Volcano eruption characteristics for a GE-level course in natural disasters.

1. Igneous Rocks & Plate Tectonics
The types and abundances of igneous rocks are closely related to tectonic
setting. Basalt and gabbro are found most abundantly on the ocean floor;
whereas, rhyolite and granite are most commonly located within continental
interiors. The vast majority of all igneous rocks are produced along plate
boundaries.

2. Magma Generation
From Tarbuck and Lutgens
• Most melting takes place between 10 and 200 km
• Decrease in density allows liquid to rise
• Believed to rise as tear-drop shaped bodies
• Rises through fractures and rock assimilation
• Solidifies as it moves into the cooler crust
• Most movement ceases upon reaching rocks of similar density or when
overlying rocks form an impenetrable barrier
• Some magma makes it to the surface where it erupts as lava

3. Magma Composition
• Magmas are a mixture of liquid, dissolved gasses, and
crystalline solids (Gasses are lost upon eruption as lava)
• H2O and CO2 are the most abundant gasses
• Consist mostly of 8 elements (0, Si, Al, Ca, Na, K, Fe, & Mg)
• Vary from mafic (basaltic) through intermediate (andesitic) to
felsic (rhyolitic) depending upon source
• Range in viscosity from highly fluid to a thick paste
• Controlled by tectonic setting, composition of the original rock,
and time
• Major processes include melting, mixing, assimilation, and
differentiation

4. Magma Composition (phases)
Magmas are a mixture of liquid, dissolved gasses, and crystalline solids

5. Magma Composition (gas)
A large volume of gas
(mostly H2O & CO2) is
expelled during an eruption.

7. Magma Composition (liquid)
Eight elements make up nearly
99% of a magma’s volume.

8. Magma Composition (solid)
Minerals that crystallize early can become separated
from the rest of the melt if they have a significantly
different density. Minerals with a higher density
sink to the bottom of the chamber, while those with
a lower density rise to the top.

9. Magma Composition (mineralogy)
Basaltic Fissure Eruption
Andesitic
(Composite)
Cone
Rhyolitic Dome
Highly fluid Very viscous (paste-like)
Magmas vary in composition from mafic (basaltic) through
intermediate (andesitic) to felsic (rhyolitic), which greatly affects the
type of volcanic eruption due to differences in viscosity.

10. Controlled by tectonic setting, composition of the original rock, and time
From Tarbuck and Lutgens
Magma Composition
(tectonic setting)
Basaltic
Magma
Andesitic &
Rhyolitic
Magma

11. Magma Composition
(source materials)
The final magma composition is influenced by:
• Composition of the source rock
• Composition of the country rock (assimilation)
• Mixing with other magmas

12. Magma Composition (time)
The longer it takes for
the magma to cool, the
more differentiated it
becomes.

13. Divergent Plate Boundaries
Divergent plate boundaries are characterized
by basaltic volcanoes and gabbroic intrusions.

14. Divergent Boundary Volcanism
Volcanism along divergent plate boundaries is basaltic in composition
and highly fluid in character.

21. Photo by W. W. Little

22. Shield Volcanoes
Because of the highly fluid nature of basaltic magma, it spreads
laterally and flows for long distances upon eruption. The result is a
broad, relatively flat structure made up of widespread, thin layers.
Photo by W. W. Little

23. Mauna Loa: World’s Largest Volcano
Photo by W. W. Little
Mauna Loa extends from a depth of approximately 10,000 feet to an
elevation of nearly 13,800 feet above sea-level.

24. Columbia Plateau Flood Basalt

25. Types of Basaltic Lava Flows

26. Pahoehoe
Flows
Pahoehoe flows are rich in volatiles (fluids) and deficient in silica and, therefore,
are highly fluid in their flow characteristics.

33. Photo by W. W. Little
Columnar Joints
Columnar joints are shrinkage cracks that form as lava cools and
contracts.

34. Photo by W. W. Little

35. Photo by W. W. Little

36. Aa Flows
Aa flows contain fewer volatiles (fluids) and, therefore, are stiffer and
less fluid in their flow characteristics.

