How Volcanoes Are Born Across the World

Learn about how volcanoes are formed and the ways they erupt

On February 20, 1943, Dionisio Pulido and his family were working in their cornfield near the city of Uruapan, in the Mexican state of Michoacán, about 200 miles west of Mexico City. For weeks there had been tremors and deep rumblings from within the Earth. At 4pm a crack opened in Dionisio’s cornfield and he watched in amazement as the ground rose up about 6½ feet, emitting ash and gas. There were loud explosions and a strong smell of rotten eggs (a clear sign of hydrogen sulfide emission). A new volcano had been born! Within hours the fissure was transformed into the small crater of the newly created Parícutin volcano. After 24 hours, the cone was 164 feet high, and in a week it reached about 492 feet. The activity of Paricutin lasted for 9 years, forming a 1,391 feet high cone and completely covering the nearby village of San Juan Pararingutíro in lava and scoria – a dark rock, relatively light due to the presence of lots of holes, a bit like a pumice, which is formed during volcanic eruptions that emit low silica magma. In the end the eruption affected an area of 90 square miles and forced hundreds of people to be relocated from their homes.

Volcanoes have fascinated humans since the beginning of our journey on Earth with their terrifying beauty and their power of destruction. The earliest humans must have known, too, that the lands around volcanoes were rich and fertile. Volcanoes play such an important role in life on Earth, even for those who have never stepped foot on one. It is no coincidence that our early ancestors evolved in Africa in the volcanic region of the East African Rift. Active volcanoes, of course, pose a threat for those living nearby, but life on Earth would have been impossible without the key role played by volcanoes in the development of the atmosphere and fertile soils, among many other benefits.

What makes a volcano? Volcanoes are not randomly distributed. They are concentrated in certain areas of the Earth, as becomes clear when you look at their distribution on a map. This distribution tells us that they are linked to plate tectonics and we find them where two plates collide, as seen in the so-called Ring of Fire that encircles the Pacific Ocean. They also occur where two plates separate, forming the mid-ocean ridge or a continental rift, if the two separating plates are continental. Finally, volcanoes can also be located in the middle of plates associated with mantle plumes, which are concentrated regions where magma rises from the deep mantle to shallow depths. Such regions are also called hot spots, a classic example of which are the Hawaiian volcanoes.

A volcano is usually high ground or a mountain, sometimes with very steep flanks, produced as a result of the eruption of magma, a mixture of molten rocks and gases, outpoured in a gentle way or in an explosive fashion from an opening in the Earth’s crust. A volcano is a complex system that can be described in three parts: the volcano edifice, usually with a crater area on top, where lava, ash and gases are emitted; a conduit, where magma rises to the surface; and a plumbing system, where the magma is stored.

Magma is a very hot mixture of molten rock, gas, and crystals, sometimes carrying also fragments of solid rock. The temperature is usually around 1,472–2,192°F. Once erupted, magma is called lava and upon cooling, it solidifies to form glassy or crystalline igneous rocks. Magma is prevalently composed of silica, the oxide of silicon (SiO2) formed by the two most abundant elements in the Earth’s mantle and crust, silicon (Si) and oxygen (O). Other important elements such as aluminium (Al), magnesium (Mg), iron (Fe), calcium (Ca), sodium (Na), potassium (K), titanium (Ti), and many more, are present in various proportions. Given that silicon is the most abundant element in the Earth’s mantle and crust, most magma and the rocks that are formed upon magma solidification are called silicates. Two of the most common rocks formed from the solidification of magmas (so called igneous rocks) in the Earth’s crust are basalt, a volcanic rock with a relatively low silica content, and granite, a plutonic rock (cooled beneath the Earth’s surface and rich in crystals) with a higher silica content than basalt. There are some unusual non-silicate magma compositions such as carbon-rich magma (carbonatite) that erupts in certain volcanoes, such as the Ol Doinyo Lengai volcano in Tanzania, and sulfur-rich melts outpoured by some fumaroles. However, these are both rather rare and the prevalent composition of magmas is silicatic.

