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Chapter 5
Volcanoes
Magma Sources and Types
• Magma sources tend to be 50 to 250 km deep
into the crust and upper mantle
• Temperatures increase as depth increases
• Some of the internal heat is left over from the
earth’s formation; more heat is generated by
the decay of radioactive elements in the earth
• Volcanoes are generated at:
– Divergent Plate Boundaries
– Convergent Plate Boundaries
– “Hot Spots”
Magma Sources and Types
• Magma compositions vary in SiO2 , iron,
magnesium, and volatile gases
• Mafic magma – low in SiO2 (45-50 %) but high
in iron, and magnesium
• Felsic magma – high in SiO2 (up to 75 %) but
low in iron, and magnesium
• Intermediate magma – intermediate range of
SiO2 (50-65 %), iron, and magnesium
• Amount of volatile gases will affect explosive
characteristics of eruptions
The nature of volcanic eruptions
• Factors determining the “violence” or
explosiveness of a volcanic eruption
• Composition of the magma
• Temperature of the magma
• Dissolved gases in the magma
• The above 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 (e.g., higher viscosity
materials flow with greater difficulty)
Factors affecting viscosity:
–Temperature – hotter magmas are less
viscous
–Composition – silica (SiO2) content
• Higher silica content = higher viscosity
(e.g., felsic lava such as rhyolite)
• Lower silica content = lower viscosity or
more fluid-like behavior (e.g., mafic lava
such as basalt
Factors affecting viscosity:
– Dissolved gases
• Gas content affects magma mobility
–Gases expand within a magma as it nears
the Earth’s surface due to decreasing
pressure
–The violence of an eruption is related to
how easily gases escape from magma
In summary:
• Fluid basaltic lavas generally produce
quiet eruptions
• Highly viscous lavas (rhyolite or
andesite) produce more explosive
eruptions
Materials extruded from a volcano
• Lava flows
• Basaltic lavas are much more fluid
• Types of basaltic flows
– Pahoehoe lava (resembles a twisted or ropey
texture)
» Lava tubes, pillow lavas,
– Aa lava (rough, jagged blocky texture)
• Dissolved gases( volatiles )
• One to six percent of a magma by weight
• Mainly water vapor and carbon dioxide
A typical aa flow
Figure 4.7 A
Pahoehoe flow
Pyroclastic materials
• “Fire fragments”
Types of pyroclastic debris
• Ash and dust – fine, glassy fragments
• Pumice – porous rock from “frothy” lava
• Lapilli – walnut-sized material
• Cinders – pea-sized material
• Particles larger than lapilli
– Blocks – hardened or cooled lava
– Bombs – ejected as hot lava – Stromboli!
A volcanic bomb
Figure 4.9 left
Figures 5.9 A-D
Volcanic Structures
• Opening at the summit of a volcano
– Crater – steep-walled depression at the
summit, generally less than 1 kilometer in
diameter
– Caldera – a summit depression typically greater
than 1 kilometer in diameter, produced by
collapse following a massive eruption
• Vent – opening connected to the magma
chamber via a pipe
Types of volcanoes
•Shield volcano
»Broad, slightly dome-shaped
»Composed primarily of basaltic lava
»Generally cover large areas
»Produced by mild eruptions of large
volumes of lava
»Mauna Loa on Hawaii is a good example
Figures 5.8 a – d
YouTube - Kilauea Volcano Erupts - Dramatic Video
Shield Volcano – Mauna Loa
Types of volcanoes
•Cinder cone
»Built from ejected lava (mainly cindersized) fragments
»Steep slope angle
»Rather small size (less than 1000ft )
»Frequently occur in groups
Sunset Crater
A cinder cone near Flagstaff, Arizona
Copyright © 2006 Pearson Prentice Hall, Inc.
Paricutin
• Observed by farmer, forming in a Mexican
cornfield , 1943
• Grew to 130 ft in first day, Day 5 – 330 ft.
• Covered a nearby village in 1944
(aa flow)
• Ended in 9 years -
Figures 5.10 A and B
Composite cones (stratovolcanoes)
»Most are located adjacent to the
Pacific Ocean (e.g., Fujiyama, Mount St.
