Magma is molten rock beneath Earth's surface that rises toward the surface due to being less dense than surrounding rock. Magma that reaches the surface erupts through volcanoes. There are different types of volcanoes that can have eruptions ranging from gentle lava flows to catastrophic explosions. The composition of magmas varies but is dominated by silica and results in three main types: basaltic, andesitic, and rhyolitic. Basaltic magma is erupted by most volcanoes and forms lava flows, while more viscous andesitic and rhyolitic magmas can lead to explosive eruptions due to gas bubbles rising slowly in the thick lava.

1. Chapter 5:Chapter 5:
Magma And VolcanoesMagma And Volcanoes
No LabNo Lab

2. Earth’s Internal Thermal EngineEarth’s Internal Thermal Engine
 Magma is molten rock beneath Earth’sMagma is molten rock beneath Earth’s
surface.surface.
 Because liquid magma is less dense thanBecause liquid magma is less dense than
surrounding solid rock, and obviously moresurrounding solid rock, and obviously more
mobile, magma, once formed, rises towardmobile, magma, once formed, rises toward
the surface.the surface.
 Magma that reaches the surface does so byMagma that reaches the surface does so by
erupting through vents we call volcanoes.erupting through vents we call volcanoes.

3. VolcanoesVolcanoes
 The termThe term volcanovolcano comes from the name ofcomes 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 andEruption 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 theThe majority of eruption never make the
news because they occur beneath the ocean,news because they occur beneath the ocean,
unobservedunobserved..

4. 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 liquidMost 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.

5. Composition of Magmas and LavasComposition of Magmas and Lavas
 The composition of magmas and lavas isThe composition of magmas and lavas is
controlled by the most abundant elements incontrolled 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 moreThree distinct types of magma are more
common than others:common than others:
– Basaltic (mafic), containing about 50 percent SiOBasaltic (mafic), containing about 50 percent SiO2.2.
– Andesitic (intermediate), about 60 percent SiOAndesitic (intermediate), about 60 percent SiO2.2.
– Rhyolitic (felsic), about 70 percent SiORhyolitic (felsic), about 70 percent SiO2.2.

6. Figure 5.1

7. Mafic or Basaltic MagmasMafic or Basaltic Magmas
 Mafic or Basaltic magmas are erupted byMafic or Basaltic magmas are erupted by
approximately 80 percent of volcanoesapproximately 80 percent of volcanoes
worldwide (the seafloor worldwide is mostlyworldwide (the seafloor worldwide is mostly
basalt).basalt).
 Magma from Hawaiian volcanoes such asMagma 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.

8. Intermediate or Andesitic andIntermediate or Andesitic and
Felsic or Rhyolitic MagmasFelsic or Rhyolitic Magmas
 Intermediate or Andesitic magmas are aboutIntermediate or Andesitic magmas are about
10 percent of the total magma.10 percent of the total magma.
– Magma from Mount St. Helens in WashingtonMagma from Mount St. Helens in Washington
State and Krakatau in Indonesia is usuallyState and Krakatau in Indonesia is usually
andesitic.andesitic.
 Felsic or Rhyolitic magmas are about 10Felsic or Rhyolitic magmas are about 10
percent of the total magma.percent of the total magma.
– Magmas erupted from volcanoes that once wereMagmas erupted from volcanoes that once were
active at Yellowstone Park are mostly rhyolitic.active at Yellowstone Park are mostly rhyolitic.

9. Figure 5.3

10. Figure 5.5

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

12. Temperature of Magmas and LavasTemperature of Magmas and Lavas
 Magma temperatures range from 1000Magma temperatures range from 1000oo
toto
12001200oo
C (1832˚F – 2192˚F).C (1832˚F – 2192˚F).
 Magma temperatures can reach 1400Magma temperatures can reach 1400oo
CC
(2552˚F) under some conditions.(2552˚F) under some conditions.

13. Viscosity of Magmas and LavasViscosity of Magmas and Lavas
 The internal property of a substance thatThe internal property of a substance that
offers resistance to flow is calledoffers resistance to flow is called viscosity.viscosity.
 The more viscous a magma, the less easily itThe more viscous a magma, the less easily it
flows.flows.
 Viscosity of a magma depends onViscosity of a magma depends on
temperature and composition (especially thetemperature and composition (especially the
silica and dissolved-gas contents).silica and dissolved-gas contents).

