Volcanic Hazards in Latin America - The University of Texas at Dallas

Сomentários

Transcrição

Volcanic Hazards in Latin America - The University of Texas at Dallas
Volcanic Hazards in
Latin America: Nueé
Ardentes & Lahars
Circum-Pacific “Ring of Fire”
Could also be called CircumPacific “Ring of Earthquakes”
“This Dynamic Earth” http://pubs.usgs.gov/publications/text/fire.html
Subduction Zone beneath W.Latin
America & Lesser Antilles
Seismogenic Zone
Earthquakes occur all along the subduction zone but those
along the ‘Seismogenic zone are most destructive
Normal (Steep) Slab
Flat Slab
Flat Slab
regions
allow for
more
stress to
build up
between
the plates
(Gutscher
et al.,
2000)
Flat Slab
segments
lack arc
volcanoes
Trans-Mexico Volcanic Belt
Central
America
Lesser
Antilles
Island Arc
FS
Andes N. Volcanic Zone
FS
FS
Andes Central
Volcanic Zone
Andes S. Volcanic Zone
Flat Slab
segments
in S.
America
(Gutscher
et al.,
2000)
Top 5 Killer Volcanic Eruptions, World
Since 1500AD, about 300,000 people have died as a result
of volcanic eruptions
Volcano
Country
Year
1) Tambora Indonesia
1815
2) Krakatau Indonesia
1883
3) Mt. Pelée Martinique
1902
4) Nevado del Ruiz Colombia1985
5) Unzen
Japan
1792
# deaths
92,000
36,000
29,000
25,000
15,000
Cause
Starvation
Tsunami
Pyroclastic flow
Mudflow
Debris avalanche
Deadliest eruptions in Latin America
Volcano
Country
Nevado del Ruiz
Colombia
Cotopaxi
Ecuador
Soufrière
St. Vincent (L. Ant.)
Mount Pelée Martinique (L. Ant.)
Santa Maria
Guatemala
El Chichón
Mexico
Nevado del Ruiz
Colombia
„
Year
1845
1877
1902
1902
1902
1982
1985
Deaths
1,000
>300
1,680
29,000
~1,500
>2,000
25,000
http://volcanoes.usgs.gov/Hazards/Effects/Fatalities.html
Martinique,
1902
Martinique,
Lesser Antilles
1902
Volcanoes of the
Lesser Antilles
http://www.mountpelee.com/arcvolc.htm
Martinique
„
„
„
„
Colonized by France
in 1635, the island has
subsequently
remained a French
possession except for
three brief periods of
foreign occupation.
1100 sq. km (424 sq
mi.)
430,000 people
Highest point is Mt.
Pelee, 1400 m
Précheur
*
M.R.
St. Pierre
M.R. = Morne Rouge
http://www.mount-pelee.com/martinique.htm
St. Pierre before 1902 eruption
Pre-1902
2002
http://www.mount-pelee.com/martinique.htm
St. Pierre before its Destruction
St. Pierre was the economic center of Martinique
http://www.mount-pelee.com/martinique.htm
Pelee chronology
„
„
„
During the year 1889, the reactivation of fumaroles is
reported at the summit.
In February 1902, strong sulfur smells are perceived,
first in the vicinity of Prêcheur, then in Fond Coré
(North Saint-Pierre).
April 1902
–
–
–
–
April 22nd, the telegraph cable between Martinique and
Guadeloupe is broken.
April 24th, a column of black smoke arises from the crater, up
to a height of 1600-1900 ft.
April 25th, the village of Prêcheur is covered with ash.
April 28th, strong rumblings are heard, as the vapor column
becomes higher.
Pelee Chronology (cont’d.)
„
May 1902
–
–
–
–
–
May 2nd, abundant ashfalls and dull rumblings can still be
heard. The black column of vapor is furrowed with lightning.
The first ashfall is reported on St.Pierre, and a thin layer of
dust covers the whole island.
May 3rd, total darkness over Prêcheur. Many inhabitants seek
refuge in St. Pierre.
May 5th, one of the sides of the Etang Sec Crater suddenly
falls apart, giving way to a devastating mudflow which destroys
the sugar mill Guérin on the coast between St.Pierre and
Prêcheur. The first 25 deaths are reported.
