Violent and devastating, legendary in history and mythology, volcanoes in action are among the most powerful forces of nature. They have shaped the Earth for billions of years and continue to do so, sometimes with deadly consequences to life and property.
Awesome to witness, erupting volcanoes are natural geologic structures providing a conduit to the Earth's surface for the extrusion of molten rock materials migrating upwards from the depths beneath the Earth's crust. Volcanoes can occur on land or on the seabed beneath the oceans.
Molten rock, or magma, to use its proper geological name for a body of molten rock found at depth, exists under pressure deep within the Earth. When overlaying solid rocks provide a channel, perhaps because of the constantly occurring internal adjustments of the surrounding rocks, the magma may escape to move upward to regions of lower pressure until eventually emerging at the surface, acquiring the new name: lava. On the other hand, magma that cannot find a outlet will become trapped at some sub-surface location for ever.
Magma and lava are similar but not the same. With the relief of pressure on the upward moving magma, much of the gaseous component is able to escape, thus changing its composition. This modified molten rock will emerge at the surface and then be called lava. In some cases the volcanic lava reaches the end of its upward journey with a volatile mixture of molten rock, gases, and water, expanding and exploding with such force that molten lava, rock fragments and ash are violently propelled into the air to great heights.
Active volcanoes release carbon dioxide and other gasses into the atmosphere, including significant amounts of water vapor. Volcanoes are not rare, and with tens of thousands of volcanoes erupting over the hundreds of millions of years of the planet's early existence, the water vapor released by volcanoes provided the source of much of the Earth's original oceans and much of its atmosphere.
Although the dramatic scenes of devastation resulting from volcanic eruptions capture our attention and imagination, volcanoes have also created many benefits for the young Earth. Volcanoes have produced gases that escape to warm the atmosphere and these gases contribute to the protective filtering of the Sun's harmful radiation. Volcanoes provide fertilizer for the soil - most of which was derived from volcanic outpourings in the first place, they provide liquid in the form of water and nutrients and a habitat that helps make life possible. Volcanoes have created and are still creating all of the sea floor of the Earth's oceans as a result of the outpourings at the mid-ocean ridges, those mountain chains that stretch around the globe, rising from the seabed. Most of the surface rocks and materials of the Earth's crust are of volcanic origin. It is estimated that there are thousands of active volcanoes and also many dormant volcanoes that may re-awaken in the future. There are also many extinct volcanoes.
Volcanoes occur all over the Earth, but many are concentrated at the edges of continents, or beneath the sea where they form underwater volcanic mountain ranges, or long chains of islands such as those of Hawaii. A large number of active volcanoes encircle the Pacific Ocean basin and have acquired the name "Pacific Ring of Fire". There is a particular reason for this configuration over thousands of miles and the explanation for this involves a well established geological theory known as Plate Tectonics - but that is a little too complex for discussion here. That same theory also explains the occurrence of volcanoes at the edges of continents.
The shape and structure achieved by volcanoes depends on the composition of the erupting lava and the amount of energy and force at their creation, with the main categories being, somewhat descriptively: cinder cones, composite volcanoes, shield volcanoes, and lava domes. Following is a brief description of these several types.
An example of a Cinder Cone is the famous Paricutin Volcano of Mexico that quite suddenly, in 1943, started to grow out of a farmer's field with explosive eruptions of molten lava into the air, forming cinders that fell back to Earth around the point of eruption. As this continued, a cone of cinders slowly took shape until reaching a height of 1200 feet. Paricutin stayed active for nine years, emitting ash that covered the surrounding area for miles and destroyed a nearby town.
Composite Volcanoes, as their name implies, are formed of a composite of alternating layers of hardened lava and rock fragments. Composite volcanoes are also called Strato volcanoes and achieve the well known high peaked form, sometimes snow covered, like the often pictured 12,400 feet high Mount Fuji, one of Japan's holy mountains. Other well-known composite volcanoes are Vesuvius and Stromboli. There are several variations of the composite shape.
