Chapter 5
Volcanoes
Most volcanic activity is concentrated
near plate boundaries
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”
Figure 5.2
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
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 5.3
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 5.4
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
Figure 5.5
Figure 5.6
Figures 5.7 a and b
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 a, b , and c
Figures 5.9 a and b
Figure 5.10 a and b
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 a and b
Figures 5. 11 a, b, c, and d
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 and b
Hazards Related to Volcanoes
• Lava, the principal hazard? But not lifethreatening generally
• Pyroclastics, more dangerous than lava flows
• Lahars, a volcanic ash and water mudflow
• Pyroclastic Flows - Nuées Ardentes
• Toxic Gases
• Steam Explosions
• Secondary Effects; Climate and Atmospheric
Chemistry
Figures 5.1 a and b
Figure 5.14
Figures 5.15 a and b
Figure 5.16
Figure 5.17
Figure 5.18
Figure 5.19
Figure 5.22
Figure 5.23a
Figure 5.25
Figure 5.26
Figure 5.27
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: 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
Figure 5.31
Figure 5.29
Figure 5.32
Figures 5.33 a and b
Figure 5.34
Figures 5.35 a and b
Fig. 5.36 Track of North America
across Yellowstone hotspot