14. Viscosity of Magmas and LavasViscosity of Magmas and Lavas
 The higher the temperature, the lower theThe 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 isformed from a very hot, very fluid lava is
calledcalled pahoehoe.pahoehoe.
 The rough-looking lava formed from a coolerThe rough-looking lava formed from a cooler
lava having a high viscosity is calledlava having a high viscosity is called aaaa (ah(ah
ah).ah).

15. Viscosity of Magmas and LavasViscosity of Magmas and Lavas
 Felsic or rhyolitic magma (70% silica) isFelsic 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 aIntermediate or Andesitic magma has a
viscosity that is intermediate between theviscosity that is intermediate between the
two (60% silica).two (60% silica).

16. How Buoyant Magma Erupts onHow Buoyant Magma Erupts on
the Surfacethe Surface
 Magma is less dense than the solid rock fromMagma is less dense than the solid rock from
which it forms.which it forms.
 The pressure is proportional to depthThe pressure is proportional to depth
(thickness of overlying rock).(thickness of overlying rock).
– Therefore, as magma rises upward, the pressureTherefore, as magma rises upward, the pressure
on it decreases.on it decreases.

17. How Buoyant Magma Erupts onHow Buoyant Magma Erupts on
the Surfacethe Surface
 Pressure controls the amount of gas aPressure 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 magmaGas dissolved in an upward-moving magma
comes out of solution and forms bubbles.comes out of solution and forms bubbles.

18. Eruption Style—Nonexplosive orEruption Style—Nonexplosive or
Explosive?Explosive?
 The difference between nonexplosive andThe difference between nonexplosive and
explosive eruptions depends largely onexplosive eruptions depends largely on
magma viscosity and dissolved-gas content.magma viscosity and dissolved-gas content.
 Low viscosity magmas and low dissolved gasLow viscosity magmas and low dissolved gas
contents produce nonexplosive eruptions.contents produce nonexplosive eruptions.

19. Eruption Style—Nonexplosive orEruption Style—Nonexplosive or
Explosive?Explosive?
 Nonexplosive eruptions may appear violentNonexplosive eruptions may appear violent
during their initial stages.during their initial stages.
– The reason is that gas bubbles in a low-viscosityThe reason is that gas bubbles in a low-viscosity
basaltic magma will rise rapidly upward, like thebasaltic 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 lavaIf a basaltic magma rises rapidly, spectacular lava
fountains will occurfountains will occur..

20. Eruption Style—Nonexplosive orEruption Style—Nonexplosive or
Explosive?Explosive?
 Because heat is lost quickly at the surface ofBecause heat is lost quickly at the surface of
the flowing lava, the surface solidifies into athe flowing lava, the surface solidifies into a
crust, beneath which the liquid lavacrust, beneath which the liquid lava
continues to flow in well-defined channelscontinues to flow in well-defined channels
called lava tubes.called lava tubes.
 The very fluid lava initially forms thinThe very fluid lava initially forms thin
pahoehoe flows.pahoehoe flows.
 With increasing viscosity the rate ofWith increasing viscosity the rate of
movement slows and the stickier lava maymovement slows and the stickier lava may
be transformed into a rough surfaced aabe transformed into a rough surfaced aa
flow that moves very slowly.flow that moves very slowly.

21. Pahoehoe and Aa LavaPahoehoe and Aa Lava

22. 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; theseformed bubbles become trapped; these
bubble preserved in the rock are calledbubble preserved in the rock are called
vesicles.vesicles.
 Vesicles filled by secondary minerals areVesicles filled by secondary minerals are
calledcalled amygdules.amygdules.

23. AmygdulesAmygdules

24. Explosive EruptionsExplosive Eruptions
 In viscous andesitic or rhyolitic magmas, gasIn viscous andesitic or rhyolitic magmas, gas
bubbles can rise only very slowly.bubbles can rise only very slowly.
 When confining pressure drops quickly, theWhen confining pressure drops quickly, the
gas in a magma expand into a froth ofgas in a magma expand into a froth of
innumerable glass-walled bubbles calledinnumerable glass-walled bubbles called
pumice.pumice.

25. Explosive EruptionsExplosive Eruptions
 In many instances, instead of formingIn many instances, instead of forming
pumice, small bubbles expanding within apumice, small bubbles expanding within a
huge mass of sufficiently gas-rich, viscoushuge mass of sufficiently gas-rich, viscous
magma will shatter the magma into tinymagma will shatter the magma into tiny
fragments called volcanic ash.fragments called volcanic ash.
 Volcanic ash is the most abundant product ofVolcanic ash is the most abundant product of
explosive eruptions.explosive eruptions.

