Volcanoes. Big, scary powerful explosive things spewing fountains of magma. Sometimes they add scenery, sometimes they add danger (don't fall in the Lava), sometimes they are the central disaster of a story. If they can go off, they will, and sometimes must be stopped with a human sacrifice, usually a virgin. Or they may be a great location for the bad guy. But how do they form, and what happens when they erupt? Read on.
Where do they come from? Where are they found?
Volcanoes on Earth can't be found just anywhere. Well, okay, they sort of can, but are much more likely in certain regions.
Ultimately, volcanoes on any planet, moon, or similar body form because the inside is much hotter than the surface, hot enough to melt whatever material the body is made from. On Earth, rock melts to magma, ice bodies release water, other material might be possible. The source of this heat depends on the body. Some may come from the decay of radioactive materials. Some comes from heat of formation of the body: gravity pulling anything together releases some energy, planetary amounts of material release a huge amount. Differentiation: heavier materials sinking to the center, lighter materials rising, releases more heat on top of this. A few moons are heated tidally: for a complete explanation, read further. Some energy might come from chemical reactions, freezing of materials or other phase changes, and other unusual sources. If enough energy is produced, and the inside of the body stays hot enough, volcanoes can form.
On Earth, it is estimated that somewhere between 1/4 and 3/4 of the energy released comes from radioactivity, almost all the rest comes from leftover heat of formation, and a little bit from the slow freezing of the outer core. (For moons or other planets, skip to the end)
Also needed is a way for heat and melted material to reach the surface. Rock, like most materials, melts at a higher temperature when under more pressure, on Earth, the pressure increase as you go towards the center is enough to keep almost all rock solid the whole way through. Rock is a decent thermal insulator, so heat conduction on its own does not bring enough energy upwards to melt anything.
Fortunately for the formation of volcanoes, the rock in Earth's interior, in a layer called the mantle, can deform and flow over long enough periods of time. A good comparison is something like fudge: acts as a solid over short periods, put put some pressure on it and wait long enough, and it will deform. (Though Earth's rock flows much more slowly than fudge would. Pop culture is often confused on this: hot material that flows suggests magma, and earth's mantle is often mistakenly shown as liquid.) On Earth, this movement allows for plate tectonics, where the upper layer (crust and upper mantle) forms plates that can break and shear, and move across the surface, while most of the interior (the rest of the mantle: Asthenosphere, transition zone, and lower mantle) convects much like a liquid does, but slower. These movements can cause melting in a number of ways:
1. Subduction zones: If plates push together, and at least one of them is an ocean plate, then the ocean plate will sink into the mantle (the denser of the two, if they're both ocean plates). Ocean plates, being under the ocean have absorbed a lot of water, they also have accumulated carbonate sediments from shells of sea creatures. Inside the hotter mantle, this water mixes with hot rock, lowering its melting point and allowing magma to form
In fluid convection, cooler fluid sinks because it is denser than hotter fluid, subduction is this process in Earth's mantle, with wrinkles. Ocean plate can sink because ocean crust is dense, becoming denser than mantle under high pressure, helping pull the plate down. Continent is thicker and less dense, too low density to sink, continents pushing into each other compact instead and make mountains. Subducted ocean crust stays as a structure into the mantle, being an important part of it's convection....but that's another useful notes.
2. Mantle Plumes: At some points on the earth, a large glob or plume of hotter the normal mantle rock forms and rises to the surface. This rock comes from the bottom of the mantle: the heat comes from the core, as well as the lower mantle simply being hotter (also an important part of mantle convection, see above about this being its own Useful Notes article). When such a plume reaches the crust, it can heat the surrounding rock enough to melt it, itself melt due to lower pressures near the surface, or both.
Mantle plumes themselves were theorized, but not seen, for a few decades. Some other theories have been proposed for how volcanoes attributed to plumes might form, but plumes have now been detected underneath many volcanic areas where they were expected. It is possible they are more commonly formed around a couple mysterious large structures under Africa and the Pacific.
3. Spreading regions: At these boundaries, plates move apart. As they move, pressure is released from mantle rock beneath the spreading area; this rock is pushed upward and melts.
(There are actually a few more ways volcanoes can form, real life being complicated after all. However, the above processes create the vast majority that we see today.)
