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 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 other benefits for the young Earth by providing gases to warm the atmosphere and possibly provide protective filtering of the Sun’s harmful radiation, providing fertilizer for the soil – most of which was derived from volcanic outpourings in the first place, providing 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, and most of the surface rocks and materials 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 along 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 knobs 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 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 was witnessed live by so many with the aid of television.
There are dozens of other such events in recorded history that tell a similar story of devastation and loss of life. But now, on February 6th 2009, we are alerted by the Alaska Volcano Observatory to watch for one of the most dangerous volcanoes in the United State, the Redoubt Volcano, now under “code orange watch level alert” as a result of a recent period of growing activity and indications of a possible eruption. Redoubt last erupted in 1989.
So at the close of this article, the possibility of volcanic activity close to home for those who live in North America is a distinct possibility.