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Volcanoes

There are many active volcanoes in the world, but none as dangerous as was Mount St. Helens in Washington State. On May18,1980 the mountain erupted with a high-speed avalanche of hot ash, rock fragments, and gas called a pyroclastic flow which moved down the sides of the volcano. The explosion blew out the side of the mountain, knocked down thousands of trees, damned rivers and destroyed 27 bridges. However because of timely warnings only 57 people perished.

I have a personal interest in the 1980 Mount St. Helens volcano. I was then working as a physicist at the US Air Force Geophysics Laboratory located at Hanscom AFB in Masssachusetts. We had a magnetometer array located across the US from which we observed pertubations in the Earth’s magnetic field, which we related directly to the explosion. These perturbations were observed at stations as far away as 3122 km at Mt.Clemens, Michigan. We were able to measure the velocity of the resulting atmospheric gravity waves as about 316 M/sec, in reasonable agreement with velocities of the pressure wave that was observed on microbarometers by the Japan Meteorological Agency, from 305 to 309 M/sec.This article :

"AFGL Magnetometer Observations of Mount St. Helens Eruption by P.F.Fougere and C.W. Tsacoyeanes appeared in EOS Transactions of the American Geophysical Union, vol. 61 1209-1210, 1980."

Mt. Rainier volcano looms over the Seattle/Tacoma area. If it explodes it will endanger a population of more than three million people. In Mexico, South of Mexico City, Mt.Popocatépetl has begun to come to life again, putting a million nearby residents at risk. Another million people living in the Naples, Italy area are threatened by Mt. Vesuvius’ continued unrest.

Most volcanoes occur at tectonic plate boundaries where two plates are diverging or converging or subducting. See the tectonic plates page for more on tectonic plates

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October 11, 2009 Ash and Steam Plume, Soufriere Hills Volcano, Montserrat

NASA Earth Observatory Photo from International Space Station

More than 500 volcanoes are now active in the world.

The “Ring of Fire” which abuts the west side of the continents of North and South America and the eastern side of Asia traces the plate boundaries with hundreds of volcanoes in the Pacific ocean.

The most horrific volcanic eruptions in recorded history occurred at the island of Krakatau, near Indonesia, on Aug. 26 and 27,1883. These four explosions destroyed two thirds of the island, produced prodigious amounts of very hot pyroclastic flows which burned and killed more than 4,500 people and initiated horrible tsunamis. The explosions and the tsunamis killed over 36,000 people.

Volcanic eruptions since 1927 have built a new cone called “Anak Krakatau” which means child of Krakatau. It has been erupting in many years since; these eruptions have been mostly quite mild but serve as a reminder of the tragic events of 1883

Here are three more recent volcanoes :Kilauea,Nyiragongo and Klyuchevskaya

All these volcanoes might be called “ordinary volcanoes”. Besides these, there have been “SuperVolcanoes” which occurred before recorded history from several hundred thousand to a few million years ago. These supervolcanoes are recorded in the rocks.


Go to Kilauea Volcano

Go to Nyiragongo


Go to Klyuchevskaya

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Much earlier than Krakatau, The 79 AD volcanic eruption of Mt. Vesuvius was described by the scholar and historian Pliny the Younger who witnessed the volcanic explosion in which his uncle Pliny the Elder, historian and admiral of the Roman Fleet was killed.
This violent eruption produced a tall tree shaped cloud that rose above the volcano and consisted of very hot pyroclastic flow with ashes, rock and gasses. This flow landed on and destroyed three cities , Herculaneum, Pompeii and Stabiae and buried more than 16,000 people who remained buried for nearly 1,700 years until 1748 when archaeologists began uncovering the remains of Herculaneum.The eruption of Mt. Vesuvius and its aftermath have been described and reenacted in the beautiful book:

"Pompeii: A Novel " by Robert Harris.

Here is an eye-witness account by Pliny the Younger.

