Out of all the practical options we elaborate about, volcanoes are probably the least likely for activists to be able to affect. However, since volcanism is such a powerful element, we couldn’t totally disregard it, (at the end of this text we address the unlikeliness issue). But first a very important clarification must be made – volcanoes' potential is held not within their immediate destruction power which, as enormous as it can be, is still relatively local, but rather within their climatic long term effects, which in some cases can be global. But in order to explain the global potential effect of volcanoes, a brief explanation of their nature, locations and types, is required.

The force with the most powerful potential on earth is found within it. The earth is getting hotter with depth, reaching about 7000°C in the center of its core, which is hotter than the surface of the sun. This heat, generated from the decay of radioactive materials in the core, heats the rocky surrounding layer called the mantle. The heat creates a movement in the mantle which correspondingly causes a movement in the layer above it – the earth’s crust (the surface of the earth). The crust is not a continuance shell but is built of giant pieces called tectonic plates, and the mantle movement causes these plates to move.

The plates’ movement involves several types of interactions between them with several different geological outcomes. Two of them involve volcanism:

Plates Divergence – Is when two plates diverge from one another due to the mentioned movement in the mantle. This movement also causes hot material from the mantle to rise and as it does, this rocky material melts creating magma (molten rock). The magma emerges through the crack that was formed by the separation of the two plates and fills it. When the magma reaches the surface of the earth it is called lava.
An example for tectonic plates divergence, is a mid-oceanic ridge, such as the Mid-Atlantic Ridge, which contains volcanoes.

Plate Convergence – Is when two plates are moving towards each other. When the two are continental plates they collide creating mountains, but when an oceanic plate meets another oceanic plate or a continental plate, it moves beneath it creating what is called a subduction zone.
The sinking oceanic plate is full of water and when it reaches a certain depth, these water lower the melting point of the surrounding solid mantle (a process called flux melting), which creates magma (molten rock). This magma, hotter and less dense than the surrounding rocks, rises up towards the surface and erupts as an intense volcano.
An example for tectonic plates convergence, and probably the most interesting and relevant area to our interest, is the volcanoes in the Ring of Fire which encircle the basin of the Pacific Ocean.

There is another volcanic activity, which is not caused by the tectonic plates movement (and so doesn’t form next to tectonic plates interaction) called a Hotspot. In this case hot material rises fast from the deep mantle or even the core-mantle boundary, towards the earth’s crust. This hot mantle plume is so intense that it burns a hole in the crust and tremendous volumes of lava erupt.
The most famous Hotspots on earth are the one who formed the state of Hawaii (which is actually a group of islands created by volcanoes), and the Yellowstone volcano in Wyoming USA.

There are around 1,510 active volcanoes in the world. About 75% of them are located in the mentioned Ring of Fire since the Pacific Ocean basin is comprised of active tectonic plate boundaries.

As powerful as volcanic eruptions are, it is not the destruction potential held by the bursting but rather the global climatic aftereffect that is relevant.
The desirable worldwide impact of an eruption is largely the result of gases release. It appears that eruptions of global effect, at first lead to an instant cooling period and then to a warming period.
The global cooling is a result of global dimming, which is caused mostly by sulfur compounds that are released from the volcano and high into the atmosphere and simply block the sun. The warming effect that follows is caused mostly by the CO2 gas that is released during the eruption and acts as a greenhouse gas.
The reason that the warming effect follows the cooling period even though the effecting compounds are released at the same time, is that the sulfuric compounds stay in the atmosphere for a limited time while CO2 stays in the atmosphere for a much longer time and accumulates.

The Indian volcanic outburst is an example for the global destructive potential of volcanism. Some scientists claim today, that this outburst caused the extinction of the dinosaurs around 65 million years ago and not an asteroid hit, which its impact was only the trigger that set off the volcanic eruption that was the main element that drove to the extinction of almost 75% of the species alive at that time.

