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Mayon Volcano-Explained
Report from
American
Museum of Natural History
website
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Why is a Volcano Here? Mayon
Volcano stands 150 miles south of Manila, the Philippine capital, and looms
above the bustling city of Legazpi. More than 2400 meters (~8,000 feet)
tall, Mayon is the most active of the numerous volcanoes dotting the
Philippine Islands. The volcanoes of the Philippines, in turn, are part of a
much longer chain of volcanoes, called the Ring of Fire, that encircles the
Pacific Ocean from South America up to Alaska, across to Russia, and down
through Japan and Indonesia.
The Ring of Fire. Intense tectonic activity occurs all around the
perimeter of the Pacific Ocean. Along this length, the Pacific Plate and
various smaller oceanic plates collide, scrape and subduct, or sink
below, one another and the adjacent continental plates. This subduction -
actually a string of subduction zones - generates the chain of volcanoes
known as the Ring of Fire. The Philippines, part of the Ring of Fire, lie at
the meeting point of at least three tectonic plates: the Eurasian, the South
China Sea, and the Philippine plates.
The squeeze is on. The Philippine Islands are all that remain of a
narrow - and gradually disappearing - oceanic plate. This "microplate" is
wedged against the Eurasian and South China Sea plates to the west.
Meanwhile, the Philippine plate is slowly advancing from the east. The
microplate is caught in the middle. As a result, the islands are bounded by
trenches, laced with faults, and are rife with seismic stress; earthquakes
and volcanic eruptions are common events. The squeezing has been going on
for 20 to 30 million years.
Stuck in the middle. The Philippines sit on a microplate that is
caught between two subduction zones. The eastern zone, called the Philippine
Trench, marks the boundary of the advancing Philippine plate. This zone
gives rise to the Bicol Arc, a string of volcanoes that includes Mayon. The
western subduction zone, called the Manila trench, occurs where the
microplate meets the South China Sea plate. This zone feeds another string
of volcanoes: the Bataan Arc, which includes Mount Pinatubo, the source of
the biggest eruption of the 20th century.
Why does subduction cause volcanoes? When two tectonic plates
collide, the heavier plate subducts, or descends, beneath the other,
into the mantle below. Water in the descending plate is pressed out and
rises into the mantle above. This water causes the mantle to partially
melt.The melted rock, called magma, is more buoyant than the rock around it.
The magma rises to the Earth's surface and forms a volcano. In fact, magma
rises along the entire length of the subduction zone. The result is usually
a string, or arc, of volcanoes.
A continent grows. For geologists, the Philippines offer an
opportunity to watch new continental crust take shape - a process that is
poorly understood. Subduction along the western margin of the Philippines is
slowly pinning the islands against the Eurasian continent. In effect, the
islands are being bulldozed into an ever-growing pile. The Philippines are
being added to the landmass of the Eurasian Plate - and a large continent is
becoming even larger.
Dying gasps. Volcanic activity in the Philippines is slowly
changing. The archipelago is pinned against the Eurasian plate; it can't
move any farther. As a result, subduction along the western zone (the Manila
Trench) is slowing and eventually will stop. And when subduction stops, the
volcanoes above (including Mount Pinatubo) become extinct. The eastern
subduction zone, however, will grow: it will extend northward and join the
Ryukyu Trench as the Philippine plate continues to sink below the Eurasian
plate. So Mayon and the eastern volcanoes will keep on erupting.
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The Recent Eruptions
In mid-February, a series of small earthquakes signalled to scientists
that Mayon might soon erupt. On February 28, just before dawn, the drama
began: explosive eruptions of lava, sulfurous ash and superheated steam.
Fourteen explosions were recorded by late afternoon. The fiercest sent
ash several kilometers into the air. Dr. Ronaldo Arboleda, of the
Philippine Institute of Volcanology and Seismology, saw Mayon eject
rocks "as big as houses." By late evening, lava trailed several
kilometers down the volcano's flank.
Mayon remained active well after its initial outburst. For days, the
volcano emitted clouds of superhot steam and ash that darkened skies and
cascaded down the mountainside. These pyroclastic flows, powered
by volcanic gases within them, reached temperatures in excess of 450
degrees centigrade (850 degrees Fahrenheit) and traveled as fast as 80
km (50 miles) per hour, incinerating everything in their path.
