot, a basalt melt separates from a periodotite parent rock
and ascends to erupt at the surface in the form of mid-plate chains of
volcanoes. Pressure-release melting, however can produce magma in
other situations where relatively hot rock rises through the mantle and
crust.
In another scenario, water or other volatile compounds like carbon
dioxide catalyze the melting of otherwise ordinary rock. The presence
of water in concentrations of 1-5% by weight within a rock can lower
its solidus, the temperature at which melting starts to occur, by 200
degrees C or more. Abundant at the surface, water and CO_2 are
transported into the mantle and crust by oceanic-plate subduction and
by motion along crustal faults. Much of this water is not liquid, but
rather bound into hydrous minerals like clays and serpentine. Under
sufficiently elevated temperature and pressure, water is released from
these minerals and can promote melting in the surrounding rock.
The mode of magma generation has a profound impact on volcanic
behavior. As volatiles dissolved in the magma increase, so does the
potential violence of its eruption. Once exposed to atmospheric
pressure, water vapor and other gases emerge virtually instantaneously
from the liquid rock. If you can imagine opening a bottle of champagne
that has been heated in a microwave oven, you can appreciate the
dangers of a phreatic eruption of "wet" magma. Magmas at Hawaii are
relatively dry, as their source is a deep- seated mantle hotspot.
Magmas in the Pacific Northwest Cascades contain more dissolved water.
No magma is completely free of volatile compounds, however. Even at
Hawaii's Kilauea volcano, mild-mannered basaltic eruptions can resemble
kitchen sponges, from air bubbles, called vesicles, formed as water
vapor escapes the lava.
The viscosity of magma also influences its hazard potential. Viscosity
measures the tendency of a fluid to resist flow, and is defined as
strain rate = stress/viscosity
Subjected to an equal stress, fluids with higher viscosity will flow
less readily. The tangential traction exerted by a rolling tire on
pavement is one example of shear stress. The internal sliding forces
within a spoonful of molasses as it drips down the side of a mixing
bowl is another. Low viscosity lavas flow readily within volcanoes and
at the surface, making their behavior more predictable. Strong
resistance to flow can lead to unpredictable behavior. For instance,
if a high-viscosity magma clogs the underground plumbing of a volcano,
internal pressure can build to explosive levels before eruption
occurs. Decreasing temperature leads to an increase in magma
viscosity, partly due to the development of phenocrysts, which are
mineral crystals that solidify within a magma while it is still largely
liquid. As a magma cools towards its solidus temperature, a larger
fraction of its volume consists of these solid crystals, which impede
flow. Because the melting temperatures of minerals in a magma can be
estimated in a laboratory, the types of minerals that form phenocrysts
in a solidified lava flow can be used to determine the temperature of
its parent magma.
The composition of a magma influences its viscosity, as does its
volatile content. Partial melting of rock induces fractional
differentiation, which leads to igneous rocks of different
compositions. Partial removal of phenocrysts and successive remelting
of older igneous rocks leads to "evolved" magmas with higher silica
content and more silicic (quartz, feldspars) than mafic (pyroxene,
olivine) mineral composition, trending from basalt (~50% SiO_2) through
andesite (~57% SiO_2), dacite (~65% SiO_2) and rhyolite (> 70% SiO_2)
magmas. The highly-evolved silicic magmas that form rhyolite are less
dense and cooler than the mafic magmas that form basalt, and have lower
solidus temperatures. Higher silica content tends to correlate with
wetter magmas, but large variations are common. The tendency of silica
to form interconnected networks, even in the molten state, promotes
high viscosity in the silicic magmas. Dissolved water tends to break
apart these silica networks, decreasing viscosity somewhat. At 1200
degrees C, a fluid basaltic lava has viscosity only 30-300 times
greater than that of motor oil, and so flows readily down topographic
slopes. As a result, many basaltic volcanoes form gently-sloping
cones. A rhyolitic magma at 800 degrees C is at least a thousand times
more viscous, and forms volcanoes with steeper relief and less
symmetry.
B. Plate Tectonics and Volcanism
The theory of plate tectonics can put most volcanic activity into a
unified conceptual framework. Seafloor spreading and continental drift
are the surface expressions of a larger convection system that
transports heat upward through the mantle. As plates diverge at
oceanic ridges, mantle rock rises to fill the gap and undergoes
pressure-release melting to supply basalt to the rift. Plumes of hot
mantle, bouyant due to thermal expansion, rise from the core-mantle
boundary to supply basaltic magma to Hawaii, Iceland and a few dozen
other regions. The initial collision of a new hot spot at the base of
a plate is often followed by an outpouring of flood basalts, which can
bury thousands of square kilometers with lava flows many kilometers
thick. Most of Washington State east of the Cascades was resurfaced 12
million years ago by the Columbia River Flood Basalts. Lava erupted at
rates of several cubic kilometers per day, and spread rapidly enough to
reach the Pacific Ocean. The gap in the Cascade Range through which
the flood basalts reached the ocean is now followed by the Columbia
River, which has incised a gorge through the basalt layers. After this
initial outpouring, some geologists think that North America drifted
westward and the hotspot supplied basaltic magma along a gentle arc
through southern Idaho toward its current location in Yellowstone
National Park. In a broad, flat plateau where a thin, but rich, layer
of soil has been weathered from the volcanic flows, Idaho farmers grow
their justly-famed potatoes.