Approximately 251 million years ago, the vastest extinction known in paleontology had occurred when up to 95% of all species had gone extinct. This unprecedented proportion of extinction is largely ascribed to a volcanic event called the Siberian Flood Basalts during which vast quantities of magma and gases were poured out of a chain of volcanoes in Siberia. The Siberian eruption presumably contributed at first to an enormous cooling effect (caused by sulfur dioxide emission) and then to global warming (due to greenhouse gases as CO2). Most of the species were probably killed at that phase. Since the water temperature rose gently , marine animals were relatively unaffected. That was until the seas released their frozen methane (for further explanation please read Methane Hydrates) and then there was also a massive extinction of species from the world's oceans.

The volume of lava that erupted in Siberia, is thought to have been about 2 million cubic kilometers (480,000 cubic miles), but volcanic activities don’t have to involve such huge volumes of lava to cause such a strong impact. There is a second type of volcanic activity, more of an explosive one (rather than a flood-like type) that is known to affect the entire planet. They are called ultra Plinian eruptions, and they erupt from what are known as super volcanoes.

The nature of volcanic eruptions is determined by the magma that forms it, which is determined mostly by its chemical composition. The type of magma that generates Plinian eruptions is called Rhyolitic magma, and it is characterized by High SiO2 (silica) content (about 65-75%), comparatively lower temperature (650 - 800°C) and high viscosity which keeps the gases in, not allowing them to escape (as with basaltic magma) and so holds high amounts of gases within the magma.
The composition of the gases in the Rhyolitic magma is mostly H2O (water vapor) and some CO2 (carbon dioxide) as well as Sulfur, Chlorine, and Fluorine gases.
An Ultra Plinian eruption occurs after the pressurized magma (which builds over time as more magma collects under the enormous weight of overlying rock) pushes the overlying crust enough to create vertical fractures that extend to the planet's surface.
As pressure is reduced the volume of gas expands, making the magma burst in an explosive way (inside the magma chamber the gas is dissolved in the liquid, but when pressure is decreased as magma rises toward the surface of the earth, the gas forms a separate vapor phase, just like a shaken bottle of soda as it is opened).

Ultra Plinian eruptions are apparently the largest, most destructive kinds of eruptions on Earth. They are marked by the ejection of large amount of ash at hundreds of feet per second and very powerful continuous gas blast eruptions that rise as a massive column, as high as 30 miles (48 km), into the atmosphere. The force of a single super volcanic explosion, which propels this column of gas and ash sky-high, is million times stronger than the Hiroshima nuclear bomb and is considered to have a global climatic effect.

The world-wide effects occur when the eruption is powerful enough to reach the stratosphere (the second layer of the atmosphere) because in the first layer (the troposphere) rain clouds continually form and shower and so the potential sun blocking material (sulfuric acid) gets washed away after a short period of time. In the stratosphere these compounds are not washed away as rain rarely forms there.
Sulfur dioxide (SO2) is one of the two most significant components of Ultra Plinian eruptions, and of the varied other gases that make up the eruption, it causes the strongest immediate effect on the environment (as opposed to CO2 which causes a very strong effect in the long run). Sulfur dioxide reacts with oxygen and water to produce tiny droplets of sulfuric acid (H2SO4) which condense rapidly in the stratosphere and reflect sunlight directly back into space. Stratospheric winds spread the sulfuric acid particles until they practically cover the globe, where they can linger for a couple of years. By reflecting the sunlight, they reduce the amount of energy reaching the earth's surface and so it is getting significantly cooler.

