Download Chapter 9: Causes of Volcanic Eruptions

Chapter 9: Causes of
Volcanic Eruptions
What You Will Learn
•Describe the formation and movement of
magma.
•Explain the relationship between volcanoes
and plate tectonics.
•Summarize the methods scientists use to
predict volcanic eruptions.
• Today, even more people are living on and near active
volcanoes. Scientists closely monitor volcanoes to avoid this
type of disaster.
• They study the gases coming from active volcanoes and look
for slight changes in the volcano’s shape that could indicate
that an eruption is near
• Scientists know much more about the causes of eruptions
than the ancient Pompeiians did, but there is much more to
be discovered.
The Formation of Magma
• Understanding how magma forms helps explain why
volcanoes erupt.
• Magma forms in the deeper regions of the Earth’s
crust and in the uppermost layers of the mantle
where the temperature and pressure are very high.
• Changes in pressure and temperature cause magma
to form.
Pressure and Temperature
• Part of the upper mantle is made of very hot, puttylike rock that flows
slowly.
• The rock of the mantle is hot enough to melt at Earth’s surface, but it
remains a puttylike solid because of pressure.
• This pressure is caused by the weight of the rock above the mantle.
• In other words, the rock above the mantle presses the atoms of the
mantle so close together that the rock cannot melt.
•
Rock melts when its temperature
increases or when the pressure on the
rock decreases.
Magma Formation in the Mantle
•Because the temperature of the mantle is
fairly constant, a decrease in pressure is the
most common cause of magma formation.
•Magma often forms at the boundary
between separating tectonic plates, where
pressure is decreased.
•Once formed, the magma is less dense than
the surrounding rock, so the magma slowly
rises toward the surface like an air bubble in
a jar of honey.
Where Volcanoes Form
•The locations of volcanoes give clues
about how volcanoes form.
•A large number of volcanoes lie
directly on tectonic plate boundaries.
•In fact, the plate boundaries
surrounding the Pacific Ocean have so
many volcanoes that the area is called
the Ring of Fire.
Plate Boundaries
• Tectonic plate boundaries are areas where tectonic
plates either collide, separate, or slide past one
another.
• At these boundaries, it is possible for magma to form
and travel to the surface.
• About 80% of active volcanoes on land form where
plates collide, and about 15% form where plates
separate.
• The remaining few occur far from tectonic plate
boundaries.
When Tectonic Plates Separate
• At a divergent boundary, tectonic plates move away from each other.
• As tectonic plates separate, a set of deep cracks called a rift zone
forms between the plates.
• Mantle rock then rises to fill in the gap. When mantle rock gets closer
to the surface, the pressure decreases.
• The pressure decrease causes the mantle rock to melt and form
magma.
• Because magma is less dense than the surrounding rock, it rises
through the rifts.
• When the magma reaches the surface, it spills out and hardens,
creating new crust
When Tectonic Plates Collide
• If you slide two pieces of notebook paper into one another on a flat
desktop, the papers will either buckle upward or one piece of paper
will move under the other.
• This is similar to what happens at a convergent boundary. A
convergent boundary is a place where tectonic plates collide.
• When an oceanic plate collides with a continental plate, the oceanic
plate usually slides underneath the continental plate.
• The process of subduction, the movement of one tectonic plate
underneath another, is shown in Figure 4.
• Oceanic crust is subducted because it is denser and thinner than
continental crust.
Hot Spots
• Not all magma develops along tectonic plate
boundaries.
• For example, the Hawaiian Islands, some of the most
well-known volcanoes on Earth, are nowhere near a
plate boundary.
• The volcanoes of Hawaii and several other places on
Earth are known as hot spots.
• Hot spots are volcanically active places on the
Earth’s surface that are far from plate boundaries.
Measuring Small Quakes and Volcanic Gases
• Most active volcanoes produce small earthquakes as
the magma within them moves upward and causes
the surrounding rock to shift.
• Monitoring these quakes is one of the best ways to
predict an eruption.
• The ratio of certain gases, especially that of sulfur
dioxide, SO2, to carbon dioxide, CO2, may be
important in predicting eruptions.
Measuring Slope and Temperature
• As magma moves upward prior to an eruption, it
can cause the Earth’s surface to swell.
• The side of a volcano may even bulge as the
magma moves upward
• An instrument called a tiltmeter helps scientists
detect small changes in the angle of a volcano’s
slope.