Volcano

• 1. Introduction

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  A volcano is a vent or fissure in the crust of a planetary-mass object, a place where hot lava, ash, and gases force their way up from a magma chamber below. Most of them are hidden. Because the majority of Earth's plate boundaries sit underwater, most of the planet's volcanoes are submarine, drowned far below any human eye. Only 119 submarine volcanoes have a documented eruption from the Holocene, yet there may be more than a million geologically young ones scattered across the ocean floor. The word itself was born in the early 17th century, from Vulcano, a small volcanic island in Italy's Aeolian chain, named for the Roman god of fire. How does a single name come to cover gentle Hawaiian lava fountains, the eruption that buried Pompeii, and ice-spewing vents on the moons of Neptune? What decides whether a mountain leaks lava quietly or detonates? And how did people move from blaming gods to mapping the machinery 3000 km down?

• 2. Plates In Motion

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  Earth's lithosphere, its rigid outer shell, is broken into sixteen larger plates and several smaller ones, drifting slowly on convection in the ductile mantle beneath. Most volcanic activity follows the seams between these plates, where lithosphere is either created or destroyed. The pattern is not random, and it explains why volcanoes cluster where they do.

  At mid-ocean ridges such as the Mid-Atlantic Ridge, two plates pull apart and hot mantle rock creeps upward beneath the thinned crust. As pressure drops, the rock undergoes adiabatic expansion and partial melting, generating fresh oceanic crust. These divergent eruptions are usually non-explosive. Black smokers, the deep sea vents, mark this process, and where the ridge rises above the waves it builds islands like Iceland.

  Subduction zones tell a more violent story. An oceanic plate dives beneath a continental one, carving a deep ocean trench offshore. Water released from the sinking slab lowers the melting point of the mantle wedge above, a process called flux melting. The resulting magma is rich in silica and extremely viscous, often stalling at depth. Where it breaks through, it builds chains of volcanoes called volcanic arcs, like the Cascade Volcanoes and the Japanese Archipelago around the Pacific Ring of Fire.

  Some volcanism ignores plate edges entirely. Mantle plumes, hypothesized columns of hot rock rising from the core-mantle boundary, feed hotspots as plates drift overhead. The Hawaiian Islands and the Snake River Plain trace this motion, with the Yellowstone Caldera now sitting above the Yellowstone hotspot. Where a continent begins to tear apart, sustained upwelling drives rifting, and failed rifts like the East African Rift erupt unusual alkali lava or carbonatites. Volcanoes almost never form at transform boundaries, where plates merely slide past one another.

• 3. A Catalogue Of Forms

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  The popular image of a volcano is a conical mountain spitting lava and poison gas from a summit crater, but that describes only one shape among many. Volcanoes can wear rugged lava-dome peaks, spread into massive plateaus, or sit so low they are hard to recognize at all. Their craters do not always crown a summit; some cradle lakes, like Lake Taupō in New Zealand.

  Shield volcanoes earn their name from broad, shield-like profiles built by low-viscosity basaltic or andesitic lava that travels far from the vent. They erupt gently rather than catastrophically and dominate oceanic settings like the Hawaiian volcanic chain. Olympus Mons, an extinct martian shield volcano, is the largest known volcano in the Solar System.

  Stratovolcanoes, also called composite volcanoes, rise as tall conical mountains layered from alternating lava flows and tephra. Mount Fuji in Japan, Mayon Volcano in the Philippines, and Mount Vesuvius and Stromboli in Italy are the classic examples. Their high-silica lavas are viscous and gas-rich, a combination that drives explosive eruptions and feeds dangerous lahars. One pyroclastic surge from this type destroyed the city of Saint-Pierre in Martinique in 1902. Their slopes climb 30-35 degrees, far steeper than a shield's 5-10 degrees, and the volcanic bombs they hurl can measure more than 4 ft across and weigh several tons.

  Lava domes grow steep convex sides from slow eruptions of viscous lava like rhyolite, sometimes inside an older crater as at Mount St. Helens, sometimes alone as at Lassen Peak. Cinder cones, by contrast, pile up scoria and pyroclastics from often single, short-lived eruptions; Parícutin in Mexico and Sunset Crater in Arizona are examples, and Caja del Rio in New Mexico gathers over 60 of them in one field. Strangest of all are mud volcanoes, the largest 10 km across and 700 m high, and cryovolcanoes that erupt ice on the moons of Jupiter, Saturn, and Neptune.

