Volcanic activity sites along boundaries
Where do volcanoes occur—and why?
• Divergent [MOR] boundaries
– About 75% of erupted lava is found here
– Basaltic magma; low viscosity, therefore it flows easily!
– Oceanic crust ranges from about 2 km to 10 km thick
• Convergent plate boundaries
– Forms about 10% of all erupted magma (on continents or ocean floor)
– Ocean-continent or ocean-ocean plate collisions
– Volcanoes contain mainly andesite, intermediate in composition
• Isolated hot spots
– Under oceanic plates - Hawaii
– Under continental plates – Yellowstone NP
– At plate boundaries - Iceland
Lithosphere and asthenosphere
Volcanic features
• Composite volcano
• Basaltic lava flow
Volcanoes in Ring of Fire
Hawaiian Islands and seamounts
Distance vs age in Hawaiian Islands
Products of volcanic activity
Magma becomes lava and other products of eruption
• Lava flows
– Basalt flows faster and farther
– Higher silica content means slower flowing lava, such as
rhyolites
• Pyroclastic debris
– Due to high gas content of magma
• Volcanic gases
– Many not-so-nice gases
Components of Volcanic Eruptions
• Role of gases
• Volcanic explosivity
• Mineral composition
– Role of silica content
– Presence of different elements
Layers of basalt along Columbia River
Mauna Loa, HI, shield volcano
Cinder cone
• General structure of cinder
cone
• Sunset Crater, AZ
Composite or stratovolcano
Domes
Elden Mt in Flagstaff
Dome inside Mt St Helens
Observe flow down hillside
Caldera
Lake Atitlan in central Guatemala
Types of Volcanic Hazards
•
•
•
•
•
•
•
Gas emissions
Lava flows
Lahars
Pyroclastic flows
Directed blasts
Landslides and tsunami
Earthquakes
Gas emissions
•
•
•
•
•
•
•
Up to 9% of composition
Water vapor (ranges from 50 to >80%)
Carbon dioxide
Sulfur dioxide
Hydrogen sulfide
Lesser gases – HCl, HF, Cl
Creates vesicles when trapped in lava
Fig. 7.05a
W. W. Norton
Consider distribution of
fallout material away from
source
Lava flows
• Extruded magma creates lava
• Movement controlled by composition and
temperature
• More basaltic, greater flow due to simple
chemistry (most common type)
• Can flow hundreds of kilometers
Hawaii basalt lake
Lahars due to snowmelt
Nevado del Ruiz, Colombia
Toutle River, near Mt St Helens
Pyroclastic flows
Combination of gases, ash and other debris
Up to 100s of meters thick
Can travel at >200 km per hour
Mount Rainier, WA
Activity around Mt Rainier, WA
Directed blasts
Landslides and tsunami
Earthquakes
• Caused by rising magma
• Usually small
magnitude events
• Mostly shallow source
Human toll through history
•
•
•
•
•
•
Mount Vesuvius, Italy 79 AD
Mount Tambora, Indonesia 1815 (famine)
Krakatua, Indonesia 1883 (tsunami)
Mount St. Helens 1980 (last in continental US)
Lake Nyos 1986 – expulsion of carbon dioxide
Mount Pinatubo, Philippines 1991
Many others listed in Table 3.3 (p.72 of text)
Events are Mt St Helens
Look at the stages of development of
the eruption
What were the precursors?
Relatively few died (about 60)
Precursors to help mitigate damage
• Active, dormant, or extinct structures
• Possible signals of eruptions
– Seismic activity changes
– Surface heat
– Surface bulge
– Gas emissions
Type of
volcano
Viscosity
Volatiles
(fluidity)
(gas content)
Volume
Examples
Shield
Low
Low
Large
Hawaii
Flood basalt
Low
Low
Very large
Deccan,
Columbia
River
Scoria
Medium
Medium
Small
Paricutin,
Stromboli
Stratovolcano
(composite)
High
High
Large
Vesuvius,
Rainier,
Fujiyama
Caldera
High
High
Very large
Crater Lake,
OR, Krakatoa
Lava domes
High
Low
Small
In crater of Mt
St Helens;
Mt Elden
• Sample Questions
– Convection cells in the mantle rise and generate
what feature on the ocean floor above the cells? (a)
continental plates, (b) active subduction, (c)
convergent boundaries, (d) the mid-ocean ridge
– The concept of plate tectonics is classified as a(n):
(a) theory, (b) hypothesis,(c) law or principle, (d)
unaccepted idea.