Lava Types
INTRODUCTION
Today, there are many active volcanoes
worldwide. Is there anything we can do to
predict how and when they will erupt?
As the world’s population grows, more and
more people are living in potentially
dangerous volcanic areas. Volcanic
eruptions continue – as they have
throughout most of geologic time – posing
ever – greater threats to life and property.
Mt. Rainier looms over the Seattle/Tacoma
area, endangering a population of more
than three million people. South of Mexico
City, Popocatepetl has begun to come to
life again, putting a million nearby
residents as risk. Another million people
living in the Naples area are threatened by
Mt. Vesuvius’ continued unrest. The need
for planning is urgent.
MELTING ROCKS
Before talking about predicting eruptions,
we need to look at why they occur.
How do Volcanoes Form?
Deep inside Earth, between the molten iron
core and the thin crust at the surface, there
is the mantle, a large layer of rock that is
largely solid, but flows like plastic. When,
for various reasons, rock from the mantle
melts, it sometimes moves to the Earth’s
surface through weak spots in the crust,
releasing heat, gasses, and rock – a
volcanic eruption. But why does this solid
rock melt and come to the surface?
From Magma to Lava
Extremely high temperature and pressure
can cause the lower mantle to melt and
become liquid rock, or magma. When a
body of magma rises through the denser
rock layers toward Earth’s surface, some of
it remains liquid. Magma that has reached
the surface is called lava.
Magma comes in different “flavors,” or
compositions. Each of these will produce a
different lava, from fluid, fast-moving
basalt to slower, more viscous andesite.
Because rocks are made up of collections
of minerals that melt at different
temperatures, the makeup of the rock being
melted affects the magma that results.
DYNAMIC EARTH
Why do volcanoes erupt in
different ways?
Most volcanoes occur on plate boundaries.
Plate boundaries are areas where Earth’s
shifting plates meet or split apart, usually
with violent results.
Plate margins that are coming together are
called convergent margins, while those that
are splitting apart are called divergent. A
third type, transform-fault margins, are
sliding against each other, going in
opposite directions (like those of the San
Andreas Fault). Volcanoes can occur on
convergent or divergent plate margins or
over a hotspot, a spot inside the mantle that
heats an area of the plate above it.
Colliding Plates
Along convergent margins, when two
plates meet, sometimes one descends,
usually of oceanic composition, beneath
the other, usually of continental
composition, in a process called
subduction. As the descending plate is
forced deeper into the mantle, parts of it
begin to melt and form magma that rises to
the surface, often in explosive eruptions.
Subduction zones tend to create large,
classic, cone-shaped volcanoes called
stratovolcanoes, such as Mt. St. Helens in
Washington State, or Mt. Shasta in
California.
Separating Plates
At divergent margins, plates are coming
apart and hot rock forces its way to the
surface. Many divergent plate margins are
under the oceans, creating long undersea
rift zones that fill with lava. In some
eruptions at divergent margins, the
relatively calm, smooth flow of lava creates
volcanoes with gently sloping sides called
shield volcanoes.
Hotspots
Hotspots can also cause shield volcanoes to
form. As plates move over hotspots,
volcanoes spring up and die down in turn,
often creating an island chain. The
Hawaiian Islands are the result of a
hotspot.
JUDGING HAZARDS
What happens when a volcano erupts?
When you think of a volcanic eruption, you
probably imagine red-hot lava flowing
down the side of a volcano. Lava is a
serious hazard, but there are others—some
of them far more dangerous.
Lava Flows
Lava flows are a threat, but they are usually
slow enough that people can get out of the
way. Economic loss from lava flows—
including burned buildings and ruined
crops –is more common. Because lava
flows normally don’t cover very large
areas, the damage is usually limited.
Falling Ash
In an explosive eruption, pent-up gases
escape violently. Magma breaks into pieces
and bursts from the volcano in a column of
ash and fiery fragments. The cooled
fragments that fall back to Earth are called
tephra. In a large eruption, tephra can cover
vast areas with a thick layer of ash,
presenting a much greater hazard than lava
flows. Very large fragments can cause
significant damage on impact, colliding
with structures or setting things on fire.
Very fine particles of tephra cause
breathing difficulties and interfere with
machinery.
Glowing Avalanches
Pyroclastic flows are mixtures of very hot
gas and tephra that cascade down a
volcano’s sides at high speeds. A
pyroclastic flow covered the city of
Herculaneum in AD 79, killing many
residents. Because pyroclastic flows can
spread destruction over larges areas and
move at very high speeds, they are
extremely hazardous. Pyroclastic flows
cause damage by burial and by
incineration, and because of their speed and
gas content can also cause impact damage
and asphyxiation. Most people are not
aware that this danger exists. Pyroclastic
flows are common at stratovolcanoes.
