Chapter 5: Magma And Volcanoes
Introduction: Earth’s Internal Thermal
Engine
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Magma is molten rock beneath Earth’s surface.
Because liquid magma is less dense than
surrounding solid rock, and obviously more
mobile, magma, once formed, rises toward the
surface.
Magma that reaches the surface does so by
erupting through vents we call volcanoes.
Volcanoes
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The term volcano comes from the name of the
Roman god of fire, Vulcan.
There are different types of volcanoes.
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Eruption vary from gentle flows (Hawaii and Iceland)
to catastrophic explosions (Mount St. Helens, Mt.
Pinatubo, Soufriere Hills).
The majority of eruption never make the news
because they occur beneath the ocean,
unobserved.
Magma
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Magma has a wide range of compositions, but
silica (SiO2) always dominates the mix.
Magma has high temperatures.
Magma is fluid—it has the ability to flow. Most
magma actually is a mixture of liquid (often
referred to as melt) and solid mineral grains.
Composition of Magmas and Lavas
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The composition of magmas and lavas is
controlled by the most abundant elements in the
Earth—Si, Al, Fe, Ca, Mg, Na, K, H, and O.
Three distinct types of magma are more common
than others:
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Basaltic, containing about 50 percent SiO2.
Andesitic, about 60 percent SiO2.
Rhyolitic, about 70 percent SiO2.
Figure 5.1
Basaltic Magmas
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Basaltic magmas are erupted by approximately
80 percent of volcanoes worldwide (the seafloor
worldwide is mostly basalt).
Magma from Hawaiian volcanoes such as
Kilauea and Mauna Loa is basaltic.
The entire island of Iceland is basaltic.
Andesitic and Rhyolitic Magmas
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Andesitic magmas are about 10 percent of the
total magma.
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Magma from Mount St. Helens in Washington State
and Krakatau in Indonesia is usually andesitic.
Rhyolitic magmas are about 10 percent of the
total magma.
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Magmas erupted from volcanoes that once were active
at Yellowstone Park are mostly rhyolitic.
Figure 5.3
Figure 5.5
Gases Dissolved in Magma
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Small amounts of gas (0.2 to 3% by weight) are
dissolved in all magma.
The principal gas in water vapor, which,
together with carbon dioxide, accounts for more
than 98 percent of all gases emitted from
volcanoes.
Temperature of Magmas and Lavas
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Magma temperatures range from 1000o to
1200oC.
Magma temperatures can reach 1400oC under
some conditions.
Viscosity of Magmas and Lavas (1)
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The internal property of a substance that offers
resistance to flow is called viscosity.
The more viscous a magma, the less easily it
flows.
Viscosity of a magma depends on temperature
and composition (especially the silica and
dissolved-gas contents).
Viscosity of Magmas and Lavas (2)
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The higher the temperature, the lower the
viscosity, and the more readily magma flows.
The smooth, ropy-surfaced lava in Hawaii,
formed from a very hot, very fluid lava is called
pahoehoe.
The rough-looking lava formed from a cooler
lava having a high viscosity is called aa (ah ah).
Viscosity of Magmas and Lavas (3)
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The greater the silica content, the larger is the
polymerized group.
For this reason, rhyolitic magma (70% silica) is
always more viscous than basaltic magma (50%
silica).
Andesitic magma has a viscosity that is
intermediate between the two (60% silica).
How Buoyant Magma Erupts on the
Surface (1)
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Magma is less dense than the solid rock from
which it forms.
The pressure is proportional to depth (thickness
of overlying rock).
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Therefore, as magma rises upward, the pressure on it
decreases.
How Buoyant Magma Erupts on the
Surface (2)
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Pressure controls the amount of gas a magma
can dissolve—more at high pressure, less at low.
Gas dissolved in an upward-moving magma
comes out of solution and forms bubbles.
Eruption Style—Nonexplosive or
Explosive? (1)
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Nonexplosive eruptions occur notably in Hawaii,
Iceland, and the seafloor.
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They are relatively safe.
The difference between nonexplosive and
explosive eruptions depends largely on magma
viscosity and dissolved-gas content.
Low viscosity magmas and low dissolved gas
contents produce nonexplosive eruptions.
Eruption Style—Nonexplosive or
Explosive? (2)
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Nonexplosive eruptions may appear violent
during their initial stages.
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The reason is that gas bubbles in a low-viscosity
basaltic magma will rise rapidly upward, like the gas
bubbles in a glass of soda.
If a basaltic magma rises rapidly, spectacular lava
fountains will occur.
Eruption Style—Nonexplosive or
Explosive? (3)
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Because heat is lost quickly at the surface of the
flowing lava, the surface solidifies into a crust,
beneath which the liquid lava continues to flow
in well-defined channels called lava tubes.
The very fluid lava initially forms thin pahoehoe
flows.
With increasing viscosity the rate of movement
slows and the stickier lava may be transformed
into a rough surfaced aa flow that moves very
slowly.
