Gas pours out of volcanoes in large quantities during most eruptions. The gas is made up chiefly of steam. But it includes carbon dioxide, nitrogen, sulphur dioxide, and other gases. Most of the steam comes from a volcano's magma. But some may also be produced when rising magma heats water in the ground.
Volcanic gas carries a large amount of volcanic dust.
This combination of gas and dust looks like black smoke. These groups are based on the shape of the volcanoes and the type of material they are built of. Shield volcanoes are formed when a large amount of free-flowing lava spills from a vent and spreads widely. The lava gradually builds up a low, broad, dome-shaped mountain. The famous Mauna Loa in Hawaii is a shield volcano.