Volcanoes
Section 1
Volcanic Eruptions
The explosive pressure of a volcanic eruption can turn an entire mountain into a
billowing cloud of ash and rock in a matter of seconds.
But eruptions are also creative forces—they help form fertile farmland. They also
create some of the largest mountains on Earth.
Magma
molten rock forced to the Earth’s surface.
Lava
Magma that flows onto the Earth’s surface.
Volcanoes
are areas of Earth’s surface through which magma and volcanic gases pass.
Nonexplosive Eruptions
At this moment, volcanic eruptions are occurring around the world—on the ocean
floor and on land.
Nonexplosive eruptions are the most common type of eruption. These eruptions
produce relatively calm flows of lava
Nonexplosive eruptions can release huge amounts of lava. Vast areas of the Earth’s
surface, including much of the sea floor and the Northwest region of the United
States, are covered with lava from nonexplosive eruptions.
Explosive Eruptions
Explosive eruptions are much rarer than nonexplosive eruptions. However, the
effects of explosive eruptions can be incredibly destructive.
During an explosive eruption, clouds of hot debris, ash, and gas rapidly shoot out
from a volcano.
Instead of producing lava flows, explosive eruptions cause molten rock to be blown
into tiny particles that harden in the air.
Ash
The dust-sized particles can reach the upper atmosphere and can circle the Earth for
years.
Larger pieces of debris fall closer to the volcano. An explosive eruption can also
blast millions of tons of lava and rock from a volcano. In a matter of seconds, an
explosive eruption can demolish an entire mountainside
What Is Inside a Volcano?
magma chamber
is a body of molten rock deep underground that feeds a volcano.
Vents
Magma rises from the magma chamber through cracks in the Earth’s crust to
openings called vents. Magma is released from the vents during an eruption.
What Makes Up Magma?
Water and Magma Are an Explosive Combination
If the water content of magma is high, an explosive eruption is more likely.
Because magma is underground, it is under intense pressure and water stays
dissolved in the magma. If the magma quickly moves to the surface, the pressure
suddenly decreases and the water and other compounds, such as carbon dioxide,
become gases. As the gases expand rapidly, an explosion can result.
Silica-Rich Magma Traps Explosive Gases
Magma that has a high silica content also tends to cause explosive eruptions. Silicarich magma has a stiff consistency. It flows slowly and tends to harden in a volcano’s
vents. As a result, it plugs the vent. As more magma pushes up from below,
pressure increases. If enough pressure builds up, an explosive eruption takes place.
Stiff magma also prevents water vapor and other gases from easily escaping. Gas
bubbles trapped in magma can expand until they explode. When they explode, the
magma shatters and ash and pumice are blasted from the vent. Magma that contains
less silica has a more fluid, runnier consistency. Because gases escape this type of
magma more easily, explosive eruptions are less likely to occur.
What Erupts from a Volcano?
Lava
is liquid magma that flows from a volcanic vent.
Pyroclastic material
forms when magma is blasted into the air and hardens.
Nonexplosive eruptions produce mostly lava.
Explosive eruptions produce mostly pyroclastic material. Over many years—or even
during the same eruption—a volcano’s eruptions may alternate between lava and
pyroclastic eruptions.
Types of Lava
The viscosity of lava, or how lava flows, varies greatly.
viscosity,
remember that a milkshake has high viscosity and a glass of milk has low viscosity.
Lava that has high viscosity is stiff.
Lava that has low viscosity is more fluid. The viscosity of lava affects the surface of a
lava flow in different ways
Blocky lava and pahoehoe (puh HOY HOY) have a high viscosity and flow slowly.
Other types of lava flows, such as aa (AH AH) and pillow lava, have lower viscosities
and flow more quickly
Types of Pyroclastic Material
Pyroclastic material forms when magma explodes from a volcano and solidifies in the
air. This material also forms when powerful eruptions shatter existing rock. The size
of pyroclastic material ranges from boulders that are the size of houses to tiny
particles that can remain suspended in the atmosphere for years
Pyroclastic Flows
Pyroclastic flows are produced when enormous amounts of hot ash, dust, and gases
are ejected from a volcano.
This glowing cloud of pyroclastic material can race downhill at speeds of more than
200 km/h—faster than most hurricane-force winds!
The temperature at the center of a pyroclastic flow can exceed 700°C.
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Section 2
Effects of Volcanic Eruptions
During large-scale eruptions, enormous amounts of volcanic ash and gases are
ejected into the upper atmosphere. As volcanic ash and gases spread throughout the
atmosphere, they can block enough sunlight to cause global temperatures to drop.
Different Types of Volcanoes
Volcanic eruptions can cause profound changes in climate. But the changes to Earth’s
surface caused by eruptions are probably more familiar. Perhaps the best known of
all volcanic landforms are the volcanoes themselves.
Shield Volcanoes
Shield volcanoes are built of layers of lava released from repeated nonexplosive
eruptions. Because the lava is very runny, it spreads out over a wide area. Over
time, the layers of lava create a volcano that has gently sloping sides. Although their
sides are not very steep, shield volcanoes can be enormous. Hawaii’s Mauna Kea, the
shield volcano shown here, is the tallest mountain on Earth. Measured from its base
on the sea floor, Mauna Kea is taller than Mount Everest.
