1:3 Definitions- Pyroclastic Kim Hatcher

IntroThis assignment highlights the importance of definitions, specifically in professional writing. Definitions must be created with an intended audience in mind, so that the level of detail is appropriate. The following post defines the term “pyroclastic” for a non-technical audience using three types of definitions.

Term: PyroclasticAudience and Scenario: A tourist group is sightseeing near a volcano. An interpretive guide is explaining the type of rocks found in the area. The tourists have an interest in nature but no prior geologic knowledge.

Parenthetical DefinitionThe material surrounding Mount Oly is predominantly pyroclastic (made of erupted volcanic fragments).

Sentence DefinitionPyroclastic is a geologic term used to describe material that has been violently erupted from a volcano. Pyroclastic debris includes fine ash, welded glass and rock fragments.

Expanded DefinitionPyroclastic is a term used in geology to describe erupted volcanic material. Any debris that has been violently expelled from the earth’s surface is classified as pyroclastic. The size and shape of pyroclastic material varies, grading from large rock fragments to fine ash. The following image from Plummer et al (2013) illustrates different types of pyroclastic ejecta (ejected volcanic material). The top frame shows very fine powder-like volcanic ash. The bottom frame displays volcanic bombs, which are pyroclastic lava fragments that solidify as they are erupted from a volcano.

Plummer et al, 2013

The word pyroclastic is derived from the Greek words pyros, meaning “fire”, and klastos, meaning “broken”. Pyroclastic material originates as gas and magma deep in the earth. As magma rises to the earth’s surface, bubbles in the magma begin to expand due to the decrease in pressure (Montgomery, 2008). The quick release of built-up gas pressure causes a powerful explosion. Hot gas and lava are ejected into the air, often forming a buoyant plume. These enormous plumes suspend volcanic particles in the air until they eventually fall out as ash and rock fragments. If the mixture of gas and magma is viscous and dense, the eruption will cause the material to flow down the side of the volcano as a dense pyroclastic flow (Plummer et al, 2013). Pyroclastic flows can also result from the collapse of plumes, sending gas and debris down the volcano at incredible speeds.

MacPhie et al, 2010

The schematic drawing above illustrates these mechanisms of pyroclastic transport and deposit. Not all volcanic rocks are pyroclastic. To be classified as pyroclastic, volcanic material must have been fragmented and transported by an explosive eruption. Lava from non-explosive eruptions, commonly seen in Hawaiian volcanism, is not classified as pyroclastic. Local examples of volcanoes that have historically produced pyroclastic flows include Mount St. Helens and Mount Baker.

Pyroclastic flows and plumes pose many potential hazards to humans. The speed, temperature and projectile nature of pyroclastic flows are a threat to infrastructure and human life. Plumes can carry pyroclastic material great distances and cover massive areas. Plumes also cause economic and environmental issues. The large plumes rise and spread, injecting sulphur gas into the atmosphere and affecting air travel (Plummer et al, 2013). These hazards show the importance of understanding pyroclastic materials, as pyroclastic flows and plumes can dramatically both affect human safety and global atmospheric conditions.

Works Cited

Coe, A. L., Argles, T. W., Rothery, D. A., & Spicer, R. A. (2012). Geological field techniques. Oxford: Wiley-Blackwell

Montgomery, C. W. (2008). Environmental Geology (Eighth ed.). New York: MCGraw-Hill

Plummer, C. C., Carlson, D. H., & Hammersley, L. (2013). Physical Geology (Fourteenth ed.). Toronto: McGraw-Hill

MacPhie, J., Doyle, M., & Allen, R. L. (2010). Volcanic textures: a guide to the interpretation of textures in volcanic rocks. Hobart: ARC Centre of Excellence in Ore Depoits, University of Tasmania