May 6th, the telegraph cable between St.Pierre and St.Lucia is
broken. Numerous glowing phenomena are observed over the
crater.
May 7th, houses collapse and trees break down under the
weight of the ash.
Why didn’t the people of St. Pierre evacuate?
„
„
No volcanologists were on the island, and for
the residents the only point of reference was
the mild eruption of 1851-1852.
The governor appointed a "Volcano
Commission" with the most qualified
specialists on the island : doctors,
pharmacists, science teachers... The first
meeting was held May 7th (the day before
the city was destroyed)
First (and only) report of the St.
Pierre ‘Volcano Commission’
„
The commission, responsible for the study of the
Mount Pelée's volcanic phenomena met yesterday
evening, May 7th, under the chair of the Governor.
After a careful analysis of the facts, the commission
declares that:
–
–
–
1°All the phenomena which have occurred so far are
normal, and are commonly observed on all volcanoes
around the world;
2°Since the craters of the volcano are wide open, the
expansion of the vapors will continue with no earthquake or
rock projection;
3°According to the location of the craters and the position
of the valleys leading to the sea, the City of St. Pierre is
perfectly safe.
Some people evacuated
Some people were afraid of the powerful phenomena which
took place between April and May 7th 1902. Some
families moved away from the volcano, but those
voluntary evacuations were quite limited. The travel
records in the early morning of May 8th - a religious
holiday-- shows that there were more people traveling to
St. Pierre than people leaving the city !
The story of Captain Leboffe of the Italian bark Orsolina
suggests that the population of St. Pierre would have
been saved if they had some knowledge about explosive
volcanoes. The Italian captain was a native of Naples,
and was quite familiar with the Vesuvius volcano. He
hastily left the harbor of St. Pierre on May 7th, without his
custom clearance, warning that "if Vesuvius looked like
Mt. Pelee did, Naples would have been evacuated"..
http://www.mount-pelee.com/Ifacts.htm#Haunted
Types of Eruptions: Vulcanian
„
A vulcanian eruption is a type of
explosive eruption that ejects new lava
fragments that do not take on a rounded
shape during their flight through the air.
This may be because the lava is too
viscous or already solidified. These
moderate-sized explosive eruptions
commonly eject a large proportion of
volcanic ash and also breadcrust bombs
and blocks. Andesitic and dacitic
magmas are most often associated with
vulcanian eruptions, because their high
viscosity (resistance to flow) makes it
difficult for the dissolved volcanic
gases to escape except under extreme
pressure, which leads to explosive
behavior.
http://volcanoes.usgs.gov/Products/Pglossary/vulcanian.html
Types of eruptions: Strombolian
„
Strombolian eruptions are
characterized by the intermittent
explosion or fountaining of basaltic
lava from a single vent or crater.
Each episode is caused by the
release of volcanic gases, and they
typically occur every few minutes
or so, sometimes rhythmically and
sometimes irregularly. The lava
fragments generally consist of
partially molten volcanic bombs
that become rounded as they fly
through the air. The photo at right
is a close view of Stromboli
Volcano erupting incandescent
molten lava framgents.
http://volcanoes.usgs.gov/Products/Pglossary/strombolian.html
Types of Eruptions:
Plinian
„
„
Plinian eruptions are very explosive
events that form enormous dark
columns of tephra and gas that rise
high into the stratosphere (>11 km).
Such eruptions are named for Pliny
the Younger, who carefully
described the disastrous eruption of
Vesuvius in 79 A.D. This eruption
generated a huge column of tephra
into the sky, pyroclastic flows and
surges, and extensive ash fall.
This is a photograph of a pliniantype eruption of Mount Spurr,
Alaska. This sent an eruption column
to a height of about 18 km above sea
level
http://volcanoes.usgs.gov/Products/Pglossary/PlinianEruption.html
What is tephra?
„
Tephra is a general term for
fragments of volcanic rock
and lava regardless of size
that are blasted into the air by
explosions or carried upward
by hot gases in eruption
columns or lava fountains.