• 4. What The Earth Expels

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  Three kinds of material come out of a volcano: volcanic gases, lava, and tephra. The mixture of gases is dominated by water vapour, followed by carbon dioxide and sulfur dioxide, with hydrogen sulfide, hydrogen chloride, and hydrogen fluoride among the principal others. Magma takes the name lava only once it emerges and flows across the surface.

  Silica content governs almost everything about how lava behaves. Magma rich in silica is far more viscous and tends to trap more dissolved gas, while silica-poor magma runs hot and fluid. Felsic lavas, dacites or rhyolites carrying more than 63 percent silica, erupt as stubby domes; Lassen Peak in California is itself a large felsic lava dome. Because these magmas trap their volatiles, they tend toward explosive volcanism and deadly pyroclastic flows that can reach 850 degrees Celsius and incinerate everything flammable. Alaska's Valley of Ten Thousand Smokes, formed by the 1912 eruption of Novarupta near Katmai, is one thick ignimbrite deposit.

  Intermediate or andesitic lava, with 52 to 63 percent silica, characterizes stratovolcanoes and forms mainly at convergent boundaries. Mafic or basaltic lava, below 52 percent silica, is hotter and much less viscous, feeding mid-ocean ridges and shield volcanoes such as Mauna Loa and Kilauea. Rarest is ultramafic lava at 45 percent silica or less, known as komatiites, scarcely erupted since the Proterozoic when the planet's heat flow ran higher.

  Mafic flows show two Hawaiian-named surface textures. Aa carries a rough, clinkery skin typical of cooler basalt, while pāhoehoe forms smooth, ropey or wrinkly surfaces from more fluid flows. Pāhoehoe sometimes shifts into aa as it cools away from the vent, but never the reverse. Tephra, meanwhile, forms when hot gases blast magma apart; particles under 2 mm across are volcanic ash. These shattered volcaniclastics may have supplied as much as a third of all sedimentation in the geologic record.

• 5. Measuring The Blast

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  The Smithsonian Institution's Global Volcanism Program lists 9,901 confirmed eruptions from 859 volcanoes across the Holocene, the last 11,700 years, alongside 1,113 uncertain and 168 discredited entries. To rank their intensity, volcanologists use the volcanic explosivity index, or VEI, running from 0 for gentle Hawaiian-type eruptions up to 8 for supervolcanic ones.

  The named eruption styles climb that scale. Hawaiian eruptions are almost entirely effusive, their columns never topping 2 km. Strombolian eruptions, named for Stromboli, fire frequent short bursts hundreds of meters high and mostly produce scoria. Vulcanian eruptions, named for Vulcano, throw out lava blocks and bombs in explosions that can reach 20 km. Peléan eruptions, named for Mount Pelée, grow and collapse domes into pyroclastic flows. Plinian eruptions take their name from Pliny the Younger, who chronicled the Plinian eruption of Mount Vesuvius in 79 AD, while Ultra-Plinian eruptions are larger still and can carve great calderas.

  At the extreme sits the supervolcano, defined by an eruption that produced over 1000 km3 of deposits in a single explosive event. Such eruptions are rare; four are known from the last million years, and around 60 historical VEI 8 events appear in the geologic record. A supervolcano can devastate a continent and cool global temperatures for years. Known examples include the Yellowstone Caldera, Valles Caldera in New Mexico, Lake Taupō, Lake Toba in Sumatra, and Ngorongoro Crater in Tanzania. The eruption of Lake Toba about 70,000 years ago may have created a population bottleneck that shaped the genetic inheritance of all humans today.

• 6. Sleeping Or Dead

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  Volcanoes are described as erupting, active, dormant, or extinct, but volcanologists do not fully agree on where the lines fall, and the categories often overlap. The USGS calls a volcano erupting whenever ejected magma is visible, active when subterranean signs like earthquake swarms or high carbon dioxide appear, and dormant when it shows no unrest yet hints it could wake again. Technically, any dormant volcano is still geologically active. The term itself has fallen out of favour; the Encyclopedia of Volcanoes from 2000 does not even list it.

