Volcano

A volcano is a geological landform (usually a mountain) where magma (rock of the Earth's interior made molten or liquid by high temperature along with a reduction in pressure and/or the introduction of water or other volatiles) erupts through the surface of the planet. Although there are numerous volcanoes (some very active) on the solar system's rocky planets and moons, on Earth at least, this phenomenon tends to occur near the boundaries of the continental plates. However, important exceptions exist in hotspot volcanoes.

Smoking Bromo and Semeru volcanoes on Java in Indonesia.

The name "volcano" has its origin from the name of Vulcan, a god of fire in Roman mythology. The study of volcanoes is called vulcanology (or volcanology in some spellings).

Volcano Classification

Erupted material

Lava composition
One way of classifying volcanoes is by the composition of material erupted (lava), since this affects the shape of the volcano. Lava can be broadly classified into 4 different compositions (Cas & Wright, 1987):

If the erupted magma contains a high percentage (>63%) of silica, the lava is called felsic.

• Felsic lavas (or rhyolites) tend to be highly viscous (not very fluid) and are erupted as domes or short, stubby flows. Viscous lavas tend to form stratovolcanoes or lava domes. Lassen Peak in California is an example of a volcano formed from felsic lava and is actually a large lava dome.
• Because silicious magmas are so viscous, they tend to trap volatiles (gases) that are present, which cause the magma to erupt catastrophically, eventually forming stratovolcanoes. Pyroclastic flows (ignimbrites) are highly hazardous products of such volcanoes, since they are composed of molten volcanic ash too heavy to go up into the atmosphere, so they hug the volcano's slopes and travel far from their vents during large eruptions. Temperatures as high as 1,200 °C are known to occur in pyroclastic flows, which will incinerate everything flammable in their path and thick layers of hot pyroclastic flow deposits can be laid down, often up to many meters thick. Alaska's Valley of Ten Thousand Smokes, formed by the eruption of Novarupta near Katmai in 1912, is an example of a thick pyroclastic flow or ignimbrite deposit. Volcanic ash that is light enough to be erupted high into the Earth's atmosphere may travel many kilometres before it falls back to ground as a tuff.

If the erupted magma contains 52-63% silica, the lava is of intermediate composition.

• These "andesitic" volcanoes generally only occur above subduction zones (e.g. Mount Merapi in Indonesia).

If the erupted magma contains < 52% and > 45% silica, the lava is called mafic (because it contains higher percentages of magnesium (Mg) and iron (Fe)) or basaltic. These lavas are typically less viscous than rhyolitic lavas, depending on their eruption temperature. These lavas occur in a wide range of settings:

• At Mid-ocean ridges, where two oceanic plates are pulling apart, basaltic lava erupts as pillows to fill the gap;
• Shield volcanoes (e.g. the Hawaiian Islands, including Mauna Loa and Kilauea), on both oceanic and continental crust;
• As Continental flood basalts.

If the erupted magma contains <=45% silica, the lava is called ultramafic. Ultramafic flows are very rare and are thought to be even more fluid than common mafic lavas.

Lava Texture
Two types of lava are erupted according to the surface texture: ?A?a (pronounced IPA [?a?a]) and pahoehoe (pronounced [pa?ho?e?ho?e?]), both words having Hawaiian origins. ?A?a is characterized by a rough, clinkery surface and is what most viscous and hot lava flows look like. However, even basaltic or mafic flows can be erupted as ?a?a flows, particularly if the eruption rate is high and the slope is steep. Pahoehoe is characterized by its smooth and often ropy or wrinkly surface and is generally formed from more fluid lava flows. Usually, only mafic flows will erupt as pahoehoe, since they often erupt at higher temperatures or have the proper chemical makeup to allow them to flow at a higher fluidity.

Shape

Shield volcanoes

Hawai?i and Iceland are examples of places where volcanoes extrude huge quantities of basaltic lava that gradually build a wide mountain with a shield-like profile. Their lava flows are generally very hot and very fluid, contributing to long flows. The largest lava shield on Earth, Mauna Loa, rises over 9,000 m from the ocean floor, is 120 km in diameter and forms part of the Big Island of Hawai?i. Olympus Mons is the largest shield volcano on Mars, and is the tallest mountain in the known solar system. Smaller versions of shield volcanoes include lava cones, and lava mounds.

Quiet eruptions spread out basaltic lava in flat layers. The buildup of these layers form a broad volcano with gently sloping sides called a shield volcano. Examples of shield volcanoes are the Hawaiian Islands.

Cinder cones

Volcanic cones or cinder cones result from eruptions that throw out mostly small pieces of scoria and pyroclastics (both resemble cinders, hence the name of this volcano type) that build up around the vent. These can be relatively short-lived eruptions that produce a cone-shaped hill perhaps 30 to 400 m high. Most cinder cones erupt only once. Cinder cones may form as flank vents on larger volcanoes, or occur on their own. Paricutín in Mexico and Sunset Crater in Arizona are examples of cinder cones.

Stratovolcanoes

These are tall conical mountains composed of lava flows and other ejecta in alternate layers, the strata that give rise to the name. Stratovolcanoes are also known as composite volcanoes. Classic examples include Mt. Fuji in Japan, Mount Mayon in the Philippines, and Mount Vesuvius and Stromboli in Italy.