Spreading and Subduction erupt most magma on Earth. Spreading mostly happens under the ocean at ridges, so most land volcanoes will form from subduction areas: Around the Pacific Ocean ("ring of fire"), in Indonesia, and the Mediterranean are the most well known areas. Mantle plumes might be able to form anywhere, whether clustered or not are far less common: Hawaii and other mid pacific islands, plus Iceland, are the most famous examples, Yellowstone might be one as well (or a more complex process), and some African Rift Valley volcanoes are a mix of this and spreading.
(California is not a good example of a volcano location. It is a place where plates move past each other: since nothing is sinking or rising nearby, there is no way to heat rock, change pressure, or mix anything, and magma doesn't form as a result. The same is true for similar regions. hot spots, or possibly weirder processes, might happen there, but in practice most of California lacks volcanoes.)
Once magma is formed, it is less dense than the rock it is melted from, as a result it is pushed upward by buoyancy, finding or creating weak spots and cracks. If the magma freezes before reaching the surface, it can form underground structures. If it stays molten and reaches the surface, a volcano erupts. Many volcanoes are fed by magma chambers: large sections of rock where magma can pool before being forced to the surface. these chambers may feed any number of vents at the surface, or simply explode if conditions are right.
Of course, if a story has spirits, gods, volcano creatures, and other such beings, ignore all of the above.
What Happens When They Erupt?
Lava, landslides, burning gas, poisonous gas.....Erupting volcanoes throw out all sorts of things, many of which can kill you. What gets released changes from volcano to volcano, or eruption to eruption. Less gassy, less viscous magma produces mostly lava flows. Add more gas, and you start to get lava fountains, thrown rocks, possibly explosions. Viscous lava produces more viscous flows, and also can trap gas, resulting in the same sorts of releases. Add enough gas and lava, made thick enough, and you get a massive explosion. Subduction volcanoes are more likely to be gassy: the water and carbon dioxide (from sea floor carbonate sediments) brought by the subducting plate (and that creates the magma) provides the needed gas.
The shape of the volcano depends on what sorts of eruptions built it. Mostly lava flows, and you get a low, broad shield volcano as the lava flows long distances. More explosive eruptions, or gassy eruptions, and the volcano gets narrower and more cone like: material thrown out cannot fly as far and piles up in the same location. More powerful and explosive usually equals rarer: Hawaii's volcanoes continuously release magma with little personal danger unless you are very close, while large, explosive volcanoes can sit for a long time, than explode violently.
Despite what entertainment often shows, walking near magma can get very, very hot, and hanging out near a lava lake is a bad idea for reasons explained there. The properties of magma (how it flows, the rocks it forms) depend on the temperature and chemistry of the magma: these properties are influenced by how the magma formed, the properties of the rock that melted, and the fraction of rock melted. (Rocks do not have fixed melting points: Because they are made of lots of different chemicals, they instead have a melting range, lower melting minerals go first, higher melting dissolve more and more until the whole rock is molten.) Lava flowing over ground is quite slow, the material is heavy and viscous. In a channel it can flow faster, channels also insulate the magma, keeping it hotter for longer. If a roof forms over the magma, it gets even more insulated, forming a lava tube.
Cooling magma can produce interesting formations. Rope like flow produces twisted, rope like features. Empty lava tubes produce caves, empty channels can leave canyons. Freeze magma underwater and you see pillow shapes. Crystallize it in the right way, and hexagons or other shapes form.
Also coming from volcanoes are gases. Once again, the exact gases depend on the volcano: water and carbon dioxide are probably the most common, but hydrogen, ammonia, sulfur gases, hydrogen chloride, and lots of others leave as well. In fact, Earth's atmosphere (and most chemicals for life) is made from chemically processed volcanic gases. shorter term, though, gases are yet another thing that can kill you: hydrogen sulfide, hydrogen chloride, and a number of others can be deadly: sulfurous gases can also block sunlight and cause acid rain, carbon dioxide released can than increase global warming. This cooling has caused some unusually cold years over the past few centuries (1815 was "the year without a summer", and modern weather measurements have detected cooling from other eruptions. Much more dramatically, enormous eruption episodes are a prime suspect for several mass extinctions: volcanic eruptions over huge areas for tens to hundreds of thousands of years occur at the same times.
Combine gases with magma, and you get ballistic magma, from cool looking lava fountains to dangerous lava bombs: blocks of solid rock thrown through the air. These can, for understandable reasons, be dangerous to those nearby.
A more unusual effect of ash blown is on airplanes. It might not seem dangerous at first glance, but the ash can erode the sides of planes and interfere with the engines (abrasive material, melting in the engine and clogging it up, does not mix with expensive machinery.)