Tephra includes large dense
blocks and bombs, and small
light rock debris such as
scoria, pumice, and ash.
http://volcanoes.usgs.gov/Products/Pglossary/tephra.html
Pyroclastic flow
„
„
„
A pyroclastic flow is a ground-hugging
avalanche of hot ash, pumice, rock
fragments, and volcanic gas that rushes
down the side of a volcano as fast as 100
km/hour or more. The temperature within a
pyroclastic flow may be greater than 500° C,
sufficient to burn and carbonize wood. Once
deposited, the ash, pumice, and rock
fragments may deform (flatten) and weld
together because of the intense heat and the
weight of the overlying material.
Pyroclastic flows are sometimes called
‘Nueé Ardentes’, French for ‘glowing
avalanche’ and are associated with Plinian
eruptions
Photo at right is a pyroclastic flow sweeping
down the side of Mayon Volcano,
Philippines, 15 September 1984. Note the
ground-hugging cloud of ash (lower left)
that is billowing from the pyroclastic flow
and the eruption column rising from the top
of the volcano.
Pyroclastic flows descend the flank of Mayon Volcano, Philippines. Maximum height of the
eruption column was 15 km above sea level, and volcanic ash fell within about 50 km toward the
west. There were no casualties from the 1984 eruption because more than 73,000 people
evacuated the danger zones as recommended by scientists of the Philippine Institute of
Volcanology and Seismology. http://volcanoes.usgs.gov/Imgs/Jpg/Mayon/32923351020_caption.html
Colima (Mexico) Nueé Ardente
Aerial view of Colima Volcano showing an eruption of a pyroclastic flow. The nueé reached a
maximum distance of 4.5 km from the summit. Photograph courtesy of Abel Cortes, Colima
Volcano Observatory,University of Colima, November 22, 1998.
Pelee May Chronology (cont’d.)
May 8th, at around 3:00 am, mudflows destroy houses in
Grand-Rivière, Macouba and Basse-Pointe. At 5:00 am, a
mudflow floods Prêcheur, carrying away 800 people. At
8:02 am, St. Pierre is destroyed by a nuée ardente
(glowing avalanche) devastating an area of approx. 22 sq
miles.
All members of the ‘Volcano Commission’ were killed
Only 2 people who were actually inside the city survived the
blast : Cyparis and Leon Compère. Many others who were
on the boats anchored in the harbor, or in the
neighborhoods around St. Pierre also got caught in the
blast and killed. As a matter of fact, the only people who
survived the blast (in its direct path), had a shelter inside a
building, a jail cell, or in a boat, where the contact with the
intense heat of the nuee ardente was limited
St. Pierre before and after May 8, 1902
http://www.mount-pelee.com/Ifacts.htm#Haunted
Cyparis the Survivor
“Cyparis said that the cell he occupied in the St. Pierre prison was an
underground dungeon, which had no other window than a grated aperture
in the upper part of the door. On the morning of May 8th, while he was
waiting for breakfast, it suddenly grew very dark ; and almost immediately
afterward hot air, mixed with fine ashes, came in through the door-grating
and burned him. He rushed and jumped in agony about the cell and cried
for help ; but there was no answer. He heard no noise, saw no fire, and
smelled nothing except "what he thought was his own body, burning." The
intense heat lasted only a moment, and during that time he breathed as
little as possible. There was no smoke in the cell and the hot air came in
through the door-grating without any noticeable rush or blast. He had on,
at the time, hat, shirt, and trousers, but no shoes. His clothing did not take
fire, and yet his back was very severely burned under his shirt.”
Cyparis got a contract to show his burns in the Barnum & Bailey American
circus
http://www.mount-pelee.com/Ifacts.htm#Haunted
The Cell of Cyparis the Survivor
http://www.mount-pelee.com/Ifacts.htm#Haunted
Witness accounts of the May 8 eruption
„
„
On the edges of the pyroclastic flow, the heat wasn't high enough and the
blast wasn't strong enough to kill instantly or to cause heavy damage. The
account of Mr Lassère and Mr Simonet, travelling from St. Pierre to MorneRouge on a mule-drawn carriage:
"They saw it (the black cloud of the volcano) coming, just as they began the
ascent of the Grande Reduit, and lashed their mules into a gallop with the
hope of escaping it ; but the eastern edge of it caught them about sixty feet
from the crucifix on the top of the hill. It approached with a roaring sound,
struck them like a mighty wind, overturned and wrecked their carriage, setting
the mules free, and then swept across the summit of the Grande Reduit,
leaving them stunned, burned and half dead in the shattered vehicle. They
saw no flame or fire, did not notice anything like suffocating gas, and smelled
nothing except what they described as the "odor of smoke from lava." Both felt
the intense heat of the blast as it swept over them, but Mr. Lassère did not
realize that he was seriously burned until he crawled out of the wrecked
carriage. Darkness became quickly afterward, but they succeeded in getting
into a small deserted house near the shrine, and there they waited until it
became light enough so that they could walk to Morne Rouge. Their clothing
showed no signs of injury from heat, but their backs were badly burned or
scalded. The skin peeled off their hands so that it hung from them in strips,
and when they arrived at Morne Rouge, their shoes had to be cut from their
burned and swollen feet."
