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Volcanoes
Figure 5.4 Volcanoes of the World
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
• Volcanoes are generated at:
– Divergent Plate Boundaries
– Convergent Plate Boundaries
– “Hot Spots”
Figure 5.2 Relationships of volcanic activity to
plate tectonics.
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
Figure 5.3 Common volcanic rock types (bottom labels) and their plutonic equivalents
(top). The rock names reflect varying proportions of silica, iron, and magnesium, and
thus of common silicate minerals. Rhyolite is the fine-grained, volcanic compositional
equivalent of granite, and so on.
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
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)
Figure 5.5 Selected prominent hot spots around the world. Some
coincide with plate boundaries; most do not.
Figure 5.6 Schematic diagram of a fissure eruption. (At a spreading
ridge, the magma has generally solidified before it can spread very far
sideways at the surface, quenched by cold seawater.
Types and 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
Figures 5.7 a and b Flood basalts (A) A real extent of Columbia River
flood basalts. (B) Multiple lava flows, one atop another, can be seen in
an outcrop of these flows in Washington state.
Types and Locations of Volcanoes
• Shield volcanoes
–
–
–
–
Very large, flat, with abundant thin basalt flows
Basalt is less viscous than andesite or rhyolite
Shield like shape - larger area relative to height
Examples: Hawaiian Island chain
• 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
Figures 5.8 Shield
volcanoes and their
characteristics
(B) Very thin lava flows, like these of Kilauea in
Hawaii, are characteristic of shield volcanoes.
(A) Schematic diagram of a
shield volcano in cross
section.
(C) Fluidity of Hawaiian lavas is evident even after they have solidified. This ropytextured surface is termed pahoehoe (pronounded “pa-hoy-hoy).
Figures 5.9 Mauna Loa, an example of a shield volcano.
(B) Bird’s-eye view of Hawaii, taken by Landsat satellite, shows
its volcanic character more clearly. The large peak with
abundant relatively fresh, dark lava flows surrounding it is
Mauna Loa; the smaller one, above it, is Mauna Kea.
(A) View from low altitudes, Note
the gently sloping shape summit
caldera has been enlarged by
collapse. The peak of Mauna Kea
rises at rear of photograph.
Figure 5.10 Volcanic dome formation.
(A) Schematic of volcanic
dome formation. (B)
Novarupta dome, Katmai
National Park, Alaska.
Types and Locations of Volcanoes
• Cinder Cones
– Minor explosive volcano
– Batches of lava shot into the air as pyroclastics
– Size of pyroclastics range from ash (very fine),
cinders, bombs, or blocks (very coarse)
– Pyroclastics fall close to the vent creating a cone
shaped volcano
– Example: Particutin, Mexico
Figures 5.12
Paricutin (Mexico), a classic cinder
cone. (A) Night view shows formation
by accumulation of pyroclastics flung
out of the vent.
(B) Shape of the structure revealed by day is
typical symmetric form of cinder cones.
Figures 5. 11 Include types of pyroclastics (which sometimes are produced
even by the placid shield volcanoes). Bombs are molten, or at least hot enough
to be plastic, when erupted, and may assume a streamlined shape in the air..
(A)
Volcanic
ash from
Mount St.
Helens
(B) Bombs
from Mauna
Kea
(C) Blocks
from Kilauea
(D) is volcanic
breccia (at Mt.
Lassen) formed
of welded hot
pyroclastics
Types and Locations of Volcanoes
• Composite Volcanoes (Stratovolcanoes)
are built up of layers of lava and pyroclastics
– Mix of lavas and pyroclastic layers allows for a
tall volcano to form
– Usually associated with subduction zones
– These tend to be violent and explosive
– Example: Mount St. Helens, Cascade Range,
Northwest U.S.A.
Figures 5.13
(A) Schematic cross section of a
stratovolcano (composite volcano),
formed of alternating layers of lava
and pyroclastics.
(B) Two composite volcanoes of the Cascade
Range: Mount St. Helens (foreground) and Mt.
Rainier (rear); photograph predates 1980
explosion of Mount St. Helens.
Hazards Related to Volcanoes
•
•
•
•
•
•
•
Lava
Pyroclastics
Lahars
Pyroclastic Flows - Nuées Ardentes
Toxic Gases
Steam Explosions
Secondary Effects; Climate and
Atmospheric Chemistry
Figure 5.14 Formation of “lava trees” near Kilauea illustrates the effect
of quenching lava. As hot lava hits cooler trees – and moisture in trees
evaporates, absorbing more heat – lava is quenched and hardened.
