Brief Summary
This article provides a comprehensive overview of volcanoes, including their formation, types, features, erupted materials, eruption styles, activity levels, and impact on humans and other celestial bodies. It covers the etymology of the term "volcano," the role of plate tectonics in volcano formation, and the various hazards and benefits associated with volcanic activity.
- Volcanoes are vents or fissures in a planet's crust that allow hot lava, ash, and gases to escape from a magma chamber.
- They are commonly found at diverging or converging tectonic plate boundaries, as well as hotspots and continental rifts.
- Volcanoes are classified as active, dormant, or extinct based on their eruption history and magma source.
- Volcanic eruptions can affect atmospheric temperature and have historically caused volcanic winters and famines.
- Volcanoes exist on other planets and moons, with varying forms of volcanism, including cryovolcanism.
Etymology and Terminology
The word "volcano" comes from the Italian island Vulcano, named after the Roman god of fire, Vulcan. Volcanism refers to the processes and phenomena associated with volcanic activity, while volcanology is the study of volcanism and volcanoes.
Plate Tectonics
Earth's lithosphere is divided into plates that move due to convection in the mantle. Most volcanic activity occurs along plate boundaries where plates converge or diverge. Certain concepts, such as polygenetic and monogenetic volcanoes, have been developed to group volcanoes based on their activity and structure, which are explained by plate tectonics.
Divergent Plate Boundaries
At mid-ocean ridges, tectonic plates diverge, allowing hot mantle rock to rise and melt, causing volcanism and creating new oceanic crust. Most divergent plate boundaries are underwater, resulting in submarine volcanic activity. Iceland is an example of volcanic islands formed where the mid-oceanic ridge is above sea level.
Convergent Plate Boundaries
Subduction zones occur where two plates collide, typically an oceanic and a continental plate. The oceanic plate subducts, and water released from it lowers the melting temperature of the overlying mantle, creating magma. This magma forms volcanoes, resulting in volcanic arcs like the Pacific Ring of Fire.
Hotspots
Hotspots are volcanic areas formed by mantle plumes rising from the core-mantle boundary. As tectonic plates move over these plumes, volcanoes become inactive, and new ones form. The Hawaiian Islands and the Snake River Plain are examples of hotspot volcanism.
Continental Rifting
Sustained upwelling of hot mantle rock under a continent can lead to rifting. Early stages involve flood basalts and can progress to a divergent plate boundary. Failed rifts often result in volcanoes erupting unusual lavas like those in the East African Rift.
Volcanic Features
Volcanoes require a magma reservoir, a conduit for magma to rise, and a vent for lava to escape. The deposited material forms a volcanic edifice, such as a cone or mountain. Volcanoes vary in structure and behavior, with features like rugged peaks, plateaus, and craters that may contain lakes. Other types include mud volcanoes and cryovolcanoes.
Fissure Vents
Fissure vents are linear fractures at diverging plate boundaries where basaltic lava emerges. These volcanoes are non-explosive, and the low-viscosity lava creates gently sloping basaltic plateaus. Lakagigar in Iceland is an example of a fissure vent with a chain of volcanic cones.
Shield Volcanoes
Shield volcanoes have broad, shield-like profiles formed by low-viscosity basaltic or andesitic lava that flows great distances. They are characterized by gentle effusive eruptions and are common in oceanic settings. The Hawaiian volcanic chain and Olympus Mons on Mars are examples of shield volcanoes.
Lava Domes
Lava domes have steep, convex sides built by slow eruptions of highly viscous lava, such as rhyolite. They can form within craters of previous eruptions or independently. Like stratovolcanoes, they can produce violent eruptions, but the lava does not flow far. Mount St. Helens and Lassen Peak are examples of lava domes.
Cryptodomes
Cryptodomes form when viscous lava is forced upward, causing the surface to bulge. The 1980 eruption of Mount St. Helens is an example, where lava beneath the surface created an upward bulge that later collapsed.
Cinder Cones
Cinder cones result from eruptions of small pieces of scoria and pyroclastics that build up around the vent. These eruptions are relatively short-lived, producing cone-shaped hills. Parícutin in Mexico and Sunset Crater in Arizona are examples of cinder cones.
Stratovolcanoes
Stratovolcanoes are tall, conical mountains composed of alternating layers of lava flows and tephra. They are also known as composite volcanoes because they are created from multiple structures during different kinds of eruptions. Mount Fuji in Japan, Mayon Volcano in the Philippines, and Mount Vesuvius in Italy are classic examples.
Supervolcanoes
A supervolcano is defined as a volcano that has experienced one or more eruptions that produced over 1,000 cubic kilometers of volcanic deposits in a single explosive event. These eruptions are very rare but can cause devastation on a continental scale and cool global temperatures for many years. Yellowstone Caldera, Lake Taupō, and Lake Toba are known examples.
Caldera Volcanoes
Volcanoes that are large but not large enough to be called supervolcanoes may also form calderas. These calderas may contain active or dormant cones, or volcanic lakes.
Submarine Volcanoes
Submarine volcanoes are common features of the ocean floor. While only 119 have documented activity during the Holocene Epoch, there may be over a million geologically young submarine volcanoes. In shallow water, they may blast steam and debris above the surface, while in deep water, eruptions can be detected by hydrophones and water discoloration.