Under the Earth’s brittle surface, there is no such a thing as a magma layer or ‘ocean’ from where the magma is siphoned off to the surface. Instead, partial melting of crystalline rocks, at depth, generates magma. But how can a rock melt? Temperature and pressure increase with depth from the surface of the Earth towards the core. Despite the increase of temperature with depth, rocks stay solid because the higher the pressure the greater the melting point of the rock. As shown in the graph opposite, the geotherm (the rate at which temperature increases with depth) and the rock’s solidus line (the temperature above which the rock melts) do not cross each other under normal conditions. However, at depth the Earth’s mantle is not rigid like the overlying lithosphere (the outer shell of the Earth formed by the crust and the upper mantle with a variable thickness up to about 174 miles), but ductile. It can deform plastically and is called the asthenosphere (i.e. ductile asthenosphere). When two crustal plates are pulled apart as in the oceanic basins, the released burden allows the asthenosphere to rise upwards filling the gap in the rift. This decompression favours spontaneous melting of the mantle, because the decrease in pressure is quicker than the cooling of the rock. Thus, the rock is hotter than it would be in relation to the lower pressure, and it melts. Decompression melting is the most common melting process in the Earth and produces all the oceanic floor basalts. It is also the common mechanism that produces magmatism linked to a hot spot, such as Hawaii, where the hot plume of mantle rises to produce a chain of volcanic islands.

There is a second common mechanism that allows the mantle to melt and thus cause the production of magma. The melting temperature of rocks is decreased by the addition of water, thus the solidus line moves towards the geotherm. When they cross, the rocks start to melt. Depending on the amount of added water, the exact position of the crossing point changes. This second melting mechanism is commonly seen in a subduction zone, where water is brought to depth by the subducting slab. Slabs are composed of oceanic sediments and basalts. Both contain minerals that can contain some water and other fluids, as carbon dioxide (derived from carbonates), in their structure. When heated, these minerals can release the water, along with other elements, while the slab is pulled down into the mantle.

Finally, the third melting mechanism is simply to heat up the rocks to their melting point. The heat is usually provided by hot basaltic magma ponding at the base of the crust due to its higher density. Large volumes of silica-rich rocks (rhyolites) can be generated in this way by the melting of the thick continental crust in some cases. Basaltic rocks are by far the most common rocks and basaltic magma produced by melting of upwelling parts of the mantle is by far the most common type of magma on Earth.

When we imagine a volcano, we usually think of a big explosion with incandescent ash, gases and rocks spewing into the air and a big umbrella-like plume. Certainly, this is the most spectacular type of eruption, but there are many other types of eruptions. These include the lava flows and spectacular fire fountains of Etna, Hawaii and the recent Bárðarbunga Icelandic lava flow. Volcanic eruptions can be divided into two main groups: effusive and explosive.

Lava flows and sticky lava that forms bulging domes belong to the effusive type of eruption. There is very little gas, therefore very little or no explosion is involved in the outpouring of the magma. Hawaiian and Icelandic volcanoes are mostly characterized by effusive eruptions, but whenever lava flows this can be considered an effusive eruption. Many explosive volcanoes also have lava flows, along with explosive eruptions. Basaltic lavas are fluid and they cool down to a shiny smooth surface called pahoehoe (a Hawaiian name which means ‘on which one can walk’). Upon cooling the surface can form wrinkles and it is then known as ropy lava. The surface of pahoehoe lavas cool very quickly and can form an external carapace under which the lava flow continues to travel over tens or hundreds of kilometres in lava tubes. These form when the external surface cools down but the interior is still hot and keeps flowing, thus the external cooled surface forms a sort of tube. When the eruption finishes, the drained lava tube can survive as a spectacular tunnel. Another common type of lava flow is called aa, another Polynesian name. It is very common on both oceanic islands and continents and its name is due to the very rough, blocky and sharp surface, which makes it very difficult to walk on. Most pahoehoe lava, upon cooling but before solidifying, becomes aa lava because of the increased viscosity (or stickiness) due to the cooling process.

