111-12 - GY 111 Lecture Notes D. Haywick (2008-09) 1 GY 111...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
GY 111 Lecture Notes D. Haywick (2008-09) 1 GY 111 Lecture Note Series Extrusive Igneous Rocks Lecture Goals A) Pyro-what? Air fall volcanic rocks B) Felsic and Intermediate Extrusive Rocks C) Mafic Extrusive Rocks Reference: Press et al., 2004, Chapters 5 and 6; Grotzinger et al., 2007, Chapter 4 A) Pyro-what? The term pyroclastic is composed of two parts, both of Greek origin. Pyro means “fire”. Clastic means particles. Pyroclastic literally means particles produced from fire. Pyroclastic igneous rocks have been erupted “explosively” and are deposited as particulate material (not liquid lava) around the flanks of volcanoes. Please remember that the pyroclastic material first starts of as lava, but as it is blasted into the atmosphere (see image to right), it is fragmented and cools very quickly. Texturally, almost all pyroclastic material is glassy . Most pyroclastic igneous rocks are produced by composite volcanoes because their magmas are much more viscous and more likely to be violently erupt As examples, the volcanic eruptions of Mt. Vesuvius in AD 79, Krakatoa in the late 1880’s and Mt St. Helens 100 years later eruption were all pyroclastic. It is quite easy to get confused with some of the terminology used to describe eruptions from composite volcanoes. The term pyroclastic refers to the material that is produced from these eruptions, but some geologists (yours truly included) often them pyroclastic eruptions. This is not really correct usage of the term. Eruptions that produce vast qualities of pyroclastic material are best called plinian . Plinian eruptions blast pyroclastic material from the volcanic vent at supersonic speeds. This material can reach 25 km or more in altitude (higher than the Concorde flies). Exceptionally powerful eruptions (termed ultraplinian by some geologists) may reach 75 km. That is almost outer space. The stuff shot out of volcanoes during plinian eruptions is not just pyroclastic rock. Much of it is gas. The most common gases erupted from composite volcanoes include water vapor (H 2 0), carbon dioxide (CO 2 ), hydrogen chloride (HCl), sulfur dioxide (SO 2 ), nitrogen oxide (N 2 0), and argon (Ar). Water is omnipresent in magmas and much of it (at least along convergent plate boundaries) was derived from the subduction of wet oceanic plate. Geologists have calculated that the entire volume of the Earth’s oceans can be recycled at convergent plate boundaries in as little as 200 million years. Argon comes from radioactive decay in the Earth’s interior. There is a lot of this going on; Ar is the 3 rd most abundant gas in the Earth’s atmosphere (after nitrogen and oxygen). There is one other eruption worth mentioning here. When magma encounters groundwater near the Earth’s surface, you can get an explosion of steam and pyroclastic material. These phreatic eruptions resemble the puffs from old steam locomotives, but are on a scale 100’s or 1000’s of time larger.
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
GY 111 Lecture Notes D. Haywick (2008-09)
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 02/04/2012 for the course GLY 111 taught by Professor Haywick during the Fall '11 term at S. Alabama.

Page1 / 6

111-12 - GY 111 Lecture Notes D. Haywick (2008-09) 1 GY 111...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online