Finally, the trapped gas builds up so much pressure that it blasts the magma into fragments. Tephra includes, from smallest to largest, volcanic dust, volcanic ash, and volcanic bombs. Volcanic dust can be carried great distances. The dust was carried around the earth several times and produced brilliant red sunsets in many parts of the world. Some scientists believe that large quantities of volcanic dust can affect the climate by reducing the amount of sunlight that reaches the earth.
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. Volcanic bombs are large fragments. Most of them range from the size of a baseball to that of a basketball. Small volcanic bombs are generally called cinders. Gas pours out of volcanoes in large quantities during most eruptions.