When lava flows interact with water, as for example from an effusive eruption under the sea at the mid-ocean ridge or when the lava flows into the sea, pillow lava will form. A pillow is a round to elongate kidney-shaped basaltic body. The water cools the exterior of the lava very quickly in a sort of bulge or pillow-shape, while the interior is still hot. The continuous flowing of the lava breaks the pillow at one point allowing a second pillow to form. This, in turn, will be broken by the flowing lava and a third pillow will form attached to the second. The process continues until the end of the eruption. The result is a lava flow formed of densely packed individual pillows, each one attached to the other.

When thick lava flows or huge flood basalts cool down, fractures form on the surface and propagate inwards into the thick slowly cooling lava, forming columnar hexagonal shaped basalt. The famous Giant’s Causeway in Northern Ireland is an impressive example of columnar basalts formed in this way by a volcanic eruption that occurred around 50 to 60 million years ago.

Around 67 large cities (>100,000 inhabitants) are located on or close to an active volcano – one that has the potential to erupt either eff usively or explosively or both. Among those, there are three megacities: Tokyo in the shadow of Mt Fuji; Manila close to Pinatubo volcano and Mexico City, which is not far from the towering Popocatepétl volcano. All three volcanoes are strato- or composite volcanoes that have the classical steep cone shape. They are also very explosive volcanoes. The majority of explosive volcanoes are located along the Ring of Fire, above subduction zones. However, other types of volcanoes can also generate explosive eruptions.

When measuring the relative size of a volcanic eruption, two factors are important: the eruption column and the explosiveness. The height of the eruption column can be measured directly in observed eruptions. The extent of the dispersion of tephra around the volcano, whose volume is correlated with the size of the eruption, can be used to estimate the size and height of the eruption column, when the eruption is not directly observed. The explosiveness of a volcano is expressed by the Volcanic Explosive Index (VEI). This is a sort of intensity scale, similar to the one used for earthquakes, which takes into account a number of criteria to estimate the force of the volcanic event. The VEI runs from 0 for not-explosive or very small explosive eruption (called Hawaiian) up to 8 for the colossal ones (termed Ultra-Plinian). The VEI can also be linked with the volume of emitted material, to give a better idea of how big the eruption is, and is also loosely linked with the frequency of the eruption. Larger eruptions are less frequent than smaller ones, which is very good news for all of us.

At the low intensity end of the VEI scale we find Hawaiian eruptions, named after the style of activity currently observed in Hawaiian volcanoes. They are characterized by lava fountains that can be quite spectacular with hot, incandescent clots of magma ejected at high speed from a vent, typically rising to a height of a few tens to a few hundred metres before landing back on the ground. The emitted magma is still extremely hot (about 1,832°F) and if the fountaining activity is very intense, when the erupted material falls on the ground it can accumulate to form lava flows, as frequently observed on Hawaiian volcanoes. Any type of activity similar to this one is called a Hawaiian eruption regardless of where in the world it occurs.

Stromboli volcano in southern Italy is considered the lighthouse of the Mediterranean because of its constant activity, with minor eruptions that are visible from many points in the surrounding sea. Indeed, Strombolian eruptions consist of transient explosions of short duration that occur in sequence at intervals of a few minutes to a few hours (frequently exploding every 20 minutes or so). Each explosion generates a small plume of some hundred metres in height throwing ash, incandescent volcanic bombs and large ballistic blocks. Apart from Stromboli, there are many volcanoes that show the same Strombolian-type of activity with transient, short yet regular minor eruptions. Both Hawaiian and Strombolian eruptions are characteristic of basaltic magmas.

