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Since the Earth was young, volcanoes have hurled fiery rock from the planet's interior

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to its surface.

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In one way or another, they have affected almost every spot on the Earth's crust.

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Here, in the mid-Atlantic Ocean off the south coast of Iceland, volcanic rock and ash have

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risen from the ocean floor to form the island of Surtsey, new land on the face of the Earth.

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This loosely accumulated pile of ash could be washed away by wind and waves.

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But lava flowing over the surface has provided a protective cap for the island.

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The volcanic land building process has produced an island where before there was only sea.

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Surtsey and Iceland are located in an area of intense volcanic activity.

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Shortly after the appearance of Surtsey, violent underwater explosions caused by the meeting

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of the fiery rock and cool water announced the appearance of yet another volcano.

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Again, volcanic rock and ash, accompanied by billowing clouds of steam, shoot into the

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air and build a new island.

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And again, a battle begins against the processes of erosion.

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Of the two new islands that now stand off Iceland's coast, only one is protected by lava.

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And in a few months, only Surtsey remains.

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For man, the formation of Surtsey has been a spectacular and fascinating event, a rare

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glimpse of the process by which volcanoes change the face of the Earth.

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But in the lifetime of the planet, it is only an insignificant incident, a tiny chapter in

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a story that has been unfolding for billions of years.

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The world's active volcanoes number more than 500, most distributed in well-defined

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belts along the edges of continents.

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By examining four of them, in Hawaii, Mexico, Italy and Iceland, we can see examples of

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the basic types of volcanic eruption.

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The Hawaiian Islands, a huge range of mountains that rose from the floor of the Pacific more

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than 11 million years ago, are formed largely of volcanic material.

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Molten rock from chambers deep within the Earth continues to build the islands and change

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their shape.

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In modern times, one of the most active Hawaiian volcanoes has been Kilauea on the island of

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Hawaii.

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At Kilauea's summit, steam and other gases pour almost continuously into the atmosphere,

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giving evidence of the intense heat and activity of the molten rock below.

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The molten rock, called magma, is believed to come from a chamber beneath the volcano's

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summit.

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Most of the magma originates from deeper chambers, 50 to 100 kilometers below the surface of

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the Earth.

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Magma may travel directly upward from the reservoir, or occasionally it may flow outward,

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opening vents on the sides or flanks of the volcano.

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On the flank of Kilauea in 1969, a new eruption began when a vent opened about 10 kilometers

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from the mountain's main crater.

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Magma that reaches the Earth's surface is called lava.

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Expanding gas within the molten rock drives it upward to the vent, where it churns, bubbles,

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and erupts onto the Earth.

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In the volcano's initial stages, small eruptions, each lasting a few hours, occur repeatedly

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over a period of weeks.

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Spectacular orange fountains of lava soar more than 100 meters into the air.

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As the volcano reaches the peak of its activity, the fountains reach heights of more than 500

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meters, taller than any structure built by man.

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Expanding gas causes lava to froth and break into small pieces.

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As it comes in contact with the atmosphere, some of the molten material heartens into

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a light porous rock called pumice, which falls to the Earth in a fiery shower.

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But most Hawaiian lava remains in liquid form and pours from the site of the fountain.

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The cascading flows of molten rock spread over the land and eventually harden.

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The result of this process is the landform known as a shield volcano.

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Shield volcanoes are formed in areas where magma is very fluid.

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Repeated flows of lava form layers of rock that build mountains with broad, sloping sides.

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After more than a year of successive lava outpourings, Kilauea's flank eruption has built a new mountain,

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Manaulu. It is more than a kilometer wide and over 100 meters high.

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The small vent from which the eruption began is now about 20 times its original size.

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For the time being, spectacular lava fountains have subsided, but the eruption continues.

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Inside the vent, lava is in constant motion as expanding gas is released from within.

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Small fountains of red lava break through the hardened bluish crust of rock

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that is constantly ruptured and reformed at the surface of the lava lake.

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In this type of eruption, no lava spills out over the crater's rim.

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But lava is still flowing from the lake through lava tubes,

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naturally formed channels left beneath the field of hardened lava that surrounds the crater.

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Here, a window in a lava tube shows an underground river,

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fiery lava rushing through the tube to the edges of the mountain.

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Beneath the cover of insulating rock, the lava holds its temperature of more than 1000 degrees centigrade,

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ten times the temperature of boiling water.

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The tube follows the slope of the shield to the sea, a distance of more than 15 kilometers.

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Near the ocean, lava emerges from the tube to continue its flow over open land.

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This lava may travel as rapidly as 45 kilometers per hour,

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but as it gets farther and farther from its source, it gradually loses gas and heat.

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As it cools, the motion of the flow is reduced to a crawl.

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Two distinct types of lava are given Hawaiian names.