More-silicic magmatism can be found bordering trench-subduction zones,
where volatiles are transported by the slab to depths of 50-150 km.
Once released, they rise to promote the partial melting and further
fractionation of shallow mantle and crust, leading to chains of
volcanoes of varied composition. These parallel the trenches in island
arcs or along continental margins. The "ring of fire" that surrounds
the Pacific Ocean is a collection of island-arc and continental margin
volcanoes above subduction zones, ranging from the Andes in South
America, to the Cascade Ranges of the Pacific Northwest, to the
Aleutian Islands, the Kamchatka Peninsula and Kurile Islands in the
north, to Japan, Marianas Islands, New Hebrides Islands and the Tonga
Islands in the western Pacific. The most destructive volcanic eruptions
in human history have been associated with subduction-zone volcanism on
islands and in coastal areas.
Midcontinental volcanism can be related to plate tectonics, but tends
to exhibit special features. Continental plates are thicker than
oceanic plates, and pre-existing lenses of hydrous minerals lead to an
uneven propensity for melting. Therefore mid-continental volcanism
tends to be more diffuse, and diverse in composition. Mid-continent
plate boundaries and hot-spot tracks are often less clearly defined by
volcanism. Basaltic magma from the Yellowstone hotspot tends to melt
silicic crustal rock as it ascends, leading to a mixture of rhyolitic
and basaltic volcanism. The Basin and Range Province in the western US
is a heart-shaped region bordered by the eastern edge of the Sierra
Nevada mountains in California, the Wasatch Front in central Utah and
the Yellowstone hotspot track in southern Idaho. Geologic mapping of
past fault motion in the Basin and Range suggests that the crust is
stretching east-west. Studies of earthquake waves indicate low seismic
velocity in the underlying mantle, suggesting elevated temperature.
Geologically young volcanism is scattered throughout the region and
neighboring areas, including the Rio Grande Rift in central New
Mexico.
Geologists have speculated that the Basin and Range represents the
initial stages of a new rifted plate boundary that would, over time,
split off a portion of the North American plate, much as the East
African Rift appears to partition the African continent. Newer
speculation draws a parallel between the Basin and Range Province, the
Altiplano in South America, and the Tibetan Plateau in Asia. In the
latter regions, the collision of two plates appears to have caused
broad uplift above one segment of the plate boundary, leading to
average elevations greater than 4 km and a wide scattering of silicic
volcanism. Some 30-40 million years ago, the Basin and Range may have
been similar to the current situation in Tibet and the Altiplano. Both
explanations are consistent with pressure-release melting during
Basin-and- Range uplift, which has led to volcanism, in the absence of
either a mantle hot spot or an active subduction zone, in Long Valley
Caldera of California, Craters of the Moon in Idaho, and elsewhere.
3. Volcano Hazards
The hazard posed by a volcano depends on its eruptive style. The ways
that volcanoes bring material to the surface span a wide range between
two extremes, which we term basaltic and silicic. Basaltic volcanism
is more predictable, and typically characterizes volcanoes with
sustained activity. Silicic volcanism is more violent, less
predictable, and typically characterizes volcanoes with infrequent
activity. Each type of behavior requires a different hazard
mitigation strategy.
A. Basaltic Volcanism
The eruption of basaltic lava occurs where conditions in the uppermost
mantle, beneath the crust, allow a partial melt of mantle rock to
occur. These include the globe-encircling system of mid-ocean ridges,
upwelling mantle hot spots and trench-arc subduction zones. An
estimated 75% of global volcanism erupts as basalts at the mid- ocean
ridges, largely hidden from our view. We can observe mid-ocean
processes more directly in Iceland in the North Atlantic, where a
vigorous, buoyant mantle hot spot intersects the mid-ocean ridge and
lifts it above sea level. Hot-spot basaltic volcanism creates mid-
ocean island chains, such as the Hawaiian Islands in the Pacific Ocean
and the Mascarene Islands in the Indian Ocean. Kilauea on the big
island of Hawaii erupts 100*106 cubic meters (0.1 km^3) of basalt on
average every year, enough lava to cover 100 square kilometers of the
island with lava to a depth of 1 meter. The volcanic output of Piton
del la Fournaise on Reunion Island in the Mascarenes is about 10% of
Kilauea's. A substantial fraction of the active volcanoes that parallel
trench subduction zones are basaltic in character, including Mt. Etna
in Sicily, Italy and Klyuchevshoy in Kamchatka, Russia. Among the most
active island-arc volcanoes, the eruptive volumes of Klyuchevshoy and
Mt. Etna average 20*106 cubic meters per year.