Helens)
»Large, classic-shaped volcano
(thousands of feet high and several
miles wide at base)
»Composed of interbedded lava flows
and layers of pyroclastic debris
Composite cones
»Most violent type of activity (e.g., Mount
Vesuvius)
»Often produce a nuée ardente
»Fiery pyroclastic flow made of hot gases infused
with ash and other debris
»Move down the slopes of a volcano at speeds up
to 200 kilometers per hour
»May produce a lahar, which is a volcanic
mudflow
Mount St. Helens
• Mount St. Helens National Volcanic
Monument
• Mount St. Helens -- From the 1980 Eruption
to 2000, Fact Sheet 036-00
Mt Shasta – composite
with small parasitic cone on left
Mt Vesuvius
79 A.D. – 2m of pumice in 24hrs buried
2000, then hot gases suffocated those still
alive
Mount Etna Erupts
A composite volcano
Figure 4.10
Figure 5.11 A-B
Types and Locations of Volcanoes
• Volcanic Domes
– Composed of more viscous andesite or rhyolite
• these lavas do not flow
– Ooze out onto surface from a tube and pile up close to the
vent
– Compact, small, and steep sided
– Various locations around Pacific Ring of Fire
Figure 5.12 – domes formed in Mt. St. Helen
A lava dome on Mount St. Helens
Figure 4.25
A size comparison of the three
types of volcanoes
Figure 4.13
Other volcanic landforms
• Calderas
• Steep-walled depressions at the summit
• Size generally exceeds 1 kilometer in diameter
• produced by collapse following a massive
eruption
Deriba Caldera, western Sudan
outer – 3mi diameter about 3500 yrs ago, inner formed later
Formation
of Crater
Lake,
Oregon
Figure 4.22
• Volcanic necks (e.g., Ship Rock, New Mexico)
are resistant vents left standing after erosion
has removed the volcanic cone
• Volcanic necks (e.g., Ship Rock, New Mexico)
are resistant vents left standing after erosion
has removed the volcanic cone
Plutonic igneous activity
•Most magma is emplaced at depth in the
Earth
•An underground igneous body, once cooled
and solidified, is called a pluton
•Classified by:
•Shape
»Tabular (sheetlike)
»Massive
And:
•Orientation with respect to the host
(surrounding) rock
»Discordant – cuts across sedimentary rock
units – ex. Dike – a tabular, discordant
pluton
»Concordant – parallel to sedimentary rock
units –ex. Sill – a tabular, concordant pluton
(e.g., Palisades Sill in New York)
• Laccolith
–Similar to a sill
–Lens or mushroom-shaped mass
–Arches overlying strata upward
•Batholith
»Largest intrusive body
»Surface exposure of over 100 square
kilometers (smaller bodies are termed
stocks)
»Frequently form the cores of mountains
Figure 4.28 B
A batholith exposed by erosion
Figure 4.28 C
A sill in the Salt River
Canyon, Arizona
Figure 4.30
Magma Sources and Types
• Mafic magmas produce basalt lavas
– Intrusive equivalent is gabbro
• Intermediate magmas produce andesite
lavas
– Intrusive equivalent is diorite
• Felsic magmas produce rhyolite lavas
– Intrusive equivalent is granite
Figure 5.2
Plate tectonics and igneous activity
• Global distribution of igneous activity is not random
• Most volcanoes are located within or near
ocean basins
• Basaltic rocks are common in both oceanic
and continental settings
• granitic rocks are rarely found in the oceans?
