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Chapter 6
Volcanoes
SECTION 1: VOLCANIC ERUPTIONS
• Volcanoes are areas of Earth’s surface through
which magma and volcanic gasses pass
Nonexplosive Eruptions
• Are occurring at this very moment on earth and on the
ocean floor
• These are the most common eruptions.
• They produce relatively calm flows of lava
• 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
• Are much more rare, but the effects can be much
more destructive.
• During an explosive eruption, clouds of hot
debris, ash and gas rapidly shoot out of a
volcano.
• Instead of lava flows, explosive eruptions cause
molten rock to be blown into tiny particles that
harden in the air.
• The dust sized particles, called ash, can reach the upper
atmosphere and can circle the Earth for years.
• Larger pieces of debris fall closer to the volcano
• 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?
• A magma chamber is a body of molten rock
deep underground that feeds a volcano
• Magma rises from the magma chamber
through cracks in the Earth’s crust to openings
called vents.
• Magma is released through these vents during
an eruption.
What makes up magma?
• The composition of the magma affects how
explosive an eruption is. The explosiveness
lies in the silica, water and gas content of the
magma.
• If the water content of the magma is high, an
explosive eruption is more likely.
– Just like shaking a can of soda….The water in the
magma underground remains dissolved because
of high heat and pressure. When the magma
moves to the surface the water and other
compounds become gases. As the gases expand
rapidly, an explosion can occur.
– Because some lava is so frothy from the gases
when it hardens it becomes pumice!
Magma with high silica content is also
explosive.
• The silica-rich magma is stiff and flows slowly,
occasionally blocking the vents. As more magma
pushes up below, the pressure increases and if
enough pressure exists an explosion can occur.
• Stiff magma can also prevent water vapor and
other gases from escaping; these gases can
expand until the explode shattering the magma
and ash and pumice are blasted from the vent.
What Erupts from a Volcano?
• Lava is liquid magma that flows from a volcanic vent
– Lava is released mostly from nonexplosive volcanoes
– The viscosity of lava varies greatly. High viscosity is stiff
and low viscosity is more fluid
– Blocky lava and pahoehoe have a high viscosity and flow
slowly
– Other types such as aa and pillow lava, have lower
viscosities and flow more quickly.
Pyroclastic material forms when magma is blasted into the air
and hardens
• Produced from mostly explosive volcanoes
• Forms when magma explodes from a volcano and solidifies in the
air
• Also from eruptions that shatter existing rock
• Range in size from tiny pebbles to huge boulders the size of houses
• Four types are vocanic bombs, Lapilli, volcanic ash, and volcanic
blocks…see page 160.
• 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/hr – faster than most hurricane winds!!
• Temps at the center of this flow can exceed
700⁰C
SECTION 2: EFFECTS OF VOLCANIC
ERUPTIONS
Volcanic Eruptions and Climate
Change
• As volcanic ash and gases
spread throughout the
atmosphere they can
block enough sunlight to
cause global
temperatures to drop.
• A small shift of
temperature, such as
0.5⁰C can disrupt climates
all over the world.
Ash from the eruption of Mount
Pinatubo blocked out the sun in
the Philippines for several days.
The eruption also affected global
climate.
DIFFERENT TYPES OF VOLCANOES
Shield Volcanoes
• Are built of layers of lava released from repeated
nonexplosive eruptions
• Lava is runny and spreads over a large area, creating layers,
and eventually a volcano that has gently sloping sides
• Can be enormous
• Hawaii’s Mauna Kea is the tallest mountain on Earth, taller
than Mount Everest when you measure from its base at the
ocean floor.
http://www.youtube.com/watch?v=byJp5o49IF4
Cinder Cone Volcanoes
• Made of pyroclastic material usually produced from
moderately explosive eruptions
• Forms steep slopes
• Cinder cones are small and usually erupt for only a
short time
• Cinder cones often occur in clusters, commonly on the
sides of other volcanoes.
