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What are Volcanoes?

Volcanoes are vents in the earth's surface through which magma, gases, and other materials erupt. They are found primarily at tectonic plate boundaries, but also exist at hot spots, which are places in the earth's crust where hot mantle plumes have broken through. Some volcanoes erupt explosively, while others erupt slowly. Explosive volcanoes present many potential threats including the release of toxic gases, pyroclastic flows (flows containing fragments of hot rock and ash), nuee ardentes (fast moving clouds of extremely hot gases and fine ash) and large volumes of ash. It is common for volcanoes to trigger other natural hazards; debris flows, earthquakes, floods, landslides and fires are frequent products of volcanic eruptions.

When do volcanic eruptions occur?

The molten rock, called magma, which is lighter than the surrounding solid rock, driven by buoyancy and gas pressure forces, its way upward and can ultimately break through zones of weaknesses in the Earth's crust. If so, an eruption begins, and if magma is thin and runny, gases can escape easily from it causing the magma to flow out of the volcano. If the magma is thick and sticky, gases cannot escape easily. Pressure builds up until the gases escape violently and explode. In this type of eruption, the magma blasts into the air and breaks apart into pieces called tephra. Tephra can range in size from tiny particles of ash to house-size boulders.

Most volcanoes provide various types of warnings before eruptions begin. Although an explosive eruption could occur without warning, some premonitory events more likely will precede the next eruption.

What is the impact of volcanic eruptions?

Lava flows are one of the most characteristic manifestations of volcanism. Basaltic lava can flow at speeds varying from less than 1 m per day to 3 m per second, but the threat to human life is slight. Lava flows fastest near to its source and progressively slower at greater distances, as it cools in contact with the ground and atmosphere. Cooling can cause it to congeal and flow continues within lava tubes of solidified material.
Doming can be a serious problem in volcanoes. Material that has solidified in the volcanic vent is forced upwards and may eventually collapse onto the flanks of the volcano in the form of a hot or cold rock avalanche, or it may cause serious ground deformation and instability.
Explosively erupting volcanoes pose a serious hazard of tephra ejection. Blocks and bombs may fall in significant quantities within a 5 km radius of the eruptive centre.
Ashfall is likely to be more widespread and can cover or bury farmland, ruin crops, clog sewers and machinery, cause excessive wear to mechanical parts, suffocate fauna and create a severe weight on shallow or flat roofs.
Nuees ardentes ("glowing avalanches") involve the fluidization of a cloud of ash, dust and gas at 600o C. In the form of heavy base surges, nuees may flow at up to 100 km/hr and for distances of up to 10 km.
A larger, more directionally oriented form of nuee composed of suspended rock froth and gases is known as the ash flow or pyroclastic flow, and it can travel at 200 km/hr for distances of up to 25 km.
Air pollution and associated risks caused by volcanic eruptions are not restricted to ash emissions. Poisonous gases may be emitted (for example they are thought to have killed the elder Pliny on the beach at Pompeii in AD 79).
Large eruptions also cause atmospheric modification in the form of volcanic aerosols injected through the tropopause, which is situated at altitudes of 8-16 km.

Using a Japanese word, volcanic mudflows are called lahars, and are classified as primary when they result directly from eruption and secondary if there are other causes. They can occur at any time before, during or after an eruption and may consist of hot or cold material. Finally volcanic eruptions can cause tsunamis as described in the relevant section.

Are there any Volcanoes positive effects?

In the long run and geologic time, volcanic eruptions and related processes have directly and indirectly benefited mankind. Volcanic materials ultimately break down and weather to form some of the most fertile soils on Earth, cultivation of which has produced abundant food and fostered civilizations. People use volcanic products, the internal heat associated with young volcanic systems has been harnessed to produce geothermal energy, and most of the metallic minerals mined in the world, such as copper, gold, silver, lead, and zinc, are associated with magmas found deep within the roots of extinct volcanoes.

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Do you know that ...

About 500 active volcanoes are known on Earth, not counting those that lie beneath the sea. They are not randomly distributed over the Earth's surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea forming long mountain ranges due to the fact the Earth's outermost shell -- the lithosphere -- is broken into a series of slabs known as lithospheric or tectonic plates. These plates are rigid, but they float on the hotter, softer layer in the Earth's mantle. There are 16 major plates. As the plates move about, they spread apart, collide, or slide past each other. Volcanoes occur most frequently at plate boundaries. More than half of the world's active volcanoes above sea level encircle the Pacific Ocean to form the circum-Pacific "Ring of Fire".

Some volcanoes, like those that form the Hawaiian Islands, occur in the interior of plates at areas called hot spots. Although most of the active volcanoes we see on land occur where plates collide, the greatest number of the Earth's volcanoes is hidden from view, occurring on the ocean floor along spreading ridges.

The roots of the volcanoes, for example Mount St. Helens, are 100 to 330 kilometers (70 to 200 miles) below the Earth's surface. Here in the Earth's mantle temperatures are hot enough to melt rock and form a thick, flowing substance called magma. Lighter than the solid rock that surrounds it, magma is buoyant much like a cork in water; being buoyant, it rises.

The largest eruption in the world this century occurred in 1912 at Novarupta on the Alaska Peninsula. An estimated 15 cubic kilometers of magma was explosively erupted during 60 hours beginning on June 6 -- (which is equivalent to 230 years of eruption at Kilauea (Hawaii) or, about 30 times the volume erupted by Mount St. Helens (Washington) in 1980.

Mauna Loa (Hawaii) is the world's largest active volcano, projecting 13,677 feet above sea level, its top being over 28,000 feet above the deep ocean floor. From its base below sea level to its summit, Mauna Loa is taller than Mount Everest.