Lately Mayon has quieted down. Small tremors still occur, and the
occasional puff of ash and steam is released. By early April, volcano
experts had downgraded Mayon's status from Alert Level 5 ("Alarming") to
Alert Level 2, which means that the probability of another explosive
eruption at this point is minimal. Officials warn that mudslides and
landslides still present a danger, particularly with the onset of the
rainy season in the autumn. The 6 kilometers (4 miles) immediately
surrounding the crater, however, remain permanently off-limits to
people.
Will
it happen again?
Mayon is the most active volcano in Philippines. And because so
many people live on Mayon's slopes, Mayon's eruptions have often
wrought calamity. More than 2,000 villagers perished in 1814, during
Mayon's most violent eruption, when volcanic mudflows buried two
nearby towns. To avert such tragedies, and to improve their own
understanding of how volcanoes work, scientists at the Philippine
Institute of Volcanology and Seismology constantly monitor Mayon for
signs of new activity.
Although scientists didn't know exactly when Mayon would rewaken,
they knew eventually it would. "The volcano had been quiet since
1993 and we all knew it would be erupting again one of these years,"
says Chris Newhall, a geologist with the U.S. Geological Service who
has studied volcanoes in the Philippines extensively. The volcano
showed renewed signs of unrest last June, when it began billowing
steam and ash. Earthquakes and further ash releases in February,
just days before the latest eruption, warned experts of the coming
explosion.
By studying volcanoes like Mayon, scientists understand better
than ever how to tell whether a major volcanic eruption is pending.
The warning signs include:
Seismic activity. As magma rises toward Earth's surface, it
displaces the rock around it, causing earthquakes. The frequency,
intensity and location of earthquakes in turn can tell scientists
how close the magma is to the surface, how fast it's moving - and
roughly how soon it might erupt.
Ground tilt. As magma rises, but before it erupts, it
wells to form a lava dome at or near the surface. By measuring the
tilt of the ground at various points around the dome, and by
monitoring how quickly the tilt increases or decreases, scientists
can gauge how quickly magma is moving into the uppermost part of the
volcano - and how soon it might erupt.
Gas emissions. The magma in subduction volcanoes is rich
in dissolved gases, particularly sulfur dioxide. As the magma rises
toward Earth's surface, the pressure exerted on it by the
surrounding rock decreases. The decrease in pressure permits the
dissolved gas to escape, like bubbles escaping from a newly opened
soda can. When a volcano begins emitting more sulfur dioxide than
usual, that's often a sign that fresh magma is on the move towards
the surface.
Scientists have found that Mayon's eruptions are usually preceded
by ground tilting and an increase in earthquakes and gas emissions.
Also, the crater may begin to glow, or the volcano may give off
audible rumbling sounds. What do these events add up to? Sometimes
nothing at all happens, and the volcano returns to normal. Sometimes
the volcano emits a minor puff of ash, as it did last June. And
sometimes the volcano in fact erupts - if not within days, then
within a few months.
Will Mayon erupt again? Without a doubt. Mayon's history,
recent behavior and active tectonic setting suggest that the volcano
will continue to erupt for thousands, or even hundreds of thousands
of years to come. From a human perspective, however, there is good
news: the number of Mayon's fatalities has dropped significantly in
the past hundred years, even as the population of the surrounding
area has grown. Mayon will never be completely safe - but
volcanologists will continue to get better at forecasting its
activity.
Eruptions aren't the only danger. An eruption produces tons of
ash and debris, which settle loosely back to earth. Subsequent
earthquakes (common around volcanoes) can jolt the debris free,
causing deadly landslides. Heavy rains can cause lahars, or
mudslides. Fortunately, Mayon's recent eruption happened during the
Philippine dry season. But the danger is ever-present. In 1766,
2,000 people died when rivers of hot mud, set loose by rains months
after one of Mayon's eruptions, swept over surrounding villages.
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Volcanoes
and Climate
A single volcano can dramatically alter weather patterns around
the world. This was made clear in 1991, when Mount Pinatubo, a
volcano in the western Philippines, erupted with astonishing
force. 800 people were killed, and dust and gas were blasted
more than 18 kilomters (12 miles) into the air. The debris
formed a thin veil in the upper atmosphere, preventing some of
the Sun's heat from reaching Earth's surface. In the following
months, temperatures around the world dropped by several tenths
of a degree. Could Mayon have the same effect?
