History UsefulNotes / Volcanoes

1. Subduction zones: If plates push together, and at least on of them is an ocean plate, than the (denser, if two ocean plates) ocean plate will sink into the mantle. Ocean plates, being [[CaptainObvious 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

This is a classification system, though the terms can get somewhat fuzzy. Active volcanoes are [[CaptainObvious still active]], either erupting or have erupted recently, and are expected to o 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 can get fuzzy.

Ocean plate can sink because it is made of heavier rocks, under enough pressure, these rocks become heavier than nearby mantle rock, and this weigh pulls the plate downward. The plate itself actually remains a single structure, sometimes to several hundred miles down, sometimes all the way to the core boundary, and this sinking is an important part of how mantle convection works...but that's an entire useful notes all on its own. Continent is too light to sink like this: if two continents hit each other, they compact and make non-volcanic mountain ranges instead.

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. the upper layer is mostly basalt and granite, the next layer down is mostly a rock called peridotite. As seismic waves travel from less dense basalt and granite to more dense peridotite, they get bent and refracted: the detected boundary was used to define the crust and mantle.) 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.

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 energ, planetary amounts of material release a huge amount. Differentiatio: heavier metrials 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.

Earth being hot isn't enough on its own to produce volcanoes. like most materials, increased pressure increases rock's melting point. On Earth, though the interior is hot, it is also under high enough pressure to keep almost all of it solid. In order to melt rock, it must be either heated enough to reach a higher temperature melting point, pressure must be released somehow, and/or the composition of the rock must be changed to lower its melting point. Simple heat flow wouldn't do this on its own, rock is a good insulator and the heat would be released too slowly to cause melting.

Fortunately for the formation of volcanoes, the rock in Earth's interior 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 this is how it often gets described.) 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. These movements can cause melting in a number of ways:

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.

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 can in theory form anywhere, but 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.

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.

Volcanoes on Earth can't be found just anywhere. Well, o.k., they sort of can, but are much more likely in certain regions.

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 heat from radioacticity if it exists. Due to the SquareCubeLaw, 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; the way radioactive decay works (the more material there is, the more it is emitting), radioactive heating is higher as well if it exists.

Mercury and the moon fit this pattern, they are very small and volcanism died early. Mars is somewhere in between: a few volcanoes are recognizable, erupting somewhat recent geologically, but no activity is seen today. Smaller rocky bodies have no evidence of volcanoes whatsoever. Venus 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: a continuous plume is responsible for Mar's 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 (thoug hthe evidence is still being questioned.), 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 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), 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 take luck.) These eruptions are also far hotter than earth.

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 a 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.

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 make magma, but some outer solar system moons have similar structures made from melted water, and other materials could theoretically exist also. 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: Planetary amounts of material smashing and pulling itself together releases huge amounts of energy, and differentiation, where energy . A few moons are heated tidally: for a complete explanation, read further. Some energy might come from chemical reactions, freezing of materials, and other unusual sources. If enough energy is produced, and the inside of the body stays hot enough, volcanoes can form.

On some planets or moons, magma oceans or equivalent exist near the surface, where much of a layer is melted, and this material can flow or be pushed up cracks. On Earth, however, almost all the rocky part is solid. Rock, like most materials, melts at higher temperatures the higher pressure it is under. Though the rock inside the Earth is more than hot enough to melt, the high pressure keeps it solid. To cause melting, other processes must occur to either release pressure, heat rock further, or lower the melting point.

1. Subduction zones: If plates push together, and at least on of them is an ocean plate, than the ocean plate will sink into the mantle. Ocean plates, being [[CaptainObvious 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

Ocean plate can sink because it is made of heavier rocks, under enough pressure, these rocks become heavier than nearby mantle rock, and this weigh pulls the plate downward. The plate itself actually remains a single structure, sintimes to several hundred miles down, sometimes all the way to the core boundary, and this sinking is an important part of how the earth works...but that's an entire useful notes all on its own. Continent is too light to sink like this: if two continents hit each other, they compact and make mountains instead. If two ocean plates hit, only the denser one sinks.

