1. Volcanoes and Other
Igneous Activity
Chapter 5

2. NOT all Volcanic Eruptions are the Same

3. NOT all Volcanic Eruptions
are the Same
• Three factors determine the
“violence” or explosiveness of a
volcanic eruption:
1. Composition of the magma
2. Temperature of the magma
3. Dissolved gases in the magma

4. Viscosity
• These three factors control the viscosity
of a given magma.
• Which in turn controls the nature of an
eruption.
• Viscosity is a measure of a material’s
resistance to flow.
– Higher viscosity materials flow with great
difficulty.

5. Factors Affecting Viscosity
1. Temperature – Hotter magmas are less
viscous.
2. Composition – Silica (SiO2) content
–
Higher silica content = more SiO4
chains/structures = higher viscosity
•
–
felsic lava – rhyolite
Lower silica content = fewer SiO4
chains/structures = lower viscosity or more
fluid like behavior
•
mafic lava – basalt

6. Factors Affecting Viscosity
3. Dissolved Gases
• The violence of an eruption is related to
how easily gases escape from magma.
–
–
Gas content affects magma mobility.
Volatiles migrate upward and accumulate
near the top of the magma chamber.
enriching the upper portion of the magma
chamber with dissolved gases.
– Gases expand within a magma as it nears
the Earth’s surface due to decreasing
pressure.
– Escaping gases provide the force to propel
molten rock from the volcanic vent.

7. Factors Affecting Viscosity
3. Dissolved Gases
–
–
Very fluid basaltic
magmas allow the
gases to migrate
upwards and escape
the vent with relative
ease.
Produces lava
fountains extending
hundreds of meters
in height.

8. Factors Affecting Viscosity
3. Dissolved Gases
– Highly siliceous magmas
undergo magmatic
differentiation leaving the
upper portion of the magma
chamber enriched in silica
and dissolved gases.
– The volcano summit begins
to inflate and bulge months
to years prior to eruption.
– As the magma migrates up
the vent, the gases collect
into tiny bubbles.
– The mixture is transformed
into a gas jet containing tiny
magma fragments that are
explosively ejected.
– Produces plumes of a hot
ash-laden gases called
eruption columns that
extends thousands of meters
into the atmosphere.

9. The Nature of Volcanic
Eruptions
• In Summary:
– Fluid basaltic lavas generally produce quiet
eruptions.
• Basaltic flow rates between 30-1000 ft/hour.
• Traveling up to 90 miles (150 km) from the vent.
– Highly viscous lavas (rhyolite or andesite)
produce more explosive eruptions.
• Rhyolitic lava are much slower.
• Seldom travel more than a few kilometers from
their vents.

10. Materials Extruded
from Volcanoes
1. Lava Flows
2. Volatiles
3. Pyroclastic Material

11. Lava Flows
• Types of Basaltic Flows:
• Pahoehoe lava
resembles a
twisted or ropey
texture.
Pahoehoe Lava Flow in
Hawaii Volcanoes
National Park

12. Pahoehoe
Lava Flow
in Hawaii
Volcanoes
National
Park

13. Pahoehoe lava flows can contain lava
tubes, cave-like tunnels that were
horizontal conduits for lava.
Lava Tube in Hawaii
Volcanoes National Park

14. Lava Flows
• Types of Basaltic Flows:
• Aa lava has a rough,
jagged blocky texture.
– Cool and thick with gases
escaping forming
numerous voids and
sharp spines.
– Pahoehoe flows are hotter, richer in gases, and
travel faster than aa flows.
– Basaltic lavas can begin as pahoehoe flows and
become aa flows.

15. Lava Flows
• Types of Basaltic Flows:
– Pillow lavas – lava that formed
underwater.
• Occur along oceanic ridges.
• Lava cools quickly forming
an outer skin.
• Lava advances by breaking
through the outer rind.
• Forms elongated structures
resembling pillows.
Pillow Basalts, Olympic National Park,
Washington

16. Dissolved Gases (Volatiles)
• Magmas contain varying amounts of
dissolved gases held in the molten rock
under confining pressure.
– Consists mainly of:
• water vapor and carbon dioxide
– Lesser amounts of:
• nitrogen, sulfur dioxide, chlorine, hydrogen,
argon
– Contribute significantly the planet’s
atmosphere (natural air pollution).
– Rise into the atmosphere and may reside
there for years potentially impacting climate.