Combine gas, heat, and pulverized lava, and you get the most dangerous output of euptions: landslides, pyroclastic flows, and lahars. All of these are caused by a combination of rock (whether solid lava, broken up pieces of the volcano structure, or erosion) with something fluid (volcanic gas, water, mud). They can move very, very fast, and the rocks within cause enormous damage, knocking down and smothering anything they run into. Pompeii and Herculaneum's preserved bodies come from such flows: quickly killing and burying people in the towns. Pyroclastic flows are rock and ash mixed with hot gas, lahars are rock mixed with water, ordinary landslides can occur as well.
Dormant, Extinct, Active. Will it erupt?
This is a classification system, though the terms can get somewhat fuzzy. Active volcanoes are still active, either erupting or have erupted recently, and are expected to do so again. Extinct volcanoes are those though to not be erupting any more. Dormant can get fuzzy: these are not expected to erupt, but have the potential to do so again. Where the boundary between dormant and the others lies has to be determined by scientists; volcano knowledge being inexact this is not a clear division.
Eruptions aren't completely predictable, but some signs are known. for explosive volcanoes, earthquakes, higher pressures wherever the magma is, more full magma chambers are all signs. Earthquakes may even cause an eruption; Mt St Helens erupted when material holding lava in slid down the volcano: the pressure released, the volcano exploded. Changes in gases released even when a volcano is calm have also been used.
Once they stop erupting, volcanoes act like any other mountain, slowly eroding away.
Geysers, hydrothermal vents, and other hot stuff
Even if magma doesn't reach the surface, it can still release gas, and heat things. Heat water, and you get a geyser (on land), or a hydrothermal vent (undersea.) They are caused by water seeping down to hot areas, being heated, and rising back up. Geysers are cool to look at: hydrothermal vents release chemicals that power unique life, creating an unusual ecosystem. These things don't last forever, changes in the circulation pattern, amount of heat available, or any other part can cut off one vent, or open another somewhere else.
Less eruptive versions create hot springs and hot pools.
More unusual as ashphalt volcanoes: formed when heat combines with an oil deposit. This creates a mountain of tar, and also its own unique types of life under the ocean.
Add water yourself, or take advantage of existing water, and you get geothermal power. Like any other heat source, geothermal heat can create steam and power a steam turbine. Like many renewable energy sources, this is expensive to set up (drilling is expensive), but very cheap to operate once started. Iceland owes very cheap electricity to such a power source.
Rocks and Resources. What good are volcanoes anyway?
Rocks formed from frozen magma are called igneous rocks. The most common of these are basalts and granites, and they make up the outer layer of the Earth. (The crust is in fact defined by the rocks it is made from. A few miles down, the most common rock becomes a denser rock called peridotite. This change is detectable by seismic waves, the boundary defines where the mantle starts.) These two rocks create continents and oceans: lighter granite is pushed up by bouyant forces, allowing continents, heavier basalt stays low, allowing oceans to form. Basalt is heavy enough to sink when compressed, allowing subduction, granite istoo light to do this: collisions instead result in continents getting squeezed and forming mountains. Basalt fmainly forms from melted mantle material, granite can form from remelted surface rocks.
Rocks can forms on the surface, or in magma intrusions that stay underground (most granite forms from these). Different conditions and magma compositions give a range of other rocks. the most well known of these are probably volcanic glasses (including obsidian), formed when rocks cool too quickly to form crystals, and pumice, formed is a lot of gas is mixed into the rock, forming bubbles and holes when the rock solidifies.
Intrusive magma, volcanoes, and hydrothermal vents are responsible for valuable mineral deposits (long dead eruptions that is, the ores in question were formed through Earth history): the partial freezing, melting, and heat allows different minerals to separate out, and the volcano brings material that would be dispersed on the surface. Similar effects means that soil near volcanoes can be very fertile: volcanic ash adds minerals that are rare elsewhere. (though if you are living near one and it goes off...)
Most unusually, volcanoes can bring up mantle material. Peridotite rocks are valuable to geologists; this is though to be the main rock in the upper mantle. Valuable to everyone are diamonds and jadeite: These are created by the high temperatures and pressures within the mantle, and brought to the surface using eruptions.
Valuable on a much longer time scale are volcanic gases. Our atmosphere is formed from outgassed material: carbon dioxide provides the carbon for life, water helped form oceans, nitrogen formed from released ammonia. Over millions to tens of millions of years, they help control the amount of carbon dioxide in the air: carbon is taken up by life, deposited on the ocean floor (either as carbonate shells, or as whole dead organisms), subducted, and erupted. This cycling helps stabilize the carbon dioxide in the air (Over millions of years, burning fossil fuels today releases it much, much faster.) Water is cycled as well.
