Chapter 5: Magma And Volcanoes

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Chapter 5 Magma And Volcanoes
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Introduction Earths Internal Thermal Engine

• Magma is molten rock beneath Earths 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.

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Volcanoes

• The term volcano comes from the name of the Roman
  god of fire, Vulcan.
• There are different types of volcanoes.
• 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.

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Magma

• Magma has a wide range of compositions, but
  silica (SiO2) always dominates the mix.
• Magma has high temperatures.
• Magma is fluidit has the ability to flow. Most
  magma actually is a mixture of liquid (often
  referred to as melt) and solid mineral grains.

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Composition of Magmas and Lavas

• The composition of magmas and lavas is controlled
  by the most abundant elements in the EarthSi,
  Al, Fe, Ca, Mg, Na, K, H, and O.
• Three distinct types of magma are more common
  than others
• Basaltic, containing about 50 percent SiO2.
• Andesitic, about 60 percent SiO2.
• Rhyolitic, about 70 percent SiO2.

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Figure 5.1
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Basaltic Magmas

• 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.

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Andesitic and Rhyolitic Magmas

• Andesitic magmas are about 10 percent of the
  total magma.
• 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.
• Magmas erupted from volcanoes that once were
  active at Yellowstone Park are mostly rhyolitic.

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Figure 5.3
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Figure 5.5
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Gases Dissolved in Magma

• 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.

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Temperature of Magmas and Lavas

• Magma temperatures range from 1000o to 1200oC.
• Magma temperatures can reach 1400oC under some
  conditions.

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Viscosity of Magmas and Lavas (1)

• 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).

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Viscosity of Magmas and Lavas (2)

• 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).

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Viscosity of Magmas and Lavas (3)

• 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).

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How Buoyant Magma Erupts on the Surface (1)

• Magma is less dense than the solid rock from
  which it forms.
• The pressure is proportional to depth (thickness
  of overlying rock).
• Therefore, as magma rises upward, the pressure on
  it decreases.

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How Buoyant Magma Erupts on the Surface (2)

• Pressure controls the amount of gas a magma can
  dissolvemore at high pressure, less at low.
• Gas dissolved in an upward-moving magma comes out
  of solution and forms bubbles.

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Eruption StyleNonexplosive or Explosive? (1)

• Nonexplosive eruptions occur notably in Hawaii,
  Iceland, and the seafloor.
• 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.

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Eruption StyleNonexplosive or Explosive? (2)

• Nonexplosive eruptions may appear violent during
  their initial stages.
• 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.

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Eruption StyleNonexplosive or Explosive? (3)

• 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.

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Vesicles and Amygdules

• When lava finally solidified to rock, the
  last-formed bubbles become trapped these bubble
  preserved in the rock are called vesicles.
• Vesicles filled by secondary minerals are called
  amygdules.

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Explosive Eruptions (1)

• 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.

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Explosive Eruptions (2)

• 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.

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Eruption Columns and Tephra Falls (1)

• 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.

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Eruption Columns and Tephra Falls (2)

• 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.

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Pyroclastic Flows (1)

• 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).

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Pyroclastic Flows (2)

• 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.

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Lateral BlastMount St. Helens

• In 1980, Mount St. Helens, a volcano in
  Washington, erupted violently.
• As magma rose under the volcano, the mountains
  north flank began to bulge upward and outward.
• The initial blast was sideways rather than
  upward.
• 600 km2 of trees in the once-dense forest were
  leveled.

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Figure 5.10
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Volcanoes

• There are two broad families of volcanoes
• 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.

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Central-vent Volcanoes

• Based on their size and shape, there are three
  broad classes of central-vent volcanoes
• Shield volcanoes.
• Tephra cones.
• Stratovolcanoes.

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Shield Volcanoes (1)

• 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.

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Shield Volcanoes (2)

• 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).

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Figure 5.11
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Figure 5.13
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Shield Volcanoes (3)

• 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.

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Tephra Cones

• Tephra cone is built by shower of pyroclastic
  debris around a volcanic vent.
• The slopes of tephra cones are steep, typically
  about 30o.

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Statovolcanoes (1)

• 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
• Large.
• Conical.
• Steep-sided.

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Statovolcanoes (2)

• Near the summit, a stratovolcanos 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.

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Statovolcanoes (3)

• 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.

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Other Features of Central Eruptions (1)

• Craters
• Funnel-shaped depressions with steep-sided walls
  that open upward.
• Craters form in two ways
• 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.

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Other Features of Central Eruptions (2)

• Lava domes
• If the magma is very viscous (as in a rhyolitic
  or andesitic magma), it squeezes out to form a
  lava dome.

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Figure 5.16
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Other Features of Central Eruptions (3)

• Calderas
• 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.

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Figure 5.19
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Other Features of Central Eruptions (4)

• Resurgent domes
• Often, more magma enters the chamber and lifts
  the collapsed caldera floor to form a resurgent
  dome.
• Diatremes
• 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.

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Fissure Eruptions (1)

• 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.
• Such lavas are called plateau basalts.

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Figure 5.21
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Fissure Eruptions (2)

• 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.
• 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).

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Fissure Eruptions (3)

• Pillow basalts
• 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.

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Figure 5.18
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Fissure Eruptions (4)

• 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.
• The erupted product covered an area in excess of
  200,000 km2.

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Figure 5.22
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Posteruption effects

• 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.
• A thermal spring that intermittently erupts water
  and steam is a geyser.
• Most of the worlds geysers outside Iceland are
  in New Zealand and in Yellowstone National Park.

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Figure B5.2
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Figure B5.3
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Volcanic Hazards (1)

• Volcanic eruptions are not rare on land, and are
  essentially continuous on the seafloor.
• Every year about 50 volcanoes erupt on the
  Earths continents.
• Most eruptions are basaltic.
• Tephra eruptions from andesitic or rhyolitic
  stratovolcanoes like Mount St. Helens and
  Krakatau can be disastrous.

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Volcanic Hazards (2)

• Eruptions present five kinds of hazards
• Hot, rapidly moving pyroclastic flows and
  laterally directed blasts can overwhelm people
  before they can evacuate.
• Mont Pelee in 1902 and Mount St. Helens in 1980.
• Tephra and hot poisonous gases can bury or
  suffocate people.
• 79 Mount Vesuvius in A.D. 79.

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Volcanic Hazards (3)

• Mudflows, called lahars, can be devastating.
• 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.
• Violent undersea eruptions can cause powerful sea
  waves called tsunamis.
• Krakatau, in 1883, killed more than 36,000 on
  Java and nearby Indonesia islands.
• A tephra eruption can disrupt agriculture,
  creating a famine.

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Figure 5.24
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Figure 5.25
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Plates and Volcanoes (1)

• The distribution of active volcanoes around the
  world is strongly influenced by plate margins.
• Most of the worlds volcanism happen beneath the
  sea, along the 64,000 km midocean ridge.
• About 15 percent of all active volcanoes are
  located along spreading centers.
• Iceland, the Azores, and the East African Rift
  Valley.

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Plates and Volcanoes (2)

• Most of the worlds 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.

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Plates and Volcanoes (3)

• 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.
• Hawaiian volcanoes.

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Submarine Volcanism and the Composition of
Seawater

• 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.