17. Volcanoes and Climate
• Explosive eruptions emit huge quantities
of gases and fine-grained debris into the
atmosphere which
• Filter out and reflect a portion of the
incoming solar radiation.
• This can cause cooling on a global scale.
• Examples of volcanism affecting climate:
• Mount Tambora, Indonesia – 1815
• Krakatau, Indonesia – 1883
• Mount Pinatubo, Philippines – 1991

18. Pyroclastic Materials –
“Fire Fragments”
• Ash and Dust – Fine, glassy fragments
compose tuffs and welded tuffs.
• Pumice – Porous rock from “frothy” lava
or rhyolitic composition.
• Scoria – Vesicular rock of typically
basaltic composition.
• Cinders – Glassy vesicular fragments 4 32 mm.

19. Pyroclastic Materials –
“Fire Fragments”
• Lapilli – pyroclastic fragments 2-64 mm in size.
• Particles larger than lapilli:
• Blocks – pyroclasts more
than 64 mm in diameter
ejected in the solid state
(rock torn from the vent
wall).
• Bombs – ejected blob of
hot lava streamlined
during flight (more than
64 mm in diameter).
Bomb is approximately 10 cm long

20. Anatomy of Volcanoes

21. Anatomy of Volcanoes
• General Features
– Magma chamber is connected to the surface by
a conduit or pipe that terminates at a surface
vent.

22. Anatomy of Volcanoes
• General Features
– Opening at the summit of a volcano:
• Crater – steep-walled, circular depression at the
summit, generally less than 1 km diameter.
– Produced by explosive excavation of rock during eruptions.
• Caldera – a summit depression typically greater than
1 km diameter.
– Produced by collapse following a massive eruption.

23. Anatomy of Volcanoes
• General Features
– Mature volcanoes develop fissures along the
flanks and base producing parasitic cones and
fumaroles (emit only gases and smoke).

24. Types of Volcanoes and
Their Characteristics

25. Plate Tectonics and Igneous Activity
• Global distribution of igneous activity is not random.
• Most volcanoes are located within or near ocean
basins or along continental margins.
• Each type of plate tectonic boundary produces a
specific type of igneous activity.
Insert Plate Tectonic
Boundary Features
Animation #66

26. Plate Tectonics and
Igneous Activity
• Basaltic igneous activity is common in
both oceanic and continental settings.
• Granitic igneous activity is rarely found
in the oceans.

27. Not all Volcanoes are the Same
• Factors determining the size
and shape of volcanoes:
1. Volume of lava erupted
2. Viscosity of lava:
• Composition of the magma
• Temperature of the magma
• Dissolved gases in the magma

28. Shield Volcanoes
•
•
•
•
Broad, slightly domed-shaped.
Composed primarily of basaltic lava.
Generally cover large areas.
Produced by mild eruptions of large volumes of
lava.
Mauna Loa

29. Shield Volcanoes
• Most grow up from the seafloor to form islands or sea
mounts.
• From ocean floor to summit, Mauna Loa is over 6 miles
high (higher than Mt. Everest [~32,000 feet]).
• Large steep-walled calderas occupy the summit.
• Mature volcanoes erupt lava from the summit and rift
zones that develop along the slopes (flank).

30. Cinder
Cones Haleakala
Maui
• During late stages, shield
volcanoes produce clusters of
cinder cones in summit area.

31. Shield Volcanoes
and Plate Tectonics
• Shield volcanoes are a product
of intra-plate volcanism in
oceanic crust.
– Activity within a tectonic plate.
– Associated with plumes of heat
upwelling in the mantle – mantle
plume.
– Form localized volcanic regions
in the overriding plate called a
hot spot.
– Produces basaltic magmas
creating:
• Shield volcanoes
• Volcanic chains and seamounts

32. Volcanism on a
Tectonic Plate
Moving over a
Hot Spot
Produces Shield
Volcanoes
(basaltic) and
Volcanic Island
and Seamount
Chains

33. Global distribution of flood basalt provinces (black) and associated hotspots (red dots). Red dashed lines are hot spot tracks, which appear as lines
of volcanic structures on the ocean floor. The Keweenawan and Siberian
Traps formed in failed continental rifts where the crust had been greatly
thinned. Whether there is a connection between the Columbia River basalts
and the Yellowstone hot spot is still a matter of ongoing research.