Summit eruptions occur at a central caldera at the top of the volcano.
In addition, large volumes of lava are typically erupted on the slopes
of the cone, in flank eruptions. Most, though not all, basaltic
volcanoes are underlain by a magma chamber in the shallow crust, which
stores molten rock for later eruptions. Whether this magma erupts at
the summit or on the flank of a volcano depends on several factors,
primarily the hydraulic pressure on the magma within the chamber. In
exceptional cases sufficient magma erupts into the central caldera to
form a lava lake, a seething, glowing pool of molten rock. A lava lake
persisted in the Kilauea caldera for much of the 1800's, and became a
tourist attraction. In 1866 Mark Twain recorded his observations of
this lava lake in the guest book of the visitor's center.
A large eruption drains the magma chamber. This can cause a downward
collapse of the volcano's caldera at the surface. Similarly, a
refilling of the chamber causes an upward bulging of the caldera and
surrounding volcano. Although individual eruptions occur sporadically,
basaltic volcanism often appears as a steady-state process when
averaged over 10-20 years. Lavas spill from the caldera or from flank
fissures and spread downhill over wide areas. The surface quickly
forms a crust of solidified, vesicular, shiny-black basalt, under which
liquid lava can flow, thermally insulated from the air. Large lava
flows can form solid-basalt lava tubes that channel the flow. Relict
lava tubes, whose smooth inner surfaces record the passage of molten
rock, are open to the public in Hawaii, the Snake River Plain in Idaho,
and other locales. Lava solidifies in two characteristic patterns at
the surface. Pahoehoe flows have smooth black surfaces which resemble
a bunched-up carpet. In contrast, aa flows resemble piles of rubble,
and their dull black mounds project glassy shards that can quickly tear
apart a pair of sneakers. (Aa is pronounced "ah ah," the sound a
barefoot Hawaiian makes while walking over it.)
Though basaltic volcanism is typically effusive in character,
conditions sometimes promote eruptive violence. Basaltic magma has
relatively low levels of volatiles, but it can encounter them at or
near the surface. Grimsvotn in Iceland erupts under a glacial ice cap,
and its lava melts the overlying ice in a volume ratio of roughly
1:10. The eruptions if Grimsvotn are often followed by jokulhlaups, or
glacier-bursts, which flood surrounding areas with a cubic kilometer of
more of meltwater. When lava meets liquid water at atmospheric
pressure, much of the latter is turned into steam, expanding with much
violence. When a Hawaiian lava flow reaches the sea, much is torn apart
to solidify as fine particles, which can form black sand beaches. When
the Icelandic island Surtsey began its life as an undersea volcano, the
pressure of overlying water prevented steam from forming. Once Surtsey
breached the sea-surface in November 1963, however, massive steam
explosions shattered the magma into glassy fragments. Such behavior
can occur on land as well, either in lakes or where rainwater
percolates downward to contact a shallow magma body. In such cases the
emergence of lava is not effusive, but phreatic, involving steam
explosions. Solid material that erupts explosively is called tephra,
which falls to the surface to form pyroclastic (fire-broken) deposits.
Tephra consists of particles that vary widely in size, from
volleyball-sized bombs to fine, glassy ash. Eruptions of steam and
lava fragments can form fountains hundreds of meters high, termed
Strombolian eruptions after a volcanic island north of Sicily.
The shape of a basaltic volcano reflects its eruptive style. The most
volatile-poor, low- viscosity basaltic lavas flow down shallow slopes
to form broad shield volcanoes like Kilauea, Hawaii. The increased
volatile content of island-arc lavas leads to a mix of effusive and
phreatic eruptions. These form the steeper slopes of stratovolcanoes,
which interleave layers of ashfalls and lava flows. Strombolian
eruptions form black cinder cones of sand- and pebble-sized basalt
fragments.
Hazards to life and property from basaltic volcanoes are great. If
properly monitored, a basaltic lava flow, though fluid, can be avoided
by a timely evacuation of the communities in its path. In 1669 the
Sicilian port city Catania was destroyed by an eruption of nearby Mt.
Etna. More dangerous are less frequent, but more violent, eruptive
phenomena. The formation of the Kilauea lava lake was preceded in 1790
by an explosion that excavated the caldera. Ash falls from this
eruption preserve the footprints of an army of Hawaiian warriors that
died from breathing the fiery ash-laden air blown from the caldera.
The residents of Catania rebuilt their city atop the 1669 lava flow,
but an estimated 18000 of its 24000 inhabitants perished in 1693 when a
large earthquake shuddered the ground beneath Mt. Etna.