• Usually erode quickly because the pyroclastic material
is not cemented together.
http://www.youtube.com/watch?v=aS_xl3nu_mY
Composite Volcanoes
• Sometimes called stratovolcanoes
• Are the most common types of volcanoes
• Form from explosive eruptions of pyroclastic material
followed by quieter flows of lava, forming alternating layers
• Have broad bases and sides that get steeper towards the
top
• Include Mount Hood, Mount Rainier, Mount Shasta, and
Mount St. Helens
http://www.youtube.com/watch?v=1u1Ys4m5zY4
Other Types of Volcanic landforms
• Craters are funnel
shaped pits found
around the
central vent at the
top of many
volcanoes
• Calderas are large,
semicircular
depressions that form
when the chamber that
supplies magma to a
volcano partially
empties and the
chamber’s roof
collapses
• Lava Plateaus are landforms that result
from repeated eruptions of lava spread
over a large area (usually from fissures or
rifts)
The Columbia River Plateau formed from a massive outpouring
of lava that began 17 million years ago.
SECTION 3: CAUSES OF VOLCANIC
ERUPTIONS
The Formation of magma
• 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
• Rock melts when its temperature increases or when the
pressure on the rock decreases
• A decrease in pressure is the most common cause of
magma formation, often forming at the boundary between
separating tectonic plates where pressure is decreased
The curved line indicates the melting point of a rock. As pressure
decreases and temperature increases, the rock begins to melt.
Where Volcanoes Form
• A large number of volcanoes lie directly on
tectonic plate boundaries
Tectonic plate boundaries are likely places for volcanoes to form. The
Ring of Fire contains nearly 75% of the world’s active volcanoes on land.
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 the gap. When
mantle rock gets closer to the surface, pressure
decreases causing 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.
• Lava that flows from undersea rift zones produces
volcanoes and mountain chains called mid-ocean ridges.
• Most volcanic activity on Earth occurs at mid-ocean ridges.
• Iceland was created from by lava from the Mid-Atlantic
Ridge and in 1963 enough lava poured out of the MidAtlantic Ridge near Iceland to form a new island called
Surtsey.
When Tectonic Plates Collide
• A convergent boundary is a place where tectonic plates collide
• When an ocean plate collides with a continental plate, the ocean
plate usually slides underneath because it is more dense and
thinner than continental crust
• The process of subduction is the movement of one tectonic plate
underneath another
• Subduction causes an increase in temperature and pressure causing
the water in the oceanic crust to be released; it 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
(example are the Hawaiian islands)
• Two theories are that hot spots are directly above
columns of rising magma called mantle plumes,
or that they are the results of cracks in the Earth’s
crust.
• Hot spots usually produce a long chain of
volcanoes.
Predicting Volcanic Eruptions
Volcanoes are classified in three categories:
1. Extinct Volcanoes – have not erupted in recorded
history and probably will never erupt again
2. Dormant Volcanoes – are currently not erupting, but
the record of past eruptions suggests that they may
erupt again
3. Active Volcanoes – are currently erupting or show
signs of erupting in the near future.
http://www.youtube.com/watch?v=D4QT215zYgY
• Most active volcanoes produce small earthquakes
as the magma within them moves upward and
causes the surrounding rock to shift.
• Before an eruption, the number and intensity of
the earthquakes increase and the occurrence of
quakes may be continuous. Monitoring these
quakes may be the best way to predict a volcanic
eruption.
• Can also study the volume
and composition of volcanic
gases. Changes in the sulfur
dioxide to carbon dioxide
ratios may indicate changes
in the magma chamber
below.
• As magma moves upward
prior to an eruption, it can
cause the Earth’s surface to
swell, sometimes causing
the sides of a volcano to
bulge as the magma moves
upward.
As if being this close to an active
volcano is not dangerous enough, the
gases being collected are extremely
poisonous
• An instrument called a tiltmeter helps scientists
detect small changes in the angle of a volcano’s
shape
• Scientists use GPS to detect the changes in a
volcano’s slope that may signal an eruption
• Also using satellite images can predict eruptions
by recording changes in surface temperatures
and gas emissions of a volcano over time. If the
site is getting hotter, the magma below is
probably rising.
The Mount St Helens Story, 22:58
http://www.youtube.com/watch?v=FnDT_6V4qVw&feature=related