volcanism and volcanic hazards

7/27/2019 Volcanism and Volcanic Hazards

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Volcanism and Volcanic

Hazards

Bill Menke

September 21, 2005


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Questions to think about

Which is the most dangerous volcano in

the world?

What group of people are most

endangered by volcanoes


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Summary

What is a volcano?

Where does lava come from and why do

volcanoes erupt?

How is size quantified?

Where are volcanoes occur?

How frequently do volcanoes erupt? How do volcanoes cause damage?


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What is an volcano ?

An volcano is a mountain from which lavaand hot gasses erupt


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But I have a semantic problem here

But I dont like the word mountain,

because sometimes lava erupts from

places that have little, if any, topographic

relief

And I dont like the definition leaving out

the notion of temporal persistence, that the

eruptive activity is long-lasting


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So maybe I should use the phrase

Volcanic Center

A region of the earth in which lava and hotgasses have persistently erupted from the

ground over many thousands, or evenmillions, of years.

A single volcanic center may includeseveral related and closely-spacedvolcanoes, or it may contain none.


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Why do Volcanoes Erupt ?


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What is Magma ?

Molten rock with in earth

Chemical composition matters!

Low silica rocks (e.g. basalts) very fluid

High silica rocks (e.g. rhyolites) very viscous

Concentration of gasses (main H20 and CO2)

matters!Gasses under pressure cause explosions

Gasses are poisonous


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Volcano Size

The Volcanic Explosivity

Index, or VEI, was

proposed in 1982 as away to describe the

relative size or magnitude

of explosive volcanic

eruptions. It is a 0-to-8

index of increasing

explosivity. Each increase

in number represents an

increase around a factor

of ten. The VEI uses

several factors to assign a

number, including volume

of erupted pyroclastic

material (for example,ashfall, pyroclastic flows,

and other ejecta), height

of eruption column,

duration in hours, and

qualitative descriptive

terms.


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V

EI

Descriptio

n

Plume

HeightVolume Classification How often Example

0non-

explosive< 100 m 1000s m3 Hawaiian daily Kilauea

1 gentle 100-1000 m 10,000s m3Haw/Stromboli

andaily Stromboli

2 explosive 1-5 km 1,000,000s m3Strom/Vulcani

anweekly Galeras, 1992

3 severe 3-15 km10,000,000s

m3Vulcanian yearly Ruiz, 1985

4 cataclysmic 10-25 km100,000,000s

m3Vulc/Plinian 10's of years

Galunggung,

1982

5 paroxysmal >25 km 1 km3 Plinian 100's of yearsSt. Helens,

1981

6 colossal >25 km 10s km3Plin/Ultra-

Plinian100's of years Krakatau, 1883

7super-

colossal>25 km 100s km3 Ultra-Plinian

1000's of

yearsTambora, 1815

8mega-

colossal>25 km 1,000s km3 Ultra-Plinian

10,000's of

years

Yellowstone, 2

Ma


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Global distribution of Volcanoes

About 1500 volcanoes worldwide


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How long do eruptions

last?

Most eruptions

last 10 - 1000days

Less than 20%

over within 72

hours

Median is 7

weeks (1176

hours)

0.1 1 10 100 1000 10000

Duration (days)

Eruptions 600

1000

200


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How Often do Eruptions Occur?

Eruptions obey Gutenberg-Richter

Statistics: lots of small ones, just

a few big ones.

The graph at the left are for

Explosive eruptions in Kamchatka

During the last 10,000 years

Cumulat

ivenumberoferuptions

Witheru

ptivevolumesequaltoor

Greaterthanthegivena

mount


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Divergent Plate Boundaries

Mid-ocean ridges

strong mantle upwelling to fill in hole, so

lots of basaltic magmatism (almost all of

which is under water)

Continental RiftingThicker crust suppresses mantle melting

somewhat, but can add volatiles


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Convergent Plate Boundaries

Subduction Zones

dewatering of subducted lithosphere (string at100-150 km depth) suppresses the melting point

of mantle rock to produce basaltic magma

Continental Collision

Strong thickening of crust can cause once-

shallow, water-rich crustal rocks to be buried.They can heat up and melt to produce graniticmelts


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Mantle Plumes

Oceanic Hotspots

rising mantle plumes lead to melting that

produces basaltic magmas and Hawaii-

type volcanic islands

Continental Hotspots

Thick lithosphere leads to greater degreeof chemical variability, broader range oflava types.


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Hazard 1: Lava Inundation(rare, but it happens)

Case of Goma, Congo

a city with a population 500,000, locatednear Rwanda border, on shore of Lake

Kivu, affected by lava from NyiragongoVolcano

Jan 17, 2002, 45 people died, 50,000displaced. Eruptions with deaths in 1977,too.


