In which US state is the world’s largest volcano?
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U.S. state Volcano List of volcanoes in the United States
A state of the United States of America is one of the 50 constituent political entities that shares its sovereignty with the United States federal government. Due to the shared sovereignty between each state and the federal government, Americans are citizens of both the federal republic and of the state in which they reside.  State citizenship and residency are flexible, and no government approval is required to move between states, except for persons covered by certain types of court orders (e.g., paroled convicts and children of divorced spouses who are sharing custody).

States range in population from just under 600,000 (Wyoming) to over 38 million (California), and in area from  (Rhode Island) to  (Alaska). Four states use the term commonwealth rather than state in their full official names.

States are divided into counties or county-equivalents, which may be assigned some local governmental authority but are not sovereign. County or county-equivalent structure varies widely by state. State governments are allocated power by the people (of each respective state) through their individual constitutions. All are grounded in republican principles, and each provides for a government, consisting of three branches: executive, legislative, and judicial. 

States possess a number of powers and rights under the United States Constitution; among them ratifying constitutional amendments. Historically, the tasks of local law enforcement, public education, public health, regulating intrastate commerce, and local transportation and infrastructure have generally been considered primarily state responsibilities, although all of these now have significant federal funding and regulation as well. Over time, the U.S. Constitution has been amended, and the interpretation and application of its provisions have changed. The general tendency has been toward centralization and incorporation, with the federal government playing a much larger role than it once did. There is a continuing debate over states' rights, which concerns the extent and nature of the states' powers and sovereignty in relation to the federal government and the rights of individuals.

States and their residents are represented in the federal Congress, a bicameral legislature consisting of the Senate and the House of Representatives. Each state is represented by two Senators, and at least one Representative, while additional representatives are distributed among the states in proportion to the most recent constitutionally mandated decennial census.  Each state is also entitled to select a number of electors to vote in the Electoral College, the body that elects the President of the United States, equal to the total of Representatives and Senators from that state. 

The Constitution grants to Congress  the authority to admit new states into the Union. Since the establishment of the United States in 1776, the number of states has expanded from the original 13 to 50. Alaska and Hawaii are the most recent states admitted, both in 1959.

The Constitution is silent on the question of whether states have the power to secede (withdraw from) from the Union. Shortly after the Civil War, the U.S. Supreme Court, in Texas v. White, held that a state cannot unilaterally do so.  

States of the United States

The 50 U.S. states (in alphabetical order), along with state flag and date each joined the union:

Governments

As each state is itself a sovereign entity, it reserves the right to organize its individual government in any way (within the broad parameters set by the U.S. Constitution) deemed appropriate by its people. As a result, while the governments of the various states share many similar features, they often vary greatly with regard to form and substance. No two state governments are identical.

Constitutions

The government of each state is structured in accordance with its individual constitution. Many of these documents are more detailed and more elaborate than their federal counterpart. The Constitution of Alabama, for example, contains 310,296 words — more than 40 times as many as the U.S. Constitution. In practice, each state has adopted a three-branch system of government, modeled after the federal government, and consisting of three branches (although the three-branch structure is not required): executive, legislative, and judicial. 

Executive

In each state, the chief executive is called the governor, who serves as both head of state and head of government. The governor may approve or veto bills passed by the state legislature, as well as push for the passage of bills supported by the party of the Governor. In 43 states, governors have line item veto power. 

Most states have a "plural executive" in which two or more members of the executive branch are elected directly by the people. Such additional elected officials serve as members of the executive branch, but are not beholden to the governor and the governor cannot dismiss them. For example, the attorney general is elected, rather than appointed, in 43 of the 50 U.S. states.

Legislative

The legislatures of 49 of the 50 states are made up of two chambers: a lower house (termed the House of Representatives, State Assembly, General Assembly or House of Delegates) and a smaller upper house, always termed the Senate. The exception is the unicameral Nebraska Legislature, which is composed of only a single chamber.

Most states have part-time legislatures, while six of the most populated states have full-time legislatures. However, several states with high population have short legislative sessions, including Texas and Florida. 

In Baker v. Carr (1962) and Reynolds v. Sims (1964), the U.S. Supreme Court held that all states are required to elect their legislatures in such a way as to afford each citizen the same degree of representation (the one person, one vote standard). In practice, most states choose to elect legislators from single-member districts, each of which has approximately the same population. Some states, such as Maryland and Vermont, divide the state into single- and multi-member districts, in which case multi-member districts must have proportionately larger populations, e.g., a district electing two representatives must have approximately twice the population of a district electing just one. If the governor vetoes legislation, all legislatures may override it, usually, but not always, requiring a two-thirds majority.

In 2013, there were a total of 7,383 legislators in the 50 state legislative bodies. They earned from $0 annually (New Mexico) to $90,526 (California). There were various per diem and mileage compensation. 

Judicial

States can also organize their judicial systems differently from the federal judiciary, as long as they protect the federal constitutional right of their citizens to procedural due process. Most have a trial level court, generally called a District Court or Superior Court, a first-level appellate court, generally called a Court of Appeal (or Appeals), and a Supreme Court. However, Oklahoma and Texas have separate highest courts for criminal appeals. In New York State the trial court is called the Supreme Court; appeals are then taken to the Supreme Court's Appellate Division, and from there to the Court of Appeals.

