Volcanic ash is made up of fragments less than 1/5 inch (0.5 centimeter) in diameter. Most volcanic ash falls to the surface and becomes welded together as rock called volcanic tuff. Sometimes, volcanic ash combines with water in a stream and forms a boiling mudflow. Mudflows may reach speeds of 60 miles (97 kilometers) per hour and can be highly destructive.
Volcanic bombs are large fragments. Most of them range from the size of a baseball to that of a basketball. The largest bombs may measure more than 4 feet (1.2 meters) across and weigh up to 100 short tons (91 metric tons). Small volcanic bombs are generally called cinders.
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.