Volcanic ash, pumice, cinder, lapilli, blocks and bombs are all examples of pyroclastic material. Rocks and magma are pulverized by volcanic explosions to create the different types of materials.
Pyroclastic materials combine with gases to create pyroclastic flows, which can move very fast and cause dramatic destruction. Some pyroclastic flows have been known to move as fast as 200 meters per second, with smaller flows moving 10 to 30 meters per second.
There are two types of pyroclastic flows. Ignimbrite flows primarily contain vesiculated material, which means the rocks are pitted and have cavities both on the surface and inside the rock. The second type of flow is called nuee ardente, which means glowing cloud, and is comprised of denser materials.
Pyroclastic flows are very hot. Temperatures of the pyroclastic flows from the Mount St. Helens' volcanic eruption of 1980 reached 350 degrees Fahrenheit. Mount Pelee’s 1932 eruption had flows that reached 1075 degrees Fahrenheit. With such high temperatures, pyroclastic flows are one of the most dangerous aspects of volcanic eruptions. The gases and ash can asphyxiate people, and the highly heated materials can burn most anything they come into contact with. Pyroclastic flows are also dangerous because they can cause destruction over great distances.