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An volcano is a mountain from which lava and hot gasses erupt ... Lava from Nyiragongo volcano, inundates Goma, Congo. Goma airport closed by lava flow ...

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volcanism and volcanic hazards

Volcanism and Volcanic Hazards

Bill Menke

September 21, 2005

questions to think about
Questions to think about
  • Which is the most dangerous volcano in the world?
  • What group of people are most endangered by volcanoes
  • 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
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 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
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.
what is magma
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.

global distribution of volcanoes
Global distribution of Volcanoes

About 1500 volcanoes worldwide

how long do eruptions last
How long do eruptions last?

Most eruptions last 10 - 1000 days

Less than 20% over within 72 hours

Median is 7 weeks (1176 hours)











Duration (days)

how often do eruptions occur
How Often do Eruptions Occur?

Cumulative number of eruptions

With eruptive volumes equal to or

Greater than the given amount

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

divergent plate boundaries
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
Convergent Plate Boundaries

Subduction Zones

dewatering of subducted lithosphere (string at 100-150 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 granitic melts

mantle plumes
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 degree of chemical variability, broader range of lava types.

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

Case of Goma, Congo

a 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 Divergent

Plate Boundary

Nyiragongo Volcano


City of




reasons for deaths
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
Hazard 2:Mudflow = Lahar

Mt. Rainier, Washington State

Lots of water locked up in that

summit glacier …


Nevado del Ruis Volcano

Note glacier at summit, valley that channels water


Part of town that was in river valley

Was innundated by several meters

of mud

reason for deaths
Reason for Deaths

Being buried alive in thick, gooey mud

hazard 3 pyroclastic flow nuee ardente
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
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)

May 8, 1902:

Pyroclastic flow

Destroying the

town of St. Pierre


St. Pierre

After the

Pyroclastic flow


(Aside) … The famous Pelean Spine,

350 meters high, was a rock spire

that was pushed up out

Of the crater of Mt. Pele in 1902 as

new lava rose beneath the volcano.

hazard 4 tsunami
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.


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

August 27, 1883:

extremely large explosion

collapse of the volcanic edifice

30 meter high tsunami hits coast of

Java and Sumatra, killing

36,000 people

165 villages totally destroyed


Steamship washed

Inland by 1883 tsunami

reasons for deaths52
Reasons for Deaths


Being crushed by floating debris

hazard 5 ash falls volcanic ash fine rock particles
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
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!

Hazard 6: Global Cooling

Aerosols (particularly sulfate) injected high is the atmosphere

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

Earth’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,


Extremely large VEI=6

eruption in 1815.

May have been the

Largest in 10,000 years

Perhaps 90,000 people

Died worldwide, from


people and volcanoes
People and Volcanoes

Very local hazards

Lava inundation

pyroclastic flow

regional hazards


Explosion induced tsunami

global hazards

Global cooling

Chemical pollution by ash

Island collapse

questions to think about62
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