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 (see above about this being its own UsefulNotes article). When such a plume reaches the crust, it can either heat the surrounding rock enough to melt it, or melt itself due to lower pressures near the surface.

3. Spreading regions: At these boundaries, plates move apart. As they move, mantle rock is exposed, pressure is released, and it is pushed upward and melts.

(There are actually a few more ways volcanoes can form, life being complicated after all. However, these 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 can in theory form anywhere, but 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 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, and rises, finding or creating weak spots and cracks. If the magma freezes before reaching the surface, it can form other 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.

You could try the old standbys, praying, offerings to a volcano god, appeasing spirits, earth or fire magic, [[AppeaseTheVolcanoGod throwing someone in]]. Or, in a different work, use you [[{{Technobabble}} 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 be releasing building gas and lowering pressure. Th 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 makes 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, such weaknesses [[HoistByHisOwnPetard 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.

Despite [[ConvectionSchmonvection what entertainment often shows]], walking near magma can get very, very hot, and hanging out near a lava llake 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.

Lava, landslides, burning gas, poisonous gas.....Erupting cvolcanoes 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 volcanoesare 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.

Despite [[ConvectionSchmonvection whatentertainment often shows]] walking near magma can get very, very hot, and hanging out near a lava llake 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.

Volcanoes. Big, scary powerful explosive things spewing fountains of magma. Sometimes they add scenery, sometimes they add danger ([[ConvectionSchmonvection don't fall in the Lava]]), sometimes they are the entral disaster of a story. If they can go off, [[ChekhovsVolcano they will]], and sometimes must be stopped with a [[AppeaseTheVolcanoGod human sacrifice, usually a virgin]]. Or they may be a [[{{Mordor}} great location]] [[VolcanoLair for the bad guy]]. But how do they form, and what happens when they erupt? Read on.

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 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 a hard to deform but not completely rigid material (liquids, hot rock, not so cold ice), 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 take luck.) These eruptions are also far hotter than earth.

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. Pompeei 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.

!!Other Stuff
''insert later''

''insert later''

Whether a volcano releases flowing lava, explodes, fountains magma into the air, or anything in between, depends on the type of magma, what is in it, how big the eruption is, and a number of other conditions. Generally, the more gas and the thicker the lava, the more explosive the eruptions. Gas (Water vapor, carbon dioxide are the most common) can be released from the lava in the same way as carbonation is released from a drink, and with similar effects. Too much gas, and the magma explodes. Some gas, and the magma is thrown up and fountains. Little gas, and the magma simply flows. Thinner magma can reach the surface more easily, and flows out. Thicker magma can get stuck, and trap gas if gas exists, [[OhCrap until something lets it go]] and causes explosions or other huge eruptions.

Subduction volcanoes, because the lava contains so much water, plus carbon dioxide (from sea sediments) are more likely to erupt explosively.

Volcanoes formed from flowing lava are usually flatter: flowing lava can spread out over a wide area. Volcanoes in Hawaii form shield volcanoes, named for the wide, gentle hill formed. Eruptions at spreading sites may not form obvious volcanoes at all: new rock forms almost everywhere as the plates spread. Lava flowing on the ground usually moves slowly: it is viscous compared to, say, water. However, if the lava forms a channel, the material in the middle can flow much faster. If the top of the channel or lava flow freezes into a hard roof, the rest can stay liquid for quite a long distance. These tubes or channels can form caves if eruptions end without them being filled in, they can also be dangerous in erupting areas if the roofs are weak. If lava erupts into water, the outer layer cools much faster than in air. Channels for underwater as well, and structures called pillow lavas can form as globs of lava are quickly cooled and frozen. If molten lava stays at the entrance, it can form a lava lake.

More explosive or gaseous eruptions release a range of other materials. Volcanoes formed from mostly explosive eruptions tend to be narrower and taller: material thrown into the air can't travel as far, and accumulated into the central cone we are all familiar with. Fountaining or exploding volcanoes can form lava bombs: large rocks made from magma thrown up and freezing in the air. Exploding volcanoes ....''(to be continued)''