26. Eruption Columns and Tephra FallsEruption Columns and Tephra Falls
 The largest and the most violent eruptionsThe largest and the most violent eruptions
are associated with silica-rich magmas havingare associated with silica-rich magmas having
a high dissolved-gas content.a high dissolved-gas content.
 This hot, turbulent mixture rises rapidly inThis hot, turbulent mixture rises rapidly in
the cooler air above the vent to form anthe cooler air above the vent to form an
eruption column that may tower as high aseruption column that may tower as high as
45 km in the atmosphere.45 km in the atmosphere.

27. Eruption Columns and Tephra FallsEruption Columns and Tephra Falls
 A violent eruption of this kind is called aA violent eruption of this kind is called a
plinian eruption, named after the Romanplinian eruption, named after the Roman
author and statesman, Pliny, who lost his lifeauthor 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 tephraThe particles of debris rain down in a tephra
fall and eventually accumulate on the groundfall and eventually accumulate on the ground
as tephra deposits.as tephra deposits.

28. Pyroclastic FlowsPyroclastic Flows
 When the mixture of hot gases andWhen the mixture of hot gases and
pyroclasts is more dense than thepyroclasts is more dense than the
atmosphere, the turbulent mixture flowsatmosphere, the turbulent mixture flows
down the side of the volcano rather thandown the side of the volcano rather than
forming an eruption column.forming an eruption column.
 A hot, highly mobile flow of tephra thatA hot, highly mobile flow of tephra that
rushes down the flank of a volcano during arushes down the flank of a volcano during a
major eruption is called a pyroclastic flowmajor eruption is called a pyroclastic flow
(the most devastating and lethal forms of(the most devastating and lethal forms of
volcanic eruption).volcanic eruption).

29. Pyroclastic FlowsPyroclastic Flows
 Pyroclastic flows are also known asPyroclastic flows are also known as nunuééee
ardenteardente (glowing cloud).(glowing cloud).
 Historic observations indicate thatHistoric observations indicate that
pyroclastic flows can reach velocities of morepyroclastic flows can reach velocities of more
than 700 km/h.than 700 km/h.
 In 1902, a pyroclastic flow rushed down theIn 1902, a pyroclastic flow rushed down the
flanks of Mont Pelee Volcano at an estimatedflanks of Mont Pelee Volcano at an estimated
speed of 200 KM/h, instantly killing 29,000speed of 200 KM/h, instantly killing 29,000
peoplepeople..

30. Lateral Blast—Mount St. HelensLateral Blast—Mount St. Helens
 In 1980, Mount St. Helens, a volcano inIn 1980, Mount St. Helens, a volcano in
Washington, erupted violently.Washington, erupted violently.
 As magma rose under the volcano, theAs magma rose under the volcano, the
mountain’s north flank began to bulgemountain’s north flank began to bulge
upward and outward.upward and outward.
 The initial blast was sideways rather thanThe initial blast was sideways rather than
upward.upward.
– 600 km600 km22
of trees in the once-dense forest wereof trees in the once-dense forest were
leveled.leveled.

31. Figure 5.10

32. VolcanoesVolcanoes
 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 theCentral-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.

33. Central-vent VolcanoesCentral-vent Volcanoes
 Based on their size and shape, there areBased on their size and shape, there are
three broad classes of central-ventthree broad classes of central-vent
volcanoes:volcanoes:
– Shield volcanoes.Shield volcanoes.
– Tephra cones.Tephra cones.
– Stratovolcanoes.Stratovolcanoes.

34. 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 surfaceshaped mountain with an average surface
slope of only a few degrees.slope of only a few degrees.
 Low-viscosity basaltic lavas can flow forLow-viscosity basaltic lavas can flow for
kilometers down gentle slopes.kilometers down gentle slopes.
 The accumulated lava from repeatedThe accumulated lava from repeated
eruptions of low-viscosity lava build a shielderuptions of low-viscosity lava build a shield
volcano.volcano.

