poudrette

Poudrette is a geological term referring to a type of igneous rock. Specifically, it is a volcanic breccia characterized by its composition of angular fragments of lava cemented together. These fragments, or clasts, are typically less than 2 millimeters in size and can vary in composition, often reflecting the material ejected during a volcanic eruption. The cementing material is usually a fine-grained matrix of ash or finer volcanic debris.

The term "poudrette" is sometimes used interchangeably with the broader term volcanic breccia, though poudrette often implies a finer grain size for the constituent fragments. These rocks form when viscous lava, ash, and other volcanic materials are ejected from a volcano, fragment, and then re-solidify. The angular nature of the fragments is a key characteristic, distinguishing them from more rounded volcanic conglomerate.

Poudrettes can be found in volcanic regions worldwide where explosive eruptions have occurred. Their study can provide valuable insights into the eruptive history and processes of a particular volcano, including the conditions under which fragmentation and subsequent cementation took place. The appearance of poudrette can vary greatly depending on the minerals present in the original volcanic material, ranging in color from dark gray and black to lighter shades of brown and red.