Vesicles and Amygdules
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When lava finally solidified to rock, the lastformed bubbles become trapped; these bubble
preserved in the rock are called vesicles.
Vesicles filled by secondary minerals are called
amygdules.
Explosive Eruptions (1)
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In viscous andesitic or rhyolitic magmas, gas
bubbles can rise only very slowly.
When confining pressure drops quickly, the gas
in a magma expand into a froth of innumerable
glass-walled bubbles called pumice.
Explosive Eruptions (2)
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In many instances, instead of forming pumice,
small bubbles expanding within a huge mass of
sufficiently gas-rich, viscous magma will shatter
the magma into tiny fragments called volcanic
ash.
Volcanic ash is the most abundant product of
explosive eruptions.
Eruption Columns and Tephra Falls (1)
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The largest and the most violent eruptions are
associated with silica-rich magmas having a high
dissolved-gas content.
This hot, turbulent mixture rises rapidly in the
cooler air above the vent to form an eruption
column that may tower as high as 45 km in the
atmosphere.
Eruption Columns and Tephra Falls (2)
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A violent eruption of this kind is called a plinian
eruption, named after the Roman author and
statesman, Pliny, who lost his life in the A.D. 79
eruption of Mt. Vesuvius.
The particles of debris rain down in a tephra fall
and eventually accumulate on the ground as
tephra deposits.
Pyroclastic Flows (1)
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When the mixture of hot gases and pyroclasts is
more dense than the atmosphere, the turbulent
mixture flows down the side of the volcano
rather than forming an eruption column.
A hot, highly mobile flow of tephra that rushes
down the flank of a volcano during a major
eruption is called a pyroclastic flow (the most
devastating and lethal forms of volcanic
eruption).
Pyroclastic Flows (2)
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Pyroclastic flows are also known as nuée ardente
(glowing cloud).
Historic observations indicate that pyroclastic
flows can reach velocities of more than 700 km/h.
In 1902, a pyroclastic flow rushed down the
flanks of Mont Pelee Volcano at an estimated
speed of 200 KM/h, instantly killing 29,000
people.
Lateral Blast—Mount St. Helens
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In 1980, Mount St. Helens, a volcano in
Washington, erupted violently.
As magma rose under the volcano, the
mountain’s north flank began to bulge upward
and outward.
The initial blast was sideways rather than
upward.
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600 km2 of trees in the once-dense forest were leveled.
Figure 5.10
Volcanoes
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There are two broad families of volcanoes:
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Those formed by eruptions from a central vent.
Those that erupt through a long fissure.
Central-vent eruptions build mounds of the kind
most people associate with volcanoes.
Fissure eruptions build plateaus.
Central-vent Volcanoes
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Based on their size and shape, there are three
broad classes of central-vent volcanoes:
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Shield volcanoes.
Tephra cones.
Stratovolcanoes.
Shield Volcanoes (1)
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A shield volcano produces a broad, dome-shaped
mountain with an average surface slope of only a
few degrees.
Low-viscosity basaltic lavas can flow for
kilometers down gentle slopes.
The accumulated lava from repeated eruptions
of low-viscosity lava build a shield volcano.
Shield Volcanoes (2)
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The farther lava flows down the flank, the cooler
and more viscous it becomes, so the steeper the
slope must be for it to flow.
Large shield volcanoes rise as islands in the
ocean (Hawaiian Islands, Tahiti, Samoa, the
Galapagos, and many others).
Figure 5.11
Figure 5.13
Shield Volcanoes (3)
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Mauna Loa volcano, for example, rises to a
height of 4169 m above sea level, but if measured
from the seafloor the height is 10,000 m, making
Mauna Loa the tallest mountain on Earth.
Tephra Cones
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Tephra cone is built by shower of pyroclastic
debris around a volcanic vent.
The slopes of tephra cones are steep, typically
about 30o.
Statovolcanoes (1)
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Some volcanoes (andesitic composition) emit
both viscous lava flows and tephra.
The emissions tend to alternate, forming
alternating strata of lava and tephra, building a
stratovolcano.
Stratovolcanoes are:
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Large.
Conical.
Steep-sided.
Statovolcanoes (2)
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Near the summit, a stratovolcano’s slope may
reach 40o.
Toward the base, the slope flattens to about 6o to
10o.
As a stratovolcano develops, lava flows act as a
cap to slow erosion of the loose tephra.
Statovolcanoes (3)
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The volcano becomes much larger and steeper
than a typical tephra cone.
Mount Fuji (Japan), Mount Rainier, Mount
Baker in Washington State, Mount Hood in
Oregon, Mt Mayon in the Philippines are
stratovolcanoes.
Other Features of Central Eruptions (1)
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Craters:
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Funnel-shaped depressions with steep-sided walls that
open upward.
Craters form in two ways:
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By the collapse of the steep sides of the vent.
By an explosive eruption.
In subsequent eruptions, pressure blasts open the
vent, removing both the solidified magma from the
previous eruption and part of the crater wall.
A crater can grow slowly larger, eruption by eruption.
