magma and volcanoes/epcc/lm4

Chapter 5: Chapter 5: Magma And VolcanoesMagma And Volcanoes

No LabNo Lab

Earth’s Internal Thermal EngineEarth’s Internal Thermal Engine

Magma is molten rock beneath Earth’s Magma is molten rock beneath Earth’s surface.surface.

Because liquid magma is less dense than Because liquid magma is less dense than surrounding solid rock, and obviously more surrounding solid rock, and obviously more mobile, magma, once formed, rises toward mobile, magma, once formed, rises toward the surface.the surface.

Magma that reaches the surface does so by Magma that reaches the surface does so by erupting through vents we call volcanoes.erupting through vents we call volcanoes.

VolcanoesVolcanoes The term The term volcanovolcano comes from the name of comes from the name of

the Roman god of fire, Vulcan.the Roman god of fire, Vulcan. There are different types of volcanoes.There are different types of volcanoes.

– Eruption vary from gentle flows (Hawaii and Eruption vary from gentle flows (Hawaii and Iceland) to catastrophic explosions (Mount St. Iceland) to catastrophic explosions (Mount St. Helens, Mt. Pinatubo, Soufriere Hills).Helens, Mt. Pinatubo, Soufriere Hills).

The majority of eruption never make the The majority of eruption never make the news because they occur beneath the ocean, news because they occur beneath the ocean, unobservedunobserved..

MagmaMagma

Magma has a wide range of compositions, Magma has a wide range of compositions, but silica (SiObut silica (SiO22) always dominates the mix.) always dominates the mix.

Magma has high temperatures.Magma has high temperatures. Magma is fluid—it has the ability to flow. Magma is fluid—it has the ability to flow.

Most magma actually is a mixture of liquid Most magma actually is a mixture of liquid (often referred to as melt) and solid mineral (often referred to as melt) and solid mineral grains.grains.

Composition of Magmas and LavasComposition of Magmas and Lavas

The composition of magmas and lavas is The composition of magmas and lavas is controlled by the most abundant elements in controlled by the most abundant elements in the Earth—Si, Al, Fe, Ca, Mg, Na, K, H, and O.the Earth—Si, Al, Fe, Ca, Mg, Na, K, H, and O.

Three distinct types of magma are more Three distinct types of magma are more common than others: common than others: – Basaltic (mafic), containing about 50 percent Basaltic (mafic), containing about 50 percent

SiOSiO2.2.

– Andesitic (intermediate), about 60 percent SiOAndesitic (intermediate), about 60 percent SiO2.2.

– Rhyolitic (felsic), about 70 percent SiORhyolitic (felsic), about 70 percent SiO2.2.

Figure 5.1

Mafic or Basaltic MagmasMafic or Basaltic Magmas

Mafic or Basaltic magmas are erupted by Mafic or Basaltic magmas are erupted by approximately 80 percent of volcanoes approximately 80 percent of volcanoes worldwide (the seafloor worldwide is mostly worldwide (the seafloor worldwide is mostly basalt). basalt).

Magma from Hawaiian volcanoes such as Magma from Hawaiian volcanoes such as Kilauea and Mauna Loa is basaltic.Kilauea and Mauna Loa is basaltic.

The entire island of Iceland is basaltic.The entire island of Iceland is basaltic.

Intermediate or Andesitic and Intermediate or Andesitic and Felsic or Rhyolitic MagmasFelsic or Rhyolitic Magmas

Intermediate or Andesitic magmas are about Intermediate or Andesitic magmas are about 10 percent of the total magma. 10 percent of the total magma. – Magma from Mount St. Helens in Washington Magma from Mount St. Helens in Washington

State and Krakatau in Indonesia is usually State and Krakatau in Indonesia is usually andesitic.andesitic.

Felsic or Rhyolitic magmas are about 10 Felsic or Rhyolitic magmas are about 10 percent of the total magma. percent of the total magma. – Magmas erupted from volcanoes that once were Magmas erupted from volcanoes that once were

active at Yellowstone Park are mostly rhyolitic.active at Yellowstone Park are mostly rhyolitic.