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Fast-flowing, ropy lavas are called pahoehoe.

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Moving across the land, much like an amoeba,

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pahoehoe advances by extruding molten toes of lava beneath a thin, flexible crust.

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The thick, blocky flows that form as lava loses heat and gas are called a'a.

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The crust is brittle and rough.

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The flows are massive, and their motion can be so slow and ponderous that they appear to be standing still.

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As lava advances across the island's surface, some cools and hardens,

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adding another layer to the shield volcano.

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But vast, billowing clouds of steam announce that other flows have reached the sea.

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The molten lava solidifies. New land is formed.

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The boundaries of the island are extended outward.

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Manuulu, the new mountain, has revealed the gradual land-building process

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by which shield volcanoes have built the Hawaiian islands over a period of millions of years.

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Not all volcanoes build the land so gradually.

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Five thousand kilometers east of Hawaii, there is a different type of volcanic eruption

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in another zone of volcanic activity, Mexico.

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This was once the site of a Tarascon Indian community.

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Today, the streets, shops, and dwellings of more than 500 people lie buried beneath lava.

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A short distance away, a steep-sided cinder cone marks the spot where Pericotin volcano

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heaved rock and ash from the earth for a period of nine years.

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In 1943, thick, sticky lava, driven by large volumes of gas, exploded from Pericotin's vent.

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The material that was blasted into the ground was then used to make the lava.

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The air cooled and solidified. Much of it fell back around the vent, forming a cone-shaped mountain of cinders.

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The rest, carried by wind, spread a choking blanket of ash and dust over an area of 25 square kilometers.

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At night, the fiery glow of falling cinders clearly showed the building of the cone.

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While the lava that buried the village poured from several small vents around the base of the new mountain.

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Pericotin erupted from a place where no volcano had been before.

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Now it is dormant. Its activity seems to have come to an end.

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But we know that some volcanoes have remained inactive for hundreds, even thousands of years,

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and then unexpectedly erupted again.

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One such volcano can be found in the Mediterranean region, another area with a history of volcanic activity.

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Here, the city of Naples, Italy, stands in the shadow of Mount Vesuvius.

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Vesuvius is a composite cone, or stratovolcano, built by many eruptions of ash, cinders, and lava.

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Long periods of time can pass between eruptions.

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After lying dormant for 40 years, Vesuvius erupted violently in 1944.

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The whole world watched in awe as newsreels showed villages at the foot of the mountain, crumbling before the advancing lava.

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Today, again, the mountain is quiet. The lava flows of the 1944 eruption tower over the city.

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But man continues to live and build on the side of the volcano.

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The crater of Mount Vesuvius shows no evidence of active volcanism.

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But the volcano has appeared peaceful and quiet many times in the past, and just as many times, it has exploded.

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Vesuvius's earliest recorded eruption is now its most famous.

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In the year AD 79, with little or no warning, Vesuvius' volcano erupted.

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It erupted and completely buried the ancient city of Pompeii.

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For two days, the mountain shot volcanic material into the air, spreading a blanket of ash over the landscape.

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Pompeii, located seven kilometers from the volcano, was buried under five meters of ash.

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The devastation was so complete that it was not until 1600 years later that the city was discovered.

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About three-fifths of Pompeii has now been excavated.

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Evidence of the religion, culture, and everyday life of the Pompeians, all abandoned in the sudden panic of the volcano's catastrophe, are remarkably well preserved.

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And everywhere, there is the shadow of Vesuvius and reminders of its sudden and brutal eruption.

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Even the forms of the citizens remain. Their bodies, cast in plaster from the cavities they left in the volcano's ash, lie much as they did on the day of their death, almost 20 centuries ago.

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The disaster that struck the people of Pompeii is ancient history, but volcanoes continue to affect man.

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In 1973, Hemaia Island, a fishing center off the coast of Iceland, became a modern-day Pompeii.

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After 6,000 years of dormancy, a volcano on the outskirts of the city of Vestmonar erupted without warning.

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The eruption occurred in early morning. Within a matter of hours, it deposited a cover of black volcanic ash over the island.

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The 5,000 inhabitants were forced to abandon their homes and businesses.

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Their thriving community, the product of decades of human activity on this lush and fertile island, was wiped away by the volcano in less than a week.

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People of Pompeii knew next to nothing about volcanoes.

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Twenty centuries later, modern man knows much more.

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But the knowledge of scientists was of little use to the people of Hemaia Island.

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Their homes, like the homes of the Pompeians, are gone, taken from them by the earth without a hint of warning.

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How can we apply what we know about volcanoes to benefit the millions of people who live in their shadows?

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How can we predict with more certainty when and where they will erupt?

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Will it ever be possible for us to turn them to our advantage by harnessing their awesome power?

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These questions, and many others, remain to be answered.

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Thank you for watching.