Other Features of Central Eruptions (2)
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Lava domes:
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If the magma is very viscous (as in a rhyolitic or
andesitic magma), it squeezes out to form a lava
dome.
Figure 5.16
Other Features of Central Eruptions (3)
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Calderas:
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Caldera is from the Spanish word for cauldron.
A roughly circular, steep-walled basin about a
kilometer in diameter or larger.
Calderas are created by collapse of the surface rock
following an eruption and partial emptying of the
underlying magma chamber.
Crater lake in Oregon occupies a circular caldera 8
km in diameter.
Figure 5.19
Other Features of Central Eruptions (4)
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Resurgent domes:
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Often, more magma enters the chamber and lifts the
collapsed caldera floor to form a resurgent dome.
Diatremes:
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Volcanic pipes filled with a rubbles of broken rock.
The walls are vertical, or very nearly so.
A famous diatreme is the diamond mine in Kimberly,
South Africa.
Fissure Eruptions (1)
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Fissure eruptions extrude lava along an elongate
fracture in the crust.
– When fissure eruptions occur on land, the
low-viscosity basaltic lava tends to spread
widely and to create flat lava plains.
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lavas are called plateau basalts.
Figure 5.21
Fissure Eruptions (2)
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The Laki eruption, in Iceland in1783, occurred
along a 32 km long fracture. Lava from it flowed
64 km from one side of the fracture and nearly
48 km from the other, covering 588 km2.
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The Laki eruption is the largest lava flow of any kind
in historic times.
Famine followed and more than 9000 died (20 percent
of the Icelandic population).
Fissure Eruptions (3)
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Pillow basalts:
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When the basaltic magma erupts under the ocean,
seawater cools it so rapidly that pillow-shaped masses
of basalt, ranging from a few centimeters to a meter
or more in greatest dimension form.
Fissure eruptions of andesitic or rhyolitic
magma are much less common than fissure
eruptions of basaltic lava.
Figure 5.18
Fissure Eruptions (4)
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Sometimes the pyroclasts in the tephra are so hot
that the fragments form welded tuff.
Some 40 to 50 million years ago, huge ash-flow
eruptions happened in Nevada.
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The erupted product covered an area in excess of
200,000 km2.
Figure 5.22
Posteruption effects
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When active volcanism finally ceases, rock in
and near an old magma chamber may remain
hot for hundreds of thousands of years.
Thermal spring at many volcanic sites (Italy,
Japan, and New Zealand) have become famous
health spas and sources of energy.
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A thermal spring that intermittently erupts water and
steam is a geyser.
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Most of the world’s geysers outside Iceland are in New
Zealand and in Yellowstone National Park.
Figure B5.2
Figure B5.3
Volcanic Hazards (1)
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Volcanic eruptions are not rare on land, and are
essentially continuous on the seafloor.
Every year about 50 volcanoes erupt on the
Earth’s continents.
Most eruptions are basaltic.
Tephra eruptions from andesitic or rhyolitic
stratovolcanoes like Mount St. Helens and
Krakatau can be disastrous.
Volcanic Hazards (2)
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Eruptions present five kinds of hazards:
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Hot, rapidly moving pyroclastic flows and laterally
directed blasts can overwhelm people before they can
evacuate.
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Mont Pelee in 1902 and Mount St. Helens in 1980.
Tephra and hot poisonous gases can bury or suffocate
people.
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79 Mount Vesuvius in A.D. 79.
Volcanic Hazards (3)
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Mudflows, called lahars, can be devastating.
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Violent undersea eruptions can cause powerful sea
waves called tsunamis.
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In 1985, the Colombian volcano Nevado del Ruiz
experienced a small, nonthreatening eruption. But, when
glaciers at the summit melted, massive mudflows of volcanic
debris moved swiftly down the mountain , killing 20,000.
Krakatau, in 1883, killed more than 36,000 on Java and
nearby Indonesia islands.
A tephra eruption can disrupt agriculture, creating a
famine.
Figure 5.24
Figure 5.25
Plates and Volcanoes (1)
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The distribution of active volcanoes around the
world is strongly influenced by plate margins.
Most of the world’s volcanism happen beneath
the sea, along the 64,000 km midocean ridge.
About 15 percent of all active volcanoes are
located along spreading centers.
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Iceland, the Azores, and the East African Rift Valley.
Plates and Volcanoes (2)
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Most of the world’s visible and active volcanoes
are located where two plates collide and one is
subducted beneath the other.
Water released from the subducted plate leads to
the formation of andesitic magma by wet partial
melting of mantle rock.
Plates and Volcanoes (3)
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The Pacific Ocean is ringed on three sides by
subducting plate margins.
About 5 percent of all active volcanoes are
located in the interiors of plates.
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Hawaiian volcanoes.
Submarine Volcanism and the
Composition of Seawater
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Magnesium and sulfur are removed from
seawater by the hot rocks.
Calcium and the other chemical elements are
added.
The hot rocks of the midocean ridge are a major
factor in maintaining the composition of
seawater.