37. Photo by W. W. Little Photo by W. W. Little

38. Photo by W. W. Little

40. Fissure Eruptions
Fissures are similar to shield volcanoes, except that the vent is a long
fracture rather than a crater.

42. Cinder Cones
Photo by W. W. Little
Cinder cones are small, steep-sided (~330) volcanoes formed by the
vertical accumulation of spatter that is shot out of the top of the
volcano by escaping gasses.

43. Spatter

45. Volcanic Bombs
As volcanic spatter moves through the air, it sometimes becomes
streamlined, forming a feature called a volcanic bomb.

47. Photo by W. W. Little

50. Phreatic Cinder Cones

53. Intrusive Rock Bodies
As magma makes its way toward
the surface, it cuts through other
rock bodies.
• If it cuts across layers as a planar unit, its
called a dike.
• If it is squeezed between layers, its called
a sill.
• When the layers become arched over the
sill, its referred to as a laccolith.
• Magma that cools within the volcano
forms a neck.
• Magma that cools within the chamber
becomes a stock or a batholith, depending
upon size.

54. Volcanic Necks
Photo by W. W. Little
A volcanic neck forms when magma cools within the throat of a volcano.
It is more resistant to erosion than the outer part of the volcano.

55. Photo by W. W. Little

59. Dikes
Dikes are tabular igneous rock bodies that cut
across the layers of other rock units.

62. Sills
Sills are tabular igneous rock bodies that are squeezed between
other rock layers.

64. Inverted Valleys
Basalt is fluid and flows down hill where it fills stream channels. Because
basalt is more resistant to erosion than most rocks that form valley walls,
it can be preferentially preserved and, eventually, cap the hill after the
other rocks have been removed.

65. Underwater Eruptions

68. Pillow Structures
Underwater basalt flows cool almost immediately on the outer surface but remain
fluid on the inside, forming a blob or pillow shape with a glassy outer shell and
aphanitic interior. In some cases, the interior remains hollow.

70. Formation of Pillow Structures
As the fluid center continues to move, it breaks through the outer, brittle shell to
form another pillow.

75. Structure of Oceanic Crust
The oceanic crust is relatively thin and
simple in it’s structure and lithology.
Ultramafic rocks of the upper mantle are
overlain by gabrroic intrusions, which are
overlain by sheeted dikes that filled volcanic
fissure vents. The succession is capped by
pillow basalt extruded onto the ocean floor
and overlying pelagic sediment.
Oman Ophiolite

76. Oman Ophiolite

77. Moho
MANTLE CRUST

78. Convergent Plate Boundaries
Convergent plate boundaries are characterized by andesitic
and/or rhyolitic volcanoes and dioritic and/or granitic
intrusions.

79. Ocean/Continent Boundaries
From Tarbuck and Lutgens
Ocean/continent convergent plate boundaries are characterized by
both andesitic and rhyolitic volcanoes and accompanying intrusions.
The type is determined by the degree of crustal assimilation and the
cooling history.

80. Composite Volcano
Photo by D. W. Little
Composite volcanoes form large, steep-sided mountains that are
composed of a variety of materials, including rhyolite, andesite, tuff,
breccia, and obsidian.

81. Composite Volcanoes
Because of a high silica content, composite volcanoes are characterized
by violent eruptions of ash and other pyroclastic material.

83. Photos by W. W. Little
Mount Shasta, CA

88. Pyroclastic Flows
Avalanches of ash and breccia are known as pyroclastic flows. These
accumulate on the volcano slope as dipping layers.

91. Ash Falls
Ash is shot tens of thousands
of feet into the air and settles
to the earth to form tuff.

113. Lahars
A lahar is a flood/debris flow caused by a
volcanic eruption.

114. Photo by W. W. Little

115. Photo by W. W. Little

116. Rhyolite Plugs
Following pyroclastic activity, composite volcanoes often begin to
rebuild through rhyolite flows that plug the volcanic vent.

