Transcript Volcanism and Volcanic Hazards

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.
V
EI
Descriptio
n
Plume
Height
Volume
Classification
How often
Example
0
nonexplosive
< 100 m
1000s m3
Hawaiian
daily
Kilauea
1
gentle
100-1000 m
10,000s m3
Haw/Stromboli
an
daily
Stromboli
2
explosive
1-5 km
1,000,000s m3
Strom/Vulcani
an
weekly
Galeras, 1992
3
severe
3-15 km
10,000,000s
m3
Vulcanian
yearly
Ruiz, 1985
4
cataclysmic
10-25 km
100,000,000s
m3
Vulc/Plinian
10's of years
Galunggung,
1982
5
paroxysmal
>25 km
1 km3
Plinian
100's of years
St. Helens,
1981
6
colossal
>25 km
10s km3
Plin/UltraPlinian
100's of years
Krakatau, 1883
7
supercolossal
>25 km
100s km3
Ultra-Plinian
1000's of
years
Tambora, 1815
8
megacolossal
>25 km
1,000s km3
Ultra-Plinian
10,000's of
years
Yellowstone, 2
Ma
Global distribution of Volcanoes
About 1500 volcanoes worldwide
How long do eruptions
last?
1000
Eruptions
•
600
200
0.1
1
10 100 1000 10000
Duration (days)
•
•
Most eruptions
last 10 - 1000
days
Less than 20%
over within 72
hours
Median is 7
weeks (1176
hours)
Cumulative number of eruptions
With eruptive volumes equal to or
Greater than the given amount
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
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 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 onceshallow, water-rich crustal rocks to be buried.
They can heat up and melt to produce granitic
melts
Mantle Plumes
Oceanic Hotspots
rising mantle plumes lead to melting that
produces basaltic magmas and Hawaiitype 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)
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
Goma
Lake
Kivu
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 that
summit glacier …
Crate Lake, Oregon
lots of water in that lake ...
Tacoma, Wa
If all that ice
on Mt. Ranier
suddenly
melted, where
would the
water and
mud go?
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
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 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
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 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 Deaths
drowning
Being crushed by floating debris
Hazard 5: Ash Falls
volcanic 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 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,
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
People and Volcanoes
Very local hazards
Lava inundation
pyroclastic flow
regional hazards
mudflow
Explosion induced tsunami
global hazards
Global cooling
Chemical pollution by ash
Island 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