Most states base their legal system on English common law (with substantial indigenous changes and incorporation of certain civil law innovations), with the notable exception of Louisiana, a former French colony, which draws large parts of its legal system from French civil law.

Only a few states choose to have the judges on the state's courts serve for life terms. In most of the states the judges, including the justices of the highest court in the state, are either elected or appointed for terms of a limited number of years, and are usually eligible for re-election or reappointment.

Relationships

Among states

Each state admitted to the Union by Congress since 1789 has entered it on an equal footing with the original States in all respects. With the growth of states' rights advocacy during the antebellum period, the Supreme Court asserted, in Lessee of Pollard v. Hagan (1845), that the Constitution mandated admission of new states on the basis of equality.

Under Article Four of the United States Constitution, which outlines the relationship between the states, each state is required to give full faith and credit to the acts of each other's legislatures and courts, which is generally held to include the recognition of legal contracts and criminal judgments, and before 1865, slavery status. Regardless of the Full Faith and Credit Clause, some legal arrangements, such as professional licensure and marriages, may be state-specific, and until recently states have not been found by the courts to be required to honor such arrangements from other states. 

Such legal acts are nevertheless often recognized state-to-state according to the common practice of comity. States are prohibited from discriminating against citizens of other states with respect to their basic rights, under the Privileges and Immunities Clause. Under the Extradition Clause, a state must extradite people located there who have fled charges of "treason, felony, or other crimes" in another state if the other state so demands. The principle of hot pursuit of a presumed felon and arrest by the law officers of one state in another state are often permitted by a state. 

With the consent of Congress, states may enter into interstate compacts, agreements between two or more states. Compacts are frequently used to manage a shared resource, such as transportation infrastructure or water rights. 

With the federal government

Every state is guaranteed  a form of government that is grounded in republican principles, such as the consent of the governed.  This guarantee has long been at the fore-front of the debate about the rights of citizens vis-à-vis the government. States are also guaranteed protection from invasion, and, upon the application of the state legislature (or executive, if the legislature cannot be convened), from domestic violence. This provision was discussed during the 1967 Detroit riot, but was not invoked.

Since the early 20th century, the Supreme Court has interpreted the Commerce Clause of the Constitution of the United States to allow greatly expanded scope of federal power over time, at the expense of powers formerly considered purely states' matters. The Cambridge Economic History of the United States says, "On the whole, especially after the mid-1880s, the Court construed the Commerce Clause in favor of increased federal power."  In Wickard v. Filburn , the court expanded federal power to regulate the economy by holding that federal authority under the commerce clause extends to activities which may appear to be local in nature but in reality effect the entire national economy and are therefore of national concern. 

For example, Congress can regulate railway traffic across state lines, but it may also regulate rail traffic solely within a state, based on the reality that intrastate traffic still affects interstate commerce. In recent years, the Court has tried to place limits on the Commerce Clause in such cases as United States v. Lopez and United States v. Morrison.

Another example of congressional power is its spending power—the ability of Congress to impose taxes and distribute the resulting revenue back to the states (subject to conditions set by Congress).  An example of this is the system of federal aid for highways, which include the Interstate Highway System. The system is mandated and largely funded by the federal government, and also serves the interests of the states. By threatening to withhold federal highway funds, Congress has been able to pressure state legislatures to pass a variety of laws. An example is the nationwide legal drinking age of 21, enacted by each state, brought about by the National Minimum Drinking Age Act. Although some objected that this infringes on states' rights, the Supreme Court upheld the practice as a permissible use of the Constitution's Spending Clause in South Dakota v. Dole .

Admission into the Union

Article IV, Section 3, Clause 1 of the U.S. Constitution grants to Congress the authority to admit new states into the Union. Since the establishment of the United States in 1776, the number of states has expanded from the original 13 to 50. Each new state has been admitted on an equal footing with the existing states. It also forbids the creation of new states from parts of existing states without the consent of both the affected states and Congress. This caveat was designed to give Eastern states that still had Western land claims (there were 4 in 1787), to have a veto over whether their western counties could become states, and has served this same function since, whenever a proposal to partition an existing state or states in order that a region within might either join another state or to create a new state has come before Congress.

Most of the states admitted to the Union after the original 13 have been created from organized territories established and governed by Congress in accord with its plenary power under Article IV, Section 3, Clause 2.  The outline for this process was established by the Northwest Ordinance (1787), which predates the ratification of the Constitution. In some cases, an entire territory has become a state; in others some part of a territory has.

When the people of a territory make their desire for statehood known to the federal government, Congress may pass an enabling act authorizing the people of that territory to organize a constitutional convention to write a state constitution as a step towards admission to the Union. Each act details the mechanism by which the territory will be admitted as a state following ratification of their constitution and election of state officers. Although the use of an enabling act is a traditional historic practice, a number of territories have drafted constitutions for submission to Congress absent an enabling act and were subsequently admitted. Upon acceptance of that constitution, and upon meeting any additional Congressional stipulations, Congress has always admitted that territory as a state.

