Volcanoes form as a result of geological processes occurring deep within the Earth. There are over 1500 volcanoes that have erupted in the past 10,000 years on land alone. Whether an eruption is explosive or mild depends on the type of magma, with felsic magma producing explosive eruptions and mafic magma producing milder eruptions. Volcanic activity can create different landforms and also pose various hazards such as volcanic blasts, earthquakes, pyroclastic flows, lava flows, mudflows, and ashfall. Proper monitoring and preparedness is needed to mitigate risks from volcanic eruptions.

1. Volcanoes
Earth’s Interior in Action!

2. Volcanism:
all phenomena associated with the origin and
movement of molten rock
⇐A volcanologist
geared up for work

3. What Constitutes An
“Active” Volcano?
• Not fully agreed upon among scientists
• Intervals between eruptions may be thousands
of years
• On land, close to 1500 have erupted in the past
10,000 years
– Millions, if you count undersea eruptions
• Over a thousand active magma systems have
been identified on land
• An accurate count of the world’s volcanoes
remains elusive

4. Chaiten Volcano
(southern Chile)
May 3, 2008—Chaiten volcano erupts May 2nd and thousands flee; ash from
the volcano reaches Esquel, Argentina; last recorded eruption: over 9000
years ago (7420 BC ± 75 yrs)

5. What’s that stuff on the ground?
• Lava
– Magma that makes it to the
surface
• Pyroclastic material
– When lava erupts into the air
and cools quickly, it can form
material the size of ash up to
large rocks
– Any of this solidified lava
“spew” is called pyroclastics
(pyro = fire; clast = rock)

6. Eruptions

7. Eruptions
• Whether a volcanic eruption is explosive
or mild depends on the type of magma
chemistry that is involved.

8. Eruptions
• Whether a volcanic eruption is explosive
or mild depends on the type of magma
chemistry that is involved.

9. Eruptions
• Whether a volcanic eruption is explosive
or mild depends on the type of magma
chemistry that is involved.
– Felsic = explosive

10. Eruptions
• Whether a volcanic eruption is explosive
or mild depends on the type of magma
chemistry that is involved.
– Felsic = explosive

11. Eruptions
• Whether a volcanic eruption is explosive
or mild depends on the type of magma
chemistry that is involved.
– Felsic = explosive
– Mafic = mild

12. Structures Associated With
Extrusive Volcanism

13. Structures Associated With
Extrusive Volcanism

14. Structures Associated With
Extrusive Volcanism
• Volcanic peaks

15. Structures Associated With
Extrusive Volcanism
• Volcanic peaks

16. Structures Associated With
Extrusive Volcanism
• Volcanic peaks
• Caldara

17. Structures Associated With
Extrusive Volcanism
• Volcanic peaks
• Caldara

18. Structures Associated With
Extrusive Volcanism
• Volcanic peaks
• Caldara
• Volcanic neck

19. Structures Associated With
Extrusive Volcanism
• Volcanic peaks
• Caldara
• Volcanic neck

20. Structures Associated With
Extrusive Volcanism
• Volcanic peaks
• Caldara
• Volcanic neck
• Flood basalt

21. Volcanic Peaks

22. Volcanic Peaks
• The type of peak that forms depends on
the type of magma

23. Volcanic Peaks
• The type of peak that forms depends on
the type of magma
– Felsic

24. Volcanic Peaks
• The type of peak that forms depends on
the type of magma
– Felsic
• Forms composite (strato) volcanoes

25. Volcanic Peaks
• The type of peak that forms depends on
the type of magma
– Felsic
• Forms composite (strato) volcanoes
– Magmas don’t flow far

26. Volcanic Peaks
• The type of peak that forms depends on
the type of magma
– Felsic
• Forms composite (strato) volcanoes
– Magmas don’t flow far
– Tall, steep-sided cones

27. Volcanic Peaks
• The type of peak that forms depends on
the type of magma
– Felsic
• Forms composite (strato) volcanoes
– Magmas don’t flow far
– Tall, steep-sided cones
– Mafic

28. Volcanic Peaks
• The type of peak that forms depends on
the type of magma
– Felsic
• Forms composite (strato) volcanoes
– Magmas don’t flow far
– Tall, steep-sided cones
– Mafic
• Forms shield volcanoes

29. Volcanic Peaks
• The type of peak that forms depends on
the type of magma
– Felsic
• Forms composite (strato) volcanoes
– Magmas don’t flow far
– Tall, steep-sided cones
– Mafic
• Forms shield volcanoes
– Magmas flow for long distances

30. Volcanic Peaks
• The type of peak that forms depends on
the type of magma
– Felsic
• Forms composite (strato) volcanoes
– Magmas don’t flow far
– Tall, steep-sided cones
– Mafic
• Forms shield volcanoes
– Magmas flow for long distances
– Broad, gently-sloping cones