Main mass of fluid lava flows on, leaving the lava trees.
Figures 5.15 Impact of lava flows on Heimaey, Iceland. (A) May
showing extent of lava filling the harbor of Heimaey after 1973 eruption.
(B) Lava flow control efforts on Heimaey.
Figure 5.16 Aftermath of Mount St. Helens eruption, 18 May, 1980
Figure 5.17 Volume of
pyroclastics ejected
during major explosive
eruptions. (numbers of
casualties, where
available, are given in
parentheses).
Figure 5.18 The combination of large volumes of ash and heavy
typhoon rains at Mount Pinatubo in 1991 proved too much weight for
many buildings to bear, In fact, roof collapse was responsible for most
of the casualties.
Figure 5.19 Town of Amero was destroyed by lahars from Nevado del
Ruiz in November 1985; more than 23,000 people died.
Figure 5.22 St. Pierre, Martinique, West Indies, was destroyed by a
nuee ardente (pyroclastic flow) from Mont Pelee, 1902
Figure 5.23 (A) by the time this ash cloud loomed over Plymouth on 27
July 1996, the town had been evacuated; the potential for pyroclastic
eruptions of Soufriere Hills volcano was well recognized.
Figure 5.25 Ash and
gas from Mount
Pinatubo was shot into
the stratosphere, and
had an impact of climate
and atmospheric
chemistry worldwide.
Eruption of 12 June
1991
Figure 5.26 Satellites tracked the path of the airborne
sulfuric-acid mist formed by SO2 from Mount Pinatubo;
winds slowly spread it into a belt encircling the earth
Figure 5.27 Effect of 191 eruption of Pinatubo on near-surface
(lower-atmosphere) air temperatures. Removal of ash and dust
from the air was relatively rapid; sulfate aerosols persisted longer.
The major explosive eruption occurred in mid-June 1991.
Predicting Volcanic Eruptions
• Classification by activity
– Active: erupted in recent history
– Dormant: no historic erupts but not badly
eroded
– Extinct: no historic eruptions and badly
eroded
• Volcanic Precursors
– Seismic activity
– Bulging, tilting or uplift
– Monitoring gas emissions around volcano
Figure 5.28
Present and Future Volcanic
Hazards in the United States
•
•
•
•
Hawaii
Cascade Range
The Aleutians
Long Valley and Yellowstone Calderas
Figure 5.21 Pyroclastic flow
from Mount St. Helens
Figure 5.31The cascade Range volcanoes and their spatial
relationship to the subduction zone and to major cities. Shaded area is
covered by young volcanic deposits less than 2 million years old.
Volcanic symbols on chart at right indicate dates of significant
eruptions.
Figure 5.29
Restricted-access zones
established by the Washington
Department of Emergency
Services before 18 May 1980
eruption of Mount St. Helens.
Red zone: No access except by
scientist, law-enforcement
officials, and search-and-rescue
personnel. Blue zone: Logging
permitted, and residents with
permits allowed access, but no
overnight stays. Shaded area is
national forest land. Casualties
would have been still fewer if
unauthorized people had not
sneaked into the restricted
areas.
Figure 5.32
Over the last 5000 to
6000 years, huge
mudflows have poured
down stream valleys
and into low-lying
areas, tens of miles
from Mount Rainier’s
summit. Even Tacoma
or Seattle might be
threatened by renewed
activity on a similar
scale.
Figures 5.33 a and b
The Aleutians are a
region of active
volcanism – fortunately,
a rather sparsely
populated one. (A) Map
of southwestern Alaska
showing major volcanic
peaks. Mount Spurr is
showing signs of
renewed activity as this
is written. (B) Mount
Veniaminof in eruption;
note lahars formed
when hot ash meets
snow.
Figure 5.34
Map of Mammoth
Lakes area, showing
Long Valley Caldera,
site of recent
earthquakes, and
area of recent uplift
beneath which
magma is rising.
Figures 5.35 a and b (A) Continuing thermal activity at
Yellowstone National Park is extensive, (B) The size of the
caldera and scale of past eruptions cause concern about
the future of the region.