Can you stop an eruption? How?
You could try the old standbys — praying, offerings to a volcano god, appeasing spirits, earth or fire magic, throwing someone in, etc. Or, in a different work, use your geotectonic thermal positronic stabilizer. In our world, stopping or preventing eruptions isn't happening yet.
In theory, an eruption could be stopped by cooling the magma, possibly by releasing building gas and lowering pressure. The first problem: this would take an enormous amount of cooling for a volcano of interest; drilling or similar work is already complicated enough, a project the size needed would make it that much more expensive, complex, and difficult. The second problem: to provide cooling would involve drilling or otherwise reaching near where the magma is; this creates weaknesses and pathways in the rock, and such weaknesses are exactly the sort of thing that sets off an eruption, or allows one.. Getting the engineering and science right would be very tricky and risky indeed.
So, for now, the best way to handle eruptions is still to get out of the area when one is coming.
Still, when Eldfell
, located on an island off Iceland, erupted in 1973, they were actually able to hold the lava at bay by spraying water on it, preventing it from spreading and closing off the island's important harbour.
Other Planets
Other solid bodies can and do have volcanoes. Earth is the most active planet in our solar system, some outer planet moons are more active. Volcanoes are expected on exoplanets, though detection isn't good enough to see them on any planet if they exist.
All else equal, bigger, younger bodies are expected to be more volcanic. Bigger bodies mean more heat produced in the body's formation, and more radioactivity increases proportionally to the body's size. Due to the Square-Cube Law, bigger bodies have less area proportionally to release this heat, more concentrated heat = more volcanoes for longer. Younger bodies have less time to cool from formation. Radioactivity is also higher: radioactive materials have not decayed, so more exist, and more energy is emitted. Sometimes, young bodies contain extremely radioactive materials that have long decayed in older bodies.
Mercury and the moon fit this pattern, they are very small and volcanism died early. Mars also fits: between Mercury and Earth in size, a few volcanoes are recognizable. They erupted somewhat recent geologically, but no activity is seen today. Smaller rocky bodies have no evidence of volcanoes whatsoever. Venus, a bit smaller than Earth, should be about as volcanic: It indeed has a lot of volcanic features, and some evidence of possible eruptions today has been seen. The form of these volcanoes is very different: Spider like features, enormously long lava channels, pancake shapes. How these formed is not understood.
No solar system world has Earth style plate tectonics, greatly changing how volcanoes form: Why only Earth is still a scientific question. Mantle plumes probably still exist: One or a small number of continuous plumes are probably responsible for Mars' giant but small in number volcanoes, a small number of plumes would continually produce magma at the same spots with no surface motion to spread the eruptions out. Venus, in addition to plumes, appears to have had a period of planetwide, intense volcanism about 500 million years ago (though the evidence is still being questioned and worked out.), and far less overall movement since. How and why this might happen is still not understood.
On outer planet moons, another source of heating occurs, producing a few enormously active worlds. Tidal heating is caused by the noncircular orbits of some moons. Like tides on earth, gas giants produce bulges on their moons, pulling material underneath and on the opposite side of the planet upward, and pulling material at 90 degrees to the planet inward. (This makes the body slightly egg shaped.) If the moon orbits in a noncircular orbit, the tidal force increases and decreases, and the pull (and site of the bulge) moves a bit over its surface. If the moon is made of something that can deform but resists it (liquids, hot rock, not so cold ice can all do this), the stretching caused heats the body, and can feed volcanoes. The most active of these moons is Io: about the size of Earth's moon, but far more volcanically active even than earth. (Any spacecraft that goes past sees a number of eruptions. Even on active Earth, catching a surface eruption with the same amount of observation would be lucky.) These eruptions are also far hotter than earth. Io's spectacular volcanism is responsible for its vivid colours, which has often been compared to pizza; its surface is constantly being remodelled by eruptive events, meaning that its appearance is likely to change at any time.
Outer solar system moons also produce the cryovolcano: similar structure, formed in a similar way, but erupting water or other non-rock material instead. Evidence of such eruptions is seem on Europa, Enceledas, and Triton, geysers have been seen erupting on Europa and Enceladus. Titan and Pluto may have some as well: this is far less certain. Enceladus and Europa are fed by tidal heating, Triton likely was in the past.
Whatever a planet is made of, if solid and enough heat exists, some version of volcanoes probably will also.