Supervolcanoes

Supervolcano is the popular term for large volcanoes that usually have a large caldera and can potentially produce devastation on an enormous, sometimes continental, scale. Such eruptions would be able to cause severe cooling of global temperatures for many years afterwards because of the huge volumes of sulfur and ash erupted. They can be the most dangerous type of volcano. Examples include Yellowstone Caldera in Yellowstone National Park and Lake Toba in Sumatra, Indonesia. Supervolcanoes are hard to identify given their enormous areas covered. Large igneous provinces are also considered supervolcanoes because of the vast amount of basalt lava erupted.

Submarine volcanoes

Submarine volcanoes are common features on the ocean floor. Some are active and, in shallow water, disclose their presence by blasting steam and rocky debris high above the surface of the sea. Many others lie at such great depths that the tremendous weight of the water above them prevents the explosive release of steam and gases, although they can be detected by hydrophones and discoloration of water due to volcanic gases. Even large submarine eruptions may not disturb the ocean surface. Submarine volcanoes often form rather steep pillars and in due time, may break the ocean surface as new islands. Pillow lava is a common eruptive product of submarine volcanoes.

Subglacial Volcanoes

Subglacial volcanoes develop underneath icecaps. They are made up of flat lava flows atop extensive pillow lavas and palagonite. When the icecap melts, the lavas on the top collapse leaving a flat-topped mountain. Then, the pillow lavas also collapse, giving an angle of 37.5 degrees. Very good examples of this can be seen in Iceland. These volcanoes are also called table volcanoes or mobergs.

Classifying Volcanic Activity

Volcanoes are usually situated either near the boundaries between tectonic plates or over geologically active hotspots. Volcanoes may be either dormant (having no activity) or active (currently erupting) or extinct (no longer active at all).

Surprisingly, there is no real consensus among volcanologists on how to define an "active" volcano. The lifespan of a volcano can vary from months to several million years, making such a distinction sometimes meaningless when compared to the lifespans of humans or even civilizations. For example, many of Earth's volcanoes have erupted dozens of times in the past few thousand years but are not currently showing signs of eruption. Given the long lifespan of such volcanoes, they are very active. By our lifespans, however, they are not. Complicating the definition are volcanoes that become restless (producing earthquakes, venting gasses, or other non-eruptive activities) but do not actually erupt.

Scientists usually consider a volcano active if it is currently erupting or showing signs of unrest, such as unusual earthquake activity or significant new gas emissions. Many scientists also consider a volcano active if it has erupted in historic time. It is important to note that the span of recorded history differs from region to region; in the Mediterranean, recorded history reaches back more than 3,000 years but in the Pacific Northwest of the United States, it reaches back less than 300 years, and in Hawaii, little more than 200 years. The Smithsonian Global Volcanism Program's definition of 'active' is having erupted within the last 10,000 years.

Dormant volcanoes are those that are not currently active (as defined above), but could become restless or erupt again. Confusion however, can arise because many volcanoes which scientists consider to be active are referred to as dormant by laypersons or in the media.

Extinct volcanoes are those that scientists consider unlikely to erupt again. Whether a volcano is truly extinct is often difficult to determine. Since "supervolcano" calderas can have eruptive lifespans sometimes measured in millions of years, a caldera that has not produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct.

For example, the Yellowstone Caldera in Yellowstone National Park is at least 2 million years old and hasn't erupted violently for approximately 640,000 years, although there has been some minor activity relatively recently, with hydrothermal eruptions less than 10,000 years ago and lava flows about 70,000 years ago. For this reason, scientists do not consider the Yellowstone Caldera extinct. In fact, because the caldera has frequent earthquakes, a very active geothermal system (i.e., the entirety of the geothermal activity found in Yellowstone National Park), and rapid rates of ground uplift, many scientists consider it to be an active volcano.

Resources about Volcano

USGS Cascades Volcano Observatory
The U.S. Geological Survey's Cascades Volcano Observatory Website, which focuses on hazards, activity, history, and monitoring of volcanoes, with emphasis on volcanoes of the Western United States.

The Electronic Volcano
Summarizes volcano-related information on the world wide web.

Alaska Volcano Observatory
To monitor and study Alaska's hazardous volcanoes, to predict and record eruptive activity, and to mitigate volcanic hazards to life and property.

Capulin Volcano National Monument
In this great volcanic arena, lava erupted in three flows, each separated by long periods of inactivity. The last series of eruptions created Capulin Volcano, whose conical form rises more than 1,000 feet above its base to 8,182 feet above sea level...

Volcano: EnchantedLearning.com
volcano is a place on the Earth's surface (or any other planet's or moon's surface) where molten rock, gases and pyroclastic debris erupt through the earth's crust...

Smithsonian Institution: Global Volcanism Program
seeks better understanding of all volcanoes through documenting their eruptions - small as well as large - during the past 10,000 years.

Volcano Watch
These satellite images show some of the world's most active volcanoes. Additional volcano information can be obtained from the Michigan Tech Geology Department.

Mount St. Helens VolcanoCam
Mount St. Helens National Volcanic Monument.

NOVA | Volcano's Deadly Warning
Read an interview with seismologist Bernard Chouet, who has come up with a new method for predicting volcanic eruptions; explore the anatomy of a volcano; find a teacher's guide and program transcript; and more.

How Volcanoes Work
A comprehensive educational resource that describes the science behind volcanoes and volcanic processes.

Michigan Technological University Volcanoes Page
Aims to provide information about volcanoes to the public and to complement other informational sites on the web.