It can be thin and runny, or thick and sticky. Mudflow A flow of hot mud, ash and rock caused when freshly erupted volcanic materials mix with streams, lakes or glacier ice and snow. Explosive volcanic activity breaks up magma and rock, resulting in a pyroclastic flow. According to the USGS, pyroclastic flows are extremely hot, often glowing mixtures of volcanic pieces and gases that sweep along near the ground. Depending on the amount of material, proportion of solids to gas, temperature, and slope, a flow can travel as fast as 450 miles an hour.
Cracks form in the surface of the Earth because of the action of tectonic plates.
If you think of the Earth like a giant egg that has been broken and fitted carefully back together, the pieces of eggshell are called tectonic plates. These huge pieces of the Earth's crust slowly slide over and under, together and apart from each other. Tephra Magma blasts high into the air and breaks up into pieces called tephra. The USGS states that tephra can range in size from tiny particles of ash to house-size boulders.