The gradual accumulation of lava flows on the slope of a volcano can
lead to unstable steep slopes, which can fail in destructive
landslides. The summit caldera of Mt. Etna has grown in recent decades
as its walls have progressively collapsed. Ash falls are particularly
vulnerable to landslides after episodes of intense rainfall. Basalt
flows emplaced atop a pre- existing layer of soft sediments can lead to
catastrophic slope failures. Roughly 5000 years ago the eastern
portion of Piton de la Fournaise on Reunion Island slid into the Indian
Ocean along a 7-km-wide chasm called the Grand Brule. A related, but
more ominous, situation may exist beneath Kilauea volcano in Hawaii,
where the shield volcano has been built atop thick Pacific Ocean
sediments. In 1975 an earthquake with magnitude 7.2 shook Hawaii.
Analysis of seismograms from around the world indicated that the fault
associated with this event was centered beneath the community of
Kalapana, on the southeast flank of Kilauea. The fault motion was
consistent with a lateral slide of a 50-km long slice of the volcano
flank into the Pacific Ocean. Although near-term collapse of this land
area does not seem likely, studies of the surrounding seafloor have
revealed the wreckage of similar spectacular landslides surrounding the
Hawaiian Islands.
B. Silicic Volcanism
The hazard posed by a silicic volcano is often masked by long intervals
of dormancy. The natural human weakness to assume that tomorrow will
invariably resemble yesterday causes communities to discount the
potential dangers of long-quiet volcanoes. The most lethal and most
spectacular phreatic eruptions of the 20th century have involved
"dormant" volcanoes.
The behavior of a silicic volcano trends toward greater violence as
the composition of its magma departs more from that of a Hawaii-like
volcano. The high volatile content and high viscosity of the most
silicic magmas induce major differences in eruptive style. Volatile
gases like H2O, CO2, SO2 and H2S belch from fissures in the volcano,
pressure-released as magma ascends from depth toward the surface.
Drastic exsolution and expansion of the dissolved gas when magma
reaches the surface makes violent explosions more likely. Such
explosions are not limited to the production of airborne ash from
magma, but can pulverize the surrounding country rock as well. The
most violent eruptions send a column of hot gas and tephra 10-30 km
tall, injecting fine ash and sulfurous gas into the stratosphere.
These are called Plinian eruptions, after the ancient Roman author
Pliny the Younger, who described how Vesuvius destroyed Pompeii and
other Roman cities in 67 AD. Tremendous volumes of ash fall to form a
rock called tuff, sometimes many meters thick, which can preserve
ripple marks and other depositional features. Gradual destruction can
come from burial by ash, but instant devastation can arrive with
avalanches that descend the volcano. A lahar is a mud flow of water,
tephra, soil and organic debris, often triggered when magma or hot ash
melts snow and ice near the volcano summit. A nuee ardentee, or
pyroclastic surge, is a red-hot mixture of tephra, hot gas and debris
that can travel at speeds in excess of 300 km/hr. A nuee ardentee can
be triggered directly by lateral blasts from the erupting caldera.
They are launched indirectly when the vertical blast column of a
Plinian eruption, which can persist for hours, collapses partly or
completely.
The most violent eruptions often do not involve liquid lava flows,
because the magma is delivered primarily in the form of tephra. Silicic
magma tends to erupt as a highly viscous liquid afterwards in the
smoldering crater created by phreatic blasts, gradually building rough
domes of rhyolite, dacite or andesite hundreds of meters high. As it
solidifies to block the path of later magma, a dome completes the
eruptive cycle, preparing the way for another phreatic eruption
centuries or millenia in the future.
Silicic volcanoes often erupt enormous volumes of ash, which spreads in
prevailing winds to carpet large areas. Vesuvius belched more than 4
km^3 of material onto Pompeii and its surrounding towns in 67 AD. Mt.
St. Helens erupted a cubic kilometer of magma on one day in May 1980,
and expelled a much larger volume of its pre-existing flank in a deadly
landslide. Mt. Pinatubo may have sent 10 km^3 of tephra into the
atmosphere during its 1991 eruption. Smaller phreatic eruptions are
not necessarily less dangerous. The 1985 eruption of Nevado del Ruiz
in Columbia, discussed below, involved less than 0.0005 km^3 of magma,
but launched lahars that obliterated several towns and their
inhabitants. When ash settles thickly on the roofs of buildings,
collapses are common. Ash particles are glassy splinters of silicate
rock, which pose a severe hazard to machinery. Jet aircraft that fly
through ash clouds risk great damage to their engines. Air-traffic
controllers therefore must be aware of volcanic activity, especially at
the remote Aleutian Island and Kamchatka volcanoes that parallel the
busy airline routes between North America and the Far East. Volcanic
ash can benefit farmers, however, if it is not too silica-rich. After
its initial damage to agricultural areas recedes, the high
volume-to-surface ratio of ash particles mediates a rapid replenishment
of essential nutrients, like iron, potassium and calcium, to farm soil
leached by centuries of rainfall and tillage.
