A volcano is a site on a planet or moon’s crust where melted material from the interior reaches the surface. On Earth, this material is melted rock and is mostly made of silica and other rock forming minerals. On other moons and planetary bodies, the melted material may take a different form. For example, water is erupted on the surface of icy moons. Volcanoes on Io – one of Jupiter’s moons and the most geologically active object in the Solar System – erupt lava flows of sulfur material.
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Sources of Heat for Volcanoes
Heat and thermal energy must by applied to solid material in order for melting to occur. Here on Earth, there are four major sources of heat that melt the crust and keep the mantle in a generally liquid state. These sources of heat are:
- Long-lived radioactivity
- Short-lived radioactivity
- Accretional heating
- Core Formation
- Dissipation of tidal energy
This heat melts enough material to cause volcanic activity all over the Earth, sometimes violently. The volcanic activity witnessed on the surface of the Earth erupts about two cubic kilometers of magma per year. Volcanic activity under the ocean erupts as much as 20 cubic kilometers of magma per year.
Long-lived and Short-lived Radioactivity
The word “radioactivity” is often associated with man made hazards produced by nuclear weapons. However, radioactive isotopes naturally occur in the planet’s interior. Four of these isotopes are mostly responsible for the production of heat that melts rocks. These isotopes are uranium-235, uranium-238, thorium-232 and potassium-40 and each of them produce heat differently.
The degree to which they are able to heat the Earth’s interior is dependent on their half-lives and abundance. Uranium-235 is much more effective as a heat producer, but it’s half-life is shorter than the others by at least 590 million years. Potassium-40, on the other hand, may not generate as much heat, but comes from an element that is far more abundant in the Earth’s material than the other isotopes and has a half-life of 1.3 billion years
These isotopes include aluminum-26, iodine-129 and plutonium-244. These isotopes have half lives that range in the hundreds of thousands of years – too short for them to be generating energy today. However, scientists believe that these isotopes are largely responsible for the generation of heat early in the formation of the Solar System. Some of the remnants of this energy still exists in the planets and moons of the Solar System today, including Earth, but their effects are not measurable.
Accretional Heating and Core Formation
When the current Solar System was forming, there were many particles accreting to form the planets and moons we have today. Further accretional activity from larger bodies smashing into each other are evident today in the meteor impact sites of Earth, the moon and many other objects in the Solar System. A few stray chunks of material were locked into orbit and formed asteroid belts. The kinetic energy produced from this accretional activity was especially prominent at the center of the planetary body, as compression generated more heat.
As planets continue to spin and more material is added, it is separated based on its density. Denser materials, such as iron and other metals, migrate inwards to form the core. Lighter material such as silica migrates upwards to form the mantle and crust. On Earth this process took place within only a few million years but the energy released by the formation of the core played a permanent role in Earth’s volcanism.
Dissipation of Tidal Energy
It is common knowledge that tidal energy is important for understanding the waves and tides of the oceans, but it is also a source of heat on our planet. Tidal energy works to slow the rotation of the Earth as the moon gets farther away. The friction caused by this “braking” produces a measurable amount of heat – about 3 X 10^19 Joules per year. Most of this heat is concentrated inside the Earth, where it can be added to the other sources of heat and melt rock.
