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 motionPlate 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

• 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)

• Gently sloped shield-shaped volcano several km Gently sloped shield-shaped volcano several km high and 10s of km in diameterhigh and 10s of km in diameter– Built up of many overlapping basalt lava flowsBuilt up of many overlapping basalt lava flows– Mauna Loa is taller than Mt. Everest (10 km) but Mauna Loa is taller than Mt. Everest (10 km) but

most of it is under watermost of it is under water– Common above mantle plumesCommon above mantle plumes

Shield Volcano

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)(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 Low volume, gas-rich, basaltic eruptions

create fragments of material which cool create fragments of material which cool quickly in airquickly in air– Fragments pile up into cones which exist at the Fragments pile up into cones which exist at the

angle of stability for a random mass of stuffangle of stability for a random mass of stuff– Scoria—”lava rock” used in landscaping Scoria—”lava rock” used in landscaping – Generally small <300mGenerally small <300m– Paricutín (from 2/20/43 to 1952, 424 m)Paricutín (from 2/20/43 to 1952, 424 m)

Convergent MarginVolcanism

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 volcanoesAlso called composite volcanoes

• Composed of alternating layers of lava(generally andesitic or Composed of alternating layers of lava(generally andesitic or dacitic composition) and pyroclastic (ash and blocks) materialdacitic composition) and pyroclastic (ash and blocks) material

• Conical profileConical profile

• Explosive personalitiesExplosive personalities

“Evolved” Eruptions: Explosions

• Imagine if sticky mass plugs top of volcano…

• gas builds up and

“Evolved” Eruptions: Columns

• Eruption ColumnsEruption Columns– Gases separate from magma during ascent due Gases separate from magma during ascent due

to decrease in pressureto decrease in pressure– Expand to a frothExpand to a froth– Unlike in basalt, viscous magma prevents Unlike in basalt, viscous magma prevents

complete escapecomplete escape– Gas explodes violently at ~surface, propels Gas explodes violently at ~surface, propels

particles of all sizes into atmosphereparticles of all sizes into atmosphere– Column propelled upward because it is buoyant—Column propelled upward because it is buoyant—

high temp + propelling force of exploding gasseshigh temp + propelling force of exploding gasses

Pyroclastic Material

• Anything shot out of a volcano

• Range of particle sizes

Particles of All Sizes

BlockAsh-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)

Rhyolite

Dacite

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?