Cinder Cone Volcanoes
Cinder cone volcanoes are made of pyroclastic material usually produced from
moderately explosive eruptions. The pyroclastic material forms steep slopes, as
shown in this photo of the Mexican volcano Paricutín. Cinder cones are small and
usually erupt for only a short time. Paricutín appeared in a cornfield in 1943 and
erupted for only nine years before stopping at a height of 400 m. Cinder cones often
occur in clusters, commonly on the sides of other volcanoes. They usually erode
quickly because the pyroclastic material is not cemented together.
Composite Volcanoes
Composite volcanoes, sometimes called stratovolcanoes, are one of the most
common types of volcanoes. They form from explosive eruptions of pyroclastic
material followed by quieter flows of lava. The combination of both types of eruptions
forms alternating layers of pyroclastic material and lava. Composite volcanoes, such
as Japan’s Mount Fuji (shown here), have broad bases and sides that get steeper
toward the top. Composite volcanoes in the western region of the United States
include Mount Hood, Mount Rainier, Mount Shasta, and Mount St. Helens
Other Types of Volcanic Landforms
Craters
Around the central vent at the top of many volcanoes is a funnel-shaped pit called a
crater. During less explosive eruptions, lava flows and pyroclastic material can pile
up around the vent creating a cone with a central crater.
As the eruption stops, the lava that is left in the crater often drains back
underground. The vent may then collapse to form a larger crater.
If the lava hardens in the crater, the next eruption may blast it away. In this way, a
crater becomes larger and deeper.
Calderas
Calderas can appear similar to craters, but they are many times larger.
caldera
is a large, semicircular depression that forms when the chamber that supplies
magma to a volcano partially empties and the chamber’s roof collapses.
Much of Yellowstone Park is made up of three large calderas that formed when
volcanoes collapsed between 1.9 million and 0.6 million years ago. Today, hot
springs, such as Old Faithful, are heated by the thermal energy left over from those
events.
Lava Plateaus
The most massive outpourings of lava do not come from individual volcanoes. Most
of the lava on Earth’s surface erupted from long cracks, or rifts, in the crust. In this
type of eruption, runny lava can pour out for millions of years and spread over huge
areas. A landform that results from repeated eruptions of lava spread over a large
area is called a lava plateau.
The Columbia River Plateau is a lava plateau that formed between 17 million and 14
million years ago in the northwestern region of the United States. In some places,
the Columbia River Plateau is 3 km thick.
Section 3
Causes of Volcanic Eruptions
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.
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.
Rift ZOne
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
Mid-Ocean Ridges Form at Divergent Boundaries
Lava that flows from undersea rift zones produces volcanoes and mountain chains
called mid-ocean ridges.
the Earth is circled with mid-ocean ridges. At these ridges, lava flows out and
creates new crust. Most volcanic activity on Earth occurs at mid-ocean ridges.
While most mid-ocean ridges are underwater, Iceland, with its volcanoes and hot
springs, was created by lava from the Mid-Atlantic Ridge. In 1963, enough lava
poured out of the Mid-Atlantic Ridge near Iceland to form a new island called
Surtsey. Scientists watched this new island being born!
When Tectonic Plates Collide
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.
subduction,
the movement of one tectonic plate underneath another
Oceanic crust is subducted because it is denser and thinner than continental crust.
Subduction Produces Magma
As the descending oceanic crust scrapes past the continental crust, the temperature
and pressure increase. The combination of increased heat and pressure causes the
water contained in the oceanic crust to be released. The water then mixes with the
mantle rock, which lowers the rock’s melting point, causing it to melt. This body of
magma can rise to form a volcano.
Hot Spots
Hot spots
are volcanically active places on the Earth’s surface that are far from plate
boundaries. Some scientists think that hot spots are directly above columns of rising
magma, called mantle plumes. Other scientists think that hot spots are the result of
cracks in the Earth’s crust.
A hot spot often produces a long chain of volcanoes.
One theory is that the mantle plume stays in the same spot while the tectonic plate
moves over it
Another theory argues that hot-spot volcanoes occur in long chains because they
form along the cracks in the Earth’s crust. Both theories may be correct.
Predicting Volcanic Eruptions
Extinct volcanoes
have not erupted in recorded history and probably never will erupt again.
Dormant volcanoes
are currently not erupting, but the record of past eruptions suggests that they may
erupt again.
Active volcanoes
are currently erupting or show signs of erupting in the near future. Scientists study
active and dormant volcanoes for signs of a future eruption.
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. Just before an eruption, the
number and intensity of the earthquakes increase and the occurrence of quakes may
be continuous. Monitoring these quakes is one of the best ways to predict an
eruption.
scientists also study the volume and composition of volcanic gases. The ratio of
certain gases, especially that of sulfur dioxide, SO2, to carbon dioxide, CO2, may be
important in predicting eruptions. Changes in this ratio may indicate changes in the
magma chamber below.
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.
Tiltmeter
An instrument that helps scientists detect small changes in the angle of a volcano’s
slope. Scientists also use satellite technology such as the Global Positioning System
(GPS) to detect the changes in a volcano’s slope that may signal an eruption.
One of the newest methods for predicting volcanic eruptions includes using satellite
images. Infrared satellite images record changes in the surface temperature and gas
emissions of a volcano over time. If the site is getting hotter, the magma below is
probably rising!