Case histories are perhaps the best way to convey the hazards of
silicic volcanism. Mt. St. Helens is the best studied example in
North America. The devastation about its fringes can be viewed only a
few hours' drive from Seattle, Washington. At the start of March 1980,
the volcano was a quiet, graceful, symmetric cone 2950 meters high. On
May 18, 1980, its north slope collapsed and a steam explosion blew
apart the underlying magma in a Plinian eruption. 400 meters of height
were lost from the volcano's summit, and a 600-m deep ampitheatre was
gouged from its north face. The explosion was heard in Seattle.
Though its extreme violence was unexpected, the volcano had telegraphed
quite clearly that an eruption was imminent. In March 1980 earthquake
tremors, centered 5-10 kilometers beneath the volcano, were felt in
surrounding communities. The nature of the ground motion suggested
strongly the slow ascent of magma from a subterranean chamber: steady
rhythmic tremors generated by liquid flowing in a conduit, and a series
of sharp shocks that tended to rise through the crust, as though a path
was being cleared for the magma to ascend. Steam vented from the
summit, and gradually the north face of Mt. St. Helens bulged outward
from the swelling volume of magma invading the mountain. Large enough
to be noticed visually, this bulge reached a maximum height of 150
meters across a 2-km swath. Prompted by shallow tremors that were
increasing in both frequency and size, authorities evacuated a broad
area around the volcano. Only 57 people, including some who refused to
be evacuated, perished in the coming cataclysm.
When the north slope failed without further warning on May 18, it set
into motion massive landslides, nuees ardentes, and lahars. The most
widespread destruction, however, was effected by a lateral blast from
water-rich magma that was uncorked by the north slope landslide. A
600 km^2 area was flattened by this blast, which blew down dense stands
of tall trees and stripped them of their branches. On the heels of the
blast, massive debris from the north slope filled much of Spirit Lake 5
km north of the summit and launched mudflows that sped 22 km down the
North Fork of the Toutle River. As a Plinian column extended from the
summit to the stratosphere for nine hours, pulses of nuees ardentes
spread along the devastated north slope at speed of 300 km/hr and
launched simmering lahars that followed river valleys well beyond the
extent of the first mudflows.
In the aftermath of the Mt. St. Helens eruption, much collateral damage
was absorbed by the local economy. Ash clouds blocked sunlight over
eastern Washington state for days, and forced the closure of many
roads. Much of the ripening alfalfa crop in eastern Washington was
lost. Mud injected into the Toutle River invaded the lower Columbia
River, impeding commercial shipping. Autos and other machinery were
damaged. Residents downwind of the volcano wondered what to do with up
to 8 cm of volcanic ash that carpeted their homes, farms and businesses
-- one early study suggested the ash be harvested to mix into
concrete. On balance the damage to society was remarkably small,
involving costs less than a tenth of those estimated for the moderate
January 1994 Northridge, Calif. earthquake in the Los Angeles
metropolitan area. Low population density is the primary factor in
this -- roughly half the estimated $1 billion societal cost was
attributed to the loss of harvestable timber. Ecological studies
demonstrated that, though conditions in the blast and tephra-blanketed
areas were harsh, the re-establishment of plants, insects and mammals
progressed rapidly in the first years after the eruption. Efforts at
reforestation, however, encompassed only 10% of the devastated areas
after five years. Trout populations in rivers draining Mt. St. Helens
suffered initially from suspended mud and other ecosystem disruptions,
but were observed to rebound quickly. Studies of nearby workers
suggested that lung damage from breathing small amounts of ash were
reversible in a few years.
Silicic eruptions have often been less forgiving with human lives. In
May 1902 a nuee ardentes from Mount Pelee on the island of Martinique
devastated the city of St. Pierre, killing 28000 people in a few
minutes. In November 1985 lahars from an eruption of Nevado del Ruiz
in Columbian Andes killed 25000 in valley towns 60 km from the volcano
summit. An incomplete evacuation from the slopes of El Chichon in
southern Mexico led to the deaths of at least 2000 out of roughly 20000
local inhabitants. The 67 AD eruption of Vesuvius exterminated the
population of Pompeii and several surrounding Roman coastal towns.
Geologists have reconstructed at least six nuees ardentes from this
eruption, some which spread more than 30 km from the summit. The
associated ashfall preserved thousands of corpses and their former
possessions, later rediscovered in archeological excavations.
C. "Extinct" Volcanoes: The Perspective of Geologic Time
Volcanic activity has deep-seated causes within the Earth system. We
do not expect volcanic eruptions to occur in isolation, but as part of
an ongoing process in which further eruption is likely. Understanding
this process is the key to accurate forecasts of volcanic hazards.
