Our Geologic Environment Volcanoes

Volcano Anatomy Key Terms: Magma / Lava Magma Chamber Neck / Vent Crater / Caldera Intrusive (Igneous Rocks) Extrusive Lava Cinder Ash

Type I: Shield Volcano Shield Shaped Flat (low-sloping sides) Tall (10,000 m) Caldera and Flank Eruptions Wide (10-100s of km) Passive eruptions, flowing lava Mauna Kea, Hawaii

Shield Volcano Eruptions pahoehoe! Iceland a) Lava is hot b) Lava is basaltic c) Eruptions are not (very) explosive.

Type II: Cinder Cones 1. Cone Shaped 2. Short (100s of meters) 3. Narrow (1-2 km) 4. Steep (up to 30-40o) 5. Built of Rhyolitic Cinder Blocks 6. Highly explosive, sticky lava Black Butte, California

Type III: Composite Volcanoes Mt. Mayon, Philippines A mix of shield and cinder cone characteristics

Composites– The Danger Mt. Fuji, Japan Mt. Pinatubo, Philippines Long spells of inactivity Some eruptions are passive Lava: andesitic, lower temperature, more gas Pressure builds beneath surface, released in explosions

Mount St. Helens, Washington Before After May 18, 1980 Mt St Helen’s Eruption 1980

Magma Composition is Key Basaltic Andesitic Rhyolitic 1. More SiO2 (silica) = More viscous magma a) Sticky… won’t flow b) Traps gas… buoyant 2. The more viscous the magma, the more explosive the eruption

Volcanoes & the 3 Plate Boundary Types 2. Convergent: andesite volcanoes 1. Divergent: basalt flows 3. Transform: little volcanic activity

#1 Most Common Location: Mid-Ocean Ridge #2 Subduction Zones (The “Ring of Fire”) … where seafloor is being separated… volcanoes! Volcanoes Hawaii … and where it is being subducted too.

And #3: Hotspots … about two dozen spots Plates move over spots making multiple volcanoes

Volcanic hazards Incineration Burial Gases Ash (tephra) Lava Pyroclastics… nuee ardente Lahars Gases Ash (tephra) Tsunami (Krakatoa 1884 – 120 ft)

The story of Heimaey Iceland is atop a spreading center and a hotspot Heimaey Island

Haemey, Iceland, 1973 January 23 USGS

Heimaey Harbor Making lava dams New basalt flow USGS

Vesuvius Quiet for 100’s of years until…... infamous eruption 79 A.D. In Pompeii Hot poisonous gases Buried by 6 m of ash uncovered in 1700’s ~3000 lives lost Herculaneum buried by 60 m of mudflows

People and Volcanoes, ca. 79CE Casts of animal and people killed by gases, buried by ash. Preserving a snap-shot of Roman life.

Cascade Volcanoes Mt. St. Helens, 1980

Lahars can be deadly Armero, Columbia, 1985. Volcanic mudflow from Nevado de Ruiz buried >23,000 people

Mt. Rainier Lahar Map Tacoma Mt. Rainier from Tacoma Small lahars Moderate lahars Large lahars Lava/ash flows

Volcanic gas hazards CO2 gas built up in lake in volcano crater Gases escaped, heavier than air Suffocated sleeping people Lake Nyos, Cameroon, 1986. 1700 people and 3000 cattle died.

Boiling Springs Temps 180oF and up. Color from Algae minerals

Volcanic Ash Ash cloud, Ephrata WA May 18, 1980 Ephrata WA, 145 miles from Mt. Saint Helens

St Helens Ash Map

Volcanic SO2 and ash circles Earth SO2 gas forms sulfuric acid droplets. Ash and droplets in atmosphere reflect sunlight

Volcano-induced climate change Temperature change ( ºC) Mt. Pinatubo eruption Global temperature dropped by ~0.2º C for 2 years after eruption

Hazards reviewed Basaltic volcanoes: lava Andesitic volcanoes: Relatively predictable, don’t cover large area, easy to get away from Andesitic volcanoes: Explosive, nuee ardentes move rapidly, cover large area quickly Mudflows Ash (tephra)

How to reduce hazard? Volcanic hazards maps based on geologic data regulate human activity in hazard areas best thing to do is not to live so close to volcanoes Predict eruptions Earthquake swarms Unusual ground tilting, “inflation” Gas emissions

US volcanic hazards No volcanoes here! Based on past activity. Brown: high; green, blue, beige – decreasing hazard Based on past activity. Active volcano: eruption within past 500 years Dormant: eruption > 500 but < 10,000 years ago. Extinct: last eruption > 10,000 years ago