About 74,000 years ago, the Toba eruption in Indonesia led to a global cooling that caused massive population reduction among animals all over the planet. The eruption column, tens of kilometers high, spouted billions of tons of ash and gases into the stratosphere which quickly scattered all around the globe. The billions of tons of sulfuric acid caused a yellow haze that clothed the planet for about 6 years, blocking the sun's radiation, causing a complete deforestation in Southeast Asia and cooling of sea temperatures by 3–3.5°C. The eruption caused an instant Ice Age on Earth (some scientists claim it didn't cause but accelerated one) and it contributed to the glaciers expansion, so even after the sulfuric acid cloud was scattered, the ice cover reflected back the sun radiation (for more information about sun radiation reflection please read our text about Albedo). The Toba eruption drove the Earth into what is called a volcanic winter, with average global temperatures reduced by 5 to 15°C.
"It was like flipping the switch on the global climate system from hot to cold" said Michael Rampino, Professor of Earth and Environmental Sciences.

One recent, relatively small scale, example of an eruption’s climatic effect is the 1991 eruption of Mount Pinatubo in the Philippines. The eruption injected nearly 20 million tons of SO2 into the stratosphere that spread around the globe in about 3 weeks. The recorded effect was a 0.5°C (0.9°F) drop in temperature for the following two years, which might sound like a small temperature decrease, but for a global average it is considered quite significant, especially when even the slightest temperature change can affect entire weather systems. One year after the eruption, the U.S. experienced its third coldest and wettest summer in 77 years, and major flooding of the Mississippi River occurred. These observations are consistent with predictions made by climate models of the Pinatubo eruption effect.

The much larger eruption of Tambora, Indonesia, in 1815 produced the greatest volcanic effects on climate in recorded history, with a 1°C (1.8°F) global temperature decrease. Even in Europe, over the other side of the globe, that year was referred as “the year without a summer.”

Super volcano eruptions would release thousands of times more energy than the Pinatubo and Tambora eruptions. The 5 to 15°C decrease in global temperature attributed to super volcano eruptions is enough to have a major impact on the climate, and ecosystems.

Under natural conditions only about 1% of the magma beneath volcanoes is estimated to ever burst to the surface of the earth. Since much more magma is already there, there may be processes that can not only trigger an eruption but also intensify it.
Cracking of the earth’s crust might influence the magma's dynamics. New cracks may allow, otherwise trapped magma, to flow from the deep and boost the upwards flow of magma.
It is said that intense and severe seismic activity (earthquakes) might lead to the cracking of the crust. There are geologists who argue that severe seismic activity can trigger and intensify an eruption by allowing more magma to escape from deep within the planet. Underground explosions can generate seismic waves. This technique is in use for decades for research purposes, at low magnitude and with caution. Modern drilling equipment that can reach thousands of meters below the earth’s surface already exists.

Having said that, obviously this scientific area is very complicated, ambitious and currently maybe even totally inapplicable. Several of the factors which are crucial even for the initial process of triggering a volcano are highly unlikely to be met. For example, a volcano must be "primed" to erupt (with magma waiting to be "tipped" into erupting, and even showing some signs of it intruding at shallow depths, or high levels of volcanic gases, maybe even shallow earthquakes, and deformation of the volcano), to be considered as an interesting option, however, currently, even a basic element such as unequivocal information regarding the state of each volcano, is hard to obtain.
Probably an even bigger challenge is how to cause a volcano to erupt even if a potential one is found, and an even bigger challenge than that is how to cause that eruption to be an ultra plinian eruption and not the flood-like type eruption. That would require somehow finding a way to release the pressure in the magma chamber keeping the "cork" on the volcano so that bubbles could be formed with in it, and figure out how to get a lot of water into it quickly, so it would burst as high as possible, hopefully with the eruption column reaching tens of kilometers high, so it can have a long term global climatic effect. Obviously this is an extremely unlikely option.
The reason we nevertheless mention it as a practical option worth investigating, is mainly for its high potential, and since hopefully, in the future, with technological and knowledge advances, it may be a much more realistic option. The motivation is mainly to open activists’ mind to potential fields of research. Maybe other activists would think differently regarding the chances to successfully use volcanoes, or that since the combination of several practical options has a far greater potential than a single one, the powerful potential within the earth can be part of it