35. Shield Volcanoes (2)Shield Volcanoes (2)
 The farther lava flows down the flank, theThe farther lava flows down the flank, the
cooler and more viscous it becomes, so thecooler 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 theLarge shield volcanoes rise as islands in the
ocean (Hawaiian Islands, Tahiti, Samoa, theocean (Hawaiian Islands, Tahiti, Samoa, the
Galapagos, and many others).Galapagos, and many others).

36. Figure 5.11

37. Figure 5.13

38. Shield Volcanoes (3)Shield Volcanoes (3)
 Mauna Loa volcano, for example, rises to aMauna Loa volcano, for example, rises to a
height of 4169 m above sea level, but ifheight of 4169 m above sea level, but if
measured from the seafloor the height ismeasured from the seafloor the height is
10,000 m, making Mauna Loa the tallest10,000 m, making Mauna Loa the tallest
mountain on Earth.mountain on Earth.

39. Tephra ConesTephra Cones
 Tephra cone is built by shower of pyroclasticTephra 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
..

40. Statovolcanoes (1)Statovolcanoes (1)
 Some volcanoes (andesitic composition)Some volcanoes (andesitic composition)
emit both viscous lava flows and tephra.emit both viscous lava flows and tephra.
 The emissions tend to alternate, formingThe emissions tend to alternate, forming
alternating strata of lava and tephra,alternating strata of lava and tephra,
building a stratovolcano.building a stratovolcano.
 Stratovolcanoes are:Stratovolcanoes are:
– Large.Large.
– Conical.Conical.
– Steep-sided.Steep-sided.

41. Statovolcanoes (2)Statovolcanoes (2)
 Near the summit, a stratovolcano’s slopeNear the summit, a stratovolcano’s slope
may reach 40may reach 40oo
..
 Toward the base, the slope flattens to aboutToward the base, the slope flattens to about
66oo
toto1010oo
..
 As a stratovolcano develops, lava flows act asAs 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.

42. Statovolcanoes (3)Statovolcanoes (3)
 The volcano becomes much larger andThe volcano becomes much larger and
steeper than a typical tephra cone.steeper than a typical tephra cone.
 Mount Fuji (Japan), Mount Rainier, MountMount Fuji (Japan), Mount Rainier, Mount
Baker in Washington State, Mount Hood inBaker in Washington State, Mount Hood in
Oregon, Mt Mayon in the Philippines areOregon, Mt Mayon in the Philippines are
stratovolcanoes.stratovolcanoes.

43. Other Features of CentralOther Features of Central
Eruptions (1)Eruptions (1)
 Craters:Craters:
– Funnel-shaped depressions with steep-sidedFunnel-shaped depressions with steep-sided
walls that open upward.walls 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 openIn subsequent eruptions, pressure blasts open
the vent, removing both the solidified magmathe vent, removing both the solidified magma
from the previous eruption and part of thefrom the previous eruption and part of the
crater wall.crater wall.
– A crater can grow slowly larger, eruption byA crater can grow slowly larger, eruption by

44. Other Features of CentralOther Features of Central
Eruptions (2)Eruptions (2)
 Lava domes:Lava domes:
– If the magma is very viscous (as in a rhyolitic orIf the magma is very viscous (as in a rhyolitic or
andesitic magma), it squeezes out to form a lavaandesitic magma), it squeezes out to form a lava
dome.dome.

45. Figure 5.16

46. Other Features of CentralOther Features of Central
Eruptions (3)Eruptions (3)
 Calderas:Calderas:
– CalderaCaldera is from the Spanish word for cauldron.is from the Spanish word for cauldron.
– A roughly circular, steep-walled basin about aA roughly circular, steep-walled basin about a
kilometer in diameter or larger.kilometer in diameter or larger.
– Calderas are created by collapse of the surfaceCalderas are created by collapse of the surface
rock following an eruption and partial emptyingrock following an eruption and partial emptying
of the underlying magma chamber.of the underlying magma chamber.
– Crater lake in Oregon occupies a circularCrater lake in Oregon occupies a circular
caldera 8 km in diameter.caldera 8 km in diameter.

47. Figure 5.19

48. Other Features of CentralOther Features of Central
Eruptions (4)Eruptions (4)
 Resurgent domes:Resurgent domes:
– Often, more magma enters the chamber and liftsOften, more magma enters the chamber and lifts
the collapsed caldera floor to form a resurgentthe collapsed caldera floor to form a resurgent
dome.dome.
 Diatremes:Diatremes:
– Volcanic pipes filled with a rubbles of brokenVolcanic pipes filled with a rubbles of broken
rock.rock.
– The walls are vertical, or very nearly so.The walls are vertical, or very nearly so.
– A famous diatreme is the diamond mine inA famous diatreme is the diamond mine in
Kimberly, South Africa.Kimberly, South Africa.

