||Sizes of Eruptions
"Great" Eruptions. As large and devastating as major eruptions are, they are not the largest volcanic eruptions known. None of these largest or "great" eruptions have occurred in historic times (thank goodness!). We have only gigantic deposits of volcanic rock as eloquent evidence of their reality.
Great eruptions emerge from two types of volcanoes: fissure eruption and giant caldera. In both types, the volume of volcanic rock deposited on the surface of the earth during a single great eruption can be many hundreds of cubic miles--hundreds of times larger than the major eruptions mankind has known.
eruptions explode with violence, noise, and destruction that defies description - try to
imagine an eruption ten thousand times larger than Mount St. Helens! Several giant caldera
eruptions are known to have occurred in the United States. The Valles caldera in New
Mexico expelled about 100 cubic miles of ash that still covers much of the middle of the
state. The Long Valley caldera in southwestern Nevada ejected over 150 cubic miles of ash
that now makes up thick layers of rock in mountains all over the West. But the largest of
these eruptions in the United States came from a fairly recently recognized giant caldera
right in the middle of Yellowstone National Park. This caldera, some 30 by 50 miles in
size, covers about half of the park, as seen in the map at left.
The Yellowstone Caldera is
much larger than the crater on Mount St. Helens. The explosive ash deposit from
Yellowstone is also much larger than the deposit from Mount St. Helens, as shown at left.
The Yellowstone eruption blasted out a phenomenal 300 cubic miles of ash in a thick
blanket that extended from California to the Mississippi River. All life within hundreds
of miles must have been extinguished in less than an hour.
In giant fissure eruptions, sometimes called "flood basalt" eruptions, the earth simply cracks open and disgorges vast amounts of fluid basalt. These giant outflows are not explosive, but they may continue for weeks without a break, inexorably burying vast areas under a sea of liquid rock. Flood basalt eruptions also come in swarms: as soon as one finishes, another begins. Multiple outflows from the same set of cracks continue for thousands of years, creating enormous barren plains of rock. The largest flood basalt deposit in the United States is the Columbia River Basalt (CRB) group which covers the eastern third of the state of Washington, a quarter of Oregon and part of Idaho. One individual layer of basalt - a single eruption - covers 15,000 square miles to depths of 100 feet or more - a total volume of about 600 cubic miles. The entire CRB covers 50,000 square miles to depths of a mile or more, with a total volume of 25,000 cubic miles! All of this "stuff" came out of a group of cracks, each 30-50 yards wide and tens of miles long, in western Idaho. Flood basalt volcanoes are the largest known deposits of volcanic material.
Given what we know about what a major eruption can do to the world's environment, we can only guess at the conditions great eruptions create. However, we do know that many of the world's great flood basalt eruptions correspond in time with some of the great periods of extinction of life in the earth's past, which indicates extremely challenging climatic conditions for extended periods of time. Fortunately, the larger an eruption, the less frequently it occurs. Many small eruptions occur around the world every year, large eruptions occur every year or so, and major eruptions occur on scales of decades to centuries, while great eruptions.... Well, the CRB, the world's youngest major flood basalt deposit, is about 15 million years old. The Long Valley and Valles Calderas blew up, respectively, about 11 million and 700 thousand years ago. The Yellowstone caldera erupted about 600 thousand years ago. Certainly no great eruptions have occurred during recorded history. It would seem that humanity is unlikely to experience a great eruption anytime soon. On the other hand, the Yellowstone blast is only the latest in a string of eruptions in that area that occur every 600 thousand years or so. And its about that time again....
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