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Geographic Location
The geographic location of a volcano provides the opportunity for an eruption. Most volcanoes lie along the edges of the 16 major continental plates on Earth. The plates exhibit pulling and pushing forces, which open up long-distance cracks over geographic areas called fracture zones. Plates that move under or over each other are called subduction zones. When plates pull apart, instead of pushing together, they leave deep rifts or valleys in the Earth's crust called rift zones. A weakened state in the crust must exist for a volcano to erupt.
Magma
If a plate plunges too deeply into the mantle, which is the second major layer of rock underneath the Earth's crust, it creates external pressure and friction. The mantle becomes hot enough to solidify into molten magma, a form of super-heated liquefied rock, which seeks out a chamber or area large enough to contain it. With the addition of water, which causes steam and expansion, a chamber can be increased in size, due to internal pressure. The magma becomes buoyant (less dense than the surrounding rocks), making it mobile. Due to its heat, it rises upward and outward, seeking the path of least resistance.
Dissolved Gases
Magma contains a certain amount of water, with some types of magma containing a higher saturation than others, such as andesitic magma. Andesitic magma is a heavier and stickier type of magma, associated with very explosive volcanoes. When the magma contains about 5 percent of its weight in dissolved water, it moves up through vents, or escape tunnels, in the rock and loses its solubility. Excess water separates from the magma by producing air bubbles. The higher upward the magma flows, the more gas is produced, until the volume of bubbles nears 75 percent. Upon reaching 75 percent dissolved gases, the magma breaks down into pyroclasts -- a mixture of solid and molten material. The result causes an explosive reaction.
Magma Injection
Although heat and pressure force dissolved gases and magma upward, another ingredient for an eruption involves the constant feeding of the chamber with the same type of magma, or that of a different composition, from the molten zone. Different magma that contains different amounts of sulfur dioxide and carbon dioxide can interact with the original magma and can make it thinner and run faster, or thicker and slower moving. The extra induced pressure further propels the magma and gases upward, while the chamber becomes replenished with new magma.
Vents and Fissures
Vents and fissures must be present to allow the flow and pressure release of magma. Old volcanoes contain conduits and chimneys from past activity that have become plugged with solidified magma. It takes only the newly heated magma, under extreme pressure, to melt the plug and allow the magma to reach the surface. The Hawaiian volcano Kilauea, which has been active since 1983, produces a daily output of thin lava which constantly releases pressure under its vast magma-chamber network. As the hot spots move in the Kilauea volcano, so does the location of the magma eruptions, following old or creating new cracks and fissures.