31. Composite Volcano
(also called a Stratovolcano)
• Felsic magma, tall, steep-sided cone formed of
alternating layers of lava flows and pyroclastic
material
Mount Mayon
Philippines, Island of Luzon, province of
Albay, Bicol region, 15 km NW of Legazpi City

32. Shield Volcano
• Mafic magma (basaltic), gentle slope (looks like
a warrior’s shield laid on the ground)
Mauna Loa Shield Volcano

33. Other Types of Volcanic Peaks

34. Other Types of Volcanic Peaks
• Lava Dome—Formed from felsic magmas,
like a bulge on the surface

35. Other Types of Volcanic Peaks
• Lava Dome—Formed from felsic magmas,
like a bulge on the surface
– Common where previous felsic eruptions have
occurred

36. Other Types of Volcanic Peaks
• Lava Dome—Formed from felsic magmas,
like a bulge on the surface
– Common where previous felsic eruptions have
occurred
– Material not well fused together (easily eroded)

37. Lava dome
New lava dome forming inside Mount St. Helens

38. Other Types of Volcanic Peaks
• Cinder Cone
– Usually less than 150 ft. high
– Associated with flood basalts and shield
volcanoes
– Frothy, (usually) mafic magma under high
pressure at a narrow vent
– Tephra rains around the vent, forming small,
rounded volcanoes made of ash and cinders

39. Cinder cones
Wizard Island—Crater Lake, OR

40. Caldera
• Collapsed and/or
exploded volcanic
peak, producing a
crater

41. Long Valley Caldera in California

42. Volcanic Neck
(Shiprock, NM)
• Solidified magma within the “neck” of a volcano
• Surrounding material may later be eroded away,
leaving a tall, standing structure of igneous rock

43. Flood Basalts
(also called Fissure Eruptions)
• No dome or cone-shaped structure
• Magma flows out of breaks in the crust, usually
in high volume, in layers that may be hundreds
of feet deep and thousands of miles wide
– Deccan Plateau in India
• 200,000mi2
– Columbia Plateau, crossing WA, OR, and ID borders
• 50,000mi2

44. Flood Basalt
Columbia Gorge, along the Columbia River in the
Cascade Mountains of Washington and Oregon

45. Hazards Associated with
Extrusive Volcanism

46. Hazards Associated with
Extrusive Volcanism
• Volcanic blast

47. Hazards Associated with
Extrusive Volcanism
• Volcanic blast
• Earthquakes

48. Hazards Associated with
Extrusive Volcanism
• Volcanic blast
• Earthquakes
• Avalanches and
Debris Flows

49. Hazards Associated with
Extrusive Volcanism
• Volcanic blast
• Earthquakes
• Avalanches and
Debris Flows
• Mudflows and
Lahars

50. Hazards Associated with
Extrusive Volcanism
• Volcanic blast • Eruption column
• Earthquakes and ashfall
• Avalanches and
Debris Flows
• Mudflows and
Lahars

51. Hazards Associated with
Extrusive Volcanism
• Volcanic blast • Eruption column
• Earthquakes and ashfall
• Avalanches and • Pyroclastic flows
Debris Flows
• Mudflows and
Lahars

52. Hazards Associated with
Extrusive Volcanism
• Volcanic blast • Eruption column
• Earthquakes and ashfall
• Avalanches and • Pyroclastic flows
Debris Flows • Lava flows
• Mudflows and
Lahars

53. Hazards Associated with
Extrusive Volcanism
• Volcanic blast • Eruption column
• Earthquakes and ashfall
• Avalanches and • Pyroclastic flows
Debris Flows • Lava flows
• Mudflows and • Volcanic gases
Lahars

54. Volcanic Blast
(also called Blowdown)
From Mount St. Helens:
• A 600ºF blast of rocks, ash, and gases swept
across the land at 670 miles an hour
• The force of the
blast stripped trees from
hillsides 6 mi. away

55. Earthquakes and Debris flows
• The movement of magma up through the
crust creates earthquakes
• The Mount St. Helens eruption began with
a magnitude 5.1 earthquake. The entire
north flank of the mountain broke loose in
three separate blocks, which slid down the
mountain at 100 mph.
• An avalanche of rock, ice, snow, and soil,
like this one, is called a debris avalanche

56. Debris Flow
• Rocks and mounds of
debris filled river
valleys for 14 miles,
as much as 600 feet
deep in some places,
damming streams
and forming new
lakes

57. Mudflows and Lahars
• Superheated ash and
magmas melt and mix
with snow and ice, then
speed down the volcano’s
flanks
– Nevado del Ruiz,
Colombia, 1985
– Melted ice and snow mixed
with volcanic ash and mud,
sending a 130 foot (40 m)
high mud-flow down the
Lagunilla River