34. Cinder Cone Volcanoes
Sunset Crater – a Cinder Cone
near Flagstaff, Arizona
• Built from ejected lava
fragments – pyroclastic
cinders or clinkers (glassy
vesicular fragments).
• Fragments range in size
from ash to bombs,
primarily lapilli.
• Product of gas-rich basaltic
magma (scoria).
• Steep slope angle.
• Rather small size.
• Frequently occur in groups.

35. Parasitic Cinder Cones, Mauna Kea, Hawaii

36. Late-Stage Cinder
Cones, Mauna Kea
Summit, Hawaii

37. Cinder Cone Volcanoes and
Plate Tectonics
• Cinder cone volcanoes are the product of latestage volcanism in various tectonic
environments.
– Basaltic magmas:
• Associated with hot spot volcanism in oceanic crust:
– Shield Volcano Flanks and Calderas
• Associated with hot spot volcanism in continental crust:
– Flanks of Calderas
• Associated with subduction zones between oceanic-oceanic
crust and oceanic-continental crust:
– Stratovolcano Flanks and Calderas

38. Composite or Stratovolcanoes
• Large, classic-shaped volcano (1000’s of feet high &
several miles wide at base).
• Composed of interbedded lava flows and layers of
pyroclastic debris.
• Primarily andesitic in composition with lesser basaltic
and rhyolitic lavas.

39. Composite or Stratovolcanoes
• Explosive eruptions that eject huge quantities of
pyroclastic material.
• Most are located adjacent to the Pacific Ocean
(e.g., Fujiyama, Mt. St. Helens).
Mt. St. Helens

40. Mt. St.
Helens
1980
Eruption

41. Mt. St. Helens Following
the 1980 Eruption

42. Pyroclastic Flows
• Stratovolcanoes erupt
violently…
– Often produce a nueé
ardente (glowing
avalanche):
• Fiery pyroclastic flow
made of hot gases
infused with ash,
pumice, and other
debris.
• Felsic and intermediate
magmas.
• Material ejected at high
velocities.
Nueé Ardente on Mt. St. Helens

43. Pyroclastic Flows
• Nueé ardente (glowing
avalanche):
– Forms from the collapse
(overcome by gravity) of tall
eruption columns.
– Moves down the slopes of a
volcano at speeds up to 200 km
(125 miles) per hour.
– Traveling up to more than 60
miles from the vent.
– Ground-hugging portion is rich
in particular matter suspended
by jets of buoyant gases (nearly
frictionless).

44. Lahars
• Stratovolcanoes may produce lahars, or
volcanic mudflows:
– Mixture of volcanic debris and water.
– Moves rapidly down slope (30 kph or more)
following stream valleys.
– Triggered when large volumes of ice and
snow melt during an eruption.
– Also generated when heavy rainfall saturates
weathered volcanic deposits.
– Highly destructive.

45. Lahar from Mt. St. Helens -- On March 18, 1980, an explosive eruption on Mt. St.
Helens generated a 14-kilometer-high eruption plume. Melted snow from the
eruption produced the dark-colored lahar seen in this photo. Part of the lahar flowed
into Spirit Lake (lower left). However, most of the lahar flowed into the North Fork
of the Toutle River valley (right), eventually reaching the Cowlitz River, 80
kilometers downstream. Courtesy of Thomas J. Casadevall, USGS.

