Mud Volcanoes
Volcanoes are vents or fissures in the earth's crust through which gases, molten rock, or lava, and solid fragments are discharged. Their study is called volcanology. Volcanism is the name given to the processes and phenomena associated with the surficial discharge of such material from volcanoes, geysers, and fumaroles. The term volcano is commonly applied both to the vent and to the conical mountain (cone) built up around the vent by the erupted rock materials.
Volcanoes are described as active, dormant, or extinct. The soil resulting from decomposition of volcanic materials is extremely fertile, and the ash itself is a good polishing and cleansing agent.
Active volcanoes are not scattered over the Earth randomly; instead, most occur in belts, especially in the island arcs and mountain ranges bordering the Pacific Ocean. The concept of seafloor spreading and, more broadly, the theory of plate tectonics offer a logical explanation for the location of most volcanoes.
Lava is molten rock that erupts on the earth's surface, either on land or under the ocean, by a volcano or through a fissure. It solidifies into igneous rock that is also called lava. Before reaching the Earth's surface, the mixture of solid and liquid rock, and gases, is known as magma. Lavas are composed chiefly of silica and the oxides of aluminum, iron, magnesium, calcium, sodium, and potassium. Silica, with soda and potash, predominates in the light-colored, acid felsites; iron oxides, lime, and magnesia, in the dark-colored, basic basalts . Rock froth forms on the upper part of a lava flow if bubbles solidify before the gas can escape. Light-colored, glassy froth is pumice ; dark, cindery or slaggy froth, of a coarser texture than pumice, forms what is known as scoriae.
Lava flows which solidify as a mass of blocks and fragments with a rough surface are called block lava, or aa; those which solidify with a smooth, ropy, billowy surface are known as corded lava, or pahoehoe. Lava can sometimes cover wide regions through great fissures in the earth's surface, as in the ancient Columbia River plateau of the NW United States, where it is spread over 30,000 sq mi (77,700 sq km) and is up to 5,000 ft (1,524 m) deep. Other such regions are found in the Deccan plateau of India, in E Brazil, and in Iceland.
Submarine lavas develop through volcanic activity along the mid-oceanic ridges and plate boundaries, where the mid-oceanic ridges produce more lava than any continental eruptions. Such underwater eruptions also harbor rich fauna unique to the vent area, such as red tube worms and giant clams, whose food supply is based on the hydrogen sulfide abundant in the vent waters.
Unique features include black smokers, or hot springs of mineral-rich water that belch out from the ocean ridge where it is most active. In many instances the reasons for the heat and liquidity of magma, its exact source, and the causes of its rise in the earth are not clearly known, though the volcanic activity is often related to seafloor spreading . Other volcanic areas also lie along colliding plate boundaries and around rising magma hot spots. See plate tectonics.
Volcanoes are found in association with midocean ridge systems - seafloor spreading - and along convergent plate boundaries, such as around the Pacific Ocean's Ring of Fire, the ring of plate boundaries associated with volcanic island arcs and ocean trenches surrounding the Pacific Ocean. Continental volcanoes are also associated with converging plate boundaries, such as the volcanoes of the Cascade Range along the W coast of the United States. Isolated volcanoes also form in the midocean area of the Pacific apparently unrelated to crustal plate boundaries. These sea mounts and volcanic island chains, such as the Hawaiian chain, may form from rising magma regions called hot spots.
Volcanic Cones and Craters
Shapes of volcanoes include composite cones, or stratovolcanoes, with steep concave sides such as Mt. St. Helens in the W United States; shield cones have gentle slopes and can be relatively large such as the Hawaiian Islands; and cinder cones as Parícutin in Mexico, with steep slopes made of cinderlike materials. Explosive eruptions build up steep-sided cones, while the nonexplosive ones usually form broad, low lava cones. Cones range in height from a few feet to nearly 30,000 ft (9 km) above their base.
Usually the cone has as its apex a cavity, or crater, which contains the mouth of the vent. Such craters are typically less than 1 mi (1.6 km) across, but larger craters, called calderas, ranging in diameter from 3 mi to - in a few instances - 50 mi (5-80 km), are formed by particularly large eruptions.
Craters are circular, bowl-shaped depression on the earth's surface. Simple craters are bowl-shaped with a raised outer rim. Complex craters have a raised central peak surrounded by a trough and a fractured rim.
