tephra

Tephra is a generic term for air fall material produced by a volcanic eruption regardless of composition or fragment size. It can be divided into the following classifications:

Ash Less than 2 mm diameter
Lapilli Between 2 and 64 mm diameter
Volcanic bombs or Volcanic blocks are greater than 64 mm diameter

The airborne fragments are also called pyroclasts or sometimes just clasts. Once they have fallen to the ground they are still called tephra unless they are hot enough to fuse together into pyroclastic rock or tuff. The distribution of tephra following an eruption is usually such that the largest boulders fall to the ground quickest and therefore closest to the vent, whilst smaller fragments travel further - ash can often travel for thousands of miles as it can stay in the stratosphere for several weeks. Both tephra and pyroclast are derived from Greek. Tephra means ash. Pyro means fire and klastos, broken; pyroclasts being "broken by fire".

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