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 H20 carbon dioxide CO2 hydrogen chloride HCl sulfur dioxide SO2 nitrogen oxide N20 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 3rd 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 GY 111 Lecture Notes D Haywick 2008 09 2 B Felsic Intermediate Volcanic Rocks When pyroclastic material is shot up into the Earth s atmosphere it is moving at very high velocity Almost immediately the Earth s gravity starts to pull it back down The result is a rain of pyroclastic material down wind of the eruption The further you are away from the eruption the finer the material is that falls on you see image to left The finest material is called volcanic ash and it can be a problem as it tends to plug your lungs if you breathe enough of it It has also been known to plug up airplane jet engines If you are closer to the eruption the pyroclastic material falling down on you can be composed of considerably larger fragments cm to 10 s of cm in size It is still composed of glassy material but we now call it pumice rather than ash they are however exactly the same as far as their geochemistry and textures are concerned Beds of pumice are give a specific name tephra Many geologists intermix the terms tephra and volcanic ash and it s okay for you to do that in this class Just remember though tephras can be composed of very large pumice clasts Some of the fragments shot out of volcanoes are large enough to remain molten as they arc through the air Their outer surfaces cool quickly forming a glassy chilled margin refer to tomorrows lecture but their interiors are still molten You do not want to be hit by one of these volcanic bombs see image to right Bad enough that they might weigh 50 kg or more and be dropping from the sky at 200 km hr these gumballs are liquid rock inside 1000 degrees C Your skull would be caved in and your brain would burst into flame if you were hit by a big enough volcanic bomb If time permits I ll tell you a true story of some geologists who learned first hand what it s like to be beaned by a bomb It s not a pleasant story see next page for a condensed version One of the problematic igneous rocks that you will see in the lab is called tuff Tuffs are pyroclastic but they are produced from ash that remains hot but not molten when it hits the ground Most tuffs form very close to the volcanic vent Here pyroclastic material is rather gooey and particles will stick together forming pyroclastic rocks not pyroclastic sediment Tuffs come in 3 main flavors Welded tuff as the name implies is a rock that fuses together through heat it is the textbook example of a standard tuff Flow banded tuffs display horizontal layers and elongated clasts of pumice The layers were caused by minor flowing after air fall is over or through variations in the intensity of the eruption The specimen you will see in the lab GY 111 Lecture Notes D Haywick 2008 09 3 was a production of the latter scenario When the eruption slowed finer ash rained down on the tuff When the eruption picked up again hotter and larger stuff was deposited on the ash etc Crystal tuffs are tuffs that contain a lot of phenocrysts They range in colour from white to purple to pink reflecting their mineral content and overall composition felsic and intermediate In some situations crystal tuffs look exactly like rhyolite porphyries The only way that you can tell the difference is to know how they were erupted the former is air fall pyroclastic the latter is a lava flow extrusive Volcanologists Killed in Blast An excursion to an active Indonesian volcano turned deadly last month after a group ignored basic safety guidelines On 27 July a party of seven scientists taking in the sights of Semeru Java s tallest volcano went within meters of the crater rim Just then the mountain let loose one of more than 500 explosions recorded that week