Figure 5.3

Figure 5.5

Gases Dissolved in MagmaGases Dissolved in Magma

Small amounts of gas (0.2 to 3% by weight) Small amounts of gas (0.2 to 3% by weight) are dissolved in all magma.are dissolved in all magma.

The principal gas in The principal gas in water vapor, water vapor, which, which, together with carbon dioxide, accounts for together with carbon dioxide, accounts for more than 98 percent of all gases emitted more than 98 percent of all gases emitted from volcanoes. from volcanoes.

Temperature of Magmas and LavasTemperature of Magmas and Lavas

Magma temperatures range from 1000Magma temperatures range from 1000oo to to 12001200ooC (1832˚F – 2192˚F).C (1832˚F – 2192˚F).

Magma temperatures can reach 1400Magma temperatures can reach 1400ooC C (2552˚F) under some conditions.(2552˚F) under some conditions.

Viscosity of Magmas and Lavas Viscosity of Magmas and Lavas

The internal property of a substance that The internal property of a substance that offers resistance to flow is called offers resistance to flow is called viscosity.viscosity.

The more viscous a magma, the less easily it The more viscous a magma, the less easily it flows.flows.

Viscosity of a magma depends on Viscosity of a magma depends on temperature and composition (especially the temperature and composition (especially the silica and dissolved-gas contents). silica and dissolved-gas contents).


The higher the temperature, the lower the The higher the temperature, the lower the viscosity, and the more readily magma flows.viscosity, and the more readily magma flows.

The smooth, ropy-surfaced lava in Hawaii, The smooth, ropy-surfaced lava in Hawaii, formed from a very hot, very fluid lava is formed from a very hot, very fluid lava is called called pahoehoe.pahoehoe.

The rough-looking lava formed from a cooler The rough-looking lava formed from a cooler lava having a high viscosity is called lava having a high viscosity is called aaaa (ah (ah ah).ah).


Felsic or rhyolitic magma (70% silica) is Felsic or rhyolitic magma (70% silica) is always more viscous than mafic (basaltic) always more viscous than mafic (basaltic) magma (50% silica).magma (50% silica).

Intermediate or Andesitic magma has a Intermediate or Andesitic magma has a viscosity that is intermediate between the viscosity that is intermediate between the two (60% silica).two (60% silica).

How Buoyant Magma Erupts on How Buoyant Magma Erupts on the Surface the Surface

Magma is less dense than the solid rock from Magma is less dense than the solid rock from which it forms.which it forms.

The pressure is proportional to depth The pressure is proportional to depth (thickness of overlying rock).(thickness of overlying rock).– Therefore, as magma rises upward, the pressure Therefore, as magma rises upward, the pressure

on it decreases.on it decreases.

How Buoyant Magma Erupts on How Buoyant Magma Erupts on the Surface the Surface

Pressure controls the amount of gas a Pressure controls the amount of gas a magma can dissolve—more at high pressure, magma can dissolve—more at high pressure, less at low.less at low.

Gas dissolved in an upward-moving magma Gas dissolved in an upward-moving magma comes out of solution and forms bubbles.comes out of solution and forms bubbles.

Eruption Style—Nonexplosive or Eruption Style—Nonexplosive or Explosive? Explosive?

The difference between nonexplosive and The difference between nonexplosive and explosive eruptions depends largely on explosive eruptions depends largely on magma viscosity and dissolved-gas content.magma viscosity and dissolved-gas content.

Low viscosity magmas and low dissolved gas Low viscosity magmas and low dissolved gas contents produce nonexplosive eruptions.contents produce nonexplosive eruptions.


Nonexplosive eruptions may appear violent Nonexplosive eruptions may appear violent during their initial stages.during their initial stages.– The reason is that gas bubbles in a low-viscosity The reason is that gas bubbles in a low-viscosity

basaltic magma will rise rapidly upward, like the basaltic magma will rise rapidly upward, like the gas bubbles in a glass of soda.gas bubbles in a glass of soda.

– If a basaltic magma rises rapidly, spectacular lava If a basaltic magma rises rapidly, spectacular lava fountains will occurfountains will occur..


Because heat is lost quickly at the surface of the Because heat is lost quickly at the surface of the flowing lava, the surface solidifies into a crust, flowing lava, the surface solidifies into a crust, beneath which the liquid lava continues to flow beneath which the liquid lava continues to flow in well-defined channels called lava tubes.in well-defined channels called lava tubes.