46. Lahars at
Mt. Rainer
The snow-covered peaks of the Cascade volcanoes in
Washington, Oregon, and northern California pose a clear
threat to surrounding towns and villages. Past events
suggest that a catastrophic lahar could lie in the future of
Mt. Rainier, the largest of the Cascade volcanoes. The
4,000 m high summit of Mt. Rainier contains the largest
system of alpine glaciers in the Cascade Range. The
periodic melting of glacier ice from Mt. Rainier has
generated at least 50 major lahars over the past 10,000
years. The largest of these mudflow deposits, one of the
world's largest, is the ~5700-year-old Osceola lahar,
shown in the adjacent map (green) (courtesy of USGS). The
Osceola lahar traveled down the White River, over 112 km
from its source. It then spread out at its mouth to cover an
area of over 300 square kilometers along the shoreline of
Puget Sound. The recent geologic history of Mt. Rainier
demonstrates that a major mudflow descends down the
White River once every 600 years. The younger 500-yearold Electron lahar (see map – yellow) was also generated
from Mt. Rainier. It flowed 56 kilometers down the
Puyallup River to within 15 kilometers of Tacoma,
Washington. More than 300,000 people now live in the
area covered by these extensive lahars! Unlike floods,
such catastrophic mudflows can occur with little or no
warning. Some volcanologists have predicted that Mt.
Rainier will be the site of the next Cascade eruption.
Therefore, the volcano is monitored closely, with the hope
that we can warn the local population before the next lahar
strikes.

47. Eruptive Frequency of
Cascade Range
Stratovolcanoes

48. Stratovolcanoes and
Plate Tectonics
• Stratovolcanoes are the product of subduction
zone igneous activity along oceanic-oceanic and
oceanic-continental convergent plate
boundaries.
– Occur in conjunction with deep oceanic trenches.
– Descending plate causes partial melting (wet
melting) of the mantle.
• A volcanic island arc if in the ocean – evolves
from early-stage mantle-derived basalts to
mature andesites and rhyolites.
• A continental volcanic arc if along a continental
margin – primarily andesites and rhyolites lesser
basalts.

49. Igneous
Activity
Along
Subduction
Zones

50. Stratovolcanoes and
Plate Tectonics
• Associated with the Pacific Ocean
Basin margin is known as the “Ring
of Fire”.
• Most of the
world’s
explosive
volcanoes are
found here.

51. Size Comparison of the
Three Types of Volcanoes

52. •
•
•
•
•
•
Calderas
Large collapse depressions with approximate circular diameter.
Steep-walled depressions at the summit.
Size generally exceeds 1 km in diameter.
Formed by the eruption of large volumes of magma from a shallow
underground magma reservoir.
Results in loss of structural support for the overlying rock.
Leading to collapse of the ground and formation of a large depression.

53. Calderas
Form by one of the following processes:
1.
Collapse of the summit of a large composite volcano
following the eruption of silica-rich pryroclastic
eruption (Crater Lake).
Crater Lake Caldera, Oregon (6 miles diameter)

54. Calderas
2. Collapse of the top of a shield volcano caused
by subterranean drainage from the central
magma chamber to rift zone – flank
eruptions (Mauna Loa).

55. Calderas
3.
Image courtesy of the United States Geological Survey.
Collapse of large
area, independent
of any preexisting
volcanic
structures, caused
by discharge of
colossal volumes
of silica-rich
pyroclastics along
ring fractures
(Yellowstone).
Yellowstone
Caldera

56. Calderas and Plate Tectonics
• Calderas are a product of intra-plate
volcanism in continental crust.
– Activity within a tectonic plate.
– Associated with plumes of heat upwelling in the
mantle – mantle plume.
– Form localized
volcanic regions in the
overriding plate called
a hot spot.
– Produces granitic
magmas creating:
• Calderas

57. Large Calderas in the U.S.
Yellowstone
Long Valley
Valles
• Long Valley, California
• Yellowstone, Wyoming
• Valles, New Mexico

58. Long Valley Caldera
• Located in eastern
California.
• Caldera is elliptical in
shape.
• Approximately 10 X 20
miles across.
• Last volcanic activity
~550-600 years ago at
Inyo Craters.
• Huge eruption 730,000
years ago ejected about
600 km3 of material.

59. Long Valley
Caldera,
Looking from
SW to NE
Inside the Long
Valley Caldera

60. Ash Deposits from Long
Valley Eruption, 730,000
years ago.
Potential Thickness of
Tephra from Eruption of less
than 1 km3 of Magma.

61. Fissure Eruptions
and Lava Plateaus
• Huge volumes of volcanic
material are extruded from
fractures in the crust called
fissures.
• Fluid basaltic lava extrude
via fissure eruptions (flood
basalts) building up a thick
lava plateau.
• e.g., Columbia River
Plateau (nearly 1 mile
thick of flood basalts).