Whether a volcano affects climate depends in large part on
what it ejects. The key compound is sulfur dioxide. In the upper
atmosphere, sulfur dioxide reacts with water molecules to form
chemical compounds called aerosols. Aerosols scatter sunlight
back into space, effectively cooling the earth; in fact many
man-made pollutants create aerosols. During the eruption on June
15, 1991, Pinatubo emitted 18 to 20 megatons of sulfur. Mayon,
in contrast, emitted 5,000 to 9,000 tons a day during its recent
eruption - too little to have a global impact.
To affect global temperatures, a volcano must loft sulfur
dioxide and dust particles very high into the atmosphere. The
Pinatubo eruption of 1991 sent gases and debris 20 kilometers
(12 miles) high, enough to reach the upper atmosphere. Here the
sulfur dioxide molecules reacted with OH to form aerosols.
High-altitude winds then carried the dust and aerosols around
the globe. Mayon's eruption in February sent debris only about
13 kilometers (8 miles) up - that's high, but it's not
stratospheric. In the end, it looks like Mayon won't have the
cooling effect that Pinatubo did.
A monster eruption. Legend has it that "Frankenstein"
was born, indirectly, from a climate-cooling volcano. In 1815,
the Indonesian volcano Tambora erupted violently. 90,000 people
were killed, and millions of tons of debris were blasted into
the stratosphere. The following year was cold and dark in
Europe; it became known as "the year without summer." That
summer, Shelley and her husband Percy were holed up at the house
of their friend Lord Byron; to amuse themselves, they wrote
ghost stories. Mary's effort eventually became the classic story
we know today.
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Life Around
Volcanoes
Mayon has erupted more than 40 times, several times with
fatalities, since historians began keeping track in 1616.
And yet a million people live within 14 kilometers (8 miles)
of the volcano; tens of thousands live on the volcano
itself. Worldwide, tens of millions of people live within
close proximity of an active volcano. Why would a person
choose to live on or near a volcano?
Why do people live near volcanoes? Because
volcanic soils are especially fertile. To a farmer, a thin
blanket of volcanic ash is free fertilizer from above -
though it may take months for rain and weathering to release
the ash's nutrients. The world's best coffee is grown on
volcanic soil. The region around Naples, Italy, one of the
richest farming areas in the world, is built in part on
volcanic ash from nearby Vesuvius. The lower slopes of Mayon
are covered with rice fields and coconut plantations;
tomatoes and other vegetables grow further uphill.
Farming
the soil of an active volcano is inherently risky. For
poorer residents, however, it is often the only livelihood
available. Mayon's eruption in 1993 killed 75 people - all
of them tomato farmers in the Bonga Valley, one of the
ravines declared off-limits by the government. Today nearly
20,000 locals live and farm within Mayon's Permanent Danger
Zone. Half of these farmers refused to evacuate during the
most recent eruption. Others returned surreptiously to tend
their fields and animals: without their crops, they risked
starvation.
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Researches and Studies about Mayon
Chris Newhall, a geologist with the U.S. Geological
Service, has been studying Mayon for 30 years. "I joined
the Peace Corps in 1970," Newhall says. "I wanted to
apply my geology training to humanitarian projects. They
offered me a spot in a science-teacher training program
in the Philippines. After a few months I began teaching
Geology 101 in a small college at the foot of Mayon. It
was perfect: I could study Mayon on weekends, and I
managed to interest a few of my students in doing the
same."
"Volcanoes in the Philippines are typical of
volcanoes in the Ring of Fire," Newhall says. "One can
study their magma to learn about the underlying mantle.
Or one can focus, as I do, on the processes in the
uppermost 10 kilometers (6 miles) or so, in shallow
magma reservoirs and conduits. I'm interested in
determining what leads to explosive eruptions, and how
we can forecast when and how big such eruptions will be.
This knowledge is critical for people who live at the
foot of volcanoes."
"Why
did I decide to become an Earth scientist in the first
place? I like the outdoors, I like science, and I like
to try to understand nature. Why rocks and the geosphere
instead of bugs and the biosphere? Maybe it was the
smell of sulfur from mud pots of Lassen Peak,
California, near where I lived as a child. Or the pretty
green actinolite in the metamorphic rocks of the area."
Report Coutesy of
American
Museum of Natural History
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