49. Fissure Eruptions (1)Fissure Eruptions (1)
 Fissure eruptions extrude lava along anFissure 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 andviscosity basaltic lava tends to spread widely and
to create flat lava plains.to create flat lava plains.
Such lavas are calledSuch lavas are called plateau basalts.plateau basalts.

50. Figure 5.21

51. Fissure Eruptions (2)Fissure Eruptions (2)
 The Laki eruption, in Iceland in1783, occurredThe Laki eruption, in Iceland in1783, occurred
along a 32 km long fracture. Lava from italong a 32 km long fracture. Lava from it
flowed 64 km from one side of the fractureflowed 64 km from one side of the fracture
and nearly 48 km from the other, coveringand nearly 48 km from the other, covering
588 km588 km22
..
– The Laki eruption is the largest lava flow of anyThe Laki eruption is the largest lava flow of any
kind in historic times.kind in historic times.
– Famine followed and more than 9000 died (20Famine followed and more than 9000 died (20
percent of the Icelandic population).percent of the Icelandic population).

52. Fissure Eruptions (3)Fissure Eruptions (3)
 Pillow basalts:Pillow basalts:
– When the basaltic magma erupts under theWhen 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 fewshaped masses of basalt, ranging from a few
centimeters to a meter or more in greatestcentimeters to a meter or more in greatest
dimension form.dimension form.
 Fissure eruptions of andesitic or rhyoliticFissure eruptions of andesitic or rhyolitic
magma are much less common than fissuremagma are much less common than fissure
eruptions of basaltic lava.eruptions of basaltic lava.

53. Figure 5.22

54. Figure 5.18

55. Fissure Eruptions (4)Fissure Eruptions (4)
 Sometimes the pyroclasts in the tephra areSometimes 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 ofThe erupted product covered an area in excess of
200,000 km200,000 km22
..

56. Posteruption effectsPosteruption effects
 When active volcanism finally ceases, rockWhen active volcanism finally ceases, rock
in and near an old magma chamber mayin and near an old magma chamber may
remain hot for hundreds of thousands ofremain hot for hundreds of thousands of
years.years.
 Thermal spring at many volcanic sites (Italy,Thermal spring at many volcanic sites (Italy,
Japan, and New Zealand) have becomeJapan, and New Zealand) have become
famous health spas and sources of energy.famous health spas and sources of energy.
– A thermal spring that intermittently eruptsA thermal spring that intermittently erupts
water and steam is awater and steam is a geyser.geyser.
Most of the world’s geysers outside Iceland are inMost of the world’s geysers outside Iceland are in
New Zealand and in Yellowstone National Park.New Zealand and in Yellowstone National Park.

57. Figure B5.2

58. Figure B5.3

59. Volcanic Hazards (1)Volcanic Hazards (1)
 Volcanic eruptions are not rare on land, andVolcanic 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 theEvery 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 rhyoliticTephra eruptions from andesitic or rhyolitic
stratovolcanoes like Mount St. Helens andstratovolcanoes like Mount St. Helens and
Krakatau can be disastrous.Krakatau can be disastrous.

60. Volcanic Hazards (2)Volcanic Hazards (2)
 Eruptions present five kinds of hazards:Eruptions present five kinds of hazards:
– Hot, rapidly moving pyroclastic flows andHot, rapidly moving pyroclastic flows and
laterally directed blasts can overwhelm peoplelaterally 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 orTephra 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.

61. Volcanic Hazards (3)Volcanic Hazards (3)
– Mudflows, calledMudflows, called lahars,lahars, can be devastating.can be devastating.
 In 1985, the Colombian volcano Nevado del RuizIn 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, massivewhen glaciers at the summit melted, massive
mudflows of volcanic debris moved swiftly down themudflows of volcanic debris moved swiftly down the
mountain , killing 20,000.mountain , killing 20,000.
– Violent undersea eruptions can cause powerfulViolent undersea eruptions can cause powerful
sea waves calledsea waves called tsunamis.tsunamis.
Krakatau, in 1883, killed more than 36,000 on JavaKrakatau, 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.

62. Figure 5.24

63. Figure 5.25