ENH Exam 2 Part 3 (Volcanoes)

What is A Volcano?

�Earth's internal heat escaping as
molten rock.
-Surface expression of a
deeper system.

1816: A Year Without Summer?

�Eighteen-hundred-and froze-to-death.
�Tambora erupted in 1815.
-June snow in New England.
-August lake/ river ice in PA.

Volcanic Anatomy

�Central vent, crater, pipe, neck.
-Lava: Melt at surface
�Flank vent, parasitic cone.
�Vertical or horizontal fractures.
�Melt reservoir at depth.
-Magma: Melt below surface.

Where Does the Melt Originate?

�There are no massive primordial pools of magma!
�To get magma, rocks must be melted.

�To get magma _____________

rocks must be melted.

How is Rock Melted?

1) Being heated by other rocks/melts.
-Thermal transfer
2) Lower pressure from upward convection.
-Decompression.
3) Freezing-point depressed by water addition.
-Flux.

Tectonics & Melting

�Explains repeated eruption.
�Decompression.
�Flux.
�Thermal transfer.

Changing Magma Chemistry

�By assimilating surrounding rocks.
�By partially crystallizing.
�By mixing with other magmas.

Magma contamination

magma from partial melt of wall rock miss with magma from below

assimilation

blocks of rock fall into magma and dissolve

come back and look at slide 8

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Melt Composition?

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Viscosity

how fluid the magma is - low viscosity= fluid - high viscosity- thick and pasty

Volatiles

the quantity of water vapor and other volcanic gases it contains

Volume

the type and amount of magma that erupts

Silica

differences in viscosity among the major magma types are due mainly to different percentages of silica- In general, the higher the percentage of silica, the more vicious the magma.

Major magma types:

Basalt
Andesite
Rhyolite

Generalized Products of Volcanoes:

Lava
Pyroclastic Material
Lahars

Lava:

Definition- molten magma that flows out and onto Earth's surface
General Characteristics- solidifies as coherent sheets or broken jumbles of volcanic rock

Pyroclastic Material

(Ash, Pumice, Other fragments)
Definition- Fragments of solidified magma blown out of a volcano; may be deposited by a pyroclastic flow or by air-fall ash
General characteristics- Fragments range rfom less than 2 mm ash to tens of cm across; larger pieces

Lahars:

Definition- Volcanic ash and other fragments transported downslope with water (mudflows)
General characteristics- Angular to rounded; unsorted particles from mud to boulders

Angle of Repose

The steepest angle at which any loose material is stable. Different materials stand at different angles of repose, depending mainly on the angularity and size of their grains and moisture content.
Rounded, dry sand grains on a pile will stand no steeper t

Why Do Volcanoes Erupt? (1)

�Melts rise until they approach the surface of the Earth.
�Pressure decreases.
-Lower load from overlying rocks.
�Heat loss increases.
-Less insulation from overlying rocks.

Why Do Volcanoes Erupt? (2)

�In a fully pressurized cooker,
water cannot boil.
-Stays a single fluid.
-Pressure release causes fluid separation.
�As magma approaches the surface, a volatile-rich fluid separates from the melt.
-Mostly water, some CO2, S, Cl, F.

Why Do Volcanoes Erupt? (3)

�As magma cools, crystals begin to form.
-Most of these contain no volatiles.
-Volatile content of melt increases.

Why Do Volcanoes Erupt? (4)

�End result of lower pressure
and temperature.
-Volatile-rich fluid separates and expands
�Fluid leaks upward through fractures.
-Weakens overlying rock.
�Eruptions are volatile-driven.

Viscosity & Volatiles = ?

Explosions

�Temperature and silica (wt % SiO2) control _______

magma viscosity.
Silicate tetrahedra stick together (Think spaghetti)

Basaltic vs. Andesitic vs. Rhyolitic ?

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Shield Volcanoes

�Multiple, fluid lava flows producing long gentle slopes.
(basalt)

Stratovolcanoes

Steep sided: Alternating fluid flows & pyroclastic deposits.
(andesite)

Cinder Cones

�Very steep pile of pyroclastics.
(basalt)

Calderas

Craters that collapse into emptied upper magma chamber.
�From small to very large.

Fissure Eruptions

�Eruption along a fracture.

Large Fissures: Flood Basalts

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Eruptive Material

Lava
Tephra

Lava

�May have ropy or jagged surface.

Tephra

�Also called pyroclasts (fire grains).
-Fine (<2mm): Ash.
-Coarse: Bombs.

Volcanoes & Tectonics

�Tie to divergent & convergent boundaries obvious.

Intraplate Volcanism

�A function of mantle plumes creating hot spots.