The very fluid lava initially forms thin pahoehoe The very fluid lava initially forms thin pahoehoe flows.flows.

With increasing viscosity the rate of movement With increasing viscosity the rate of movement slows and the stickier lava may be transformed slows and the stickier lava may be transformed into a rough surfaced aa flow that moves very into a rough surfaced aa flow that moves very slowly. slowly.

Pahoehoe and Aa LavaPahoehoe and Aa Lava

Vesicles and AmygdulesVesicles and Amygdules

When lava finally solidified to rock, the last-When lava finally solidified to rock, the last-formed bubbles become trapped; these formed bubbles become trapped; these bubble preserved in the rock are called bubble preserved in the rock are called vesicles.vesicles.

Vesicles filled by secondary minerals are Vesicles filled by secondary minerals are called called amygdules.amygdules.

AmygdulesAmygdules

Explosive EruptionsExplosive Eruptions

In viscous andesitic or rhyolitic magmas, gas In viscous andesitic or rhyolitic magmas, gas bubbles can rise only very slowly.bubbles can rise only very slowly.

When confining pressure drops quickly, the When confining pressure drops quickly, the gas in a magma expand into a froth of gas in a magma expand into a froth of innumerable glass-walled bubbles called innumerable glass-walled bubbles called pumice.pumice.

Explosive Eruptions Explosive Eruptions

In many instances, instead of forming In many instances, instead of forming pumice, small bubbles expanding within a pumice, small bubbles expanding within a huge mass of sufficiently gas-rich, viscous huge mass of sufficiently gas-rich, viscous magma will shatter the magma into tiny magma will shatter the magma into tiny fragments called volcanic ash.fragments called volcanic ash.

Volcanic ash is the most abundant product of Volcanic ash is the most abundant product of explosive eruptions.explosive eruptions.

Eruption Columns and Tephra Falls Eruption Columns and Tephra Falls

The largest and the most violent eruptions The largest and the most violent eruptions are associated with silica-rich magmas having are associated with silica-rich magmas having a high dissolved-gas content.a high dissolved-gas content.

This hot, turbulent mixture rises rapidly in This hot, turbulent mixture rises rapidly in the cooler air above the vent to form an the cooler air above the vent to form an eruption column that may tower as high as eruption column that may tower as high as 45 km in the atmosphere.45 km in the atmosphere.

Eruption Columns and Tephra Falls Eruption Columns and Tephra Falls

A violent eruption of this kind is called a A violent eruption of this kind is called a plinian eruption, named after the Roman plinian eruption, named after the Roman author and statesman, Pliny, who lost his life author and statesman, Pliny, who lost his life in the A.D. 79 eruption of Mt. Vesuvius.in the A.D. 79 eruption of Mt. Vesuvius.

The particles of debris rain down in a tephra The particles of debris rain down in a tephra fall and eventually accumulate on the ground fall and eventually accumulate on the ground as tephra deposits.as tephra deposits.

Pyroclastic FlowsPyroclastic Flows When the mixture of hot gases and When the mixture of hot gases and

pyroclasts is more dense than the pyroclasts is more dense than the atmosphere, the turbulent mixture flows atmosphere, the turbulent mixture flows down the side of the volcano rather than down the side of the volcano rather than forming an eruption column. forming an eruption column.

A hot, highly mobile flow of tephra that A hot, highly mobile flow of tephra that rushes down the flank of a volcano during a rushes down the flank of a volcano during a major eruption is called a pyroclastic flow major eruption is called a pyroclastic flow (the most devastating and lethal forms of (the most devastating and lethal forms of volcanic eruption).volcanic eruption).

Pyroclastic Flows Pyroclastic Flows

Pyroclastic flows are also known as Pyroclastic flows are also known as nunuéée e ardenteardente (glowing cloud). (glowing cloud).

Historic observations indicate that Historic observations indicate that pyroclastic flows can reach velocities of more pyroclastic flows can reach velocities of more than 700 km/h.than 700 km/h.