Residents of Spokane, WA need not live in dread of a repeat of the
Columbia River Flood Basalts, because the source of these magmas
presumably has moved two states away. On the other hand, the
construction of a vacation home on a century-old Hawaiian lava flow
entails definite risks. It is essential to reconstruct the history of
eruptive activity in order to assess hazard intelligently.
Records of recent volcanism can be historical, as for Etna and Vesuvius
in Italy, but in most places must be reconstructed from the rock
record. Radioactive dating gives volcanic stratigraphy an
age-precision that ordinary sedimentary studies rarely match. The ages
of individual lava flows on the big island of Hawaii can be found via
radiocarbon dating of the burnt vegetation beneath. Ashfalls from
large phreatic eruptions can be dated by K-Ar decay methods. In
Yellowstone National Park there are three major calderas formed by
massive explosions at 2.0, 1.3 and 0.6 million years ago. Each sent
250-2500 km^3 of ash airborne, falling in significant quantities as far
away as the Mississippi River. Since the last caldera blast, an
estimated total of 1000 km^3 of rhyolitic lava, presumably less
volatile-rich, was erupted 150, 110 and 70 thousand years ago.
Although no signs of similar eruptions have been noticed recently,
there is no reason to believe that basaltic magma from the underlying
mantle hot spot has ceased production. In New Mexico, early
Native-American cliff-dwellings were carved into the Bandelier Tuff, a
rhyolitic ashfall from phreatic explosions roughly 1 million years ago
in the Valles Caldera. A rhyolite dome has since built up in the 20-km
diameter caldera, but no magmatic activity has been apparent in the
last 150 thousand years. The 40-km Long Valley Caldera near Mammoth
Mountain, CA was formed by a phreatic eruption 0.76 million years ago.
In this case, the subsequent episodes include the eruption of the Inyo
dome 550 years ago.
In the Cascade Range volcanism has been more recent, with many peaks
active in the last 10,000 years. 7000 years ago the volcano that is
now Crater Lake, Oregon hurled 50 km^3 of material into the
atmosphere. There were no glaciers on the slopes of Mt. St. Helens,
suggesting that the pre-1980 volcano had formed since the last episode
of glacial climate, 8000 years ago. Similarly, Klyuchevskoy volcano in
Kamchatka may be younger than 10,000 years old, and hence may not have
witnessed the first waves of "Native" Americans as they migrated to
Alaska from Eurasia.
Silicic volcanism is characterized by long dormant intervals. To
verify the potential for eruption, researchers have tried imaging the
magma chambers 5-10 km beneath volcanoes using seismic techniques.
Using tomography to reconstruct where in the crust seismic waves
travel relatively faster and slower, large pockets of "slow" material
beneath Mt. St. Helens, Mt. Rainier, Long Valley Caldera, and Unzen
volcano in Japan have been observed. Here and elsewhere the magma
chambers persist, awaiting their next eruptive pulse.
D. Volcanic Gases and Aerosols
Volcanoes vent gas as well as molten rock. Some of this material is
rainwater that filters downward through cracks to meet the hot rock
surrounding a magma chamber, where it heats, expands and returns to the
surface to form geysers and fumaroles. These can make volcanic areas
look like boiling caldrons. Other volatile compounds travel with the
magma as it ascends. Vented water wapor is not by itself dangerous,
but volcanoes can also vent carbon dioxide, sulphur dioxide and
hydrogen sulfide gas. A large area downwind of Kilauea caldera in
Hawaii is a "desert," despite adequate rainfall, because SO_2 gas
poison most plant life. Near Long Valley Caldera in California, trees
on the slopes of Mammoth Mountain are dying, because CO_2 has seeped
from an underlying magma body into the soil. Volcanic CO_2 emissions
can erupt suddenly, as happened at Lake Nyos in Cameroon on 21 August
1986. An estimated 0.5 - 1.0 km^3 of CO_2 gas spread from the lake to
surrounding valleys, hugging the ground to suffocate 1700 people and
6000 cattle.
Worldwide climate change can be effected by phreatic eruptions that
vent large amounts of sulfurous gas and expel large volumes of fine ash
particles into the atmosphere. Sulfate gas react in the atmosphere to
form sulfuric acid droplets. Together with ash aerosols, these
suspended droplets can partly screen the Earth surface from sunlight,
leading to worldwide cooling. Plinian eruptions extend to the
stratosphere, where aerosol particles can remain suspended for 2-3
years. The 1815 Tambora, Indonesia eruption caused "the year without
a summer" of failed crop harvests in the northeastern US and Europe.
The 1991 Mt. Pinatubo eruption generated 20-30 million tons of sulfuric
acid droplets in the stratosphere. In the following year the steady
global warming trend paused, as average temperatures declined 0.6^o C.