119. Rhyolite Flows
Rhyolite has a very high in silica content, making it very viscous. It
flows more like a paste than a liquid and forms steep-sided domes.

121. Big Southern Butte, ID
Big Southern Butte is a rhyolite dome that formed on the Snake River Plain
300 ka.

123. Volcano Explosivity Index

125. Granitic Batholiths
Granitic batholiths are very large rock bodies that form
through subsurface solidification of magma. As a result of
faulting and isostatic adjustment, they can be brought to
the surface for observation.

126. Ocean/Ocean Boundaries
From Tarbuck and Lutgens
Ocean/ocean convergent plate boundaries are characterized by
andesitic volcanoes and dioritic intrusions.

127. Continent/Continent Boundaries
Continent/Continent convergent plate boundaries typically lack
volcanoes and igneous intrusions.

128. Hot Spots and Mantle Plumes
Magma sources from the mantle can become stationary, forming hot
spots over which the plate moves.

132. Caldera Eruptions
Caldera eruptions form in
association with composite
volcanism and are the most
violent of all volcanic eruptions.
So much magma is removed
from the chamber that it can no
longer support the weight of
overlying rock and collapses
inward into the chamber.

134. http://www.youtube.com/watch?v=ZzFeZTVxDeY

139. Summary of Volcano Types

140. Summary of Magma Characteristics
Basaltic
• Very Hot (1000 °C)
• Very Fluid (low viscosity)
• High Density
• Rich in Ca, Fe, & Mg
• Dark-colored
Andesitic Granitic
• Relatively Cool (700 °C)
• Pasty (high viscosity)
• Low Density
• Rich in Si, Al, Na, & K
• Light-colored
Intermediate Properties

141. Volcanic Hazards
Deadliest Volcanic Eruptions Since 1500 A.D.
Eruption Year Casualties Major Cause
Nevado del Ruiz, Colombia 1985 25,0001,3 Mudflows3
Mont Pelée, Martinique 1902 30,0001 (29,025)2 Pyroclastic flows2
Krakatau, Indonesia 1883 36,0001 (36,417)2 Tsunami2
Tambora, Indonesia 1815 92,0001,2 Starvation2
Unzen, Japan 1792 15,0001 (14,030)2 Volcano collapse,
Tsunami2
Lakagigar (Laki), Iceland 1783 9,0001 (9,350)2 Starvation2
Kelut, Indonesia 1586 100,001
Other Notable Eruptions
Mount Pinatubo, Philippines 1991 3503 Roof Collapse3
Mount St. Helens, Washington 1980 573 Asphyxiation from ash
Kilauea, Hawaii 1924 11 Falling rock1
Lassen Peak, California 1915 4
Mount Vesuvius, Italy 79 A.D. 33,602 Pyroclastic Flow2
Sources
1 Tilling, Topinka, and Swanson, 1990, Eruptions of Mount St. Helens: Past, Present, and Future: U.S. Geological Survey General Interest Publication, 56p.
2 Blong, R.J., 1984, Volcanic Hazards: A Sourcebook on the Effects of Eruptions: Orlando, Florida, Academic Press, 424p.
3 Wright and Pierson, 1992, Living With Volcanoes: The U.S. Geological Survey's Volcano Hazards Program: U. S. Geological Survey Circular 1073, 57p.
4 Spall, H. (ed.), 1980, Earthquake Information Bulletin: July-August, 1980, v.12, no.4, 167p.

142. Infrastructure Burial
Lava flows
Ash Falls Scoria

143. Pyroclastic Flows

144. Flooding & Lahars

145. Benefits of Volcanism

147. Mitigation

149. Mammoth Mtn, eastern California
Trees killed by volcanic CO2 gas

151. CO2 from subsurface magma can
accumulate in depressions