In 1902, a pyroclastic flow rushed down the In 1902, a pyroclastic flow rushed down the flanks of Mont Pelee Volcano at an estimated flanks of Mont Pelee Volcano at an estimated speed of 200 KM/h, instantly killing 29,000 speed of 200 KM/h, instantly killing 29,000 peoplepeople..

Lateral Blast—Mount St. HelensLateral Blast—Mount St. Helens

In 1980, Mount St. Helens, a volcano in In 1980, Mount St. Helens, a volcano in Washington, erupted violently.Washington, erupted violently.

As magma rose under the volcano, the As magma rose under the volcano, the mountain’s north flank began to bulge mountain’s north flank began to bulge upward and outward.upward and outward.

The initial blast was sideways rather than The initial blast was sideways rather than upward. upward. – 600 km600 km2 2 of trees in the once-dense forest were of trees in the once-dense forest were

leveled.leveled.

Figure 5.10

Volcanoes Volcanoes

There are two broad families of volcanoes:There are two broad families of volcanoes:– Those formed by eruptions from a central vent.Those formed by eruptions from a central vent.– Those that erupt through a long fissure.Those that erupt through a long fissure.

Central-vent eruptions build mounds of the Central-vent eruptions build mounds of the kind most people associate with volcanoes.kind most people associate with volcanoes.

Fissure eruptions build plateaus.Fissure eruptions build plateaus.

Central-vent VolcanoesCentral-vent Volcanoes

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

Shield Volcanoes (1)Shield Volcanoes (1)

A shield volcano produces a broad, dome-A shield volcano produces a broad, dome-shaped mountain with an average surface shaped mountain with an average surface slope of only a few degrees.slope of only a few degrees.

Low-viscosity basaltic lavas can flow for Low-viscosity basaltic lavas can flow for kilometers down gentle slopes.kilometers down gentle slopes.

The accumulated lava from repeated The accumulated lava from repeated eruptions of low-viscosity lava build a shield eruptions of low-viscosity lava build a shield volcano.volcano.


The farther lava flows down the flank, the The farther lava flows down the flank, the cooler and more viscous it becomes, so the cooler and more viscous it becomes, so the steeper the slope must be for it to flow.steeper the slope must be for it to flow.

Large shield volcanoes rise as islands in the Large shield volcanoes rise as islands in the ocean (Hawaiian Islands, Tahiti, Samoa, the ocean (Hawaiian Islands, Tahiti, Samoa, the Galapagos, and many others).Galapagos, and many others).

Figure 5.11

Figure 5.13


Mauna Loa volcano, for example, rises to a Mauna Loa volcano, for example, rises to a height of 4169 m above sea level, but if height of 4169 m above sea level, but if measured from the seafloor the height is measured from the seafloor the height is 10,000 m, making Mauna Loa the tallest 10,000 m, making Mauna Loa the tallest mountain on Earth. mountain on Earth.

Tephra ConesTephra Cones

Tephra cone is built by shower of pyroclastic Tephra cone is built by shower of pyroclastic debris around a volcanic vent.debris around a volcanic vent.

The slopes of tephra cones are steep, The slopes of tephra cones are steep, typically about 30typically about 30oo..

Statovolcanoes (1)Statovolcanoes (1) Some volcanoes (andesitic composition) emit Some volcanoes (andesitic composition) emit

both viscous lava flows and tephra.both viscous lava flows and tephra. The emissions tend to alternate, forming The emissions tend to alternate, forming

alternating strata of lava and tephra, building alternating strata of lava and tephra, building a stratovolcano.a stratovolcano.

Stratovolcanoes are:Stratovolcanoes are:– Large.Large.– Conical. Conical. – Steep-sided.Steep-sided.

Statovolcanoes (2)Statovolcanoes (2)

Near the summit, a stratovolcano’s slope Near the summit, a stratovolcano’s slope may reach 40may reach 40oo..

Toward the base, the slope flattens to about Toward the base, the slope flattens to about 66o o toto 1010oo..

As a stratovolcano develops, lava flows act as As a stratovolcano develops, lava flows act as a cap to slow erosion of the loose tephra. a cap to slow erosion of the loose tephra.

