There are two primary types of volcanic activity. They are explosive eruptions and quiet flows of lava (Leet, pp. 53-77). Examples of both types of activity are illustrated in the geologic cross-section on the right. The source of heat, gases, and molten rock is the underlying magma chamber. The following types of emissions result from explosive eruptions:
Water vapor is the major component of volcanic gases. Typically, water makes up 80 per cent or more of the gas. Other typical components of volcanic gases include carbon dioxide, hydrogen sulfide, sulfur, sulfur dioxide, hydrochloric acid, and ammonium chloride.
In the illustration the magma chamber eventually cools and becomes solid rock and is known as a batholith. Molten rock in the vents cool and become solid rock structures known as dikes.
Solid and liquid materials emitted vary with the composition of the molten rock in the magma chamber. The composition of most magma chambers (from an examination of batholiths, which are cooled and solidified magma chambers) has been found to be granite or granodiorite.
Some Useful Links
Following are some hyperlinks that can further assist with the evaluation and description of volcanoes:
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