  The trouble is telling a sleeping volcano from a dead one. Yellowstone runs a repose and recharge cycle of around 700,000 years, and Toba around 380,000. Vesuvius was described by Roman writers as covered with gardens and vineyards before its eruption of 79 CE buried Herculaneum and Pompeii. Pinatubo was an inconspicuous, unmonitored peak before its catastrophic 1991 eruption.

  Volcanoes presumed extinct have repeatedly proven otherwise. The Soufrière Hills volcano on Montserrat resumed activity in 1995, turning its capital Plymouth into a ghost town. Fourpeaked Mountain in Alaska erupted in September 2006 after silence since before 8000 BCE. Chaitén erupted unexpectedly in 2008. The Taftan volcano in southwestern Iran, last erupting an estimated 710,000 years ago, began uplifting near its summit around June 2023. Truly extinct volcanoes are those with no remaining magma supply, like Shiprock in New Mexico or the Castle Rock beneath Edinburgh Castle in Scotland. Yet even an extinct cone in a monogenetic field offers no guarantee a brand-new volcano will not erupt close by with little warning.

• 7. Living With Fire

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  Volcanic eruptions threaten human civilization, yet the same activity has handed people some of their most valuable resources. The hazards are wide-ranging: pyroclastic flows, lahars, sector collapses, and gases that can poison and choke. Sulfur dioxide from the eruption of Huaynaputina may have caused the Russian famine of 1601-1603. Excessive fluoride salts have poisoned livestock in Iceland on multiple occasions. Ash carries its own danger to jet aircraft, melting in the engines and warping turbine blades.

  Climate bears the deepest scars. Volcanic gases reaching the stratosphere form sulfuric acid aerosols that reflect sunlight and cool the surface. The 1815 eruption of Mount Tambora produced the climate anomalies known as the Year Without a Summer across North America and Europe. The freezing winter of 1740-41, which brought famine to northern Europe, may also trace to a volcanic eruption.

  The gifts are just as real. Tuff, a soft rock formed from volcanic ash, has served construction since ancient times; the Romans used it widely, and the Rapa Nui people carved most of the moai statues of Easter Island from it. Volcanic ash and weathered basalt make some of the world's most fertile soil, rich in iron, magnesium, potassium, calcium, and phosphorus. Volcanism emplaces metal ores and drives heat flow that can be tapped as geothermal power, while volcano tourism has grown into a worldwide industry.

  Monitoring has changed the arithmetic of survival. On well-watched volcanoes, warnings of imminent eruption can arrive hours or days ahead, enough for evacuation. The Mount Pinatubo evacuation of 1991 is believed to have saved 20,000 lives. For Mount Etna, a 2021 review counted 77 deaths since 1536 but none since 1987, though warning is never guaranteed; in March 2017, lava met snow during a presumed predictable Etna eruption, and the resulting phreatic explosion injured ten people.

• 8. From Gods To Geology

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  Hominina left footprints in East African volcanic ash dated 3.66 million years old, proof that humans met active volcanoes early. Long before any written record, that encounter carried religious and social weight, surfacing in oral traditions: Athabascan stories of people living inside mountains, Pele's migration through the Hawaiian island chain, and Javanese tales of a king dwelling in Mount Merapi and a queen at a beach 50 km away along a fault that interacts with the volcano. The earliest known supernatural account is a neolithic goddess at Çatalhöyük.

  Natural explanations arrived slowly, and wrongly at first. In the fifth century BC, Anaxagoras blamed a great wind. By 65 CE, Seneca the Younger proposed combustion, an idea later taken up by the Jesuit Athanasius Kircher, who witnessed eruptions of Mount Etna and Stromboli, visited the crater of Vesuvius, and pictured the Earth in Mundus Subterraneus as a central fire with volcanoes acting as safety valves. Johannes Kepler imagined volcanoes as ducts for Earth's tears, while in 1650 René Descartes proposed an incandescent core.

  The modern picture assembled piece by piece. Lazzaro Spallanzani showed by 1794 that steam explosions could drive eruptions, and the 1886 eruption of Mount Tarawera let observers separate phreatomagmatic and hydrothermal eruptions from dry explosive ones in a single event. By 1928, Arthur Holmes had drawn together radioactive heat generation, mantle structure, decompression melting, and magma convection. That synthesis eventually led to the acceptance of plate tectonics, the framework that now ties together every vent from the Mid-Atlantic Ridge to the methane-spewing cryovolcano Cassini-Huygens found on Saturn's moon Titan.