This cooling was reversed in certain regions, however, due to shifts in
weather patterns. Studies of weather records from central Russian
suggest that large volcanic eruptions cause a 2^o C increase in
temperatures the following winter.
Volcanic aerosols in the stratosphere, particularly sulfuric acid
droplets, catalyze the chemical reactions that destroy ozone, the
oxygen molecule that absorbs solar ultraviolet radiation dangerous to
life. Following the Mt Pinatubo eruption, the 1992 ozone "hole" over
Antarctica became unusually large and deep. The ozone layer in the
Northern Hemisphere thinned as well, by up to 10%. These effects
persist as long as sulfate aerosols remain in the stratosphere.
Volcanoes contribute a significant amount of carbon dioxide to the
atmosphere, and therefore must be included in the global budget of this
important greenhouse gas. Direct measurements of CO_2 emissions from
volcanoes are few, and vary greatly from volcano to volcano. Mt Etna
on Sicily is currently the leading source of CO_2, diffusing 25 million
tons per year through its flanks, equivalent to four large
1000-megawatt coal-fired power plants.
4. Volcano Hazard Mitigation
A. Near-Term Monitoring Methods
Although the timing and severity of an eruption is difficult to
determine precisely, many observable phenomena track well its
preparatory stages. The location of earthquake sources tend to follow
the cracking of rock to allow the upward progression of magma.
Earthquake locations can be determined with an array of seismometers
surrounding the volcano. Harmonic tremors, unlike normal earthquakes,
vibrate the ground at characteristic frequencies for minutes rather
than seconds, and are thought to represent magma flowing through
conduits. An increase of tremor intensity is common hours or days
before large eruptions. Since eruptions often occur at night or beneath
a cloud of steam and ash, seismic data is often the most convenient
record of an eruptive sequence.
As magma rises toward an eruption, it tends to inflate the volcano.
This inflation can be measured with geodetic monitoring. As intrusion
proceeds, the surface of the volcano stretches and tilts, which can be
measured by strainmeters and tiltmeters. The absolute location of a
point on the volcano can be monitored with Global Positioning System,
or GPS, equipment. These can obtain 1-10 millimeter precision in
location with reference to orbiting satellites. It is also possible to
detect centimeter-scale volcano movements from space with satellite
radar interferometry, which produces a contour map of surface motion by
comparing data from successive satellite passes. Satellite thermal
surveys can detect unusually warm surface areas on a volcano, from
steam vents or perhaps underlain by shallow magma.
Direct samples of vented gas and erupted lava and ash can be used to
monitor eruptive behavior. Small eruptions of mafic lavas from a
silicic volcano may indicate that basaltic magma from the mantle has
risen to invade a shallow magma chamber. Similarly, the presence of
3He in vented gas suggests magma from the mantle. This lightest
natural isotope of helium easily escapes the Earth's gravity to space
and is nearly absent near the surface. As a "wet" rhyolitic magma
nears the surface, steam escapes into the overlying rock, melting any
snowcover and warning of an explosive eruption. Although the volume
and composition of vented gas is difficult to measure on the ground
once an eruption appears imminent, the SO_2 emissions can be estimated
using satellites that were originally designed to monitor atmospheric
ozone levels.
B. Approaches to Hazard Mitigation
The May 1980 eruption of Mt. St. Helens inaugurated a period of intense
worldwide volcanic activity. Volcanic hazard mitigation in this period
has not always been successful. Common ingredients in a successful
mitigation effort include 1) a professional government earth-science
agency whose outlook and actions are motivated by long-term scientific,
not political, concerns; 2) a stable and trustworthy central
government; and 3) effective communication of the likely hazards posed
by the volcano. For violent silicic volcanism, the time interval
between the first stirrings of a "dormant" volcano and a Plinian
eruption can be only 2-3 months, leaving little time to overcome the
lack of proper infrastructure. Authorities must recognise the risks
posed by a potential eruption, and must persuade local inhabitants to
evacuate dangerous areas. If government officials lack technical
expertise, realistic contingency plans or authority in the eyes of the
public, the citizens most at risk may discount the likelihood of an
impending catastrophe.
These weaknesses were evident prior to the eruption of Nevado del Ruiz
and its devastating lahar in November 1985. As tremors and steam
emissions from the volcano became more alarming, the Columbian
government relied on various sources of scientific advice, from
international nonprofit agencies and individuals. Rival assessments of
the volcano's likely activity competed for attention from politicians
within the government. Without a scientific consensus hammered out in
technical debate among geologists, officials delayed action.
Reassuring public statements were issued, and responsibility for
further action passed on to local governments, whose perspectives and
resources were limited. Evacuation plans were ordered, but not
developed fully. A week before the eruption, an insurrection in the
capital city further distracted the government. When the order to
evacuate the towns in the lahars path finally arrived, it was nighttime
and raining. Most stayed home to watch a soccer game, only to be
buried by a 40-meter wall of mud travelling 30 km/hr.
