Volcanoes
Molten rock
reaches Earth’s
surface
Depending on
viscosity and
temperature, it
either flows out or
explodes
Why do volcanoes
happen?
• Subsurface materials heat up for
various reasons
• Liquid rock is less dense than solid, so it
rises
• Upward force from rising magma and
melting from hot rock meeting cold
produce gaps in overlying rock
• When magma reaches surface, it is
more dense than air, so it stays and
cools
Lava types
• Basaltic lava -- very hot, not very
viscous
– Flood basalts - large areas covered by
basaltic lava, e.g. Columbia River
basalts, Deccan Traps, lunar maria
• Granitic lava -- colder, more
viscous
– Tends to produce explosive eruptions
Columbia River Basalts, WA & OR
Devil’s Tower, WY
Basaltic flows
• Pahoehoe (means "ropy") - highly
fluid lava which has thin, glassy
skin under which hot lava flows
• Aa - forms after gases have
departed and cooling has begun.
Skin is big and chunky -- very
sharp
Basaltic flows
• Pillow basalt -- evidence of
underwater eruptions -- surface
chills quickly, but flow continues
• Bubbles -- or vesicles -- gases exist
in lava but stay in solution under
pressure under earth
Lava Flow, Hawaii
Lava Toe, Hawaii
Pyroclastic Eruptions
• Gas is trapped in magma, but
magma is too viscous to flow
through cracks
• When pressure is released and gas
comes out of magma, whole
mountaintop can explode
• Pyroclasts -- fire rocks
Pyroclastic Eruptions
• Includes ash and fine material, but
can be a lot bigger (one house
sized piece traveled 10 km in one
eruption)
• Ash can stay aloft, entering upper
atmosphere (e.g. Pinatubo)
• If particles settle while still hot,
they form tuffs -- welded together
bits
Pyroclastic Eruptions
• Pyroclastic flow -- big hazard near
continental volcanoes - e.g. Japan,
Mont Pelee on Martinique (1902)
• Pyroclastic flow can be very hard
to predict
– Prof. Landes: "The Montagne Pelee
presents no more danger to the
inhabitants of Saint Pierre than does
Vesuvius to those of Naples" -- died
next day in eruption
Mont Pelee, West Indies 1902
Nuee
Ardente
Pyroclastic
Eruption
Eruption Styles
• Lava Eruptions -- lava cone built by
successive flows from central vent
• Basalt -- creates shield volcanoes
like Mauna Loa - big, broad gentle
slopes
• Rhyolite -- creates small dome in
crater, plugs up areas below
• Pyroclastic eruptions create
concave cone with a summit vent
Mount
Saint
Helens
Washington
Erupted
May 18,
1980
Mount St. Helens
Mount Saint Helens
• Stratovolcano - mixture of lava
eruptions and explosive ash
eruptions
• 1980 eruption was very explosive
• Mountain lost its top 400 meters of
elevation within minutes
Before the eruption
After the Eruption
Mount Saint Helens Mud Flow
Krakatau
August 26, 1883
• Phreatic eruption of an entire
island (English name is Krakatoa)
• Loudest noise in recorded history
(Heard in Australia 2000 km away)
• Eruptive force of 100 million tons
of TNT
• 5000 times the force as the first
atomic bombs
• 36000 people drown in Tsunamis
Anak Krakatau