62. Flood Basalts and Plate Tectonics
• Flood basalts are a product of
intra-plate volcanism in
continental crust.
– Activity within a tectonic plate.
– Associated with plumes of heat
upwelling in the mantle –
mantle plume.
– Form localized volcanic regions
in the overriding plate called a
hot spot.
– Produces basaltic magmas
creating:
• Flood basalts in continental
environments (Columbia
Plateau).

63. Global distribution of flood basalt provinces (black) and associated hotspots (red dots). Red dashed lines are hot spot tracks, which appear as lines
of volcanic structures on the ocean floor. The Keweenawan and Siberian
Traps formed in failed continental rifts where the crust had been greatly
thinned. Whether there is a connection between the Columbia River basalts
and the Yellowstone hot spot is still a matter of ongoing research.

64. • Spreading Centers
– The greatest volume
of volcanic rock is
produced along the
oceanic ridge system.
– Continental or oceanic
rifting.
– Results in partial
melting of mantle
(decompression
melting).
– Large quantities of
basaltic magma are
produced.

65. Lava Domes
Lava Dome on Mt. St. Helens
• Bulbous mass
of congealed
lava.
• Form in the
summit crater.
• Most are associated with composite cones that
produce explosive eruptions of silica- and gas-rich
lavas.
• Typify late stages if mature, chiefly andesitic
cones.

66. Climbing Mount St. Helens

67. Climbing Mount St. Helens

68. Climbing Mount St. Helens

69. Volcanic Pipes
•
•
•
•
Most volcanic pipes are conduits that connect a magma chamber
to the surface.
Pipes may extend in a tube-like manner to depth exceeding 200
km (rare).
Enables ultramafic rocks from the mantle to reach the surface
that have undergone very little alteration.
Diamond-bearing kimberlite dikes.

70. Volcanic Necks
• Erosion of the outer flanks of the
volcanic cone leave behind the moreresistant neck.
Shiprock, NM
– A Volcanic
Neck

71. Columnar Joints
• Form as igneous rocks cool at or near the surface.
• Shrinkage fractures formed by tensional forces that
cracks the rock as it contracts during cooling.
• Produces elongated, pillar-like columns.
• Common is lava flows and sills.

72. Plutonic Igneous Activity

73. Plutonic Igneous Bodies
• Most magma is emplaced at depth in the
Earth.
• An underground igneous body, once
cooled and solidified, is called a pluton.

74. Classification of Plutons
•
Classification of Plutons:
1. Shape
•
•
Tabular (sheetlike)
Massive
3. Size
2. Orientation with
respect to the host
(surrounding) rock:
– Discordant – cuts
across existing
structures or rock
units.
– Concordant –
parallel to existing
structures of rock
units.

75. Types of Intrusive Igneous Features
• Dike – a tabular, discordant pluton where
magma has injected into fractures.
– Pathways that fed ancient lava flows.
– Occur in clusters – radiating around neck.

76. Vertical
Dike Near
Granby,
Colorado

77. Types of Intrusive Igneous Features
• Sill – a tabular, concordant pluton where magma
has injected along sedimentary bedding surfaces.
– Occur in shallow environments less force required to move
overlying rock layers.
– Frequently cut across layers and resume concordant nature at a
higher level.

78. A Sill in the Salt River Canyon,
Arizona

79. Types of Intrusive Igneous Features
• Laccolith
– Similar to a sill (near-surface environment).
– Lens or mushroom-shaped mass.
– Generated by accumulation of more viscous
magma.
– Arches
overlying
strata
upward.

80. Types of Intrusive Igneous Features
• Two Types of Plutons (based on size)
– Batholiths
• Largest intrusive body.
• Surface exposure of 100+ square
kilometers.
• Frequently
form the
cores of
mountains.

81. Types of Intrusive Igneous Features
• Two Types of Plutons (based on size)
– Stocks
• Smaller
• Surface exposure less than 100 square
kilometers.
• Batholiths consist of
large numbers of
distinct plutons
(including stocks) that
were intruded over
millions of years.

82. Batholiths of Western
North America
The Sierra Nevada Batholith exposed in
Yosemite Natl. Park