In contrast, hazard mitigation efforts for the 14 June 1991 Mt Pinatubo
eruption in the Philippines were largely successful. After a moderate,
but unexpected, phreatic eruption on April 2, the Philippine Institute
of Volcanology raced to map the volcano's geology and estimate its
eruptive potential. The presence of Clark Air Force Base, an important
US military installation, on the slopes of the volcano focussed US
technical resources on monitoring the ascent of magma to the surface.
A video describing the likely consequences of an eruption was quickly
filmed and distributed to local residents. In the week prior to the
Plinian eruption, nearly 60000 residents were evacuated from Pinotubo's
slopes. Only 300 died as a direct result of the eruption. Although
property damage was immense -- Clark Air Force base, buried by ash, was
abandoned soon afterwards -- the shock absorbed by Philippine society
was greatly lessened by effective hazard mitigation.
The presence of a superpower military asset is not required for
effective mitigation procedures. On 19 September 1994 the Vulcan
Crater near the port town of Rabaul, Papua New Guinea erupted
massively, after a decade of precursory signals. Over 50,000 people
were evacuated successfully, and fewer than a dozen deaths resulted.
Communities in the shadow of basaltic volcanoes must adjust to their
near-continuous activity, which can resurface a significant portion of
the mountain every century. Myopic land-use planning allowed housing
developments on Hawaii to be built downslope of Kilauea in the 1960's.
Many homes disappeared under lava flows in the 1980's and 1990's.
Italian authorities have tried to redirect lava flows Mt. Etna with
levees. Icelanders have tried to halt lava flows with cooling streams
of water. Such measures can buy time for a generation or more. In
the long term, however, real estate on the slopes of such volcanoes
must be considered impermanent.
Proper hazard mitigation near a dormant, but not extinct, silicic
volcano is more difficult to formulate. It is also more difficult to
justify to land developers, farmers and citizens drawn by the volcano's
natural beauty and by the fertile soil that often surrounds it. The
glacier-clad slopes of Mt. Rainier rise to an altitude of nearly 4400
meters from the hills of western Washington, visible from Seattle on
clear days. Radiocarbon dates indicate that 150 years ago a small
event erupted 10^6 m^3 of tephra. Steam and H2S gas currently vent
near the summit, and the crust beneath has frequent small earthquakes.
There is no geologic evidence of massive explosive eruptions like the
1980 Mt. St. Helens eruption. Nevertheless, at least 100,000 people
live atop lahars and other avalanche debris launched by the volcano in
the last 10000 years. The longest lahar deposit is the 5000-year old
Osceola Mudflow, which extends into the city of Tacoma, 110 km from the
volcano. Snow and ice volumes exceed 4 km^3 on the slopes of the
volcano, raising the flood danger from jokulhlaups, or glacier-bursts.
Warmed by the volcano, water circulates through porous ash layers
between lava flows, weathering the ash to clay. Because clay does not
resist sliding well, the collapse of the volcanos steep slopes becomes
more likely. In the last 45 years, significant floods and avalanches
have occurred roughly once every two years in the absence of volcanic
activity, with destructive effects mostly confined within Mount Rainier
National Park.
Although it is impractical to abandon the existing homes and businesses
along the river valleys that drain Mt. Rainier, sensible controls on
development can follow from careful risk assessments. Geologic mapping
indicates that lahars that extend 20-km from the volcano occur, on
average, once every 1000 years. Insurance premiums would rise in areas
most at risk. Government zoning regulations can further limit the
scope of future development. As for monitoring the volcano, a
long-term perspective is essential. Networks of seismometers and GPS
equipment around the volcano are desirable, as well as regular sampling
and analysis of its gaseous emissions. However, care must be taken not
to disturb the high-altitude ecosystem of the National Park, and not to
interfere with the ongoing commercial management of the forest that
surrounds it. Regular briefing of public officials by scientists on
current hazard research has been recommended, as well as educational
videos and pamphlets for schools in the affected areas. As further
geological investigation refines risk estimates and sharpens monitoring
techniques, an informed community response to the hazards of Mt.
Rainier will continue to evolve.
5. Summary
-- The location and magma type of volcanoes are largely determined by
their position within the global system of plate tectonics.
-- The behavior of a volcano is largely determined by the type of magma
it erupts. Low- silica magma is called basaltic, and is typically more
fluid and less volatile-rich. High-silica magma is called silicic, and
is typically more viscous and volatile-rich. The rapid exsolution of
dissolved gases (H2O, SO_2, CO_2, H_2S) from silicic magmas often leads
to phreatic (steam-driven) explosions. Phreatic explosions can also
occur when magma encounters rainwater, ice and snow.
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