Statovolcanoes (3)Statovolcanoes (3)

The volcano becomes much larger and The volcano becomes much larger and steeper than a typical tephra cone.steeper than a typical tephra cone.

Mount Fuji (Japan), Mount Rainier, Mount Mount Fuji (Japan), Mount Rainier, Mount Baker in Washington State, Mount Hood in Baker in Washington State, Mount Hood in Oregon, Mt Mayon in the Philippines are Oregon, Mt Mayon in the Philippines are stratovolcanoes.stratovolcanoes.

Other Features of Central Other Features of Central Eruptions (1)Eruptions (1)

Craters: Craters: – Funnel-shaped depressions with steep-sided walls Funnel-shaped depressions with steep-sided walls

that open upward.that open upward.– Craters form in two ways:Craters form in two ways:

By the collapse of the steep sides of the vent.By the collapse of the steep sides of the vent.By an explosive eruption.By an explosive eruption.

– In subsequent eruptions, pressure blasts open the In subsequent eruptions, pressure blasts open the vent, removing both the solidified magma from the vent, removing both the solidified magma from the previous eruption and part of the crater wall. previous eruption and part of the crater wall.

– A crater can grow slowly larger, eruption by eruption.A crater can grow slowly larger, eruption by eruption.


Lava domes: Lava domes: – If the magma is very viscous (as in a rhyolitic or If the magma is very viscous (as in a rhyolitic or

andesitic magma), it squeezes out to form a lava andesitic magma), it squeezes out to form a lava dome.dome.

Figure 5.16


Calderas: Calderas: – CalderaCaldera is from the Spanish word for cauldron. is from the Spanish word for cauldron.– A roughly circular, steep-walled basin about a A roughly circular, steep-walled basin about a

kilometer in diameter or larger. kilometer in diameter or larger. – Calderas are created by collapse of the surface rock Calderas are created by collapse of the surface rock

following an eruption and partial emptying of the following an eruption and partial emptying of the underlying magma chamber.underlying magma chamber.

– Crater lake in Oregon occupies a circular caldera 8 Crater lake in Oregon occupies a circular caldera 8 km in diameter.km in diameter.

Figure 5.19


Resurgent domes:Resurgent domes:– Often, more magma enters the chamber and lifts Often, more magma enters the chamber and lifts

the collapsed caldera floor to form a resurgent the collapsed caldera floor to form a resurgent dome. dome.

Diatremes:Diatremes:– Volcanic pipes filled with a rubbles of broken rock.Volcanic pipes filled with a rubbles of broken rock.– The walls are vertical, or very nearly so.The walls are vertical, or very nearly so.– A famous diatreme is the diamond mine in A famous diatreme is the diamond mine in

Kimberly, South Africa.Kimberly, South Africa.

Fissure Eruptions (1)Fissure Eruptions (1)

Fissure eruptions extrude lava along an Fissure eruptions extrude lava along an elongate fracture in the crust.elongate fracture in the crust.– When fissure eruptions occur on land, the low-When fissure eruptions occur on land, the low-

viscosity basaltic lava tends to spread widely and viscosity basaltic lava tends to spread widely and to create flat lava plains.to create flat lava plains.

Such lavas are called Such lavas are called plateau basalts.plateau basalts.

Figure 5.21


The Laki eruption, in Iceland in1783, occurred The Laki eruption, in Iceland in1783, occurred along a 32 km long fracture. Lava from it along a 32 km long fracture. Lava from it flowed 64 km from one side of the fracture flowed 64 km from one side of the fracture and nearly 48 km from the other, covering and nearly 48 km from the other, covering 588 km588 km22..– The Laki eruption is the largest lava flow of any The Laki eruption is the largest lava flow of any

kind in historic times.kind in historic times.– Famine followed and more than 9000 died (20 Famine followed and more than 9000 died (20

percent of the Icelandic population).percent of the Icelandic population).


Pillow basalts:Pillow basalts:– When the basaltic magma erupts under the When the basaltic magma erupts under the

ocean, seawater cools it so rapidly that pillow-ocean, seawater cools it so rapidly that pillow-shaped masses of basalt, ranging from a few shaped masses of basalt, ranging from a few centimeters to a meter or more in greatest centimeters to a meter or more in greatest dimension form.dimension form.

