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Exam 2Notes (98% on the test)

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Louisiana State University
GEOL 1001

Geology 1001Exam Two NotesChapter FiveVolcanic EruptionsVolcanoesA volcano is an erupting vent through which molten rock surfaces a mountain built from magmatic eruptions Volcanoes are caused by tectonic activity They are hazardous to humans in Pompeii Italy erupted in 79 CE This Mount Vesuvius volcanic eruption produced earthquakes dark ash clouds turning the day into night Fine ash and choking fumes enveloped the town The pyroclastic flows destroyed Pompeii and killed around 20000 people The town literally vanished beneath 6m of pyroclastic debris Volcanoes are unpredictable and dangerous They can build large mountains but at the same time they can also blow mountains to bits Eruptions can also provide highly productive soils to feed civilization they can extinguish a civilization in a matter of minutes and they affect the climate Volcanic EruptionsVolcanic eruptions transfer materials from inside the Earth to the planets surface The products of volcanic eruptions take three forms lava flows molten rock that moves underground pyroclastic debris fragments blown out of a volcano and volcanic gases vapor and aerosols that exit a volcano Lava can be thin and runny or thick and sticky Not all lava flow looks or behaves the same Flow style depends in viscosity which depends on composition especially silicaSiO Iron and magnesium 2temperature gas content and crystal content Lava FlowsLava containing low silica and high Iron and Magnesium are either mafic or basaltic Lava containing moderate silica iron and magnesium are either intermediate or andesitic Lava containing high silica and low iron and magnesium are either silicic felsic or rhyolitic Basaltic lava flows are very hot have low silica and a low viscosity They are often thin and fluid they can flow up rapidlyup to 100kmhr They can also flow for long distances up to several 100km Surface texture reflects the timing of freezing relative to movement Rapid cooling of the surface of lava flow occurs as it emerges to the surface creating a cooled crust This crust serves as insulation allowing the hot interior to flow and remain liquid for some distance The lava tube is the insulated tunnellike conduit within a flow through which lava moves Tubes prevent cooling facilitating flow for miles Eventually it will drain and become empty funnels caves where they can transmit water Once lava flow has covered the land and freezes it becomes a solid blanket of rock The Pahoehoe is a Hawaiian word describing basalt with a glassy ropy texture Pahoehoe forms when extremely hot basalt forms a skin and with flow this skin is rolled into ropy ridges and furrows The Aa is a Hawaiian word describing basalt that solidifies with a jagged sharp angular texture Aa forms when hot flowing basalt cools and thickens with flow the lava crumbles into shards and fragmentsleaving a rubbly deposit Columnar jointing is solidified flows that contract with vertical fractures that are hexagonal in cross section create hexagonal columns They typically develop in dikes and sills as flow cools and cracks Pillow basalt is rounded blobs of basalt that are cooled underwater Underwater basalt cools rapidly and cannot flow The pillow surface is cracked quenched glass rapid freezingsolidification Lava pressure ruptures a pillow to form the next blob It is very common in the MOR Andesitic lava flows has higher SiO which makes andesitic lavas viscous Unlike 2basalt they do not flow rapidly Instead they mound around the vent and flow slowly It is characterized as a lumpy flow with bulbous snout The outer crust fractures creating rubble Andesitic flows remain close to the vent Rhyolitic lava flows contain the highest SiO content which makes it the most viscous lava It is the coolest and it rarely 2flows Lava plugs the vent as a lava dome domelike mass Lava domes can be blown to smithereens by subsequent violent eruptions Sometimes rhyolitic lava freezes while still in the vent then pushes upward as a columnlike spire or spine up to 100km above the vent If and when rhyolite lava flows will have broken and blocky surface Volcaniclastic DepositsVolcaniclastic deposit is a general term for accumulation of fragmented igneous material Pyroclastic debris is the lava that freezes flying through the air Preexisting rock fragments around volcano vent Landslide debris Mudflows Basaltic pyroclastic debris is formed when basaltic magma may contain dissolved volatiles and as it approaches the surface will form bubbles When the bubbles reach the surface they burst and eject clots and drops of molten magma upward to form dramatic fountains Glass shards and fragmented lava range in sizes Basaltic eruptions generate a lot of spatter Lapilli are a pea to plumsized material Blocks and bomb is apple to refrigeratorsized Peles hairs after the Hawaiian goddess of volcanoes are strands of glass created by flying lava droplets Bombs are the streamlined fragments of ejected lavaExplosive pyroclastic debris is made of intermediate and felsic magma They are more viscous than basaltic magma and contain more gas They also produce large quantities of volcanic ash tiny shards of glass from instantaneous freezing Pyroclastic flows also called nuee ardentes is an avalanche of hot ash pulled down the side of a volcano by gravity They move up to 300kmhr incinerating everything in its pathThey are immediately deadly killing everything quickly Tephra are unconsolidated deposits of pyroclastic grains regardless of size Tuff is lithified ash or ash mixed with lapilli formed by burying and transforming into rock Airfall tuff is accumulations of ash that fell like snow Ignimbrite welded tuff is the sheet of tuff that is deposited while hot Hot pyroclastic flow material It fuses together while cooling The force of volcanic explosions can fragment and disperse preexisting volcanic rock that has built up around the volcano Blocks are chunks of preexisting rock fragments by eruption They are blown out of a volcanic vent blocks pile up nearby They create unstable slopes that easily fail Volcanic debris flows result from rain or with the mixing of melting snow and tephra Lahars are wetslurry mixture of ash coarser debris and waterLahars are deadly they move rapidly have the consistency of wet cement and have a distinct hazard to people living in volcanic valleysVolcanic gases are found in most magma and are comprised of dissolved gasses such as water carbon dioxide and sulfur dioxide Magma composition controls gas content Lavas with more silica contain a greater proportion of gas Mafic magmas tend to consist of a smaller percentage of volatiles Gases are expelled as magma rises due to concomitant P drop SO reacts with water 2
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