Volcano

All magma forms through melting of pre-existing rock. Generally, this occurs in one of two ways: (1) by convection of rock upwards through the mantle until it melts, or (2) by melting rock at a subduction zone. Mantle convection occurs because deep within the earth, radioactive decay raises the temperature of rock, making it expand. This expansion lowers the rock's density, causing it to rise, or convect. As the rock rises through the mantle, the surrounding pressure decreases and eventually the convecting rock melts as a result. Geologists call this pressure-relief melting. The magma moves upward and erupts to form either an oceanic ridge or a hot spot volcano. At subduction zones, volatile compounds (especially water) escape from the subducting plate and lower the melting temperature of the overlying mantle rocks. This triggers melting and magma forms as a result.

Magma comes from a variety of sources and may have a complicated history. For example, as magma rises in the mantle and crust, it undergoes a process known as fractional crystallization. Each mineral in a rock has its own crystallization (or melting) temperature. Because different minerals crystallize at different temperatures, certain minerals form from magma earlier than others. This produces a magma with a composition different The July 22, 1980, eruption of Mount St. Helens in southern Washington. JLM Visuals. Reproduced by permission.
from that when the minerals first began to crystallize. Therefore, the minerals that crystallize later, and the rocks that they form, will be of a different composition than those that form earlier. Fractional crystallization is thought to be one way of producing rocks of different compositions from the same magma. Partial melting and magma contamination are also important.

If a rock is not exposed to a high enough temperature to melt all of its minerals, only some minerals will melt. This is known as partial melting. If a rock melts only partially, the magma produced will have a different chemical composition than the rock from which the magma originated. As magma rises toward the earth's surface it may also cause rocks in the overlying crust to partially melt, contaminating the magma with molten rock of a different composition. The composition of magma therefore depends on many factors, including original magma composition resulting from partial melting, fractional crystallization, and magma contamination.

Volcanic rocks produced from partially melted continental crust usually appear red, brown, or gray in color and are known as felsic rocks. Felsic rocks such as rhyolite are rich in the minerals feldspar and quartz, both of which contain abundant silica. Lava formed by melting of mantle rocks contains abundant iron- and magnesium-rich minerals, which are poorer in silica than quartz and feldspar, and produces mafic volcanic rocks such as basalt. Lava with a chemical composition that falls between these two extremes is said to be of intermediate composition. Andesite is an example of a volcanic rock of this type.