MATERIALS, HAZARDS, AND ERUPTIVE
TYPES, BEHAVIOR, AND RISKS
• Volcanism. Refers to the rise
of magma onto the earth’s
• Magma - a mixture of liquid
rock, mineral crystals, and
• Volcanoes are conical or dome
shaped landforms built by the
emission of magma and its
contained gasses onto the
Volcanic activity is controlled by plate tectonics, because plate
movements relate to where sources of magma originate inside the
earth. Nearly all active volcanoes are located in one of three plate
•Subduction zones at convergent plate boundaries
Example: Volcanoes lining the trenches of the Pacific Ocean,
forming the Pacific “Ring of Fire”.
•Rifting and sea floor spreading at divergent plate boundaries,
Example: Volcanic eruptions at mid-ocean ridges, and in some
rift zones on the continents, like the East African Rift
Example: The Hawaiian Islands and the Galapagos Islands. 3
Locations of Volcanic Activity and
Map of the World’s Active Volcanoes
Note that the majority of above sea active volcanoes
(about 66%) occur in the Pacific Ring of Fire.
Magma and the Driving Force Behind Eruptions
• Magma may be ejected onto the
earth’s surface as:
– Pyroclastics or tephra- flour
sized to boulder sized
particles which are thrown in
the air due to the built up
pressure of gasses.
• The violence of a volcanic
eruption depends on the magma’s
viscosity and gas content. The
more viscous (thick) and more
gaseous the magma, the more
explosive the eruption.
Above: Gigantic eruption cloud of tephra
Above: aa lava
Right: pahoehoe lava
Viscosity is a measure of a fluid’s resistance to flow. The main factor that
determines the viscosity of magma is its silica (SiO2) content. The more silica in
the magma, the more viscous it is. The more viscous (silica-rich) the magma, the
more violent the eruption.
We are concerned, then, with three types determined by the chemical composition
of the magma.
1.Mafic or Basaltic magma – low silicon and oxygen, high iron and magnesium.
Simple silicate minerals. Dark magmas. Low Viscosity.
2.Intermediate or andesitic magma - intermediate silicon and oxygen,
intermediate iron and magnesium. High Viscosity.
3.Felsic or rhyolitic magma - high silicon and oxygen, low iron and magnesium.
Complex silicate minerals. Pale magmas. High Viscosity.
Left: The basic building block of all silicate minerals -
the silica tetrahedron. Four oxygen atoms surrounding
a single atom of silicon
The gas content of a magma also relates to its behavior. A magma with low
gas content will tend to flow out of a volcano as relatively quiet lava. A
magma with high gas content will tend to blow apart violently upon
erupting. The higher the gas content, the more violent the eruption.
The composition of the gases in magma are:
Minor amounts of sulfur dioxide, hydrogen sulfide, chlorine, and flourine
But it is the amount of dissolved water that typically inspires a volcano to
Steps to a Volcanic Eruption
• Volcanoes form wherever rock melts at depth and the
magma can rise to erupt at the surface. Rock deep in the
earth may melt by
– Increasing its temperature
– Decreasing its pressure
– Adding water (to lower the melting temperature).
• Magmas that are generated deep within the Earth begin to
rise because they are less dense than the surrounding solid
• As they rise they may encounter a depth (or pressure)
where the dissolved gas no longer can be held in solution in
the magma, and the gas begins to form a separate phase
(makes bubbles) and will continue to grow in size as pressure
Steps to a Volcanic Eruption:
• If the magma has a low
viscosity, the gas will easily
expand to atmospheric pressure
at the earth’s surface and
simply burst, and a non-violent
eruption will occur, usually as a
• If the magma has a high
viscosity the gas will not be able
to expand very easily creating a
high pressure inside which will
cause them to burst explosively
on reaching atmospheric
pressure. This will cause and
explosive volcanic eruption.
Volcanic Explosivity Index
The violence and size of a volcano’s eruption is expressed by the
Volcanic Explosivity Index (VEI). Values for the VEI range from 0
to 8, and are based on:
1. the volume of material
(lava and particles)
2. the height of the
3. how long the eruption
The larger the VEI value,
the larger the
This figure plots VEI values versus the eruption
Volcanic Explosivity Index
Two main factors
explosivity of a
1. The amount
of time that has
passed since the
This figure shows that
individual eruptions become
more violent as the time
gap between eruptions
Volcanic Explosivity Index
2. The viscosity (thickness) and gas content of the magma. Both
relate to plate tectonic setting:
• Divergent boundaries (MOR) and hot spots both draw their
magmas from the upper mantle. This magma is called MAFIC
(basaltic) MAGMA, and is characterized by relatively low silica and
low gas content. (the eruptions have low VEI).
