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Andesite
Andesite
A type of lava whose properties are halfway between rhyolite and basalt. Andesite is the most common form of lava found in explosive volcanoes. Its name comes from its association with the active Andean volcanoes.
Andesite makes up the main material of a composite volcanic cone. Andesitic lava contains many cavities. It may be light colored, but is normally dark, especially brown. Although it is not a particularly acid lava, it is still reasonably sticky and so forms short lava flows that usually develop into tongues that move short distances down the sides of volcanic cones. (See also: Diorite.)
Arc
A crescent pattern of volcanoes that sometimes forms when an oceanic plate collides with a continental plate and then slides down underneath it. (See also: Island arc and Subduction zone.)
Ash
A fine, gray material (less than 2mm across) that is almost dustlike. It is thrown out of
a volcano during an explosive eruption.
Ash is produced from sticky lava that is full of gases. As this sticky lava comes up the vent
of the volcano, it is under great pressure. Then, as it reaches the surface, the pressure is released, and the gases in the lava expand almost instantly, tearing the sticky lava apart, and creating a fine spray that is shot high into the air.
The tiny bits of lava cool
very quickly into solid rock to produce a variety of materials called tephra. The finest form of tephra is called ash.
When ash falls, it is still very powdery, but it slowly compacts over the years into new rock (see: Tuff).
The finest particles of ash are easily carried large distances by the wind.
Because ash comes from rising magma, the ash that is exploded out of a volcano and the lava that flows out are two forms of exactly the same material.
Good examples of ash clouds that have been formed in recent years include Mount Pinatubo in 1991 and Mount St. Helens in 1980 (see: Volcanic activity).
Mount Pinatubo produced fine ash that went straight up into the air. Much of it fell back around the volcano, blanketing the slopes and nearby landscape in light gray ash, but some of it stayed
in the air for a long time.
It is thought that it helped
block out the Sun’s rays. In the following year the average world temperature fell by 0.5°C. After this the ash was washed out of the air, and the world’s temperatures went up again.
 Ash—Ash makes up much of the visible material ejected during an explosive eruption, such as that of Mount Pinatubo, shown below. It is
fine enough to be carried some distance from an eruption. During an eruption it may settle out on roads and block them. If thick ash settles on roofs, the weight may cause the building to collapse.
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