http://www.mount-pelee.com/Ifacts.htm#Haunted
Nueé Ardente
1902,
Martinique
Pelee chronology (continued)
May 20th, a new paroxysm throws a dense ash plume over Fort-de-France.
May 26th, and 28th, new violent eruptions are reported.
June 6th, a new eruption sends a pyroclastic flow down the river Blanche valley
and a large ash plume covers the island. Ashfalls are reported in St. Lucia.
July 9th, a new nuée ardente travels on the south-western side, and it is
followed by several vertical explosions.
August 30, a new avalanche partly destroys the villages of Morne-Rouge, and
Ajoupa-Bouillon, reaching quarters in Basse-Pointe and Lorrain. The
phenomenon claims another 1,000 lives. The area destroyed is approx. 44 sq
miles.
After August 30th, volcanic activity continues with the formation of lava
spines at the summit. Other pyroclastic flows erupt, mainly down the river
Blanche valley.
The activity stopped in 1905.
St. Pierre today
Shortly after the eruption, St. Pierre was completely abandoned. But slowly families were
drawn to the ruins by the cheap price of land, and the desire to rebuild the city. The
first building to be constructed among the ruins was a hotel, mostly thanks to tourism
which developed after the disaster.
St. Pierre currently has about 6,000 inhabitants.
Deadliest eruptions in Latin America
Volcano
Country
Nevado del Ruiz
Colombia
Cotopaxi
Ecuador
Soufrière
St. Vincent (L. Ant.)
Mount Pelée Martinique (L. Ant.)
Santa Maria
Guatemala
El Chichón
Mexico
Nevado del Ruiz
Colombia
„
Year
1845
1877
1902
1902
1902
1982
1985
Deaths
1,000
>300
1,680
29,000
~1,500
>2,000
25,000
http://volcanoes.usgs.gov/Hazards/Effects/Fatalities.html
Nevado del Ruiz, Colombia
„
„
„
Located in the Andes mountains of
South America, Nevado del Ruiz is the
northernmost and highest Colombian
volcano with historical activity. With a
summit elevation of 5,389 m, the
volcano is the second highest active
volcano in the western hemisphere.
Its summit is covered with 25 km2 of
snow and ice even though it's located
only 500 km from the equator.
Beginning in November 1984, the
volcano began showing signs of unrest,
including earthquakes, increased
fumarolic activity from the summit
crater, and small explosions.
http://volcanoes.usgs.gov/Hazards/What/Lahars/RuizLahars.html
Trans-Mexico Volcanic Belt
Central
America
Nevado del
Ruiz
Lesser
Antilles
Island Arc
Andes N. Volcanic Zone
Andes Central
Volcanic Zone
Andes S. Volcanic Zone
Nevado del
Ruiz, Colombia
at the northern
end of the
Andes Northern
Volcanic Zone
Colombia
„
„
„
1,138,910 sq km
41,000,000 people
Per capita GDP $6300
Colombia
Geography
„
„
3 mountain
ranges in west:
Cordillera
Occidental,
Central, and
Oriental
Eastern
lowlands are
part of the
Amazon and
Orinoco
drainage basins
What is a lahar?
„
„
Lahar is an Indonesian word for a
rapidly flowing mixture of rock
debris and water that originates on
the slopes of a volcano. Lahars are
also referred to as volcanic
mudflows or debris flows. They
form in a variety of ways, chiefly
by the rapid melting of snow and
ice by lavas, intense rainfall on
loose volcanic rock deposits, and
breakout of a lake dammed by
volcanic deposits.
