Why Do Volcanoes Erupt? A Step by Step Guide

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NBC Nightly News
John Chancellor/Robert Bazell
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Video News Report
NBCUniversal Media, LLC.
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NBC Science correspondent Robert Bazell explains how volcanoes form, and why Mt. St. Helens erupted tons of ash and dust instead of lava flows.



"Why Do Volcanoes Erupt? A Step by Step Guide." Robert Bazell, correspondent. NBC Nightly News. NBCUniversal Media. 27 May 1980. NBC Learn. Web. 23 January 2015.


Bazell, R. (Reporter), & Chancellor, J. (Anchor). (1980, May 27). Why Do Volcanoes Erupt? A Step by Step Guide. [Television series episode]. NBC Nightly News. Retrieved from https://archives.nbclearn.com/portal/site/k-12/browse/?cuecard=5666


"Why Do Volcanoes Erupt? A Step by Step Guide" NBC Nightly News, New York, NY: NBC Universal, 05/27/1980. Accessed Fri Jan 23 2015 from NBC Learn: https://archives.nbclearn.com/portal/site/k-12/browse/?cuecard=5666


Why Do Volcanoes Erupt? A Step by Step Guide


Mt. St. Helens, today, continued sending steam into the Washington sky and the plumes reaches 9,000 to 17,000 feet. Scientists say they were no earth tremors on the mountain overnight and earlier today, tremors that might indicate another eruption was on the way. Robert Bazell has prepared this special segment report for us on what causes volcano eruptions and why earth tremors precede them.

Unidentified Man: Frrequency 1…phase 1…

ROBERT BAZELL, reporting:

Soon after they detected the first rumblings inside Mt. St. Helens last March, dozens of scientists gathered to listen, watch, and measure as the mountain came alive.

Unidentified Man #2: …best plume we’ve had.

Unidentified Man #3: Now, this is all pioneering stuff?

BAZELL: As the crater on top grew larger and larger.

Dr. TIM HAIT (U.S. Geological Survey): Geologists are plenty excited about the ancient geologic happenings they have seen. When they actually get to participate in and witness geological happenings in their lifetime of this magnitude, you can hardly describe the excitement.

BAZELL: From these studies and what was already known, we can understand some of the causes of the May 18th eruption. Mt. St. Helens, in the foreground, is one of a dozen volcanoes in the Cascade Range. Mt. Rainer in the background and Mt. Hood are among the others. These were formed tens of millions of years ago when huge sections of the Earth’s surface called plates started moving together in the Pacific Northwest at the rate of a few inches a year. As this cross-section of the Earth shows, one plate slides beneath another. Intense pressure, 60 miles beneath the Earth’s surface, turns the bottom plate into molten rock. Every so often, it can be every 25 years or every thousand years, no one knows why. Some of the molten rock, called magma, rises up into the mountain and the mountain is ready to erupt. When the volcanoes in Hawaii, like this one, erupt, the magna, the molten rock, flows out like lava, but when the scientists studied Mt. St. Helens, they found that the magma was very thick and gooey. It couldn’t flow out so enormous pressure built up. The geologists knew Mt. St. Helens was ready in the first weeks of May. Constant earthquakes told them the magma was inside. Pressure from it swelled one side of the mountain. Steam and small amounts of ash were flowing out, but the geologists had thought there would be some warning, such as a very strong earthquake, hours or even days before the actual eruption.

Dr. HAIT: This was clearly a big surprise. Nobody, in any way, predicted that the blast would take the – have the character that it did.

BAZELL: Dr. David Johnston, a geologist, was watching the volcano from the north side on the morning of May 18th. He was one of those killed instantly when without warning, a force equal to a hydrogen bomb blew out the side of the mountain. With the explosion, the magma, the molten rock, became dust and ash that buried communities hundreds of miles away and will be circling the Earth in the upper atmosphere for years. The mountain expelled more than a ton of ash for every human being on Earth, even though some got more than their share. If the ash were put next to the mountain in a pile a half mile on each side, it would be nearly 13,000 feet tall. The research carried out before May 18th will help scientists to predict future volcanic eruptions. Since the eruption, there has been even more research to be done. The geologists are recording every earthquake and shift in the side of the mountain to try to understand what is happening as the volcano died down. That could take years with several more small eruptions and some minor lava flows expected. The scientists are also studying sections of the Earth near the volcano where temperatures in the ground remain above 300 degrees causing new steam geysers to shoot up daily. They will be recording changes in the land where an entire mountainside was blown away, where an old lake was destroyed and a new one is being created. Biologists will watch as life reestablishes itself in areas of devastation. Research on Mt. St. Helens and on the effects of its eruption will continue for decades. Robert Bazell, NBC News, near Mt. St. Helens, Washington.