In addition to the original 13, six subsequent states were never an organized territory of the federal government, or part of one, before being admitted to the Union. Three were set off from an already existing state, two entered the Union after having been sovereign states, and one was established from unorganized territory:
*California, 1850, from land ceded to the United States by Mexico in 1848 under the terms of the Treaty of Guadalupe Hidalgo. 
*Kentucky, 1792, from Virginia (District of Kentucky: Fayette, Jefferson, and Lincoln counties)Michael P. Riccards, "Lincoln and the Political Question: The Creation of the State of West Virginia" Presidential Studies Quarterly, Vol. 27, 1997 [http://www.questia.com/PM.qst?a
o&d=5000522904 online edition]
*Maine, 1820, from Massachusetts (District of Maine)
*Texas, 1845, previously the Republic of Texas 
*Vermont, 1791, previously the Vermont Republic (also known as the New Hampshire Grants and claimed by New York) 
*West Virginia, 1863, from Virginia (Trans-Allegheny region counties) during the Civil War 

Congress is under no obligation to admit states, even in those areas whose population expresses a desire for statehood. Such has been the case numerous times during the nation's history. In one instance, Mormon pioneers in Salt Lake City sought to establish the state of Deseret in 1849. It existed for slightly over two years and was never approved by the United States Congress. In another, leaders of the Five Civilized Tribes (Cherokee, Chickasaw, Choctaw, Creek, and Seminole) in Indian Territory proposed to establish the state of Sequoyah in 1905, as a means to retain control of their lands.  The proposed constitution ultimately failed in the U.S. Congress. Instead, the Indian Territory, along with Oklahoma Territory were both incorporated into the new state of Oklahoma in 1907. The first instance occurred while the nation still operated under the Articles of Confederation. The State of Franklin existed for several years, not long after the end of the American Revolution, but was never recognized by the Confederation Congress, which ultimately recognized North Carolina's claim of sovereignty over the area. The territory comprising Franklin later became part of the Southwest Territory, and ultimately the state of Tennessee.

Additionally, the entry of several states into the Union was delayed due to distinctive complicating factors. Among them, Michigan Territory, which petitioned Congress for statehood in 1835, was not admitted to the Union until 1837, due to a boundary dispute the adjoining state of Ohio. The Republic of Texas requested annexation to the United States in 1837, but fears about potential conflict with Mexico delayed the admission of Texas for nine years.  Also, statehood for Kansas Territory was held up for several years (1854–61) due to a series of internal violent conflicts involving anti-slavery and pro-slavery factions.

Possible new states

Puerto Rico

Puerto Rico referred to itself as the "Commonwealth of Puerto Rico" in the English version of its constitution, and as "Estado Libre Asociado" (literally, Associated Free State) in the Spanish version.

As with any non-state territory of the United States, its residents do not have voting representation in the federal government. Puerto Rico has limited representation in the U.S. Congress in the form of a Resident Commissioner, a delegate with limited voting rights in the Committee of the Whole House on the State of the Union, and no voting rights otherwise. 

A non-binding referendum on statehood, independence, or a new option for an associated territory (different from the current status) was held on November 6, 2012. Sixty one percent (61%) of voters chose the statehood option, while one third of the ballots were submitted blank.  

On December 11, 2012, the Legislative Assembly of Puerto Rico enacted a concurrent resolution requesting the President and the Congress of the United States to respond to the referendum of the people of Puerto Rico, held on November 6, 2012, to end its current form of territorial status and to begin the process to admit Puerto Rico as a State. 

Washington, D.C.

The intention of the Founding Fathers was that the United States capital should be at a neutral site, not giving favor to any existing state; as a result, the District of Columbia was created in 1800 to serve as the seat of government. The inhabitants of the District do not have full representation in Congress or a sovereign elected government (they were allotted presidential electors by the 23rd amendment, and have a non-voting delegate in Congress).

Some residents of the District support statehood of some form for that jurisdiction—either statehood for the whole district or for the inhabited part, with the remainder remaining under federal jurisdiction.

Others

Various proposals to divide California, usually involving splitting the south half from the north or the urban coastline from the rest of the state, have been advanced since the 1850s.  Similarly, numerous proposals to divide New York, all of which involve to some degree the separation of New York City from the rest of the state, have been promoted over the past several decades.  The partitioning of either state is, at the present, highly unlikely.

Other even less likely possible new states are Guam and the Virgin Islands, both of which are unincorporated organized territories of the United States. Also, either the Northern Mariana Islands or American Samoa, an unorganized, unincorporated territory, could seek statehood.

Secession from the Union

The Constitution is silent on the issue of the secession of a state from the union. However, its predecessor document, the Articles of Confederation, stated that the United States "shall be perpetual." The question of whether or not individual states held the right to unilateral secession remained a difficult and divisive one until the American Civil War. In 1860 and 1861, eleven southern states seceded, but following their defeat in the American Civil War were brought back into the Union during the Reconstruction Era. The federal government never recognized the secession of any of the rebellious states. 

Following the Civil War, the United States Supreme Court, in Texas v. White, held that states did not have the right to secede and that any act of secession was legally void. Drawing on the Preamble to the Constitution, which states that the Constitution was intended to "form a more perfect union" and speaks of the people of the United States in effect as a single body politic, as well as the language of the Articles of Confederation, the Supreme Court maintained that states did not have a right to secede. However, the court's reference in the same decision to the possibility of such changes occurring "through revolution, or through consent of the States," essentially means that this decision holds that no state has a right to  unilaterally decide to leave the Union.