Volcanoes covered with ice and
snow are especially dangerous
because a small eruption can melt a
tremendous amount of ice and
produce great lahars
A small lahar triggered by rainfall rushes down the
Nima II River near the town of El Palmar in
Guatemala. The lahar developed on the slopes
of Santiaguito volcano.
http://volcanoes.usgs.gov/Hazards/What/Lahars/RuizLahars.html
Events prior to Nov. 13, 1985
Nevado del Ruiz experienced many minor earthquakes and steam eruptions for 51 weeks
prior to the November 13 eruption. The activity was just enough to keep people nervous,
particularly because lahars had killed 1000 people in1845. Since Columbia had no
equipment to monitor the volcano, or geologists skilled in using such equipment, expertise
could only come from other countries. A scientific commission and some journalists visited
the crater in late February. By July, seismographs were obtained from several countries to
monitor earthquakes which would help plot the movement of rising magma beneath the
volcano. Money was obtained from the United Nations to help map the areas that were
thought to be at the greatest risk. The resulting report and volcanic hazards map were
finished on October 7, but only ten copies were distributed. Based on the report, the National
Bureau of Geology and Mines (INGEOMINAS) declared that a moderate eruption would
produce " . . . a 100 percent probability of mudflows . . . with great danger for Armero . . .
Ambalema, and the lower part of the River Chinchina." However, some government officials
dismissed the report as "too alarming" and authorities did not want to evacuate people until
they were assured of the necessity. Continuous earth tremors began beneath the volcano on
November 10. This prompted a group of scientists to visit the crater on November 12.
However, they saw nothing to suggest imminent danger and they did not recommend an
evacuation.
http://www.geology.sdsu.edu/how_volcanoes_work/Nevado.html
The Eruption of Nov. 13, 1985
„
Broad summit of Nevado del Ruiz. An
explosive eruption from Ruiz's summit
crater on November 13, 1985, at 9:08 p.m.
generated an eruption column and sent a
series of pyroclastic flows and surges
across the volcano's broad ice-covered
summit. Within minutes, pumice and ash
began to fall to the northeast along with
heavy rain that had started earlier in the
day. The crater was enlarged slightly by the
eruption, and the summit area was quickly
covered with layers of pyroclastic flow
deposits as thick as 8 m. This eruption was
preceded by a strong phreatic (steam)
explosion from the crater at 3:05 p.m. In
this view, the dark pyroclastic-flow
deposits are partly covered with fresh
snow.
Nov. 13 Eruption & Lahars
The volcano exploded violently on November 13, 1985. The initial blast began at 3:06 p.m.,
and two hours later pumice fragments and ash fell on Armero. However, the citizens of
Armero were placated by reassuring radio messages from the mayor and from a local
priest over the church public address system. Nevertheless, the Red Cross ordered an
evacuation of the town at 7:00 p.m. However, shortly after the evacuation order the ash
stopped falling and the evacuation was called off.
At 9:08 p.m., hot tephra began to erupt from the summit crater and quickly began to melt
the summit ice cap. Unfortunately, a storm obscured the summit area. Meltwater mixed
with the erupting pyroclastic fragments to generate a series of hot lahars. One lahar flowed
west down the River Cauca, submerging the village Chinchina and killing 1,927 people.
Other lahars followed the paths of the 1595 and 1845 mudflows. Traveling at 50 kilometers
per hour, the largest of these burst through an upstream dam on the River Lagunillas and
reached Armero two hours after the eruption began. Most of the town was swept away or
buried in only a few short minutes, killing three quarters of the townspeople.
Generation of the Nevado del
Ruiz lahar
Headwaters of Azufrado River. Hot
rock fragments of the pyroclastic
flows and surges quickly eroded and
mixed with Ruiz's snow and ice,
melting about ten percent of the
volcano's ice cover. In places,
channels 100 m wide and 2-4 m deep
were eroded into the icecap. Flowing
mixtures of water, ice, pumice and
other rock debris then poured from
the summit and sides of the volcano
into rivers draining the volcano. In
one river, scientists found a piece of
ice 2 m across about 3 km from the
crater. This view of Ruiz is from the
northeast.
http://volcanoes.usgs.gov/Imgs/Jpg/Ruiz/30410135_062_large.jpg
Generation of the Nevado del
Ruiz lahar
„
Headwaters of Gualí River.