Mud and Debris Flows
Debris flows, fragments of mud and other
debris that flow down the sides of a
volcano, are another serious and littleknown hazard. Debris flows often form
when part of the volcano collapses,
breaking up and flowing downhill. If the
collapse is a major one, the large flow that
results can travel great distances, often
burying everything in its path. These are
particularly dangerous on volcanoes that
have glaciers on top, as the eruption
instantly melts the ice, causing a massive
mud slide, much like what happened during
the Mt. St. Helens eruption in 1980.
can be mapped, making disaster planning
more effective. In addition to this type of
long-range forecasting, scientists are
becoming more and more skilled at
spotting the warning signs of an eruption.
Warning Signs
Before an eruption, magma moves into the
area beneath the volcano and collects in a
magma chamber, or reservoir. As it comes
closer to the surface, the magma releases
gases. These events can offer valuable
clues about the likelihood of an eruption.
For example, the movement of magma
produces small earthquakes and vibrations
(seismicity). Magma gathering in a
chamber causes slight swelling of the
volcano’s slopes. Gases released near the
volcano can be measured for changes in
quantity and makeup.
Monitoring Methods
Another common types of debris flow is a
lahar, or volcanic mudflow. This mixture
of mud (mainly volcanic ash from tephra
deposits) and water flows quickly down
stream valleys that drain the volcano’s
slopes.
A number of tools can be used to record
these warning signs. Seismographs can
detect small earthquakes, while tiltmeters
and geodimeters can measure the subtle
swelling of a volcano. Correlation
spectrometers can measure amounts of
sulfur dioxide—a telltale gas that is
released in increasing quantities before an
eruption. Using these and other tools, it’s
possible to closely monitor activity at an
awakening volcano.
Other Dangers
The Problem of Prediction
Lava, ash, and debris flows are the most
common and serious volcanic hazards, but
others do exist. Severe eruptions can
disrupt the climate for long periods or
cause atmospheric shock waves. Eruptions
can also cause tsunamis, volcanic
earthquakes, or the release of suffocation
gases. These hazards pose serious threats to
both life and property.
Volcanologists are becoming very skilled
at predicting the likelihood of an eruption.
Still, a number of barriers remain. It’s very
difficult to pinpoint exactly when an
eruption will happen. Often, moving
magma doesn’t result in an eruption, but
instead cools below the surface.
Monitoring potential eruptions is
expensive. With many volcanoes erupting
only every few hundred or thousand years,
it’s not possible to monitor every site. If we
set up monitoring devices, we should not
be caught off guard by disastrous eruptions.
FORECASTING
Can we predict when a volcano
will erupt?
Scientists can often find clues about past
eruptions by studying the deposits left
behind. Areas affected by lava flows,
debris flows, tephra, or pyroclastic flows
COPING WITH RISK
How Can We Reduce the Risk?
There are four general approaches to
coping with volcanic hazards. We can try
to keep the hazard from occurring—often
an impossible task. We can try to alter its
path or reduce its impact on existing
development. We can take steps to protect
future development. We can also do our
best to have disaster response plans in
place before they are needed.
Removing the Threat
Obviously, there is no way to stop an
eruption. We can, however, attempt to
reduce the eruption’s effects by reinforcing
structures (for example, strengthening roofs
to support the weight of tephra deposits) or
by building protective works (such as walls
to deflect lava flows away from developed
areas). Such efforts can be and have been
successful, but are of limited use in a largescale eruption.
Planning for the Future
Protecting future development from
volcanic hazards is a simpler task. Before
building, we should evaluate the risk. If it
seems too great, a safer location should be
found. This type of planning is very
effective, but all too often, people are
drawn to the lush, rolling terrain of a quiet
volcano.
Disaster Preparedness
When a volcano comes to life, a few weeks
may not be enough time to avert a tragedy.
Planning is the key to saving lives. Well
before the warning signs occur, people
must be educated about volcanic hazards.
Evacuation plans must be in place.
Communication between scientists,
officials, the media, and the general public
should be outlined and practiced.
Emergency measures must be thought out
and agreed upon.
BENEFITS OF
VOLCANOES
(http://library.thinkquest.org/C003603/engl
ish/volcanoes/theadvantages.shtml)
What good can come from volcanoes?
Basic Life Advantages
Volcanoes are helpful, too. Can you believe
it? Most of the rocks on the ocean floor and
our continents were all formed from
volcanoes. Early volcanoes produced steam
that later became the water that we cannot
live without, as well as the oxygen we need
to survive. Volcanic ash and lava form rich
soil for farming and agriculture. Hot
springs are harnessed for geothermal
energy to create a clean, renewable way to
produce electricity and heat in cities.
Aesthetic Uses
Lava and tuffs can be cut into blocks and
used as stone for buildings. The rocks are
very strong, lightweight, resist weathering
and are good insulators. Fine-grained ash is
used as a polishing compound, and can be
found in toothpaste and household
scrubbing compounds. Ash can also be
used in concrete. Ash is even used in cat
litter because it easily absorbs moisture.
Pumice is a stone that can be used to
exfoliate skin. Obsidian, a smooth black
shiny rock, has been carved to be used as
arrowheads and knives and even used in
eye surgery. Minerals such as diamonds,
gold, zinc, silver, copper, and lead have
been found near volcanic areas.