Part 2: Major Eruptions

In 1991, two volcanoes on the western edge of the Philippine Plate produced major eruptions.

On June 15, Mount Pinatubo spewed ash 40 km into the air and produced huge ash flows (also called pyroclastic flows) and mudflows that devastated a large area around the volcano.

Pinatubo, located 90 km from Manila, had been dormant for 600 years before the 1991 eruption, which ranks as one of the largest eruptions in this century.

Also in 1991, Japan's Unzen Volcano, located on the Island of Kyushu about 40 km east of Nagasaki, awakened from its 200-year slumber to produce a new lava dome at its summit.

Beginning in June, repeated collapses of this active dome generated destructive ash flows that swept down its slopes at speeds as high as 200 km per hour.

Unzen is one of more than 75 active volcanoes in Japan; its eruption in 1792 killed more than 15,000 people--the worst volcanic disaster in the country's history.

While the Unzen eruptions have caused deaths and considerable local damage, the impact of the June 1991 eruption of Mount Pinatubo was global.

Slightly cooler than usual temperatures recorded worldwide and the brilliant sunsets and sunrises have been attributed to this eruption that sent fine ash and gases high into the stratosphere, 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 0.5 °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 °C.

Even a year after the eruption, most of the northern hemisphere experienced sharply cooler temperatures during the summer months.

In part of Europe and in North America, 1816 was known as "the year without a summer."

Part 1: The Link
Part 3: The Damages
Part 4: Hawaii
Part 5: Scientific Observation

From: Kious and Tilling, 1996, This Dynamic Earth: The Story of Plate Tectonics: USGS Special Interest Publication