Commonwealths

Four states—Kentucky, Massachusetts, Pennsylvania, and Virginia —adopted Constitutions early in their post-colonial existence identifying themselves as commonwealths, rather than states. These commonwealths are states, but legally, each is a commonwealth because the term is contained in its constitution. As a result, "commonwealth" is used in all public and other state writings, actions or activities within their bounds.

The term, which refers to a state in which the supreme power is vested in the people, was first used in Virginia during the Interregnum, the 1649–60 period between the reigns of Charles I and Charles II during which parliament's Oliver Cromwell as Lord Protector established a republican government known as the Commonwealth of England. Virginia became a royal colony again in 1660, and the word was dropped from the full title. When Virginia adopted its first constitution on June 29, 1776, it was reintroduced.  Pennsylvania followed suit when it drew up a constitution later that year, as did Massachusetts, in 1780, and Kentucky, in 1792.

The U.S. territories of the Northern Marianas and Puerto Rico are also referred to as commonwealths. This designation does have a legal status different from that of the 50 states. Both of these commonwealths are unincorporated territories of the United States.

Origins of states' names

The 50 states have taken their names from a wide variety of languages. Twenty-four state names originate from Native American languages.  Of these, eight are from Algonquian languages, seven are from Siouan languages, three are from Iroquoian languages, one is from Uto-Aztecan languages and five others are from other indigenous languages.  Hawaii's name is derived from the Polynesian Hawaiian language.

Of the remaining names, 22 are from European languages: Seven from Latin (mainly Latinized forms of English names), the rest are from English, Spanish and French.  Eleven states are named after individual people, including seven named for royalty and one named after an American president. The origins of six state names are unknown or disputed. Several of the states that derive their names from (corrupted) names used for Native peoples, have retained the plural ending of "s".

Geography

Borders

The borders of the 13 original states were largely determined by colonial charters. Their western boundaries were subsequently modified as the states ceded their western land claims to the Federal government during the 1780s and 1790s. Many state borders beyond those of the original 13 were set by Congress as it created territories, divided them, and over time, created states within them. Territorial and new state lines often followed various geographic features (such as rivers or mountain range peaks), and were influenced by settlement or transportation patterns. At various times, national borders with territories formerly controlled by other countries (British North America, New France, New Spain including Spanish Florida, and Russian America) became institutionalized as the borders of U.S. states. In the West, relatively arbitrary straight lines following latitude and longitude often prevail, due to the sparseness of settlement west of the Mississippi River.

Once established, most state borders have, with few exceptions, been generally stable. Only two states, Missouri (Platte Purchase) and Nevada, grew appreciably after statehood. Several of the original states ceded land, over a several year period, to the Federal government, which in turn became the Northwest Territory, Southwest Territory, and Mississippi Territory. In 1791 Maryland and Virginia ceded land to create the District of Columbia (Virginia's portion was returned in 1847). In 1850, Texas ceded a large swath of land to the federal government. Additionally, Massachusetts and Virginia (on two occasions), have lost land, in each instance to form a new state.

There have been numerous other minor adjustments to state boundaries over the years due to improved surveys, resolution of ambiguous or disputed boundary definitions, or minor mutually agreed boundary adjustments for administrative convenience or other purposes. Occasionally the United States Congress or the United States Supreme Court have settled state border disputes. One notable example is the case New Jersey v. New York, in which New Jersey won roughly 90% of Ellis Island from New York in 1998. 

Regional grouping

States may be grouped in regions; there are endless variations and possible groupings. Many are defined in law or regulations by the federal government. For example, the United States Census Bureau defines four statistical regions, with nine divisions.  The Census Bureau region definition is "widely used … for data collection and analysis,""The National Energy Modeling System: An Overview 2003" (Report #:DOE/EIA-0581, October 2009). United States Department of Energy, Energy Information Administration. and is the most commonly used classification system.    Other multi-state regions are unofficial, and defined by geography or cultural affinity rather than by state lines. A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.

Earth's volcanoes occur because its crust is broken into 17 major, rigid tectonic plates that float on a hotter, softer layer in its mantle.  Therefore, on Earth, volcanoes are generally found where tectonic plates are diverging or converging. For example, a mid-oceanic ridge, such as the Mid-Atlantic Ridge, has volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates coming together. Volcanoes can also form where there is stretching and thinning of the crust's , e.g., in the East African Rift and the Wells Gray-Clearwater volcanic field and Rio Grande Rift in North America. This type of volcanism falls under the umbrella of "plate hypothesis" volcanism. Volcanism away from plate boundaries has also been explained as mantle plumes. These so-called "hotspots", for example Hawaii, are postulated to arise from upwelling diapirs with magma from the core–mantle boundary, 3,000 km deep in the Earth.  Volcanoes are usually not created where two tectonic plates slide past one another.

Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. One such hazard is that volcanic ash can be a threat to aircraft, in particular those with jet engines where ash particles can be melted by the high operating temperature; the melted particles then adhere to the turbine blades and alter their shape, disrupting the operation of the turbine. Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earth's lower atmosphere (or troposphere); however, they also absorb heat radiated up from the Earth, thereby warming the upper atmosphere (or stratosphere). Historically, so-called volcanic winters have caused catastrophic famines.

Etymology

The word volcano is derived from the name of Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn comes from Vulcan, the god of fire in Roman mythology.  The study of volcanoes is called volcanology, sometimes spelled vulcanology.

