By James Donahue

It is well known that some very large volcanic eruptions have occurred on this planet in the distant past. It has only been in recent years that scientists have had the tools to determine that there is such a thing as a “supervolcano,” a dome from where the explosion of molten lava, rock and gas can be so massive that it has the capacity to destroy all or almost all life on Earth.

It has long been speculated that Yellowstone National Park is such a supervolcano because of all the geysers, hot springs, and other strange activity constantly occurring there.

But Mt. St. Helens, which last blew its top in May, 1980, spewing smoke and ash for hundreds of miles and leaving 57 people dead, was never suspected of being any more than a normal but very active volcanic precipice. That is until recently.

A team of scientists led by Graham Hill, of GNS Science, Wellington, New Zealand, used magnetotelluric sensors set up around the mountain. The sensors revealed a column of conductive material extending downward from the volcano into a very large zone of what appears to be hot magma about 15 kilometers under the surface.

This massive zone has been found to be so large it extends to Mount Rainier, seventy kilometers to the Northeast, and Mount Adams, fifty kilometers to the East.

Magma can be detected by magnototelluric sensors which measures fluctuations in electric and magnetic fields. The fields fluctuate in response to electric currents traveling under the surface. Such currents can be caused by lightning storms and other phenomena. Stronger currents measure the presence of magma because it is a better conductor than solid rock.

Some scientists who are skeptical about the findings suggest that the presence of water and mud might produce similar findings. Hill agrees and suggests that the conductivity of the material may not be high enough for it to be pure magma, but possibly a mixture of solid and molten rock.

The scientists warn, however, that if the vast structure beneath the three volcanoes is truly a bubble of even partially molten rock, it would be comparable in size to the largest magma chambers ever found, including the one below Yellowstone.

So how dangerous is such a supervolcano? They only blow up every few hundred thousand years so the odds against it happening in our time are very low. Yellowstone last blasted off about 640,000 years ago. If one does blow, however, an eruption of that size can spew enough ash into the atmosphere to cool the world’s climate by several degrees Celsius.

So what could we expect if such a volcano exploded? One report said there would be massive earthquakes, the ground would swell, and once the magma, rock and ash is unleashed, it would be flung 50 kilometers into the atmosphere. It would be the loudest noise ever heard by man.

All life would be immediately killed within a thousand kilometers by falling ash, lava flows and the explosive force of the eruption. Volcanic ash would fall for thousands of miles. There would be enough from either Mt. St. Helens or Yellowstone to coat the entire United States with five inches of ash.

The long-term effects would be even worse. Thousands of cubic kilometers of ash would remain in the stratosphere for months if not years, blocking out the light of the sun and cooling the planet. We would experience a so-called nuclear winter. A large portion of the plant life all over the world would die from the ash and the cold. There would be such massive food shortages that people would starve. The destruction could, indeed, threaten world extinction.

Will St. Helens behave itself? Since the 1980 eruption, the mountain remained dormant for about 20 years. But recently it has been spewing steam and small bursts of smoke and heat that has geologists watching closely. An ominous new dome has appeared inside the crater left by the 1980 blast.

Experts also say new lava rocks have been found that shows fresh magma is mixing with the old. This suggests that a deeper reservoir of fresh, more explosive magma has opened and the danger of a new and large eruption is rising.

As one specialist explained, the site has yielded tangible evidence of a crack in the Earth’s crust extending for miles below the volcano. It is a weak spot that has become a path for the molten rock to follow when it rises to the surface.

“The volcano is there because magma has been rising through that zone of weakness for tens of thousands of years,” said volcanologist Dan Dzurisin.