Pumice and meltwater produced
by the hot pyroclastic flows and
surges swept into gullies and
channels on the slopes of Ruiz as
a series of small lahars. After
descending several thousand
meters and eroding loose rock
debris from the sides of the
volcano, the lahars were
funneled into all six major river
valleys leading from Ruiz. Here
in the headwaters of the Gualí
River on the north side of Ruiz, a
lahar took several paths on its
journey downstream.
Gualí River valley. Flowing downstream from Ruiz at an average speed of 60 km per hour,
lahars eroded soil, loose rock debris and stripped vegetation from river channels. By
incorporating water and debris from along river channels, the lahars grew in size as they
moved away from the volcano--some lahars increased up to 4 times their initial volumes. In
some of the narrow canyons downstream from the volcano, as shown here in the Gualí
River, lahars were as thick as 50 m!
http://volcanoes.usgs.gov/Hazards/What/Lahars/RuizLahars.html
„
Gualí River. Houses and towns located high enough above river channels escaped
damage from the lahars. In the Gualí River valley, at least two lahar pulses were
reported by eyewitnesses, separated by 5 to 15 minutes depending on distance from
the volcano. Eyewitnesses reported that the noise created by the passage of each
pulse made their houses and the ground shake and that conversation, even by
shouting, was impossible.
http://volcanoes.usgs.gov/Hazards/What/Lahars/RuizLahars.html
Former site of Armero
Three quarters of Armero’s 28,700 inhabitants perished.
„
„
„
„
Accounts from survivors indicate Armero was inundated with several pulses
of flowing material. The first arrived at 11:25 p.m. and consisted of a flood
of cold water that overflowed the Río Lagunillas channel, sweeping into
downtown Armero. Only a few centimeters deep in town, this water was
from a lake located just upstream that had been displaced when lahars
entered the lake.
The second pulse arrived at 11:35 p.m. This was the largest pulse and within
10 to 20 minutes, destroyed most of the buildings and swept away most of
the people in Armero. Flow depths of the lahar ranged from 2 to 5 m.
The third pulse arrived at 11:50 p.m. with a velocity of about half of the
second one. Then, in the next hour or so, a series of smaller pulses (6 to 8)
was experienced by survivors trapped in the mud. These pulses lifted people
floating in the mud and pushed them a few meters ahead.
One last pulse struck Armero a short time after 1 a.m. on November 14.
Lessons from Nevado del Ruiz
For the generation of lahars on ice- and snow-covered volcanoes, the deadly 1985
eruption of Nevado del Ruiz offers several key lessons for scientists, emergencyresponse professionals, and communities located downstream of such volcanoes:
1) catastrophic lahars can be generated on ice- and snow-capped volcanoes by
relatively small eruptions
2) the surface area of snow on an ice cap can be more critical than total ice
volume when considering lahar potential
3) placement of hot rock debris on snow is insufficient to generate lahars -- the
two materials must be mechanically mixed together for rapid heat transfer
4) lahars can increase their volumes significantly by entrainment of water and
eroded sediment
5) valley-confined lahars can maintain relatively high velocities and can have
catastrophic impacts as far as 100 km downstream
http://volcanoes.usgs.gov/Hazards/What/Lahars/RuizLahars.html
Mt. Rainier, Washington:
Sleepless in Seattle
„
„
Mount Rainier volcano (4,393 m) is
potentially the most dangerous of the
active volcanoes of the Cascade Range in
the Pacific Northwest. The volcano
towers more than 3 km above several
river valleys that lead to populated
regions, and it has a cover of snow and
ice that is equal in volume to that at all
the other Cascade Range volcanoes
combined.
The volcano's most recent landslidetriggered lahar happened about 500 years
ago and swept more than 50 km down
the Puyallup River valley to the Puget
Sound lowland. The lahar buried the
valley near the town of Orting with
deposits 3 to 5 m thick.
http://volcanoes.usgs.gov/About/Highlights/RainierPilot/3
0410142-011_caption.html

Documentos relacionados