• The magma at convergent boundaries comes from melting of
subducted oceanic plates. Subduction forms FELSIC (rhyolitic) to
INTERMEDIATE (andesitic) MAGMA, and is characterized by
relatively high silica and high gas content. (the eruptions have high
(Refer to pg. 147)
Bottom line: The world’s most dangerous volcanoes
are those at convergent plate boundaries!
Types of Volcanic Hazards and
The figure at right shows a
composite of the potential
hazards associated with a
The main hazards are:
• Lava is molten magma that
flows out and onto the
• Lava flows are typically
formed from low viscosity
mafic magma that erupts
at divergent boundaries
and hot spots.
Fluid basalt lava flows are called Hawaiian
type lava and can come in two varieties.
Smooth, runny pahoehoe (top) and chunky
• The lower silica content
(and therefore low
viscosity) of mafic magma
allows the lava to run down
• Lava flow eruptions are
fairly gentle and quiet. They
may cause property damage,
but rarely fatalities.
16(Refer to pg. 134-135)
Lava flows destroy whatever they
overrun. Everything in the path of an
advancing lava flow will be knocked
over, surrounded, buried, or ignited by
the extremely hot temperature of lava.
Hawaii visitor’s center
…..and after (right)
Left: An example of diversion of a lava flow. The
bulldozer is making a barrier to force a moving lava
flow away from its path toward a village in Hawaii.
Right: Barrier constructed to protect the
main tourist complex on Etna, Sicily. Note the
thirty foot (10 m) thick aa flow approaching
the barrier. The barrier remained intact until
the eruption ended.
Left: Aerial view of aa flow against the
barrier on Etna, Sicily.
Diverting Lava Flows
• Ash falls form when
an eruption column of
tephra and gas is
blown into the air by
an explosive eruption.
The eruption column
can rise up more than
20 km into the
• Tephra is a general
term for any size of
blown out of a
typically falls back to the
ground on or close to the
Pele is the goddess of the volcano. Teeny
tiny volcanic bombs are known as Pele’s tears.
Volcanic Bombs form from the rapid cooling
of lava thrown in the air. When the lava
cools in mid-air, it forms the characteristic
Volcanic ash, the smallest
tephra fragments, can travel
hundreds to thousands of
kilometers downwind from a
1980 Eruption of
Mt. St. Helen and
map of ash fall
Ash spreads in upper atmosphere around the globe. The suspended
ash can decrease the insolation from the sun and lower global
Volcanic ash can be hazardous for a
variety of reasons...
Daylight turns into darkness…
Roofs may collapse from added
Machinery and vehicles will be
abraded and engines may
Farmland will be covered…
Roads will be slippery or
Waste-water systems may clog…
Pyroclastic flows (Ash Flows) are avalanches of a very hot (1300-
1800F) mixture of hot rock particles and hot gas that are blown out of
the vent of the volcano as an eruption column which subsequently
collapses and moves very rapidly down the flanks of the volcano at
speeds from 50 to over 200 km per hour and can travel for 10’s of kms
burning, burying and suffocating everything in their path.
Above: 1997 eruption on island of
Montserrat in the West Indies.
Right: 1986 eruption of Augustine
...destroy by direct impact.
bury sites with hot rock debris….
...burn forests, crops, and
The eruption of Mount Saint
Helens in 1980 produced a
devastating pyroclastic flow,
seen here blasting out to the
right as a lateral blast. The
eruption was so violent that
the entire top of the mountain
was blown off.
The Mount Saint Helens
pyroclastic flow knocked down
vast acres of forest, stripping
the bark entirely off trees.
The Eruption of Mount Saint Helens
One of the greatest
volcanic disasters in
history was the
destruction of the city
of Saint Pierre,
Martinique, by a
pyroclastic flow from
Mount Pelee in 1902.
Before the eruption
….and after the
30,000 people died
burning, burial, and
The Eruption of Mount Pelee
Refer to page 179-181.
Pyroclastic flows have buried entire cities and their inhabitants. Perhaps the
most famous examples are Pompeii and Herculaneum, which were buried in
A.D. 79 by the eruption of Mount Vesuvius.
Cast of victims. The cast is
obtained by the filling the cavity in
the ashes with liquid chalk.
Left: Ruins of Pompeii, with Vesuvius in the
The Eruption of Mount Vesuvius
Mudflows or Lahars
Lahars have caused more fatalities than any other
volcanic hazard because they are more common, and
they can occur at any time.
Volcanic Mudflows or Lahars form by mixing water
with loose volcanic ash and debris on the flanks of a
volcano. As the mud moves downslope, it gathers
rocks of all sizes accelerating as it goes.
The water can come in several ways including:
A major rainstorm.
An eruption melts large amounts of snow and ice on
the flanks of the volcano.
