Chapter 5,Magma And Volcanoes
Introduction,
Earth’s Internal-- Thermal Engine
? 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
? 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.
Magma
? 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
? 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,
? Basaltic,containing about 50 percent SiO2.
? Andesitic,about 60 percent SiO2.
? Rhyolitic,about 70 percent SiO2.
The three main Magmas
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.
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.
Very viscous magma slow moving,from Kilauea
Volcano in Hawaii in 1989
Fast flowing lava,low viscosity basaltic lava,Havaii in 1983
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,
Temperature of Magmas and Lavas
? Magma temperatures range from 1000o to
1200oC.
? Magma temperatures can reach 1400oC under
some conditions.
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),
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 绳状熔岩,
? The rough-looking lava formed from a cooler
lava having a high viscosity is called aa (ah ah).
Viscosity of Magmas and Lavas (3)
? The greater the silica content,the larger
is the viscosity.
? 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)
? 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.
How Buoyant Magma Erupts on the
Surface (2)
? 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)
? 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.
Eruption Style— Nonexplosive 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.
Eruption Style— Nonexplosive 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,
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.
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 浮石,
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.
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.
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.
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).
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（ Caribbean island of
Martinique) at an estimated speed of 200 KM/h,
instantly killing 29,000 people.
Lateral Blast— Mount St,Helens
? 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,
? 600 km2 of trees in the once-dense forest were
leveled.
Eruption of Mont St,Helens
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.
Central-vent Volcanoes
? Based on their size and shape,there are
three broad classes of central-vent
volcanoes:
? Shield volcanoes.
? Tephra cones.
? Stratovolcanoes.
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.
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).
Shield Volcano,Mauna Kea,a 4200m high shield volcano
in Hawaii
Volcano Landscape,Tephra cones on the flanks of Mauna
Kea,Hawaii
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,
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.
Stratovolcanoes (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.
Stratovolcanoes (2)
? 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,
Stratovolcanoes (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.
Other Features of Central Eruptions
(1)
? 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.
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.
Lava Dome,in crater of Mount St,Helens,Washington,in May 1982
Other Features of Central Eruptions
(3)
? 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.
Sequence of events that forming Crater Lake following the eruption of Mount
Mazama
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.
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.
Fissure Eruption,Lava fountains rise from an eruptive fissure
on Kilauea Volcano,Hawaii,in 1983
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).
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.
Tube shaped pillows of basalt on the seafloor near Hawaii
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.
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 world’s geysers outside Iceland are
in New Zealand and in Yellowstone National
Park.
Geysers erupt steam and hot water intermittently
Geothermal Reservoir
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
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)
? 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.
? Mount Vesuvius in A.D,79.
? 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.
Volcanic Hazards (3)
Volcano Gases,Sulfurous gases stream from several vents following a
1965 eruption in Hawaii
Plates and Volcanoes (1)
? 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.
? Iceland,the Azores,and the East African Rift Valley
Plates and Volcanoes (2)
? 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)
? 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.
Introduction,
Earth’s Internal-- Thermal Engine
? 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
? 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.
Magma
? 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
? 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,
? Basaltic,containing about 50 percent SiO2.
? Andesitic,about 60 percent SiO2.
? Rhyolitic,about 70 percent SiO2.
The three main Magmas
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.
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.
Very viscous magma slow moving,from Kilauea
Volcano in Hawaii in 1989
Fast flowing lava,low viscosity basaltic lava,Havaii in 1983
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,
Temperature of Magmas and Lavas
? Magma temperatures range from 1000o to
1200oC.
? Magma temperatures can reach 1400oC under
some conditions.
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),
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 绳状熔岩,
? The rough-looking lava formed from a cooler
lava having a high viscosity is called aa (ah ah).
Viscosity of Magmas and Lavas (3)
? The greater the silica content,the larger
is the viscosity.
? 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)
? 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.
How Buoyant Magma Erupts on the
Surface (2)
? 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)
? 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.
Eruption Style— Nonexplosive 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.
Eruption Style— Nonexplosive 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,
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.
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 浮石,
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.
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.
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.
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).
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（ Caribbean island of
Martinique) at an estimated speed of 200 KM/h,
instantly killing 29,000 people.
Lateral Blast— Mount St,Helens
? 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,
? 600 km2 of trees in the once-dense forest were
leveled.
Eruption of Mont St,Helens
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.
Central-vent Volcanoes
? Based on their size and shape,there are
three broad classes of central-vent
volcanoes:
? Shield volcanoes.
? Tephra cones.
? Stratovolcanoes.
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.
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).
Shield Volcano,Mauna Kea,a 4200m high shield volcano
in Hawaii
Volcano Landscape,Tephra cones on the flanks of Mauna
Kea,Hawaii
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,
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.
Stratovolcanoes (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.
Stratovolcanoes (2)
? 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,
Stratovolcanoes (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.
Other Features of Central Eruptions
(1)
? 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.
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.
Lava Dome,in crater of Mount St,Helens,Washington,in May 1982
Other Features of Central Eruptions
(3)
? 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.
Sequence of events that forming Crater Lake following the eruption of Mount
Mazama
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.
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.
Fissure Eruption,Lava fountains rise from an eruptive fissure
on Kilauea Volcano,Hawaii,in 1983
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).
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.
Tube shaped pillows of basalt on the seafloor near Hawaii
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.
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 world’s geysers outside Iceland are
in New Zealand and in Yellowstone National
Park.
Geysers erupt steam and hot water intermittently
Geothermal Reservoir
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
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)
? 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.
? Mount Vesuvius in A.D,79.
? 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.
Volcanic Hazards (3)
Volcano Gases,Sulfurous gases stream from several vents following a
1965 eruption in Hawaii
Plates and Volcanoes (1)
? 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.
? Iceland,the Azores,and the East African Rift Valley
Plates and Volcanoes (2)
? 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)
? 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.
