||Sizes of Eruptions
Small volcanic eruptions. The volume of volcanic material tossed out in the bursts or flows of small eruptions is relatively small: a few football stadiums full (equal to a few million cubic feet or a few ten thousandths of a cubic mile). No worries, mate! After all, the inhabitants of the island of Stromboli in Italy simply ignore the virtually constant thumps of red-hot lava "bombs" being tossed out of the vent of their resident volcano. Of course, just how big or important a particular eruption appears depends largely on how close you are to it at the time. Even a small steam explosion seems huge if you happen to be standing right next to the vent when it "blows." And a single lava flow seems catastrophic if it happens to go right through your bedroom! Small eruptions may also be of concern if they are harbingers of bigger things to come--a large eruption.
Large volcanic eruptions. Large eruptions often make national and even international headlines, so you have probably heard of some of these: El Chichon, Mount Redoubt (which nearly nailed a passing jetliner!), and Mount St. Helens. Large eruptions affect areas as far as tens of miles away and typically eject a few tenths of a cubic mile of volcanic rock and lava (figure). Depending on the local population distribution, large eruptions can be very destructive of life and property. Potential hazards in these eruptions include huge blasts of superheated steam and pulverized rock, rock avalanches, massive clouds of volcanic ash, asphyxiating gases, and, in the cases of volcanoes with snow-caps or glaciers, giant mudflows. The eruption of Vesuvius (79 AD) buried the towns of Pompeii and Herculaneum in Italy and killed thousands of people with falling ash and poisonous gases. The main blast of steam and rock from the 1902 eruption of Mount Pelee on the Caribbean island of Martinique destroyed St. Pierre in less than two minutes, killing all but two of the city's 30,000 inhabitants. Ironically, warnings about the dangers of an imminent eruption were down-played by public officials anxious to keep the voting populace in town for an upcoming election. Unfortunately, the volcano exploded 3 days before the scheduled election, annihilating both electors and potential electees. Mudflows triggered by a 1985 eruption of Nevado del Ruiz in Columbia swept through a small town and killed 25,000 people. In contrast, only 57 people were killed in the 1980 Mount St. Helens eruption, largely because of its remote location and the effectiveness of public warnings.
As important as large eruptions appear locally, they are still relatively small compared to our next larger size classification of major eruptions.
Major volcanic eruptions. Mount Pinatubo in the Philippines in 1991 and Krakatoa in Indonesia in 1883 (Hollywood even made a movie about that one!) were tens to hundreds of times larger than the eruption of Mount St. Helens and are classified as major eruptions. These eruptions are often caldera-type eruptions. They eject many cubic miles of material (see size comparison figure above) and affect areas around them up to hundreds of miles away. Everything about major eruptions must be written in superlatives. Major eruptions are incredibly destructive. The very islands on which Krakatoa, Tambora (Indonesia, 1815), and Santorini (Greece, about 1500 BC) had stood simply disappeared into the sea during the eruptions. Tambora spread thick layers of ash and floating islands of pumice across 2000 miles of Indonesia. Superheated steam and ash from Krakatoa's main blast killed and burned people 30 to 40 miles away on the Sumatra coast. Krakatoa also spawned tidal waves over 100 feet high that swept the coasts of Java and Sumatra, claiming most of the 37,000 lives lost in that eruption. The destruction is so great and widespread in large and especially major eruptions that eyewitnesses often describe the eruptions and their aftermath as "the end of the world."
Major eruptions are incredibly loud. Noises from the eruption of Tambora in Indonesia in 1815 were mistaken for nearby canon fire in European settlements 200 and 750 miles away from the volcano itself. In each community, nervous military leaders sent out ships and soldiers to investigate what were thought to be attacks on nearby outposts! If the main eruption of Mount St. Helens had made as much sheer noise as Tambora, it would have been heard all over the United States.
Major eruptions can affect weather around the world. Clouds of ash and sulfur-rich particles thrown into the stratosphere by large volcanic blasts reflect part of the sunlight reaching Earth, causing cooling of the atmosphere. The larger the blast, the greater the effect. A global cooling of about 1.5 deg. F (detected by modern sensors) continued for about two years after the Mount Pinatubo eruption. Cooling was so severe following the Tambora eruption that 1816 was called "the year without a summer" or "Eighteen hundred and froze to death." Cold temperatures and killing frosts in Europe and New England in America caused extensive crop failures and resulted in famine in post-Napoleonic France. Many farmers in New England left for the frontier in New York in search of better weather, swelling the American "Move West."
Major eruptions have even been responsible at times for literally changing the course of history. The Mataram empire in Java was apparently destroyed by a major eruption of Merapi in 1006 AD. The course of Mayan civilization was changed when a major eruption of Ilopango in El Salvador destroyed the highland Mayas around 300 AD. Subsequent shifts in population, trade routes, and resultant economic power gave a great boost to the development of the "Classic Maya" civilization. And the eruption of Santorini in the middle of Bronze-age Europe apparently caused the destruction of the advanced Minoan civilization on Crete and the surrounding islands and allowed the expansion of the early Greeks. Ash clouds and tidal waves from Santorini may also have reached Egypt about the time the Hebrews were leaving the country and given rise to the stories of the biblical plagues in Egypt.
Lest we become complacent thinking that major eruptions only occur in other parts of the world, just remember that the blast that destroyed Mount Mazama to form the placid-appearing Crater Lake in Oregon occurred only about 7000 years ago. Almost any of the Cascade volcanoes could do a repeat performance at any time.
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