When moving, a lahar looks
like a mass of wet concrete
that carries rock debris
ranging in size from clay to
boulders. Most lahars travel
much too fast for people to
and after the 1991
eruption of Mt.
Pinatubo in the
Mudflows from Mount Pinatubo after the 1991
eruption destroyed roads and bridges, and
buried farmland and towns with sediment.
The Eruption of
Right: This valley (north fork of the Toutle
River) was filled by a muddy lahar several
10’s of feet thick during the 1980 eruption
of Mt. St. Helens.
Left: Mudlines high on tree trunks show the
depth of the Toutle River mudflow. Note the
person on the right for scale.
Right: A lahar carries away a bridge spanning the Toutle
River about 55 km downstream from Mount St. Helens.
Before arriving at the bridge, the lahar swept through a
logging camp and picked up thousands logs.
The Eruption of
Mount Saint Helens
Located in Washington State,
Mount Rainier is the largest
and highest volcano of the
Due to its location near heavily
populated areas including
Seattle and Tacoma, Lahar
hazard makes this
stratovolcano the most
dangerous volcano in the U.S.
The Risk of Lahars
of Mount Rainier
Start of landslide
Landslide enters valley
Landslides are large masses of
rock and soil that fall, slide, or
flow very rapidly under the
force of gravity.
A number of factors can trigger a
* intrusion of magma into a volcano
* explosive eruptions
* large earthquake
* intense rainfall that saturates a
volcano or adjacent tephra-
covered hillslopes with water
Explosions (red) begin to rip
through the landslide (green)
Exploded rock debris (red) forms a
pyroclastic surge that quickly
overtakes the landslide (green)
These illustrations show the landslide
(green) and directed blast (red) that
occurred during the first few minutes
of the eruption of Mount St. Helens
Volcanic landslides can
Above and Below: Stands of dead trees killed
by excess CO2 seeping from the ground from
magma close to the surface.
Gases emitted during
volcanic eruptions may be
toxic and/or corrosive.
The most common
hazardous gas is CO2.
Carbon Dioxide is deadly
to people, animals, and
trees in high
Being heavier than oxygen, it can pour downslope and displace
oxygen at the surface. Because it is colorless and odorless, it can
suffocate without warning.
These cattle, and
from a massive
from Lake Nyos
Africa) in 1986.
Lake Nyos lies
within a volcanic
Types of Volcanoes
They differ in
Note the size
Shield Volcano Stats
• Found along hotspots and divergent boundaries.
• Mafic (basaltic) magma
• Low viscosity
• Low gas content (no water because no subduction).
• GENTLE eruptions of lava flows.
• Landform composed almost entirely of relatively thin lava flows.
• Gently sloping due to the low viscosity of the magma which allows lava
to flow great distances before it cools.
• The largest structures on earth! 9 km from seafloor to summit!
• Basalt is the most common rock type.
Most of the lava of a shield
volcano flows down the
flanks of ridges that radiate
outward from the volcano
The big island of
Hawaii has three major
volcanoes: Mauna Loa,
Mauna Kea, and Kilauea.
Mauna Loa is Hawaii’s
largest volcano but its
activity is infrequent.
Kilauea is in the main
growth stage having
continuously since 1983.
Because they form from relatively
quiet eruptions of low viscosity, fluid
magma, shield volcanoes are very
large, with gently sloping sides and a
Left: a shield volcano in the
Galapagos Islands created over a hot
Right: Iceland created where a mid-
ocean ridge sticks up above sea level.
Left: Mt. Etna, Sicily. The largest
continental volcano on earth and the most
active volcano in Europe, Etna erupts
almost continuously, with an occasional
• Found along convergent boundaries paralleling subduction zones.
• Andesitic (intermediate) to rhyolitic (felsic) magma.
• High viscosity
• High gas content (subduction drags down water)
• VIOLENT eruptions. Dangerous and Explosive.
• Landform composed of alternating layers of tephra and lava.
• Steeply sloping due to the piling up of tephra around the central vent and
the high viscosity of the lava that glues it together.
• Can reach heights of roughly 3500 meters.
• Andesite and rhyolite are the most common rock type.
This figure shows a composite of
the main hazards associated
with a typical stratovolcano.
The main hazards are:
Poisonous gas emissions
Map of the world’s active volcanoes, showing that the majority of
above sea active volcanoes (about 66%) are stratovolcanoes
produced by subduction in the Pacific Ring of Fire.
Because the western U.S.A.
borders the Pacific Ring of Fire
volcanic hazards exist primarily in
the western states. On the map,
regions with the greatest risk of
volcanic hazards are shown in red.