Volcanic Hazards: Good News

-Less widespread than earthquakes.
-Direct risk easier to define.
-Short-term prediction easier.
-Precursors clearer.

Volcanic Hazards: Bad News

-Less frequent than earthquakes.
-Easier to ignore or forget.
-Areas impacted often larger.
-Global impact.

Flow Hazards (1): Lava

Property, not personal hazard.
-Meters/day.

Flow Hazards (2): Pyroclastics

�Hot (>400� C) gases, rock, ash traveling >400 km/h.
-Anything in the way is burned then buried.

Ocean Skimming Pyroclastics

�Dense flow sinks, creates steam.
-Lighter flow is buoyed upwards.
-Air hockey puck.

Caribbean Pyroclastic History

�Plymouth, Montserrat, 1997.
�St. Pierre, Martinique, 1902.

Flow Hazards (3): Lahars

�Volcanic mudflows.
-During eruption or years later.

Armero, Colombia: 1985

�Eruption of Nevado del Ruiz melts glaciers
�25000 lost in the middle of the night.

Flow Hazards

Lava
Pyroclastics
Lahars

Airborne Hazards (1): Tephra

�Personal hazards:
-Impact.
-Suffocation/respiratory
damage.

Tephra vs. Property

�Falls like snow, but heavier and won't melt

Air Travel Hazard?

�Airborne ash not visible to aircraft weather radar.
-1989: 747 flies into ash & engines shut down.

Airborne Hazards (2): Gases

�Volcanoes emit gases even when not erupting.
-Mostly H2O, some CO2, H2S, F, etc.

Lake Nyos, Cameroon: 1986

-Water overturn releases gas to flow down sides.
�CO2 builds up in bottom waters of caldera lake.

Airborne Hazards

Tephra
Gases

Climate Hazards

�Very large eruptions add heat, ash and gases to the atmosphere.
�Mt. Pinatubo, 1991

go back and see picture examples in slides 5ish

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Adapting (1): Estimating the Risk

�Estimating risk.
-UNESCO table.
1 for yes; 0 for no.
-Higher score =higher risk.

Adapting (2): Estimating Size (1)

�Volcanic Explosivity Index (VEI).
-Logarithmic & open ended.
-USGS rating scale.
�Based on ejecta volume, cloud height, etc.
-Logarithmic & open ended.
�Eruption magnitude scale.
-Me = log10(mass of erupted materials) - 7.
-Simplifies rating volcanoes th

Adapting (3): Monitoring

�Active study of the volcano and the surrounding area.
-Geological: Mapping & dating previously erupted materials.
�Active study of the volcano and the surrounding area.
-Geochemical: Analyzing rocks, fluids and gases.
-Geophysical: Studying rock characte

Adapting - 3 things?

Estimating the risk
Estimating size
Monitoring
Intervention
Warning
Response

Long-Range Prediction

�Eruption likely within the next 100 years.
-Planning, not warning.

Short-Range Prediction

�Eruption likely within the next 100 days
Actively useful precursors.
-Heat & gas flow.
-Earthquakes.
-Swelling.
Magma doesn't just happen.

Volcanic Seismicity

�Short period.
-Rocks fracturing from pressure.
�Long period.
-Moving fluid stopped.
�Harmonic.
-Rock fractures vibrating.

Volcanic Gases

�Eruptions are pressure driven.
-Gas exsolves from melt.
�Gas chemistry can indicate the introduction of new magma.
-Flow rates can suggest building/lowering pressure.

Volcano Inflation & Tilting

�Evidence of near-surface magma.

Adapting (4): Intervention

Manual diversion of lava flow on Sicily, 1669.
�1935 eruption of Mauna Loa flowed towards Hilo.

Heimaey, Iceland: 1973

�Fissure eruption begins in January.
-Overruns part of town, then threatens harbor.

Cooling the Lava?

Seawater sprayed on the lava to harden it.
-0.7 m3 of rock for 1.0 m3 of water.
�Net effect was debatable.

Adapting (5): Warning

Color-coded warning scales.
-Warning of hazardous conditions.
-Warning of hazardous locations.

Adapting (5): Response

�Volcanic Disaster Assistance Program (VDAP).
-Developed by USGS after Armero
USGS team on call for deployment.
�Volcanic Disaster Assistance Program (VDAP).
-Developed by USGS after Armero.
-Works/advises governments.
-'Volcano SWAT Team'.

Mt. Pinatubo, 1991

�VEI of 6.
�VDAP team advised Philippine government.
-75,000 evacuated.
�VEI of 6.
-Estimated 5000 to 20,000 lives saved.
-Total team cost: About 1.5 million dollars.

Getting Out of the Way

�Star of the Sea Catholic Church.
-Moved 45 minutes before site overrun.