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Volcanoes - Basic explanation

 

Volcanoes
Volcano - photograph

 

Beneath the seemingly quiet surface of our planet, lies a boiling mass of molten rock (magma), heaving and seething like the contents of a witches couldron. The earth's crust keeps a lid on this deadly brew, but where it can find a weak spot along the great fault lines (see my pages on Plate Tectonics and The Earth's Crust), it can erupt with a force and power unimaginable. More than 80 percent of the earth's surface is volcanic in origin. The sea floor and some mountains were formed by countless volcanic eruptions. Gaseous emissions from volcano formed the earth's atmosphere. A volcano is a mountain that opens downward to a pool of molten rock below the surface of the earth. When pressure builds up, eruptions occur. Gases and rock shoot up through the opening and spill over or fill the air with lava fragments. Eruptions can cause lateral blasts, lava flows, hot ash flows, mudslides, avalanches, falling ash and floods. The pressure blast from volcanic eruptions have been known obliterate entire forests. Extremely dangerous by-products of erupting volcanoes are- tsunamis, flashfloods, earthquakes, mudflows and rockfalls.

Examples of some of the most awesome and famous volcanic eruptions can be seen on this page.

 

Krakatoa

26th. August 1883 - Krakatoa, a triple coned volcanis island in the Sunda strait midway between Java and Sumatra, started a sequence of eruptions that culminated on the following day, with four huge explosions destroying two-thirds of the island. The third of these explosions was the loudest in recorded history, being heard 4,800km (3,000mls) away.


Minor tremours and explosions were felt in the area around Krakatoa on Sunday 20th. May 1883 and a large column of steam was pouring from a fissure near the crater.


On 11th. August, a survey team landed on the island and found a blanket covering of volcanic dust over 50cm (20in) deep, and steam was spouting from several areas on the island.


On the 26th. August the first major explosion occurred awakening the inhabitants of nearby islands. The captain of a ship passing the island at a distance of 65km (40mls)
stated "Krakatoa was a terrifying glory….an immense wall, with bursts of forked lightning darting through it". Random explosions continued throughout the day.


On the 27th. August a gigantic explosion destroyed the cone of the volcano, hurling rocks over 2m (7ft) wide over a hundred mile radius. Atmospheric shock waves circled the Earth six times. The explosion was heard as far afield as the USA and Australia. Rock, ash, dust, and pumice was blasted 80km (50mls) into the atmosphere, blocking out the light for over 450km (280mls) in circumference. This darkness lasted for 22 hours inside a radius of 210km (130mls) around the island. The enormous dust cloud was blown around the World for three years, lowering annual temperatures and creating brilliant red sunsets in every part of the globe.
The collapsing volcano turned the sea into a raging maelstrom swamping all the ships in the coastal areas. Seismic tidal waves (Tsunami) up to 130ft high raced along the coasts of Sumatra and Java destroying 295 towns in it's path. North of Krakatoa the island of Sebesi was completely submerged, drowning all of the 3,000 islanders.

At Merak, on a 135 ft high hill, hundreds of people crammed into stone houses seeking safety, were washed away by a huge wave that engulfed the hill, and everything on it. Only two people out of the total population of 3,000 survived that day of terror.The steamship 'Berouw' was ripped from it's moorings in the Terok Betong harbour in Sumatra, and washed one and a half miles inland. 2,000 miles away in Calcutta, giantic waves swamped river boats. Halfway around the world the English Channel experienced giant waves, and very high tides. Other volcanic activity was triggered in the area causing large areas of the coast of Java and the Malay Archipelago to disappear under the sea. Small islands sank and new volcanic peaks appeared.


Krakatoa itself was uninhabited, but throughout the vast area of destruction caused by the rain of rocks, ash, fire, and the toll from the Tsunami, an estimated 200,000 people were said to have perished, and 6,500 ships and boats were sunk. The island was reduced to a barren wasteland 5km (3ml) in diameter.

 

Mount St. Helens

 

18th May 1980 - There was a series of earthquakes, the mountain then exploded with tremendous force. The blast was heard as far away as Montana, Idaho, Canada and California.
From the north side which had broken away, a torrent of rock, glacial ice, boiling water, and steam roared down the mountainside at speeds scientists have calculated started at about 220 miles per hour but increased to about 670 miles per hour. The mass of debris poured into Spirit Lake and Toutle River below, collecting the water forming a giant mudflow, carrying before it trees, houses, bridges, and debris of all kind including boulders up to 60 feet in diameter. The collapse of the north face, released the pressure within the volcano, and a tremendous horizontal explosion sent a searing blast of ash which together with the avalanche scoured more than 520sq km (200sq ml).

