Transcript Volcanoes I and II

Volcanoes: Part One
New Terminology
• Viscosity
– Resistance of a material to flow
– Ex: honey vs. tar
• Volatiles
– Gases dissolved in magma
Makin’ Magma…
• Recall the
asthenosphere…
– Mostly solid—very
close to melting
point—small changes
in T and P can create
melt
• Let’s make some
Magma!
“Liquid Hot Mag-ma…”
• Ways to make magma
– Increasing T
• A solid melts when it becomes hot
enough
– Decreasing P
• Removal of P allows a solid to
expand enough to melt
– Addition of water
• Wet rock melts at lower
temperatures than dry rock
Magma Environments
• Subduction Zones
– Presence of water in subducting crust,
increasing T and (generally low P)
– Generally felsic extrusives
Magma Environments
• Spreading centers
– Ocean plates pull apart, asthenosphere rises
to fill gap—decreasing P causes mantle to
melt--mafic intrusvies and extrusives
Plate motion
Plate motion
Upwelling mantle
Mantle Plumes
• Decreasing P—hot mantle material from
core-mantle boundary rises through
surrounding mantle and melts as it rises
• Mafic extrusives
Types of Volcanoes
Sizes and Shapes
Factors Controlling Size, Shape,
and Type of Eruptive Activity
• Size
– Volume of lava
erupted
– time
• Shape
– Viscosity/composition
• Explosivity
– Viscosity/composition
– Gas content
Basalt
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High in Fe, Mg, Ca
Low in silicon
1000-1200°C
Most common type of melt
Very fluid; up to ~6 mph
Very rarely explosive, mostly “lava rivers”
Deposits generally thin (several meters)
Shield Volcano
• Gently sloped shield-shaped volcano several km
high and 10s of km in diameter
– Built up of many overlapping basalt lava flows
– Mauna Loa is taller than Mt. Everest (10 km) but
most of it is under water
– Common above mantle plumes
Basalt Eruptions: Lava
• Pahoehoe (“ropy”)
– Surface cools but internal parts stay molten
– “Skin” folds as material moves below
Like so
Basalt Eruptions: Lava
• “Aa” – commonly believed to be expletive
after walking on it
– moves faster than Pahoehoe
– Skin torn into jagged blocks which cascade
down front of flow and are over ridden—like
the tread on an earthmover
Aa lava flow
Lava Tubes
• Lavas flowing in channels
• Top slowly hardens over
• Up to 60 km
Basalt Eruptions: Pillow Lavas
• Erupted under water
– Commonly from mid-ocean ridges
– Lava in contact with water cools instantly
– Lava inside remains hot and can break out
(http://www.youtube.com/watch?v=o3BjOapOSGA)
Basalt Eruptions: Fire-fountains
• Occur when large amounts of gas rich lava
erupted
– Bubbles expand as magma moves to
surface—causes lava to surge high into the air
– Erupted lava is partially molten as it hits the
ground—spatter
– At very high eruption rates, spatter
accumulates to form spatter-fed lava flows
– Generally from linear fissures
Whee
Cinder Cone
• Low volume, gas-rich, basaltic eruptions
create fragments of material which cool
quickly in air
– Fragments pile up into cones which exist at the
angle of stability for a random mass of stuff
– Scoria—”lava rock” used in landscaping
– Generally small <300m
– Paricutín (from 2/20/43 to 1952, 424 m)
Convergent Margin
Volcanism
Evolved Lavas
• Partial melting
– Not all minerals have the same melting
temperature
– When a rock is heated, minerals with lowest
melting temperatures melt first—generation of
Na, K, Si rich magmas
– Evolved lavas—high viscosity, high gas
content
• Andesite
• Dacite
• Rhyolite
Stratovolcano
• Also called composite volcanoes
• Composed of alternating layers of
lava(generally andesitic or dacitic
composition) and pyroclastic (ash and
blocks) material
• Conical profile
• Explosive personalities
“Evolved” Eruptions: Explosions
• Imagine if sticky mass plugs top of
volcano…
• gas builds up and
“Evolved” Eruptions: Columns
• Eruption Columns
– Gases separate from magma during ascent due
to decrease in pressure
– Expand to a froth
– Unlike in basalt, viscous magma prevents
complete escape
– Gas explodes violently at ~surface, propels
particles of all sizes into atmosphere
– Column propelled upward because it is buoyant—
high temp + propelling force of exploding gasses
Pyroclastic Material
• Anything shot out of a volcano
• Range of particle sizes
Particles of All
Sizes
Block
Ash-fall
deposit
Note layering
Pyroclastic Flows: Hot stuff, indeed
• Occur when large volumes of material are
erupted--atmosphere cannot support it
– Hot rock and ash flow down sides of volcano
at speeds of up to 160 km/hr (100 mph)
– Hugs valleys and low spots
– Fast (and hot) enough to travel across water
– PF video
Caldera: Supervolcanoes
• Predominantly rhyolite
• Large volume eruptions of gas-rich
magma causes surface to collapse
– Commonly related to mantle plumes beneath
continental crust or thinned continental crust
• Long Valley Caldera – 600 km3
– A football field ~133,000 miles tall
(61% of distance between Earth + Moon)
Caldera Formation
“Evolved” Eruptions: Lava
• Low eruption temperature (compared to
basaltic lava): 600-1000°C
– Often occur after large, explosive eruptions
(the lava is “flat” like a bottle of coke left open
for too long)
– Viscous: lava often has difficulty breaching
crater—lava dome
– If crater breached, very thick lava flows-move
~10 ft/day
• Some flow fronts reach 150 m (or more)
Dacite
Rhyolite
Case Studies
Mt. St. Helens
• May 18th, 1980
• “Bulge” on northern flank—
caused by development of a
crypto-dome—a magma
chamber in the body of the
volcano
• Magnitude 5 earthquake—
unstable N. Flank collapses—
debris avalanche
• MSH height lowered by 1200’
• Left a crater 2 miles wide and
.5 miles deep
• 230 square miles of wilderness
leveled
• 58 people killed
Yellowstone
• Yellowstone national
park—three overlapping
calderas—caldera at
center of park 34 miles
wide/1,500’ deep
• Last eruption--~650,000
years ago
• Repose period-~650,000 years
• Will it erupt in our
lifetime?
• If it did, what kind of
effects would it have?