Volcanism and Volcanic Hazards

Bill Menke

September 21, 2005

Questions to think about

• Which is the most dangerous volcano in the world?

• What group of people are most endangered by volcanoes

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?

What is an volcano ?

An volcano is a mountain from which lava and hot gasses erupt

But I have a semantic problem here …

• But I don’t like the word ‘mountain’, because sometimes lava erupts from places that have little, if any, topographic relief

• And I don’t like the definition leaving out the notion of temporal persistence, that the eruptive activity is long-lasting

So maybe I should use the phrase …

Volcanic Center

• A region of the earth in which lava and hot gasses have persistently erupted from the ground over many thousands, or even millions, of years.

• A single volcanic center may include several related and closely-spaced volcanoes, or it may contain none.

Why do Volcanoes Erupt ?

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

Volcano Size

The Volcanic Explosivity Index, or VEI, was proposed in 1982 as a way 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.

VEI

Description

Plume Height

Volume Classification How often Example

0non-explosive

< 100 m 1000s m3 Hawaiian daily Kilauea

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

daily Stromboli

2 explosive 1-5 km1,000,000s m3

Strom/Vulcanian

weekly Galeras, 1992

3 severe 3-15 km10,000,000s m3

Vulcanian yearly Ruiz, 1985

4 cataclysmic 10-25 km100,000,000s m3

Vulc/Plinian 10's of yearsGalunggung, 1982

5 paroxysmal >25 km 1 km3 Plinian 100's of yearsSt. Helens, 1981

6 colossal >25 km 10s km3Plin/Ultra-Plinian

100's of years Krakatau, 1883

7super-colossal

>25 km 100s km3 Ultra-Plinian1000's of years

Tambora, 1815

8mega-colossal

>25 km 1,000s km3 Ultra-Plinian10,000's of years

Yellowstone, 2 Ma

Global distribution of Volcanoes

About 1500 volcanoes worldwide

How long do eruptions last?

• Most eruptions last 10 - 1000 days

• Less than 20% over within 72 hours

• Median is 7 weeks (1176 hours)

0.1 1 10 100 100010000

Duration (days)

Eru

ptio

ns

600

1000

200

How Often do Eruptions Occur?

Eruptions obey Gutenberg-RichterStatistics: lots of small ones, justa few big ones.

The graph at the left are forExplosive eruptions in KamchatkaDuring the last 10,000 years

Cum

ulat

ive

num

ber

of e

rupt

ions

With

eru

ptiv

e vo

lum

es e

qual

to

or Gre

ater

tha

n th

e gi

ven

amou

nt

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 Rifting

Thicker crust suppresses mantle melting somewhat, but can add volatiles

Convergent Plate Boundaries

Subduction Zonesdewatering of subducted lithosphere (string at 100-150 km depth) suppresses the melting point of mantle rock to produce basaltic magma

Continental CollisionStrong thickening of crust can cause once-shallow, water-rich crustal rocks to be buried. They can heat up and melt to produce granitic melts

Mantle Plumes

Oceanic Hotspotsrising mantle plumes lead to melting thatproduces basaltic magmas and Hawaii-type volcanic islands

Continental HotspotsThick lithosphere leads to greater degree of chemical variability, broader range of lava types.

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

Case of Goma, Congoa city with a population 500,000, located near Rwanda border, on shore of Lake Kivu, affected by lava from Nyiragongo Volcano

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

African Rift Valley

Continental DivergentPlate Boundary

Nyiragongo Volcano

LakeKivu

City ofGoma

Lava from Nyiragongo volcano, inundates Goma, Congo

Goma airport closed by lava flow

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)

Hazard 2: Mudflow = Lahar

Mt. Rainier, Washington State

Lots of water locked up in thatsummit glacier …

Crate Lake, Oregonlots of water in that lake ...

If all that iceon Mt. Ranier

suddenly melted, where

would the water and mud go?

Tacoma, Wa

Nevado del Ruis VolcanoNote glacier at summit, valley that channels water

Part of town that was in river valleyWas innundated by several metersof mud

Reason for Deaths

Being buried alive in thick, gooey mud

Hazard 3: Pyroclastic Flow= Nuee’ Ardente

Hot gasses and dust “burp” out of the volcano’s summit and cascade downhill at speeds of 50-100 mph.

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)

Martinique

May 8, 1902: Pyroclastic flowDestroying thetown of St. Pierre

St. PierreAfter thePyroclastic flow

(Aside) … The famous Pelean Spine,350 meters high, was a rock spirethat was pushed up outOf the crater of Mt. Pele in 1902 asnew lava rose beneath the volcano.

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 Agean island of Thera is an example.

Krakatau

Note location of volcano: island in center of narrow straight between two populated land masses, Java and Sumatra

• August 27, 1883:

extremely large explosioncollapse of the volcanic edifice30 meter high tsunami hits coast of

Java and Sumatra, killing 36,000 people

165 villages totally destroyed

Steamship washedInland by 1883 tsunami

Reasons for Deaths

drowning

Being crushed by floating debris

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

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 it’s a meter thick! An it becomes very heavy and slick during the rain!

Ash Plume from Mt. Pinatubo

Ash covered houses near Mt. Pinatubo

Hazard 6: Global Cooling

Aerosols (particularly sulfate) injected high is the atmospherereflect sunlight back to space, resulting in net cooling of theEarth’s surface

Unseasonably cold temperatures case crop failures

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 40’s F range were reported in Connecticut at both Hartford and New Haven. Robbins in East Windsor noted temperatures almost cold enough for a frost.

Tambora volcano (Sumbawa,Indonesia)

Extremely large VEI=6eruption in 1815.

May have been theLargest in 10,000 years

Perhaps 90,000 peopleDied worldwide, fromstarvation

People and VolcanoesVery local hazards

Lava inundationpyroclastic flow

regional hazardsmudflowExplosion induced tsunami

global hazardsGlobal coolingChemical pollution by ashIsland collapse

Questions to think about

• Which is the most dangerous volcano in the world?

• Versuvio, because of its proximity to Naples, Italy• And because of the very large eruption that

occurred 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