A second explosion, this time vertically, sent a plume of smoke, ash, and debris 12 mile high, and so huge it created it's own lightning, with a frightening display. Six hundred thousand tons of ash fell on Yokima, Washington, 145km (90 ml) east of the volcano, turning a sunny clear day, into a blackness that could not even be penetrated by headlights because of the dust particles in the air. All traffic was brought to a halt due to clogged filters. Half the States Police patrol cars were immobilized.

The eruption cost 57 lives and many injuries. Many buildings were buried and more than 200 houses and cabins were destroyed. Many tens of thousands of acres of prime forest, as well as recreational sites, roads and trails were destroyed or heavily damaged. More than 185 miles of highway and roads were destroyed or extensively damaged. The Columbia River had a 20 mile logjam caused by timber and debris, reducing the navigation channel from 40 feet to 15 feet, preventing seagoing ships passage. Four billion board feet of timber was damaged or destroyed and many animals, including dear, elk and bear were killed. Millions of salmon and trout were killed either by the overheating of the Toutle and Cowlitz Rivers, or by the tons of ash clogging their gills. Many small animals, such as rodents, frogs and crawfish managed to survive because they were below ground level or water surface. . A mobile home being driven 11 miles away was tossed 600 feet, and a lorry parked 13 miles away, had all the plastic parts melted by the searing heat of the blast.

After many months hard work by the US Army Corps of Engineers, 100 million cubit yards of debris were removed from the Toutle, Cowlitz, and Columbia Rivers at a cost of $250 million. 58 people perished and the damages amounted to $1.2 billion.

 

Other Famous Volcanoes

 

Caribbean - Mount Pelee

 

8th. May 1902 - A sea of mud buried a sugar mill and 150 workers. 50 people and 200 animals killed by snakes that swarmed into the streets from the mountainside. The massive eruption covered the town with molten lava, burning ash, gas, and large debries. Almost all the people and the buildings were gone in an instant. Only 70 people in the city survived. Among them was a prisoner, locked in a stone cell, who was found several days later, scorched but still alive. Between 30,000 and 40,000 people died in the horror.

 

Columbia - Nevado del Ruiz

 

13th. November 1985 - The eruption melted a tenth of the ice-cap, sending an avalanche of mud, rocks, and ashes, roaring down the valley of the Azufrado River. Travelling at 36 km/h ( 22mph) a 30m (100 ft) wall of mud and debris demolished the town of Armero, destroying all but 100 of the 5,000 houses there, and killing 25,000 people.

 

Iceland - Laki Mountain

 

8th. June 1783 - The Mountain burst open a 24km (15ml) fissure, disgorging the largest lava flow ever recorded. Up to 80km (50ml) long and 30m (100ft) deep, it engulfed village after village as it flowed accross the land. Molten rock was hurled 1km (3/4ml) into the air, gas explosions blasted ash, dust, and debris up to a height of 12km (7.5ml). The eruptions did not cease until February 1784, by which time the lava had spread over an area of 600sq/km (230sq/ml). Two-thirds of the country's cattle, sheep, and horses perished, and almost all of the coastline's fish. One-quarter of the population died from the famine that followed.

 

Sicily - Mount Etna

1169 - Over 15,000 people huddled in the Cathedral in Catania were killed when it collapsed, following an earthquake triggered by an eruption.

11th. March 1669 - The most violent eruption was in this year, when a river of boiling lava poured down, moving at 15ft per second engulfing everything in sight. 30,000 to 100,000 people died.

This is the most active volcano in the World, 'burning' 10 to 20 times every century. Since records began, Etna has erupted over 200 times. This does not include hundreds of ventings not qualifying as 'eruptions'.

 

 

Intensity Scale - Volcanoes

 

The Volcanic ExplosivityIndex

 

0
None Explosive (Hawaiian) Plume: <100m Volume:> 1000 m3
1
Gentle (Hawaiian) Plume: 100-1000 m Volume:> 10,000 m3
2
Explosive (Hawaiian - Strombolian) Plume: 1-5 km Volume:> 1,000,000 m3
3
Explosive (Strombolian - Vulcanian) Plume: 3-15 km Volume:> 10,000,000 m3
4
Cataclysmic(Vulcanian - Plinian) Plume: 10-25 km Volume:>100,000,000 m3
5
Paroxysmal (Plinian) Plume: >25 km Volume:> 1 km3
6
Colossal (Plinian - Ultraplinian) Plume: >25 km Volume:> 10 km3
7
Super-colossal Plume: >25 km Volume:> 100 km3
8
Mega-colossal Plume: >25 km Volume:> 1,000 km3

 

 

The largest volcano on Earth is 'Mauno Loa' in Hawaii. It is 4,170m (13,681ft) high, and began forming nearly a million years ago. It takes up almost half of the island of hawaii.

 

 

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