Plate tectonics

Divergent plate boundaries

At the mid-oceanic ridges, two tectonic plates diverge from one another as new oceanic crust is formed by the cooling and solidifying of hot molten rock. Because the crust is very thin at these ridges due to the pull of the tectonic plates, the release of pressure leads to adiabatic expansion and the partial melting of the mantle, causing volcanism and creating new oceanic crust. Most divergent plate boundaries are at the bottom of the oceans; therefore, most volcanic activity is submarine, forming new seafloor. Black smokers (also known as deep sea vents) are evidence of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example, Iceland.

Convergent plate boundaries

Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges under the continental plate forming a deep ocean trench just offshore. In a process called flux melting, water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, creating magma. This magma tends to be very viscous due to its high silica content, so it often does not reach the surface but cools at depth.  When it does reach the surface, a volcano is formed. Typical examples of this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire.

"Hotspots"

"Hotspots" is the name given to volcanic areas believed to be formed by mantle plumes, which are hypothesized to be columns of hot material rising from the core-mantle boundary in a fixed space that causes large-volume melting. Because tectonic plates move across them, each volcano becomes dormant and is eventually reformed as the plate advances over the postulated plume. The Hawaiian Islands have been suggested to have been formed in such a manner, as well as the Snake River Plain, with the Yellowstone Caldera being the part of the North American plate currently above the hot spot. This theory is currently under criticism, however.

Volcanic features

The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit; however, this describes just one of the many types of volcano.  The features of volcanoes are much more complicated and their structure and behavior depends on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have landscape features such as massive plateaus. Vents that issue volcanic material (including lava and ash) and gases (mainly steam and magmatic gases) can develop anywhere on the landform and may give rise to smaller cones such as Puu Ōō on a flank of Hawaii's Kīlauea.
Other types of volcano include cryovolcanoes (or ice volcanoes), particularly on some moons of Jupiter, Saturn, and Neptune; and mud volcanoes, which are formations often not associated with known magmatic activity. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes except when the mud volcano is actually a vent of an igneous volcano.

Fissure vents

Volcanic fissure vents are flat, linear fractures through which lava emerges.

Shield volcanoes

Shield volcanoes, so named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent. They generally do not explode catastrophically. Since low-viscosity magma is typically low in silica, shield volcanoes are more common in oceanic than continental settings. The Hawaiian volcanic chain is a series of shield cones, and they are common in Iceland, as well.

Lava domes

Lava domes are built by slow eruptions of highly viscous lava. They are sometimes formed within the crater of a previous volcanic eruption, as in the case of Mount Saint Helens, but can also form independently, as in the case of Lassen Peak. Like stratovolcanoes, they can produce violent, explosive eruptions, but their lava generally does not flow far from the originating vent.

Cryptodomes

Cryptodomes are formed when viscous lava is forced upward causing the surface to bulge. The 1980 eruption of Mount St. Helens was an example; lava beneath the surface of the mountain created an upward bulge which slid down the north side of the mountain.

Volcanic cones (cinder cones)

Volcanic cones or cinder cones result from eruptions of 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 meters high. Most cinder cones erupt only once. Cinder cones may form as flank vents on larger volcanoes, or occur on their own.  Parícutin in Mexico and Sunset Crater in Arizona are examples of cinder cones.  In New Mexico, Caja del Rio is a volcanic field of over 60 cinder cones.

Based on satellite images it was suggested that cinder cones might occur on other terrestrial bodies in the Solar system too; on the surface of Mars and the Moon.    

Stratovolcanoes (composite volcanoes)

Stratovolcanoes or composite volcanoes are tall conical mountains composed of lava flows and other ejecta in alternate layers, the strata that gives rise to the name. Stratovolcanoes are also known as composite volcanoes because they are created from multiple structures during different kinds of eruptions. Strato/composite volcanoes are made of cinders, ash, and lava. Cinders and ash pile on top of each other, lava flows on top of the ash, where it cools and hardens, and then the process repeats. Classic examples include Mount Fuji in Japan, Mayon Volcano in the Philippines, and Mount Vesuvius and Stromboli in Italy.

Throughout recorded history, ash produced by the explosive eruption of stratovolcanoes has posed the greatest volcanic hazard to civilizations. Not only do stratovolcanoes have greater pressure build up from the underlying lava flow than shield volcanoes, but their fissure vents and monogenetic volcanic fields (volcanic cones) have more powerful eruptions, as they are many times under extension. They are also steeper than shield volcanoes, with slopes of 30–35° compared to slopes of generally 5–10°, and their loose tephra are material for dangerous lahars.  Large pieces of tephra are called volcanic bombs. Big bombs can measure more than 4 feet(1.2 meters) across and weigh several tons. 

Supervolcanoes

A supervolcano usually has a large caldera and can produce devastation on an enormous, sometimes continental, scale. Such volcanoes are able to severely cool global temperatures for many years after the eruption due to the huge volumes of sulfur and ash released into the atmosphere. They are the most dangerous type of volcano. Examples include: Yellowstone Caldera in Yellowstone National Park and Valles Caldera in New Mexico (both western United States); Lake Taupo in New Zealand; Lake Toba in Sumatra, Indonesia; and Ngorongoro Crater in Tanzania. Because of the enormous area they may cover, supervolcanoes are hard to identify centuries after an eruption. Similarly, large igneous provinces are also considered supervolcanoes because of the vast amount of basalt lava erupted (even though the lava flow is non-explosive).