Subglacial Volcanoes
Subglacial volcanoes develop underneath ice caps and are made up of lava plateaus capping extensive pillow lavas and palagonite. They are also called table mountains, tuyas, or mobergs. Examples can be seen in Iceland and British Columbia.
Hydrothermal Features
Hydrothermal features, such as geysers, fumaroles, mud pools, and hot springs, involve water and geothermal or magmatic activity. These features are common around volcanoes and often indicate volcanism.
Mud Volcanoes
Mud volcanoes are conical structures created by the eruption of liquids and gases, particularly mud, water, and gases. The largest mud volcanoes can be 10 kilometers in diameter and reach 700 meters high.
Fumarole
Fumaroles are vents on the surface from which hot steam and volcanic gases erupt due to superheated groundwater, indicating volcanic activity. Fumaroles erupting sulfurous gases are also called solfataras.
Geysers
Geysers are springs that occasionally erupt and discharge hot water and steam. They indicate ongoing magmatism, where underground water is heated by hot rocks, and steam pressure builds up before being released. Almost half of all active geysers are in Yellowstone National Park, US.
Erupted Material
The material expelled in a volcanic eruption includes volcanic gases, lava, and tephra.
Volcanic Gases
Volcanic gases consist mainly of water vapor, carbon dioxide, and sulfur dioxide. Other gases include hydrogen sulfide, hydrogen chloride, and hydrogen fluoride, along with minor and trace gases.
Lava Flows
The form and style of a volcanic eruption are largely determined by the composition of the lava. Viscosity and dissolved gas content are the most important characteristics, determined by the amount of silica in the magma. Lava compositions include felsic, intermediate (andesitic), mafic (basaltic), and ultramafic.
Tephra
Tephra is made when magma inside the volcano is blown apart by the rapid expansion of hot volcanic gases. These explosions produce particles of material that can then fly from the volcano. Solid particles smaller than 2 mm in diameter are called volcanic ash.
Dissection
Through erosion, the solidified erupted material of a volcano may be stripped away, revealing its inner anatomy. When a volcano is extinct, a plug forms on its vent, and over time, the volcanic cone erodes away, leaving the resistant lava plug intact.
Volcanic Eruptions
The Smithsonian Institution's Global Volcanism Program database lists 9,901 confirmed eruptions from 859 volcanoes in the Holocene Epoch. Eruption styles are broadly divided into magmatic, phreatomagmatic, and phreatic eruptions. The intensity of explosive volcanism is expressed using the volcanic explosivity index (VEI).
Volcanic Activity
Volcanoes vary greatly in their level of activity, with eruption recurrences ranging from several times a year to once in tens of thousands of years. Volcanoes are informally described as erupting, active, dormant, or extinct, but the definitions are not uniform among volcanologists.
Erupting
The USGS defines a volcano as "erupting" whenever the ejection of magma from any point on the volcano is visible, including visible magma still contained within the walls of the summit crater.
Active
The USGS defines a volcano as active whenever subterranean indicators, such as earthquake swarms, ground inflation, or unusually high levels of carbon dioxide or sulfur dioxide, are present.
Dormant and Reactivated
The USGS defines a dormant volcano as any volcano that is not showing any signs of unrest but shows signs that it could become active again. Modern monitoring techniques and modeling have improved the understanding of why volcanoes may remain dormant for long periods and then become active again.
Extinct
Extinct volcanoes are those that scientists consider unlikely to erupt again because the volcano no longer has a magma supply. Examples include many volcanoes on the Hawaiian–Emperor seamount chain and in various countries.
Volcanic-Alert Level
To prevent people from falsely believing they are not at risk, countries have adopted new classifications to describe the various levels and stages of volcanic activity, using different numbers, colors, or words to designate the stages.
Decade Volcanoes
The Decade Volcanoes are 16 volcanoes identified by the International Association of Volcanology and Chemistry of the Earth's Interior as being worthy of particular study due to their history of large, destructive eruptions and proximity to populated areas.
Volcanoes and Humans
Volcanic eruptions pose a significant threat to human civilization, but volcanic activity has also provided humans with important resources.
Hazards
Volcanic eruptions and associated activity, such as phreatic eruptions, explosive eruptions, effusive eruptions, sector collapses, pyroclastic flows, lahars, and volcanic gas emissions, can pose a hazard to humans. Volcanic gases can affect the stratosphere and cause climate anomalies.
Benefits
Past volcanic activity has created important economic resources. Tuff is used for construction, and volcanic ash and weathered basalt produce fertile soil. Volcanic activity is responsible for emplacing valuable mineral resources and geothermal power. Tourism associated with volcanoes is also a worldwide industry.
Safety Considerations
Many volcanoes near human settlements are heavily monitored to provide advance warnings of imminent eruptions. Modern volcanology has led to better-informed governmental and public responses to volcanic activities.
Volcanoes on Other Celestial Bodies
Earth's Moon has volcanic features but no current activity. Venus has a surface that is 90% basalt, indicating major volcanism. Mars has several extinct shield volcanoes. Jupiter's moon Io is the most volcanically active object in the Solar System. Cryovolcanism is common on the moons of the outer planets.
History of Volcano Understanding
Many ancient accounts ascribe volcanic eruptions to supernatural causes. However, others proposed more natural causes. Over time, scientific understanding has evolved, leading to the acceptance of plate tectonics and modern volcanology.
