ISNS 4359 EARTHQUAKES AND VOLCANOES Spring 2005
Steve Bergman, Instructor
Lecture 13. Continental (sub-aerial) Volcano Architecture
~1,500 Terrestrial volcanoes have been ACTIVE in the last 10,000 years, mostly in well-defined belts along plate boundaries, but also within plate interiors; at any given time 10-30 volcanoes are currently erupting (<100 per year); active phases may be punctuated by hundreds to thousands of years of quietACTIVE (Historic eruptions)- DORMANT (no Historic eruptions)- EXTINCT (not expected to erupt again)
Three Main Tectonic Settings:
S Subduction (most violent), HS Hot Spot (±violent), IC Intracontinental/Rift (least violent.)
Volcanic Systems have many elements = Magma Source, Chamber, Conduit, Vent, Products
Most volcanoes are constructional (mountains); some are excavational (craters, calderas, maars).
More than half of all Volcanoes are basaltic composition:
Monogenetic-erupt once & die (sporadic source melting; small magma supply rate).
Polygenetic- repeatedly erupt (continuous melting; large magma supply rate, magma chamber).
Five Main Continental Volcano Types (forms):
Typical Volcano “Life Spans” = ~ thousands to millions of years comprised of individual eruptive phases lasting weeks to decades; Areas=hundreds to millions km2; heights=0.1-10 km
Composite volcanoes (stratovolcanoes) -steeply sided (25-35°) conical volcanoes with mixed layers of lava flows and pyroclastic rocks (cinders & ash), often with parasitic cones, most hazardous-pyroclastic flows and lahars; cyclic eruptions alternate between explosive and quiet lava (destructional and constructional); variety of compositions (rhyolite-basalt) but mainly andesite (S, IC, HS; eg. Fuji, Izalco, Cera Negro, Etna, Stromboli, Vesuvius, Shasta, Rainier, Nyiragongo, Canaries).
Lava Shield volcanoes-broad, low angle (4-8°) volcano with mainly thin basaltic lava flows and central summit calderas. (HS, IC, eg. Mauna Loa & Kilauea, Iceland, Galapagos, Nyamuragira).
Flood Basalt fields-highly voluminous and extensive basaltic lavas erupted from fissures (HS, eg. Iceland, Snake River Plain, Columbia River Plateau, Deccan, Afar, Parana).
Monogenic volcano fields-clusters or scattered small monogenic basaltic cinder cone volcanoes (10-1000), many with lava flows (S, IC, HS, eg. Auckland NZ, Eifel , Springerville AZ, Pinacate AZ).
Central volcanoes-erupt a variety of magma compositions (basalt and rhyolite), commonly with central calderas. (HS, S, IC eg. Newberry, Skye, Lihir, Taupo, Yellowstone, Iceland).
Lava Flows are the most common volcanic feature on earth.
Lava Flow Rate Ranges (composition)-0.1-100 km/hr (basalt); 0.010-0.1 km/hr (andesite-dacite)
Solidification times of 10-30 m thick basalt flows are ~0.5-4 years.
Lava Flow Surfaces: Blocky-broken; Aa-loose, clinkery, rubbly, rough surface (high flow rate, high viscosity); Pahoehoe-smooth ropey surface (low flow rate , low viscosity).
Historic Lava Flow characteristics: (lavas are typically emplaced in weeks to years, rarely days or decades)
Composition Thickness (m) Length (km) Volume (km3) Discharge Rate (m3/sec)
SiO2<55wt% basalt 3-30 <10-50 0.01-1 10-1000
Lava Domes-highly viscous small volume mounds, typically late-stage plug slowly extruded within a vent crater (rhyolite, dacite, andesite, rarely basalt); Appear meek, but hazardous-dome collapse killed n=19 in 1997 at Montserrat; 66 in 1994 at Merapi, Indonesia; 44 in 1991 at Unzen, Japan.