Submarine volcanoes

Submarine volcanoes are common features of the ocean floor. In shallow water, active volcanoes disclose their presence by blasting steam and rocky debris high above the ocean's surface. In the ocean's deep, the tremendous weight of the water above prevents the explosive release of steam and gases; however, they can be detected by hydrophones and discoloration of water because of volcanic gases. Pillow lava is a common eruptive product of submarine volcanoes and is characterized by thick sequences of discontinuous pillow-shaped masses which form under water. Even large submarine eruptions may not disturb the ocean surface due to the rapid cooling effect and increased buoyancy of water (as compared to air) which often causes volcanic vents to form steep pillars on the ocean floor. Hydrothermal vents are common near these volcanoes, and some support peculiar ecosystems based on dissolved minerals. Over time, the formations created by submarine volcanoes may become so large that they break the ocean surface as new islands or floating pumice rafts.

Subglacial volcanoes

Subglacial volcanoes develop underneath icecaps. They are made up of flat lava which flows at the top of extensive pillow lavas and palagonite. When the icecap melts, the lava on top collapses, leaving a flat-topped mountain. These volcanoes are also called table mountains, tuyas, or (uncommonly) mobergs. Very good examples of this type of volcano can be seen in Iceland, however, there are also tuyas in British Columbia. The origin of the term comes from Tuya Butte, which is one of the several tuyas in the area of the Tuya River and Tuya Range in northern British Columbia. Tuya Butte was the first such landform analyzed and so its name has entered the geological literature for this kind of volcanic formation. The Tuya Mountains Provincial Park was recently established to protect this unusual landscape, which lies north of Tuya Lake and south of the Jennings River near the boundary with the Yukon Territory.

Mud volcanoes

Mud volcanoes or mud domes are formations created by geo-excreted liquids and gases, although there are several processes which may cause such activity. The largest structures are 10 kilometers in diameter and reach 700 meters high.

Erupted material

Lava composition

Another 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 (dacites 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 siliceous 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).
**Andesitic lava is typically formed at convergent boundary margins of tectonic plates, by several processes:
***Hydration melting of peridotite and fractional crystallization
***Melting of subducted slab containing sediments
***Magma mixing between felsic rhyolitic and mafic basaltic magmas in an intermediate reservoir prior to emplacement or lava flow.
*If the erupted magma contains 45% silica, the lava is called mafic (because it contains higher percentages of magnesium (Mg) and iron (Fe)) or basaltic. These lavas are usually much less viscous than rhyolitic lavas, depending on their eruption temperature; they also tend to be hotter than felsic lavas. Mafic 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.
*Some erupted magmas contain  An estimated 500 million people live near active volcanoes.

Historical time (or recorded history) is another timeframe for active.  The Catalogue of the Active Volcanoes of the World, published by the International Association of Volcanology, uses this definition, by which there are more than 500 active volcanoes. However, the span of recorded history differs from region to region. In China and the Mediterranean, it reaches back nearly 3,000 years, but in the Pacific Northwest of the United States and Canada, it reaches back less than 300 years, and in Hawaii and New Zealand, only around 200 years.

As of 2013, the following are considered Earth's most active volcanoes: 

* Kīlauea, the famous Hawaiian volcano, has been in continuous, effusive eruption since 1983, and has the longest-observed lava lake.
* Mount Etna and nearby Stromboli, two Mediterranean volcanoes in "almost continuous eruption" since antiquity.
* Mount Yasur, in Vanuatu, has been erupting "nearly continuously" for over 800 years.

The longest currently ongoing (but not necessarily continuous) volcanic eruptive phases are: 

* Mount Yasur, 111 years
* Mount Etna, 109 years
* Stromboli, 108 years
* Santa María, 101 years
* Sangay, 94 years

Other very active volcanoes include:
* Mount Nyiragongo and its neighbor, Nyamuragira, are Africa's most active volcanoes.
* Piton de la Fournaise, in Réunion, erupts frequently enough to be a tourist attraction.
* Erta Ale, in the Afar Triangle, has maintained a lava lake since at least 1906.
* Mount Erebus, in Antarctica, has maintained a lava lake since at least 1972.
* Mount Merapi
* Whakaari / White Island, has been in continuous state of smoking since its discovery in 1769.
* Ol Doinyo Lengai
* Ambrym
* Arenal Volcano
* Pacaya
* Klyuchevskaya Sopka
* Sheveluch

Extinct
 

Extinct volcanoes are those that scientists consider unlikely to erupt again, because the volcano no longer has a magma supply. Examples of extinct volcanoes are many volcanoes on the Hawaiian – Emperor seamount chain in the Pacific Ocean, Hohentwiel, Shiprock and the Zuidwal volcano in the Netherlands. Edinburgh Castle in Scotland is famously located atop an extinct volcano. Otherwise, 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. Some volcanologists refer to extinct volcanoes as inactive, though the term is now more commonly used for dormant volcanoes once thought to be extinct.