Distribution of some of the
world’s major volcanoes
Figure 4.33
Magma at Divergent Plate Boundaries
• Magma produced at a Divergent Plate Boundary is
typically melted asthenosphere material
• Asthenosphere is extremely rich in ferromagnesian
(ultramafic) and a melt from it is mafic (or ultramafic)
• Basalt is emplaced as new seafloor at the spreading
ridge or a rift
• Rift systems in continental crust may melt granitic
crust and produce andesite or rhyolite lavas
– A bimodal suite of extrusive igneous rocks characterize rift
volcanoes
Magma at Convergent Plate Boundaries
• Magmatic activity at convergent boundaries is
complex
• The composition of the subducted plate
determines the composition of the lava
– Subducted continental crust may melt and
produce rhyolite lava
– Subducted oceanic crust may melt and produce
basalt or andesite lava
– Subduction of sediments derived from the top of
the subducted slab may produce a variety of lavas
Magma at Hot Spots
• Magmas associated with a hot spot volcano in
an ocean basin will produce a basalt lava
• Magmas associated with a hot spot volcano
under continental crust generally will produce
a felsic lava (and often an explosive one)
Volcanism on a tectonic
plate moving over a hot spot
Figure 5.7 some “hot spots”
Locations of Volcanoes
• Seafloor Spreading Ridges
– Most voluminous volcanic activity
– About 50,000 km of ridges around the world
– Mostly under the oceans - except at Iceland
– Generally, harmless mafic fissure eruptions
• Continental fissure eruptions
– Pour out of cracks in lithosphere
– Result in large volume of “flood basalts”
– Columbia Plateau (over 150,000 km2 and 1 km
thick)
– Other locations include India and Brazil
Figure 5.4 – continental fissure eruption
Figure 5.5a Columbia River flood basalts
Figure 5.6 – recently active volcanoes
Hazards Related to Volcanoes
• Lava, the principal hazard? But not lifethreatening generally
• Pyroclastics, more dangerous than lava flows
• Pyroclastic Flows - Nuées Ardentes
• Toxic Gases
• Steam Explosions - Phreatic eruption
– Krakatoa, 1883
• Secondary Effects; Climate and Atmospheric
Chemistry
Lava flow – Heimaey Is., Iceland, 1973
CVO Website - Heimaey 1973 Eruption, Iceland
Pyroclastic s – blew off over 1300 ft , Mount St. Helen’s
- 1980
Pyroclastic flows , nuée ardente on Mount St. Helens
• 5.18
1902 Mount Pelee, Martinique
almost 28,000 casualties of Nuee Ardente
Lahars, a volcanic ash and water mudflow
Armero, Colombia: 1985
Mount Pinatubo eruption, 1991,
Angeles City, Philippines
Figure 5.20 Soufriere Hills volcano
A. ash cloud over Plymouth on Montserrat,, West Indies, 1996
B. by 2003 pyroclastic flows covered much of the mountain
Toxic gases –
• Vog – volcanic fog
– Water vapor, carbon
dioxide, sulfur and
hydrochloric acid
• CO2 –
– Lake Nyos, Cameroon
• Aug. 1986 -suffocated
1700
– Nearby Lake Monoun
• 1984, 37 died
1990 – Mammoth Mountain, CA – tree kill, 1990.
Secondary Effects Climate and Atmospheric Chemistry
Figure 5.23 Mt Pinatubo, 12 June 1991
Mount Pinatubo
The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines, Fact Sheet 113-97
Airborne sulfuric-acid mist formed from SO2
Figure 5.25 Effect of Pinatubo
Predicting Volcanic Eruptions
• Classification by activity
– Active: erupted in recent history (300-500)
– Dormant: no historic erupts but not badly eroded
– Extinct: no historic eruptions and badly eroded
• Not real reliable!
– Vesuvius WAS considered extinct…until it erupted
– Pinatubo WAS dormant, it had not erupted for 400 years
Volcanic Explosivity Index
• Volcanic Precursors
– Seismic activity
Seismic – harmonic tremors – different in character
from earthquakes
– Bulging, tilting or uplift
– Monitoring gas emissions around volcano –SO2
Present and Future Volcanic Hazards in
the United States
• Hawaii: active or dormant volcanoes
• Cascade Range: a series of volcanoes in the
western United States and southwestern
Canada resides above the Pacific Northwest
subduction zone
• The Aleutians: South-central Alaska and the
Aleutian island chain sit above a subduction
zone
• Long Valley and Yellowstone Calderas
Cascade Range volcanoes
Figure 5.30 Mt Rainier - mud flow danger
Figure 5.31 - 51 active Aleutian volcanoes monitored
Redoubt Volcano, Alaska -December 15, 1989
- KLM 747, 4 engines failed, fell 13000 ft and did restart, landing safely with
231 passengers
Redoubt
Long Valley and Yellowstone- collapse calderas
Yellowstone Has Bulged as Magma Pocket Swells
Figure 5.34
Volcano Hazards Program
Volcanoes and Eruptions