GEOS 110
NATURAL DISASTERS
 
Volcanoes-I
 
Volcano Basics
 1. Global Distribution
  - most volcanoes occur at divergent and convergent plate boundaries;     also at intraplate hotspots.
  - generally classified as active, dormant, or extinct
 
 2. Magma Composition, Temperature, and Viscosity
  - Magma = molten rock; lava, pyroclastic material
 
  - Composition
   basaltic (mafic): low SiO2 (<50%), dark color
   andesitic (intermediate): SiO2 (~60%),        rhyolitic (felsic): high SiO2 (~70-80%), light color
   Volcanic Gases :
 
  - Temperature erupting magma T°'s range from 600°C to 1200°C.
  - Viscosity- (important concept for volcanic hazards!)
 
 3. Volcano Types
 
  - Shield (e.g. Hawaiian Islands, Iceland)
 
  - Composite or 'stratovolcanoes' (e.g. Mt. St. Helens)
 
  - Caldera Complexes (e.g. Yellowstone)
 
  - Volcanic Domes
 
  - Cinder Cones
 
 
Prof. Gorring         March 2, 1998
GEOS 110
NATURAL DISASTERS
 
Volcanoes-II
 
Volcanic Hazards
 1. Primary Effects: (lavas flows, pyroclastic activity, gas release)
  - Lava Flows
   Can be all compositional types, but basaltic is most abundant.     Basaltic lava flows are not generally life threatening.
 
  - Pyroclastic Hazards
   1) Ash Fall- can cover hundreds of km2, vegetation/crops/      livestock can be destroyed. Structural damage to roofs.
    Respiratory problems. Climatic changes = crop failure.
 
   2) Pyroclastic Flows - avalanche of hot volcanic debris and gas.      Moves at 100 km/hr downslope.
 
   3) Lateral Blast- explosions of gas and ash from the sides. Ejected     materials can travel at the speed of sound.
 
   4) Base Surge- ring-shaped explosion of ash and steam that travels     outward from vent. Occurs when magma reacts with water.
 
  - Volcanic Gas Hazards
   Large releases of gas (mostly CO2) can asphyxiate. SO2 and H2S can     dissolve in water vapor to produce acid rain. Lake Nyos.
 
2. Secondary Effects: (debris and mud flows = "lahars", landslides, floods)
  - Lahars
   Large volume of volcanic debris becomes water-saturated and     moves downslope at high velocity (10's of km/hr). Melting     ice or heavy rain generally are the source of H2O. River valleys on    the slopes are vulnerable. Lahars entering lakes or reservoirs can    cause large waves.
 
Prof. Gorring         Mar. 5, 1998
GEOS 110
NATURAL DISASTERS
 
Volcanoes-III
 
Some Examples of Devastating Volcanic Events
 
1. Pyroclastic Events
 - Vesuvius (79 A.D.)
 Destroyed the Roman city of Pompeii. 20,000 perished. Was intially buried by  ~3m of hot pumice. Pyroclastic flows finished off the 4,000 survivorsgathered at  the beach. Later buried by lahars.
 
 - Mont Pelée, Martinique (May 8, 1902)
 Destroyed the city of St Pierre. 30,000 perished, only 2 survivors (prisoners). Governor had an election on May 10 so there was no evacuation! Massive  pyroclastic flows.
 
2. Caldera-collapse events
 - Krakatoa, Indonesia (August, 27, 1883)
 Tremendous eruption; heard for 3,000 miles. ~20 times larger than Mt St.  Helens eruption. Generated 35 m tsunami that killed 36,000 people.
 
 - Tambora, Indonesia (1815)
 Largest eruption in recorded history (~50 times larger than Mt. St.Helens).   Caused global cooling; 1816 was "year w/o a summer". Between 50,000-90,000  deaths, mostly from tsunami
 
 - Santorini, Aegean Sea (~1620 B.C.)
 Considered to be the site of the Atlantis legend of Plato. May have been the  cause of the demise of the Minoan civilization.
 
3. Lahars
 - Nevado del Ruiz, Colombia (November 10, 1985)
 Minor pyroclastic eruption caused melting of large amounts of snow and ice at  the summit (~17,700 ft). 1-2 hours later massive, cool lahars buried the town of  Armero, killing 22,000. 5,000 managed to escape by running to high ground.
 
 
 
Prof. Gorring         Mar. 16, 1998
GEOS 110
NATURAL DISASTERS
 
Volcanoes-IV
 
Predicting Volcanic Activity
 
 - Long- (10's of years) and intermediate-range (months) prediction is quite good.
 - Short-term prediction (days, hours) is possible (ie. Pinatubo, 1991), but is rare.
 
 1. Seismic Activity
  - Earthquakes occur as magma moves upward into the volcano.
 
 
 2. Ground Deformation
  - Monitoring of changes in volcano topography.
 
 
 
 3. Monitoring of Volcanic Gases
  - Recognizing changes in gas chemistry and emission rate.
 
 
 
 4. Geophysical Monitoring
  - Changes in thermal and magnetic properties of the volcano.
 
 
 
 5. Geological History