Dormant

It is difficult to distinguish an extinct volcano from a dormant (inactive) one. Volcanoes are often considered to be extinct if there are no written records of its activity. Nevertheless, volcanoes may remain dormant for a long period of time. For example,  Yellowstone has a repose/recharge period of around 700,000 years, and Toba of around 380,000 years. Vesuvius was described by Roman writers as having been covered with gardens and vineyards before its eruption of AD 79, which destroyed the towns of Herculaneum and Pompeii. Before its catastrophic eruption of 1991, Pinatubo was an inconspicuous volcano, unknown to most people in the surrounding areas. Two other examples are the long-dormant Soufrière Hills volcano on the island of Montserrat, thought to be extinct before activity resumed in 1995 and Fourpeaked Mountain in Alaska, which, before its September 2006 eruption, had not erupted since before 8000 BC and had long been thought to be extinct.

Technical classification of volcanoes

Volcanic-alert level

The three common popular classifications of volcanoes can be subjective and some volcanoes thought to have been extinct have erupted again. To help prevent people from falsely believing they are not at risk when living on or near a volcano, countries have adopted new classifications to describe the various levels and stages of volcanic activity.   Some alert systems use different numbers or colors to designate the different stages.  Other systems use colors and words.  Some systems use a combination of both.

Volcano warning schemes of the United States

The United States Geological Survey (USGS) has adopted a common system nationwide for characterizing the level of unrest and eruptive activity at volcanoes. The new volcano alert-level system classifies volcanoes now as being in a normal, advisory, watch or warning stage.  Additionally, colors are used to denote the amount of ash produced.  Details of the U.S. system can be found at Volcano warning schemes of the United States.

Decade volcanoes

The Decade Volcanoes are 17 volcanoes identified by the International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) as being worthy of particular study in light of their history of large, destructive eruptions and proximity to populated areas. They are named Decade Volcanoes because the project was initiated as part of the United Nations-sponsored International Decade for Natural Disaster Reduction. The 17 current Decade Volcanoes are

Effects of volcanoes

There are many different types of volcanic eruptions and associated activity: phreatic eruptions (steam-generated eruptions), explosive eruption of high-silica lava (e.g., rhyolite), effusive eruption of low-silica lava (e.g., basalt), pyroclastic flows, lahars (debris flow) and carbon dioxide emission. All of these activities can pose a hazard to humans. Earthquakes, hot springs, fumaroles, mud pots and geysers often accompany volcanic activity.

Volcanic gases 

The concentrations of different volcanic gases can vary considerably from one volcano to the next. Water vapor is typically the most abundant volcanic gas, followed by carbon dioxide  and sulfur dioxide. Other principal volcanic gases include hydrogen sulfide, hydrogen chloride, and hydrogen fluoride. A large number of minor and trace gases are also found in volcanic emissions, for example hydrogen, carbon monoxide, halocarbons, organic compounds, and volatile metal chlorides.

Large, explosive volcanic eruptions inject water vapor (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen chloride (HCl), hydrogen fluoride (HF) and ash (pulverized rock and pumice) into the stratosphere to heights of 16–32 kilometres (10–20 mi) above the Earth's surface. The most significant impacts from these injections come from the conversion of sulfur dioxide to sulfuric acid (H2SO4), which condenses rapidly in the stratosphere to form fine sulfate aerosols. It is worth mentioning that the SO2 emissions alone of two different eruptions are sufficient to compare their potential climatic impact.  The aerosols increase the Earth's albedo—its reflection of radiation from the Sun back into space – and thus cool the Earth's lower atmosphere or troposphere; however, they also absorb heat radiated up from the Earth, thereby warming the stratosphere. Several eruptions during the past century have caused a decline in the average temperature at the Earth's surface of up to half a degree (Fahrenheit scale) for periods of one to three years – sulfur dioxide from the eruption of Huaynaputina probably caused the Russian famine of 1601–1603. 

Significant consequences 

One proposed volcanic winter happened c. 70,000 years ago following the supereruption of Lake Toba on Sumatra island in Indonesia.  According to the Toba catastrophe theory to which some anthropologists and archeologists subscribe, it had global consequences,  killing most humans then alive and creating a population bottleneck that affected the genetic inheritance of all humans today.  The 1815 eruption of Mount Tambora created global climate anomalies that became known as the "Year Without a Summer" because of the effect on North American and European weather.  Agricultural crops failed and livestock died in much of the Northern Hemisphere, resulting in one of the worst famines of the 19th century.  The freezing winter of 1740–41, which led to widespread famine in northern Europe, may also owe its origins to a volcanic eruption. 

It has been suggested that volcanic activity caused or contributed to the End-Ordovician, Permian-Triassic, Late Devonian mass extinctions, and possibly others. The massive eruptive event which formed the Siberian Traps, one of the largest known volcanic events of the last 500 million years of Earth's geological history, continued for a million years and is considered to be the likely cause of the "Great Dying" about 250 million years ago,  which is estimated to have killed 90% of species existing at the time. 

Acid rain 

The sulfate aerosols also promote complex chemical reactions on their surfaces that alter chlorine and nitrogen chemical species in the stratosphere. This effect, together with increased stratospheric chlorine levels from chlorofluorocarbon pollution, generates chlorine monoxide (ClO), which destroys ozone (O3). As the aerosols grow and coagulate, they settle down into the upper troposphere where they serve as nuclei for cirrus clouds and further modify the Earth's radiation balance. Most of the hydrogen chloride (HCl) and hydrogen fluoride (HF) are dissolved in water droplets in the eruption cloud and quickly fall to the ground as acid rain. The injected ash also falls rapidly from the stratosphere; most of it is removed within several days to a few weeks. Finally, explosive volcanic eruptions release the greenhouse gas carbon dioxide and thus provide a deep source of carbon for biogeochemical cycles. 