Shield Volcanoes are formed by lava that flows easily and without the potential violence of some of the other forms. They are much flatter with broad summit areas and gently sloping sides. Many of the largest volcanoes on Earth are shield volcanoes.
Probably the best example is the Hawaiian Islands, all of which are shield volcanoes and the tallest of these, measured from its base on the ocean floor, is Mauna Kea, higher at 30,000 feet than Mount Everest. The Hawaiian Islands are not like those of the Pacific Ring of Fire but are called plume volcanoes. The molten material that feeds plume volcanoes originates from very deep in the Earth's mantle, thought to be possibly from about 1900 miles below the surface, far deeper than the magma source for other types of volcanoes.
Lava Domes are formed from very thick lava that flows slowly with difficulty, cooling and crystallizing before traveling far from its exit vent and are often comprised of more than one flow, forming lumpy chunks of hardened lava. Lava domes often occur in the craters or on the sides of composite volcanoes.
Volcanic eruptions are most often remembered for the devastation they have caused and many eruptions have become especially infamous. Almost everyone has heard of the Indonesian island of Krakatoa where in 1883 two thirds of the island vanished with an explosion estimated to be 10,000 times more powerful than the atomic bomb explosion over Hiroshima at the end of World War Two. This awesome Krakatoa event was followed by a deadly tsunami.
Another, equally well known event, is the eruption in Italy of Mount Vesuvius in 79 AD where the population of nearby Pompeii was overwhelmed and buried almost instantly to be found and excavated centuries later, becoming an attraction for historians and ordinary visitors.
Also in the Mediterranean, about 1600 BC, the island of Santorini and its civilization, now revealed by extensive archaeological excavations, was almost destroyed in the largest volcanic eruption in the last 10,000 years. It is believed to have killed more than a million people and wiped out the entire Minoan civilization on the island of Crete. Santorini may also have been the inspiration for Plato's island of Atlantis.
Other similarly devastating eruptions were: Tambora, Indonesia, in 1815, with the aftermath causing 92,000 deaths. The eruption of Mount Pelee in the Caribbean in 1902 destroyed the city of St.Pierre and claimed the lives of all 29,000 inhabitants except for one person who was a prisoner in an underground jail cell.
And in recorded history there have been dozens of other volcanic events that tell a similar story of devastation and loss of life.
And of course, in the United States in 1980 in the State of Washington, the beautiful composite Mount St. Helen's lost about1200 feet of its summit height in a spectacular eruption that, with the aid of television, was witnessed live by so many of us.
And more recently in the United States, in Alaska, where there are many volcanoes, the Mount Redoubt Volcano that had been under close observation for many months, finally erupted on March 22nd of this year, 2009, with considerable violence that sent a massive cloud of volcanic ash to 50,000 feet into the stratosphere. No doubt there will be other such eruptions throughout the world.
Some volcanoes erupt explosively. One such, referred to above, being that of Mount St. Helen, when on May 18, 1980, one of the largest eruptions in the recent history of North America occurred. While not always erupting with such violence, volcanoes have played a major part in shaping the Earth's crust over the course of time. Some extreme eruptions, given the name "super-volcanoes", have, at different prehistoric times millions of years ago, created vast volumes of volcanic rock over huge areas in such regions as Yellowstone in the United States, Japan, the North Island of New Zealand, the deccan traps of India and the traps of Siberia, among others. Such stupendous volcanic events as those, are thought to have been major contributing causes in several of the mass extinctions that have occurred throughout the Earth's four and a half billion year history, including the well known mass extinction event associated with the demise of the dinosaurs 65 million years ago.
Jim Robinson, the author of this article, has an interest in science and the Earth sciences, mainly as a reader rather than as a student.
Now retired, Jim Robinson has recently launched a blog-site targeted to persons diagnosed with diabetes, of which he is one, offering information and experience from the perspective of the diabetic patient rather than that of the health care professional, in a disease in which the control of blood sugars and the management of the condition is so much in the hands of the individual diabetic. He invites you to visit at his diabetic menu blog-site, and check it out.
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