The monsters of the world - Historical disaster by volcanoes
Dr. Claus Rink,
Geoscientist and Special correspondent
of Pattaya Mail and Chiang Mai Mail
How many active (erupting)
volcanoes are known?
20 each year, 50-70 each decade, about 160 historical
eruptions, about 550 known Holocene eruptions (last 10,000 years) and about
1300 known before.
Types of volcanoes (USGS)
Typical phenomena happen at a
volcano eruption (USGS)
Global Cooling by volcanoes
Global cooling often has been linked with major volcanic eruptions. The
year 1816 often has been referred to as “the year without a summer”. It was
a time of significant weather-related disruptions in New England and in
Western Europe with killing summer frosts in the United States and Canada.
These strange phenomena were attributed to a major eruption of the
Tambora volcano in 1815 in Indonesia. The volcano threw sulfur dioxide gas
into the stratosphere, and the aerosol layer that formed led to brilliant
sunsets seen around the world for several years.
Tambora is on Sumbawa Island along the east Sunda Arc. It
lies some 300 kilometers behind the Sunda Trench, but the subduction zone in
that area has a shallow dip and is less than 200 kilometers deep beneath
Tambora; it is a large stratovolcano. Before its eruption in 1815, Tambora
might have been in repose for as much as 5,000 years.
Tambora
At least 6 months and probably about 3 years of increased
steaming and small phreatic eruptions preceded the 1815 Tambora eruption,
the largest in historical time. A moderately large explosive eruption
occurred on 5 April 1815, from which ash fell in east Java and thunder like
sounds were heard up to 1,400 kilometers away.
A still larger eruption occurred on 10-11 April, ultimately ejecting
about 50 cubic kilometers of magma. The eruption left a deep summit caldera
where previously a much higher stratovolcano had stood. Earthquakes were
felt as far away as Surabaya (500 kilometers), possibly reflecting the
caldera collapse.
Year without summer
Among the more prominent theories of events that have
triggered global climatic changes is that we have increases in volcanism
that could have thrown more airborne volcanic material into the
stratosphere, thereby creating a dust veil and lowered temperatures.
The years 1980, 1981, and 1982, for example, saw several
major volcanic eruptions adding large quantities of particulate volcanic
material and volatiles to the stratosphere, including the catastrophic
eruption of Mount St. Helens, Washington, on May 18, 1980, and a large
eruption of Mount Hekla, Iceland, on August 17, 1980.
The 1982 series of eruptions from El Chichon volcano,
Mexico, caused death and destruction in the populated area around the
volcano, but a further reaching impact may result from the effect on Earth’s
climate because of the enormous ejection of volcanic material into the
stratosphere.
The potential climatic effect of the Laki volcanic
eruption in Iceland in 1783, the largest effusive (lava) volcanic eruption
in historic time, was huge.
The catastrophic eruption of the Tambora volcano,
Indonesia, in 1815 was followed by a so-called “year-without-a-summer.” In
New England, for example, frost occurred during each of the summer months in
1816.
Mount Pinatubo
Mount Pinatubo is one of a chain of composite volcanoes
that constitute the Luzon volcanic arc. Before the eruption, more than
30,000 people lived in small villages on the volcano’s flanks.
Mount Pinatubo
The June 1991 eruption of Mount Pinatubo was global.
Slightly cooler than usual temperatures recorded worldwide have been
attributed to this eruption that sent fine ash and gases high into the
stratosphere for 10 weeks, forming a large volcanic cloud that drifted
around the world. The sulfur dioxide (SO2) in this cloud - about 22 million
tons - combined with water to form droplets of sulfuric acid, blocking some
of the sunlight from reaching the Earth and thereby cooling temperatures in
some regions by as much as 1.5 degrees C.
An eruption the size of Mount Pinatubo could affect the
weather for a few years. A similar phenomenon occurred in April of 1815 with
the cataclysmic eruption of Tambora Volcano in Indonesia, the most powerful
eruption in recorded history. Tambora’s volcanic cloud lowered global
temperatures by as much as 3 degrees C. Even a year after the eruption, most
of the northern hemisphere experienced sharply cooler temperatures during
the summer months, causing the “year without a summer” in parts of Europe
and in North America, as mentioned above.
74,000 years ago: Toba
One eruption happened long ago and far away - about
74,000 years ago on the island of Sumatra. The eruption of the Toba Volcano
was the largest eruption in the last two million years, and, according to
some theories, almost wiped out our human ancestors.
The scale of the Toba eruption is difficult to
comprehend. Pyroclastic flows (hot flows of ash and pumice) covered an area
of at least 20,000 square kilometers (7,700 sq mi), with deposits as thick
as 600 m (2,000 ft) near the vents.
Ash fall was widespread over much of southeast Asia. An
ash layer approximately 15 cm (6 in) thick was deposited over the entire
Indian subcontinent. Our appreciation of the magnitude of this eruption
continues to grow as Toba ash is recognized farther and farther from the
source.
The volume of the Toba eruption is estimated at 2,800
cubic kilometers (670 cu mi). To give some comparison with more recent
eruptions, the 1980 eruption of Mount St. Helens produced less than 1 cubic
kilometer (0.25 cu mi). Vesuvius (A.D. 79) erupted about 5 cubic kilometers
(1.2 cu mi), and Krakatoa in Indonesia (1883) about 12 cubic kilometers (3
cu mi). Closer to home, the volume of Kilauea’s ongoing eruption is about
2.6 cubic kilometers (0.6 cu mi), erupted over the last 22 years.
The most widespread hazard from such an eruption is its
effect on global climate. Large, explosive eruptions eject huge amounts of
volcanic ash and gas that reach the stratosphere. Sulfur dioxide gas reacts
with atmospheric moisture to form tiny droplets of sulfuric acid. The
droplets and ash particles both absorb heat and reflect solar radiation,
cooling the lower atmosphere. While ash tends to settle out of the
stratosphere within months, the aerosol of sulfuric acid can remain in the
stratosphere for 2-3 years before dissipating.
Climate catastrophes, which have occurred many times in
the geological past, caused the extinction of large or small populations of
animals and plants. Changes in the terrestrial and marine biota caused by
the catastrophic climate changes undoubtedly resulted in considerable
fluctuations in the global carbon cycle and atmospheric gas composition.
Primarily, carbon dioxide and other greenhouse gas contents were affected.
The study of these catastrophes allows a conclusion that climate system is
very sensitive to relatively small changes in climate-forcing factors
(transparency of the atmosphere, changes in large glaciations, etc.).
To be continued…
