Volcanoes can emit gases, molten rock, or solid particles. Volcanic gases are composed mainly of water vapor, hydrogen, hydrogen chloride, hydrogen sulfide, carbon monoxide, and carbon dioxide.

The molten rock, or magma, that rises in a volcano is called lava after it flows out at the surface. Cooled and hardened lava may have either a smooth or a rough surface. Smooth lava, called pahoehoe, has a ropy, folded surface. Rough clinkery lava is called aa. Both words are of Hawaiian origin. The gas remaining in lava after its eruption forms bubbles, which leave small holes, or vesicles, in the hardened rock. Scoria is hardened lava that contains many vesicles.

Peices of magma that are hurled into the air are the third product of volcanic eruptions. The largest, which may be several feet in diameter, are called volcanic bombs. Smaller fragments are called lapilli and volcaninc cinders are the size of sand grains. The smallest particles are volcanic ash, which may be carried hundreds of miles through the air. In 1883 volcanic ash from the eruption of Krakatoa in the East Indies fell all over the world. Most of the debis of such violent eruptions is derived from hardened lava that was deposited previously, rather than from magma directly. Fragmental volcanic material is called pyroclastic, from the Greek pyr and Klastos. Light-colored pumice, so filled with small holes that it floats in water, is a pyroclastic rock that is mined for use an an abrasive.

Magma Composition and Origin

The kind of material erupted depends on the nature of the eruption, which in turn is related to the magma composition. Magmas are classified by the amount of silica that they contain. Eruptions of this material are usally quiet outporuings of lava that cools to form a dark colored rock called basalt. Acid magmas contain about two thirds silica. They are stiff and viscious, and gases are released explosively after building up enough pressure to shatter the magma.

The earth's interior, below the thin crust, was once thought to be entirely molten. It is now known that most of the earth is solid rock, and that only part of the central core is liquid. Most magma comes from pockets in the upper mantle--the thick, normally solid layer between the crust and the core. The material erupted from Hawaii's volcanoes is from a depth of about 40 mi.