The famous 79 AD Pompeii eruption of Mt Vesuvius in southern Italy caught the local population by surprise. The vivid account by Pliny the Younger, a Roman administrator and nephew of Pliny the Elder a naturalist, philosopher and author of the 37-volume Naturalis Historia, who lost his life during this eruption, gives a clear picture of the devastating consequences of the unexpected large explosive eruption. We now have a much better knowledge of volcanic behaviour and can recognize the signals of unrest that precede an eruption, but at the time the long period of inactivity of Vesuvius gave a false sense of security. The eruption started on the morning of August 24 with a volcanic plume rising above the vent up to 20 miles and a dense rain of ash and pumice, which was not necessarily lethal. Indeed, the devastation arrived around midnight when the eruptive column started to collapse generating the first devastating pyroclastic flow, an avalanche of hot ash, pumice, rock fragments and volcanic gases. This first pyroclastic flow rolled down the flank of Vesuvius at a speed of up to 62 miles per hour, wiping out everything in its path including the city of Herculaneum on the coast. The city of Pompeii suffered a similar fate only a few hours after Herculaneum, due to a second pyroclastic flow. A layer over 10 feet thick of hot volcanic material covered the two cities and the devastation can still be seen today after the archaeological excavation revealed the casts of thousands of human bodies. It is unknown exactly how many died but it was a catastrophe. Most of the people died due to suffocation by volcanic gases and heat and they are eerily preserved as they were in their last moments.

Eruptions like the one at Pompeii are called Plinian eruptions, named after Pliny the Younger, and characterized by a jet of magma and volcanic gases emerging at high speed – about 224–1,342 miles per hour from the vent and forming the classical umbrella-shaped head eruptive column. The eruption can last for hours or days. The convective plume sucks up the surrounding air into the jet and expands into the atmosphere reaching a height of about 34 miles. Depending on the intensity of the eruption, Plinian eruptions are also subdivided into Ultraplinian if they are larger than Plinian and Subplinian if the eruption is slightly smaller.

When magma interacts with water, eruptions get even more explosive and are called hydromagmatic or phreatomagmatic. Magma can interact with water in a wide range of environments such as seawater, lakes and glaciers. The island of Surtsey off the south coast of Iceland, was born between 1963 and 1965 due to the interaction of a submarine volcano and the shallow marine environment. The eruption started on 14 November 1963, when the top of the volcano was about 32 feet below the water surface. A dense black cloud of ash and steam rising in a few hours to 213 feet above the sea surface was the first sign of the eruption. By the next day, Surtsey was well above sea level and continued to grow. This type of eruption has been known as Surtseyan ever since.

Volcanoes erupt explosively because of the gases (mainly gaseous water and carbon dioxide) dissolved in the magma. However, the amount of dissolved gases (also called volatiles) depends on a variety of factors among which pressure and magma composition, particularly viscosity, are the most important. When the magma rises towards the surface, pressure decreases and the gases tend to escape. Bubbles begin to form in a process called vesiculation or gas exolution. The bubbles start growing and coalesce together leaving the magma in the interstices between the bubbles. This process is called fragmentation because the magma is fragmented by the growing bubbles. At this point the explosive eruption will occur. In this way, the amount of dissolved gases represents the driving force of an explosive eruption. However, the viscosity of the magma is equally important in determining if the eruption will be explosive or not. Indeed, in low viscosity magma, like basalt, the gas can readily escape producing an effusive or low explosive Hawaiian eruption. At the other extreme, high viscosity magma, like rhyolite, can be very explosive because it contains a lot of gas that cannot easily escape. The gas is trapped in the viscous magma but the bubbles continue to grow and expand until there are enough bubbles to fragment the magma and eject it explosively.

Read more in Earthquakes and Volcanoes, which is available from Smithsonian Books. Visit Smithsonian Books’ website to learn more about its publications and a full list of titles. 

Excerpt from Earthquakes and Volcanoes © The Trustees of the Natural History Museum, London, 2019