Mav Mark
2/6/12
How does the silica content affect the overall
viscosity of a particular lava flow?
Volcanic Eruptions
Notes
Magma
• A volcano has a pocket of magma
below the surface, called a magma
chamber. A long tube, called a pipe,
connects the magma chamber to the
surface.
• At the top of a pipe is an opening called
a vent (most volcanoes have one
primary vent, and one or more
secondary vents). Magma leaves the
volcano through the vent and becomes
lava. They are covered by lava as it
pours out of a vent and called a lava
Primary Vent
Side Vent
Lava Flow
Pipe
Magma Chamber
Magma Reaches
Earth’s
Surface
• Remember, molecules of a liquid are closer
together than those of a gas. If the molecules
are allowed to spread out, a liquid will
spontaneously turn into a gas.
• As magma gets closer to the surface, the
pressure on the magma decreases. This allows
the gases in the magma to begin to bubble out.
These gas bubbles take up more space than the
dissolved gases. The bubbles force magma out
of the vent, like the bubbles that force warm
Coke out of a bottle.
Under Pressure
Pressure Released,
gas expands
•
•
Magma Reaches
Earth’s Surface
When a volcano erupts, the force of the expanding gases pushes magma
from the magma chamber through the pipe until it flows or explodes out of
the vent.
The gases in most magma include:
Water Vapor (H2O)
Carbon Dioxide (CO2)
Sulfur Dioxide (SO2)
Hydrogen (H2)
Carbon Monoxide (CO)
Hydrogen Sulfide (H2S)
Hydrochloric Acid (HCl)
Kinds of Volcanic
Eruptions
• When magma pours out of a volcano, it
is called a volcanic eruption. An
eruption can happen slowly and quietly.
Or an eruption can happen all at once
with an explosion. How a volcano
erupts depends on the magma.
Kinds of Volcanic
Eruptions
• A volcano erupts quietly if the magma is
low in silica (low friction) and flows
easily (low viscosity). The lava may flow
for many kilometers before it starts to
harden into rock.
Kinds of Volcanic
Eruptions
• A volcano erupts with an explosion if
the magma is high in silica (high
friction) and does not flow easily (highly
viscous). Magma builds up in the pipe
until it explodes out of the vent. The
lava cools quickly.
The following slides are from the
BAJIHO Volcano Eruption in CHILE,
July 11, 2011,
and are unedited.
That is not snow, it’s ash from the volcano!
Satellite imagery from above Chile, South America.
Kinds of Volcanic Eruptions
• The pieces that fly away from an
explosive volcano are classified into 4
sizes:

Ash - particles smaller than 2 mm
(0.08 inches) in diameter

Lapilli or volcanic cinders - between 2
and 64 mm (0.08 and 2.5 inches) in
diameter

Volcanic bombs - larger than 64 mm
(2.5 inches) in diameter.
Kinds of Volcanic
Eruptions
• A pyroclastic flow is a fast-moving current of
superheated gas (which can reach temperatures
of about 1,000 °C) and rock, which reaches
speeds moving away from a volcano of up to
700 km/h. The flows normally hug the ground
and travel downhill. They are a common and
devastating result of certain explosive volcanic
eruptions.
Stages of Volcanic
Activity
• A volcano is active when it is erupting or showing signs
that it will soon erupt.
•
•
•
A volcano is dormant when it is no longer erupting but is
likely to erupt again in the future. A dormant volcano many
not erupt for thousands of years. But it can become active
at any time.
A volcano is extinct when it is no longer likely to erupt,
even in the future. Sometimes it is hard to distinguish
dormant volcanoes from extinct ones, since sometimes
thousands to hundreds of thousands of years can pass
between eruptions.
Supervolcanoes can have a dormancy of millions of years.
Stages of Volcanic
Activity
• The Jackson Dome is an extinct
volcano that was once a small island in
the middle of the ocean that covered
most of the southeastern United States
some 70 million years ago.
Supervolcanoes
• A supervolcano is a volcano capable of
producing a volcanic eruption with an ejecta
volume greater than 1,000 cubic km. This is
thousands of times larger than most historic
volcanic eruptions. Super-volcanoes can occur
when magma in the Earth rises into the crust
from a hotspot but is unable to break through the
crust. Pressure builds in a large and growing
magma pool until the crust is unable to contain
the pressure.
•
•
Supervolcanoes
Supervolcanic eruptions typically cover huge areas with lava
and volcanic ash and cause a long-lasting change to weather
(such as the triggering of a small ice age) sufficient to threaten
species with extinction.
There are currently 6 listed potential supervolcanoes on the
planet:

Yellowstone, Wyoming

Long Valley, California

Valles Caldera, New Mexico

Lake Toba, Indonesia

Taupo Volcano, New Zealand
Monitoring Volcanoes
• Scientists try to predict when a volcano
will erupt. They watch for signs that
magma is moving upward. Scientists
often can predict when a volcano will
erupt. But they cannot predict what kind
of eruption or how strong the eruption
will be.