Many of the largest craters are formed by the impact of meteorites . Impacting at speeds in excess of 10 mi/sec (16 km/sec), a meteorite creates pressures on the order of millions of atmospheres, producing shock waves that blast out a circular hole and often destroy the meteorite. Meteor, or Barringer, Crater, near Winslow, Arizona, c. 34 mi (1 15 km) in diameter and 600 ft (180 m) deep, is probably the best-known crater of this type. Of the more than 120 impact craters identified on earth, the largest are at Manicouagan, Canada; Vredefort, South Africa; and Chicxulub (off the coast of the Yucatán peninsula), Mexico. Others include Chubb Crater, Quebec; Lake Bosumtwi, Ghana; and Brent Crater, Ontario. Two major impact events have occurred in the 20th cent., both in Siberia. In 1908 in the Tunguska Basin near Lake Baykal one occurred that caused vast destruction of timber from its blast, and the other in 1947 at Sikhote-Alin also caused great damage. Craters that have been obliterated by erosion over thousands of years, leaving only a circular scar on the earth's surface, are called astroblemes .
Craters are also commonly formed at the surface opening, or vent, of erupting volcanoes , particularly of the type called cinder cones, where the lava is extruded rather explosively. Virtually all volcanoes display a crater, called a sink, around the vent; this is believed to be a collapse feature caused by molten lava subsiding as an eruption phase diminishes. Volcanic craters formed in these ways are relatively small, usually less than 1 mi (1.6 km) in diameter, and represent only a small fraction of the cone's diameter at the base.
A caldera is a much larger crater, typically ranging from 3 to 18 mi (5-30 km) in diameter, and represents a considerable fraction of the volcano's basal diameter. In a few instances, however, tremendous volcanic eruptions have left calderas 50 mi (80 km) or so, such as that that forms much of Yellowstone National Park or the basin of Lake Toba, Sumatra, Indonesia. Most calderas are formed by the collapse of the central part of a cone during great eruptions. A few small calderas have been formed by explosive eruptions in which the top of a volcano was blown out. Some volcanic craters are created by a combination of these events. Formed thousands of years ago, the caldera that contains Crater Lake, Oreg., is 6 mi (9.7 km) in diameter. In recent times, caldera-producing eruptions occurred at Krakatoa, Indonesia, in 1883 and Katmai, Alaska, in 1912.
Volcanic Eruptions
More than 500 volcanoes are known to have erupted on the earth's surface since historic times, and many more have erupted on the ocean floor unobserved by humans. Fifty volcanoes have erupted in the United States, which ranks third, behind Indonesia and Japan, in the number of historically active volcanoes. Of the world's active volcanoes, more than half are found around the perimeter of the Pacific, about a third on midoceanic islands and in an arc along the south of the Indonesian islands, and about a tenth in the Mediterranean area, Africa, and Asia Minor.
Evidence of extraterrestrial volcanic activity also has been found. Space probes have detected the remnants of ancient eruptions on earth's moon, Mars (which has the largest volcano in the solar system, Olympus Mons, 340 mi/550 km across and 15 mi/24 km high), and Mercury; these probably originated billions of years ago, since these bodies are no longer capable of volcanic activity.
Triton (a satellite of Neptune), Io (a satellite of Jupiter), and Venus are the only bodies in the solar system besides earth that are known to be volcanically active. The volcanic processes that occur in the outer portion of the solar system are very different from those in the inner part. Eruptions on earth, Venus, Mercury, and Mars are of rocky material and are driven by internal heat. Io's eruptions are probably sulfur or sulfur compounds driven by tidal interactions with Jupiter. Triton's eruptions are of very volatile compounds, such as methane or nitrogen, driven by seasonal heating from the sun.
Terrestrial volcanic eruptions may take one or more of four chief forms, or phases, known as Hawaiian, Strombolian, Vulcanian, and Peleean. In the Hawaiian phase there is a relatively quiet effusion of basaltic lava unaccompanied by explosions or the ejection of fragments; the eruptions of Mauna Loa on the island of Hawaii are typical. The Strombolian phase derives its name from the volcano Stromboli in the Lipari, or Aeolian, Islands, N of Sicily. It applies to continuous but mild discharges in which viscous lava is emitted in recurring explosions; the ejection of incandescent material produces luminous clouds.
A more explosive volcanic eruption is the Vulcanian, where the magma (lava before emission) accumulates in the upper level of the vent but is blocked by a hardened plug of lava that forms between consecutive explosions. When the explosive gases have reached a critical pressure within the volcano, masses of solid and liquid rock erupt into the air and clouds of vapor form over the crater. The Peleean, derived from Mt. Pelée, is the most violent, emitting fine ash; hot, gas-charged fragments of lava; and superheated steam in an incandescent 'cloud' that travels downhill at great speed. Eruptions are often accompanied by torrential rains caused by the condensation of steam. The erupted fragments vary in size, including minute particles of volcanic dust and ash, lapilli (cinders or pellets), bombs (rounded or ellipsoidal masses of hardened magma), and huge masses called blocks.