"My uncle was stationed at Misenum, in active command of the fleet. On 24 August, in the early afternoon, my mother drew his attention to a cloud of unusual size and appearance. He had been out in the sun, had taken a cold bath, and lunched while lying down, and was then working at his books. He called for his shoes and climbed up to a place which would give him the best view of the phenomenon. It was not clear at that distance from which mountain the cloud was rising (it was afterwards known to be Vesuvius); its general appearance can best be expressed as being like an umbrella pine, for it rose to a great height on a sort of trunk and then split off into branches, I imagine because it was thrust upwards by the first blast and then left unsupported as the pressure subsided, or else it was borne down by its own weight so that it spread out and gradually dispersed. In places it looked white, elsewhere blotched and dirty, according to the amount of soil and ashes it carried with it.

“My uncle's scholarly acumen saw at once that it was important enough for a closer inspection, and he ordered a boat to be made ready, telling me I could come with him if I wished. I replied that I preferred to go on with my studies, and as it happened he had himself given me some writing to do.

“As he was leaving the house he was handed a message from Rectina, wife of Tascus whose house was at the foot of the mountain, so that escape was impossible except by boat. She was terrified by the danger threatening her and implored him to rescue her from her fate. He changed his plans, and what he had begun in a spirit of inquiry he completed as a hero. He gave orders for the warships to be launched and went on board himself with the intention of bringing help to many more people besides Rectina, for this lovely stretch of coast was thickly populated.

“He hurried to the place which everyone else was hastily leaving, steering his course straight for the danger zone. He was entirely fearless, describing each new movement and phase of the portent to be noted down exactly as he observed them. Ashes were already falling, hotter and thicker as the ships drew near, followed by bits of pumice and blackened stones, charred and cracked by the flames: then suddenly they were in shallow water, and the shore was blocked by the debris from the mountain.

“For a moment my uncle wondered whether to turn back, but when the helmsman advised this he refused, telling him that Fortune stood by the courageous and they must make for Pomponianus at Stabiae. He was cut off there by the breadth of the bay (for the shore gradually curves round a basin filled by the sea) so that he was not as yet in danger, though it was clear that this would come nearer as it spread. Pomponianus had therefore already put his belongings on board ship, intending to escape if the contrary wind fell. This wind was of course full in my uncle's favour, and he was able to bring his ship in. He embraced his terrified friend, cheered and encouraged him, and thinking he could calm his fears by showing his own composure, gave orders that he was to be carried to the bathroom. After his bath he lay down and dined; he was quite cheerful, or at any rate he pretended he was, which was no less courageous.

“Meanwhile on Mount Vesuvius broad sheets of fire and leaping flames blazed at several points, their bright glare emphasized by the darkness of night. My uncle tried to allay the fears of his companions by repeatedly declaring that these were nothing but bonfires left by the peasants in their terror, or else empty houses on fire in the districts they had abandoned. Then he went to rest and certainly slept, for as he was a stout man his breathing was rather loud and heavy and could be heard by people coming and going outside his door. By this time the courtyard giving access to his room was full of ashes mixed with pumice stones, so that its level had risen, and if he had stayed in the room any longer he would never have got out. He was wakened, came out and joined Pomponianus and the rest of the household who had sat up all night.

“They debated whether to stay indoors or take their chance in the open, for the buildings were now shaking with violent shocks, and seemed to be swaying to and fro as if they were torn from their foundations. Outside, on the other hand, there was the danger of failing pumice stones, even though these were light and porous; however, after comparing the risks they chose the latter. In my uncle's case one reason outweighed the other, but for the others it was a choice of fears. As a protection against falling objects they put pillows on their heads tied down with cloths.

“Elsewhere there was daylight by this time, but they were still in darkness, blacker and denser than any ordinary night, which they relieved by lighting torches and various kinds of lamp. My uncle decided to go down to the shore and investigate on the spot the possibility of any escape by sea, but he found the waves still wild and dangerous. A sheet was spread on the ground for him to lie down, and he repeatedly asked for cold water to drink.

“Then the flames and smell of sulphur which gave warning of the approaching fire drove the others to take flight and roused him to stand up. He stood leaning on two slaves and then suddenly collapsed, I imagine because the dense, fumes choked his breathing by blocking his windpipe which was constitutionally weak and narrow and often inflamed. When daylight returned on the 26th - two days after the last day he had been seen - his body was found intact and uninjured, still fully clothed and looking more like sleep than death.”

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