The volcanoes of the Cascade Range,
which extends from northern
California north into Oregon and
Washington, are related to the
presence of the Cascadia Subduction
The Volcanoes of the
violent. They can
remain dormant for
tens to hundreds of
Mt. St. Helens
Above: Mount St. Helens eruptive
sequence on May 18, 1980
Below: Events leading up to Mount St. Helens eruption
on May 18. 1980.
Lava domes are steep sided structures that form when viscous lava is
erupted slowly and piles up in and around a volcanic vent. As it grows
its outer surface cools and hardens, then shatters, spilling loose
fragments down its sides.
Below: Dome extrusion often follows
explosive eruptions. This lava dome in
the crater of Mount St. Helens built by
periodic lava extrusion between 1980-
Above: Dome collapse can produce
pyroclastic flows. Shown is a pyroclastic flow
sweeping down the eastern flank of the lava
dome of Soufrière Hills volcano on
Montserrat on January 16, 1997.
• Cinder cones form when fluid
basaltic magma rises along a
fracture and encounters
groundwater. The steam
generated blows fragments of
lava violently into the air which
solidify and fall as cinders around
the vent forming a circular or
• Cinder cones are commonly found
on the flanks of shield volcanoes
• Most cinder cones erupt for only
a few months to a few years and
rarely rise more than 300-500 m
above their surroundings. Being
unconsolidated they tend to
Above: The photos the eruption of Paricutin Volcano,
Mexico, a classic example of a cinder cone.
Above: View of cinder cones near the
summit of Mauna Kea
Above: This cone is one of two cinder cones called
the Red Cones located south of Long Valley
Caldera in California. These basaltic cones were
erupted about 5,000 years ago.
Above: This cinder cone on the flank of
Mount Etna is surrounded by a younger
basaltic lava flow.
Some eruptions of stratovolcanoes are
so violent that after the eruption ends
the volcano collapses into the empty
magma chamber beneath, leaving a large
crater behind called a caldera.
The above photo shows Crater Lake in Oregon.
The basin that holds Crater Lake formed when
Mount Mazama destroyed itself in a gigantic
eruption 7,700 years ago.
Supervolcanoes – Giant
‘Supervolcanoes’ are giant
rhyolite volcanoes that form
over continental hotspots.
Resurgent caldera eruptions are
by far the largest and
presumably most destructive of
all types of volcanic eruptions.
Bottom Image: The Yellowstone Volcano
is the relic of three monstrous eruptions
that occurred 2mya, 1.3mya, and
600,000ya. Each eruption left collapse
calderas ~50km across. As magma
continues to rise beneath the caldera, it
raises a resurgent dome in its surface.
Will Yellowstone Erupt Again?
occur as a
heated to its boiling
temperature in a
confined space (ex. a
forces water and
steam up through the
fractures and onto
dome and the
According to a report from the U.S.
Geological Survey. “Recurring earthquake
swarms, swelling and falling ground, and
changes in hydrothermal features are
cited in the report as evidence of unrest
The Associated Press. May 9, 2005
A magma body beneath the caldera is
about 60 km long NE-SW and up to
40 km wide. The two resurgent domes
are where the magma body is closest
to the surface, ~5-6 km.
surface of the
Mitigation and Prediction of Volcanic
There is no effective way to mitigate most volcanic
hazards after an eruption has occurred.
One exception is lava
flows, which have been
mitigated by diverting the
moving lava and/or by
chilling the lava with sea
water. Chilling with water
was done successfully in
Iceland in 1973 to save a
town from being overrun
Mitigation before an eruption can be done by assessing a region for
its potential volcanic hazards and creating a hazard-zone map, which
indicates the type and degree of risk in particular areas.
Involves studying the
geology of the volcano to
determine the types of
hazards posed by the volcano
and the frequency at which
these types of hazards have
occurred in the past
(radiometric age dating).
Right: An example of a volcanic
hazard zone map, showing lava
flow hazard zones for the big
island of Hawaii.
Prediction of Volcanic Hazards
To predict an eruption, geologists depend on various precursors:
events that occur prior to an eruption. The main precursors that
signal an impending volcanic eruption are:
1. Tilting and swelling of the
volcano’s sides. As magma
rises below a volcano, it
forces the earth’s surface
upward and outward, causing
it to bulge.
Below: Tiltmeter site on crater
floor of Mount St. Helens
2. Increased seismic activity.
Many small earthquakes --
called earthquake swarms or
harmonic tremors -- caused
by fresh magma rising below
3. Seismic exploration. Monitoring the
movement of the s-wave shadow zone to
determine the position and movement of
4. Increased steam and gas emissions caused by rising magma
emitting gas as it approaches the surface.
5. Changes in Groundwater System - As magma enters a volcano
it may cause the water table to rise or fall and cause the
temperature of the water to increase.
Direct gas sampling
from fumeroles with