Fissure eruptions of andesitic or rhyolitic Fissure eruptions of andesitic or rhyolitic magma are much less common than fissure magma are much less common than fissure eruptions of basaltic lava.eruptions of basaltic lava.

Figure 5.22

Figure 5.18


Sometimes the pyroclasts in the tephra are Sometimes the pyroclasts in the tephra are so hot that the fragments form welded tuff.so hot that the fragments form welded tuff.

Some 40 to 50 million years ago, huge ash-Some 40 to 50 million years ago, huge ash-flow eruptions happened in Nevada.flow eruptions happened in Nevada.– The erupted product covered an area in excess of The erupted product covered an area in excess of

200,000 km200,000 km22..

Posteruption effectsPosteruption effects When active volcanism finally ceases, rock in When active volcanism finally ceases, rock in

and near an old magma chamber may remain and near an old magma chamber may remain hot for hundreds of thousands of years.hot for hundreds of thousands of years.

Thermal spring at many volcanic sites (Italy, Thermal spring at many volcanic sites (Italy, Japan, and New Zealand) have become Japan, and New Zealand) have become famous health spas and sources of energy. famous health spas and sources of energy. – A thermal spring that intermittently erupts water A thermal spring that intermittently erupts water

and steam is a and steam is a geyser.geyser.Most of the world’s geysers outside Iceland are in New Most of the world’s geysers outside Iceland are in New

Zealand and in Yellowstone National Park.Zealand and in Yellowstone National Park.

Figure B5.2

Figure B5.3

Volcanic Hazards (1)Volcanic Hazards (1) Volcanic eruptions are not rare on land, and Volcanic eruptions are not rare on land, and

are essentially continuous on the seafloor.are essentially continuous on the seafloor. Every year about 50 volcanoes erupt on the Every year about 50 volcanoes erupt on the

Earth’s continents.Earth’s continents. Most eruptions are basaltic.Most eruptions are basaltic. Tephra eruptions from andesitic or rhyolitic Tephra eruptions from andesitic or rhyolitic

stratovolcanoes like Mount St. Helens and stratovolcanoes like Mount St. Helens and Krakatau can be disastrous.Krakatau can be disastrous.

Volcanic Hazards (2)Volcanic Hazards (2)

Eruptions present five kinds of hazards:Eruptions present five kinds of hazards:– Hot, rapidly moving pyroclastic flows and Hot, rapidly moving pyroclastic flows and

laterally directed blasts can overwhelm people laterally directed blasts can overwhelm people before they can evacuate. before they can evacuate.

Mont Pelee in 1902 and Mount St. Helens in 1980.Mont Pelee in 1902 and Mount St. Helens in 1980.– Tephra and hot poisonous gases can bury or Tephra and hot poisonous gases can bury or

suffocate people.suffocate people.79 Mount Vesuvius in A.D. 79.79 Mount Vesuvius in A.D. 79.

Volcanic Hazards (3)Volcanic Hazards (3)– Mudflows, called Mudflows, called lahars,lahars, can be devastating. can be devastating.

In 1985, the Colombian volcano Nevado del Ruiz In 1985, the Colombian volcano Nevado del Ruiz experienced a small, nonthreatening eruption. But, experienced a small, nonthreatening eruption. But, when glaciers at the summit melted, massive when glaciers at the summit melted, massive mudflows of volcanic debris moved swiftly down the mudflows of volcanic debris moved swiftly down the mountain , killing 20,000.mountain , killing 20,000.

– Violent undersea eruptions can cause powerful Violent undersea eruptions can cause powerful sea waves called sea waves called tsunamis.tsunamis.

Krakatau, in 1883, killed more than 36,000 on Java Krakatau, in 1883, killed more than 36,000 on Java and nearby Indonesia islands.and nearby Indonesia islands.

– A tephra eruption can disrupt agriculture, A tephra eruption can disrupt agriculture, creating a famine.creating a famine.

Figure 5.24

Figure 5.25

magma and volcanoes/epcc/lm4

Education

magmamagma magma

mixture of liquidmost

basaltic magmas mafic

dense thanbecause liquid

percent sio2

basaltic magmasmafic

hawaiian volcanoes

different types of volcanoes