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African Rift

Valley

Continental

Divergent

Plate

BoundaryNyiragongo Volcano


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Lake

Kivu

City of

Goma


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Lava from Nyiragongo volcano,

inundates Goma, Congo


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Goma airport closed by lava flow


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Reasons for deaths

Burns from coming in contact with lava or

hot rock, or from buildings set afire.

High concentrations of poison gases that are

being emitted from the lava.

Destruction of infrastructure that supplies

vital services (water, etc)


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Hazard 2:Mudflow = Lahar

Mt. Rainier, Washington State

Lots of water locked up in that

summit glacier


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Crate Lake, Oregon

lots of water in that lake ...


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If all that ice

on Mt. Ranier

suddenlymelted, where

would the

water andmud go?

Tacoma, Wa


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Nevado del Ruis Volcano

Note glacier at summit, valley that channels water


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Part of town that was in river valley

Was innundated by several meters

of mud


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Reason for Deaths

Being buried alive in thick, gooey mud


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Hazard 3: Pyroclastic Flow

= Nuee Ardente

Hot gasses and dust burp out of the

volcanos summit and cascade downhill at

speeds of 50-100 mph.


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St. Pierre, Martinique

Town of 29,000 people on the shore of a

volcanic island in the Carribean

May 8, 1902: Pyroclastic flow from Mt.

Pele volcano destroys the entire town,

leaning only 1 survivor (a guy in jail)


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Martinique


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May 8, 1902:Pyroclastic flow

Destroying the

town of St. Pierre


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St. Pierre

After the

Pyroclastic flow


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(Aside) The famous Pelean Spine,

350 meters high, was a rock spire

that was pushed up outOf the crater of Mt. Pele in 1902 as

new lava rose beneath the volcano.


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Hazard 4: Tsunami

Explosive volcanism at volcanic island

caused a tsunami in nearby water

Local tsunamis, like the one following the

Krakatau eruption, are the most common.

But ocean-crossing tsunamis are possible.

The ca. 1625 BCE eruption of the Ageanisland of Thera is an example.


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Krakatau


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Note location of

volcano: island in

center of narrow

straight between

two populated land

masses, Java and

Sumatra


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August 27, 1883:

extremely large explosion

collapse of the volcanic edifice

30 meter high tsunami hits coast ofJava and Sumatra, killing

36,000 people

165 villages totally destroyed


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Steamship washed

Inland by 1883

tsunami


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Reasons for Deaths

drowning

Being crushed by floating debris

H d 5 A h F ll


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Hazard 5: Ash Fallsvolcanic ash = fine rock particles

Ash falls over broad area, like snow

weight of ash collapses house

ash makes road impassible

ash-covered grass poisonous

to livestock (e.g. Iceland, 1783)larger chucks of rock (=bombs) also fall


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Luzon, Phillipines

Near Mt Pinatubo

800 people killed by housing collapse

when ash covered their houses

Plenty of warning, but ash does not initially

appear all that dangerous. But wait till its

a meter thick! An it becomes very heavy

and slick during the rain!


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Ash Plume from Mt. Pinatubo


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Ash covered houses near Mt. Pinatubo


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Hazard 6: Global Cooling

Aerosols (particularly sulfate) injected high is the atmosphere

reflect sunlight back to space, resulting in net cooling of the

Earths surface

Unseasonably cold temperatures case crop failures


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1816: The Year without a Summer

June 9-10, 1816: On the 9th, frost

was reported as far south as

Worcester, Massachusetts and on

the 10th to East Windsor,

Connecticut.

July 6, 1816: Temperatures in the

40s F range were reported in

Connecticut at both Hartford andNew Haven. Robbins in East

Windsor noted temperatures

almost cold enough for a frost.


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Tambora volcano (Sumbawa,Indonesia)

Extremely large VEI=6

eruption in 1815.

May have been the

Largest in 10,000 years

Perhaps 90,000 people

Died worldwide, from

starvation


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People and Volcanoes

Very local hazardsLava inundation

pyroclastic flow

regional hazardsmudflow

Explosion induced tsunami

global hazards

Global coolingChemical pollution by ash

Island collapse


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Questions to think about

Which is the most dangerous volcano inthe world?

Versuvio, because of its proximity to Naples, Italy

And because of the very large eruption thatoccurred in 79 CE

What group of people are most

endangered by volcanoes Volcanologists I know several of whom have

died, and nearly all of us have had close calls

volcanism and volcanic hazards

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