Historical Volcanoes
Notable eruptions within historic times have been those of Vesuvius, in Italy (AD 79, 1906, and other times); Tambora, in Indonesia, where between 30 and 50 cu mi (125-210 cu km) of molten and shattered rock were blown into the air (1815); Krakatoa, near Java, material from which was sent 17 mi (27 km) into the atmosphere (1883); Parícutin, in Mexico, the volcano that began in a cornfield (1943); Hibok Hibok, on Camiguin island in the Philippines, which killed 84 people (1948); Besymianny, in Kamchatka, where 2 cu mi (8 cu km) of material were hurled into the air (1956); the peak of Tristan da Cunha, whose eruption caused the entire settlement to be evacuated (1961); Agung, in Bali, which killed 1,100 people (1963); Mt. St. Helens in Washington, which exploded with an energy equivalent to 10 million tons of TNT, killing 35, with 25 missing (1980); El Chichon in Mexico, which expelled about 500 million tons of ash and gas (1982); and Mt. Pinatubo in the Philippines, which killed over 500 people and ejected over 2 cu mi (8 cu km) of material (1991). Other notable volcanoes are Cotopaxi and Chimborazo (Ecuador), Iztaccihuatl and Popocatépetl (Mexico), Lassen Peak and Katmai (United States), and Etna (Sicily).
Mauna Loa (Hawaii) is the world's largest active volcano, projecting 13,677 ft (4,170 m) above sea level and over 29,000 ft (8,850 m) above the ocean floor; from its base below sea level to its summit, Mauna Loa is taller than Mt. Everest. In 1963 the birth of the volcanic island Surtsey near Iceland was observed. In November of that year events began with a submarine eruption along the Mid-Atlantic Ridge. Eruption followed eruption until June, 1967, by which time the island stood 492 ft (150 m) above sea level and covered an area of almost 2 sq mi (3 sq km). The island has not grown since the last eruption, and it is presently volcanically quiet.
Mud volcanoes are a geological phenomenon that has largely escaped the attention of the general public. Yet there are hundreds of thousands of them, scattered all over the globe. Over the past few decades scientists from all over Europe have been researching the mud volcanoes on the bottom of the Mediterranean Sea in an international project called 'Medmud'.
Mud volcanoes - which come in different sizes, from knee high to as big as a mountain - are often shaped like normal volcanoes, but instead of lava they expel a mixture of mud, rocks and gasses. Most of the time they just bubble away gently, but they can be dangerous. When a mud volcano ejects large amounts of gas suddenly, there is a risk of asphyxiation for humans and animals in the immediate vicinity. The gas plume can also catch fire. Luckily most mud volcanoes are found on the sea floor where they cause little harm.
But what causes mud volcanoes? According to John Woodside, geologist at the Free University in Amsterdam and one of the leading researchers in the 'Medmud' project, they can be seen as open pressure valves in the earth's crust. The tectonic plates which form the surface of our planet rub together all the time. As a result they produce large amount of debris in the form of sediment. This sediment builds up on the crack between two plates and gradually increases the pressure beneath it. At some point the pressure becomes so great that gas, water and sand is pushed out; a mud volcano is born.
Underwater volcanoes - Because most of the joins between tectonic plates are located under the oceans, most mud volcanoes are also submarine. But it's not always the case. There are a lot of mud volcanoes in the US, Azerbaijan and China, and an ancient Greek settlement was wiped out when a dormant mud volcano explosively came to life.
The Medmud project has concentrated on the ecological effects of mud volcanoes. On the one hand they provide a unique ecosystem for bacteria on the sea floor, with their abundant heat and minerals, but they also add to the Greenhouse Effect. The methane gas they often produce in large amounts is a very potent greenhouse gas. While it is broken down to carbon dioxide fairly quickly in the atmosphere, that gas also has a greenhouse effect.
Gas release Of course mud volcanoes have been around since the dawn of time, so you may well ask what the fuss is about, but there is a catch. We humans can make mud volcanoes accidentally. If, while searching for oil or gas, a hole is drilled in the crack between tectonic plates this is common practice for Russian geologists, its easy drilling - the bore hole can collapse, resulting in a ragged opening that's hard or even impossible to close again. In such a case huge amounts of methane can suddenly enter the atmosphere. There is no telling what effect that may have on our climate, and, because mud volcanoes are a good indicator of the presence of oil and gas, simply not drilling in such areas is not an option.
For these reasons Dr Woodside regards his work as far from over. Better understanding of the geological processes which produce mud volcanoes is needed, and the means of preventing sudden and uncontrollable leaks induced by drilling need to found.
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