58. Mudflows and Lahars
• Superheated ash and
magmas melt and mix
with snow and ice, then
speed down the volcano’s
flanks
– Nevado del Ruiz,
Colombia, 1985
– Melted ice and snow mixed
with volcanic ash and mud,
sending a 130 foot (40 m)
high mud-flow down the
Lagunilla River

63. Mudflows—Mt. St. Helens
• …sloshing from side-to-side as it rushes through
forests and clearcuts, ripping trees, houses, and
bridges from the ground, devastating
downstream environments and communities

64. Eruption Column and Ashfall

65. Eruption Column and Ashfall
• Can reach the stratosphere,
where it is transported long
distances

66. Eruption Column and Ashfall
• Can reach the stratosphere,
where it is transported long
distances
• Can block insolation, altering
weather and climate

67. Eruption Column and Ashfall
• Can reach the stratosphere,
where it is transported long
distances
• Can block insolation, altering
weather and climate
• Ash and gases mix with water
in atmosphere, producing acid
precipitation

68. Eruption Column and Ashfall
• Can reach the stratosphere,
where it is transported long
distances
• Can block insolation, altering
weather and climate
• Ash and gases mix with water
in atmosphere, producing acid
precipitation
• May produce enough ash to
bury the landscape and kill
residents

69. Eruption Column and Ashfall
• Can reach the stratosphere,
where it is transported long
distances
• Can block insolation, altering
weather and climate
• Ash and gases mix with water
in atmosphere, producing acid
precipitation
• May produce enough ash to
bury the landscape and kill
residents
• Can cause electrical failure in
jet engines

70. Mt. Mayon, Island of
Luzon, Phillipeans,
1984
Pyroclastic Flows
(Nueé Ardant)
• Swift, destructive cloud of hot ash and
gases that flows rapidly downhill and
burns all in its path
– Ash weighs down the gases, which would
otherwise rise into the atmosphere
– Nueé Ardant—French for “glowing avalanche”
• Mont Pelée in Martinique

71. Mt. Pelée, Island of Martinique, Caribbean
May 8,1902—The town of St. Pierre was obliterated by
a nuée ardante; over 28,000 lost their lives

72. Mt. Pelée, Island of Martinique, Caribbean
May 8,1902—The town of St. Pierre was obliterated by
a nuée ardante; over 28,000 lost their lives
St. Pierre today….

73. Pyroclastic Flow
From Mount St. Helens:
• Within a few hours of
the lateral blast, hot
mixtures of volcanic
gas, pumice, and ash
swept down the north
flank of the volcano at
speeds up to 100
miles an hour and
temperatures of over
1200ºF

74. Soufriere Hills Pyroclastic Flow
The Soufrière Hills Volcano,
Montserrat, West Indies,
began erupting on July 18,
1995.

75. Soufriere Hills Pyroclastic Flow
The Soufrière Hills Volcano,
Montserrat, West Indies,
began erupting on July 18,
1995.

76. Lava Flows
• Can travel long
distances, burning and
burying everything
• Lava does not need to
actually touch an object
to set it on fire

77. Volcanic Gases
• Volcanic gases, some of which are colorless and
odorless like CO2, can cause suffocation, killing
plants, animals, and humans alike

78. Volcanic Gases
• Volcanic gases, some of which are colorless and
odorless like CO2, can cause suffocation, killing
plants, animals, and humans alike

79. Volcanic Gases
• Such an incident happened at Mammoth
Mountain in early 2006, where CO2
emanating from faults at the edge of Long
Valley Caldera killed one member of the
local ski patrol

80. Volcanic Gases
• Ash and gases (such as SO2—sulfur dioxide) can mix
with cloud droplets near the ground to form “vog” (a
volcanic fog that causes health problems)
• Hydrogen from lava combines with chlorine in sea water
to form hydrochloric acid, which becomes airborne as
steam, forming corrosive lava haze, or “laze”
Hawaiian vog

81. Hazards Associated with
Extrusive Volcanism

82. Structures Associated With
Intrusive Volcanism
• Pluton--a massive body of intrusive igneous
rock which solidifies deeply within the crust
• Batholith • Dike
• Laccolith • Sill
• Stock • Vein

83. Intrusive Igneous Structures
and Their Formation

84. Structures Associated With
Intrusive Volcanism
• Batholith— >40 mi2 (100 km2) in diameter, amorphous,
forms deep in the crust
– granite often forms batholiths
• Sierra Nevada Mountains
• Stock—A few mi2 in diameter, amorphous, forms deep in
the crust, may be an offshoot of a batholith
• Laccolith—Similar to a stock, but intrudes just beneath
the surface, warping surface rocks and forming a hill;
may form the base of small mountain chains
– The Black Hills, South Dakota
– Devil’s Tower, WY

85. Batholiths in
Western North America

86. Laccolith:
Devil’s Tower, WY

87. Dike