Gas emissions from volcanoes are a natural contributor to acid rain. Volcanic activity releases about 130 to 230 teragrams (145 million to 255 million short tons) of carbon dioxide each year.  Volcanic eruptions may inject aerosols into the Earth's atmosphere. Large injections may cause visual effects such as unusually colorful sunsets and affect global climate mainly by cooling it. Volcanic eruptions also provide the benefit of adding nutrients to soil through the weathering process of volcanic rocks. These fertile soils assist the growth of plants and various crops. Volcanic eruptions can also create new islands, as the magma cools and solidifies upon contact with the water.

Hazards 

Ash thrown into the air by eruptions can present a hazard to aircraft, especially jet aircraft where the particles can be melted by the high operating temperature; the melted particles then adhere to the turbine blades and alter their shape, disrupting the operation of the turbine.  Dangerous encounters in 1982 after the eruption of Galunggung in Indonesia, and 1989 after the eruption of Mount Redoubt in Alaska raised awareness of this phenomenon.  Nine Volcanic Ash Advisory Centers were established by the International Civil Aviation Organization to monitor ash clouds and advise pilots accordingly.  The 2010 eruptions of Eyjafjallajökull caused major disruptions to air travel in Europe.

Volcanoes on other celestial bodies

The Earth's Moon has no large volcanoes and no current volcanic activity, although recent evidence suggests it may still possess a partially molten core.  However, the Moon does have many volcanic features such as maria (the darker patches seen on the moon), rilles and domes.

The planet Venus has a surface that is 90% basalt, indicating that volcanism played a major role in shaping its surface. The planet may have had a major global resurfacing event about 500 million years ago,  from what scientists can tell from the density of impact craters on the surface. Lava flows are widespread and forms of volcanism not present on Earth occur as well. Changes in the planet's atmosphere and observations of lightning have been attributed to ongoing volcanic eruptions, although there is no confirmation of whether or not Venus is still volcanically active. However, radar sounding by the Magellan probe revealed evidence for comparatively recent volcanic activity at Venus's highest volcano Maat Mons, in the form of ash flows near the summit and on the northern flank.

There are several extinct volcanoes on Mars, four of which are vast shield volcanoes far bigger than any on Earth. They include Arsia Mons, Ascraeus Mons, Hecates Tholus, Olympus Mons, and Pavonis Mons. These volcanoes have been extinct for many millions of years,  but the European Mars Express spacecraft has found evidence that volcanic activity may have occurred on Mars in the recent past as well.

Jupiter's moon Io is the most volcanically active object in the solar system because of tidal interaction with Jupiter. It is covered with volcanoes that erupt sulfur, sulfur dioxide and silicate rock, and as a result, Io is constantly being resurfaced. Its lavas are the hottest known anywhere in the solar system, with temperatures exceeding 1,800 K (1,500 °C). In February 2001, the largest recorded volcanic eruptions in the solar system occurred on Io.  Europa, the smallest of Jupiter's Galilean moons, also appears to have an active volcanic system, except that its volcanic activity is entirely in the form of water, which freezes into ice on the frigid surface. This process is known as cryovolcanism, and is apparently most common on the moons of the outer planets of the solar system.

In 1989 the Voyager 2 spacecraft observed cryovolcanoes (ice volcanoes) on Triton, a moon of Neptune, and in 2005 the Cassini–Huygens probe photographed fountains of frozen particles erupting from Enceladus, a moon of Saturn.   The ejecta may be composed of water, liquid nitrogen, dust, or methane compounds. Cassini–Huygens also found evidence of a methane-spewing cryovolcano on the Saturnian moon Titan, which is believed to be a significant source of the methane found in its atmosphere.  It is theorized that cryovolcanism may also be present on the Kuiper Belt Object Quaoar.

A 2010 study of the exoplanet COROT-7b, which was detected by transit in 2009, studied that tidal heating from the host star very close to the planet and neighboring planets could generate intense volcanic activity similar to Io. 

Traditional beliefs about volcanoes

Many ancient accounts ascribe volcanic eruptions to supernatural causes, such as the actions of gods or demigods. To the ancient Greeks, volcanoes' capricious power could only be explained as acts of the gods, while 16th/17th-century German astronomer Johannes Kepler believed they were ducts for the Earth's tears.   One early idea counter to this was proposed by Jesuit Athanasius Kircher (1602–1680), who witnessed eruptions of Mount Etna and Stromboli, then visited the crater of Vesuvius and published his view of an Earth with a central fire connected to numerous others caused by the burning of sulfur, bitumen and coal.

Various explanations were proposed for volcano behavior before the modern understanding of the Earth's mantle structure as a semisolid material was developed. For decades after awareness that compression and radioactive materials may be heat sources, their contributions were specifically discounted. Volcanic action was often attributed to chemical reactions and a thin layer of molten rock near the surface. A list of volcanoes in the United States of America and its territories.

Alaska

American Samoa

Arizona

California

Colorado

Hawaii

Idaho

Mississippi

Missouri

Nevada

New Hampshire

New Mexico

Northern Mariana Islands

Oregon

South Dakota

Texas

Utah

Virginia

Washington

Wyoming
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