Lecture 5 Igneous Activities & Plate Tectonics PDF

Evolution of Earth and life ECS 1213 Lecture – 5 Igneous activity and tectonic boundaries  Why does the rock melt Bulk of the planet is solid (except the outer core) Temperature decreases towards the surface of the planet. Why should the rock m...

Evolution of Earth and life ECS 1213 Lecture – 5 Igneous activity and tectonic boundaries  Why does the rock melt Bulk of the planet is solid (except the outer core) Temperature decreases towards the surface of the planet. Why should the rock melt at the crustal level? How do we study? -Experimental petrology Important factors: -Temperature - Pressure - Water  Temperature and melting Heat transfer from magma Effect on local geotherm Localized melting Partial melting -Upon melting, rocks rarely dissolve completely - Instead only a portion of the rock melts -Which portion??  Pressure and melting Decompression melting Convection Effect on the geotherm Basaltic melt If the raised geothermal gradient becomes higher than the initial melting temperature at any pressure, then a partial melt will form. Liquid from this partial melt can be separated from the remaining crystals because, in general, liquids have a lower density than solids.  Fluid and melting Fluid induced melting Role of CO2 , H2O Hydrous minerals Sediment pores  Plate tectonics and magmatic activity B A C E D A. Convergent plate volcanism (oceanic-ocenic) B. Divergent plate volcanism (oceanic ridge) C. Intraplate volcanism D. Convergent plate volcanism (oceanic-continental) E. Divergent plate volcanism (continental rifting) Nature of geotherm  Divergent plate B volcanism A B C E D F Oceanic ridge Continental rift Iceland Mt. Kilimanjaro, Africa  Mechanism of melting at spreading centers High rate of volcanic rock generation 19 km3/year Composition and physical state of mantle Original composition: Peridotite – Olivine+ Pyroxene+Garnet Decompression melting Final composition: Basalt and gabbro (rich in SiO2+Fe) Convergent plate B A volcanism C E A. D D. Mt. Augustine, Alaska Chaitén Volcano, Chile  Mechanism of melting at subduction zones Low rate of volcanic rock generation 1 km3/year Composition of the subducting plate Source of water Fluid induced melting Final composition: Basaltic, Andesitic Varying composition Richer in SiO2 Intraplate volcanism B A Decompression melting C Source of the melt E D C. Hawaiian volcanoes  Igneous structures: Intrusive Plutons: The igneous structures that form under the surface. -They can only be studied if they are uplifted and exposed above the surface. - They could cut through the existing rocks (discordant) or form parallel layers (concordant). -Three main types: - Dikes - Sills - Batholiths  Intrusive structure Dike: Discordant bodies produced when magma is injected into fractures. Sills: Concordant bodies produced when magma is injected along sedimentary bedding surface. Batholiths: Largest intrusive igneous bodies A. Batholith B. Dike C. Sill http://reynolds.asu.edu/blocks/intpdst.htm  What determines the “explosiveness” of a volcano? Primary factors: 1. Temperature Mobility / Viscosity 2. Composition More viscous the material, the greater 3. Dissolved gas resistance it has against the flow. Low viscosity High viscosity Temperature Mafic Composition (% SiO2) Felsic Dissolved gas (H2O) 1. With increasing temperature, magma flows more easily. 2. Silica content determines the viscosity. Felsic magma is more viscous than their mafic counterpart. 3. Dissolved water makes magma less viscous.  Triggering a volcanic eruption Eruption column Lava fountains Felsic magma Mafic magma Low Expanding volatiles (giant bubbles) Drop in pressure Pressure, Enrichment in Temperature volatiles Separation of Fe, Mg rich minerals Rising magma High What kind of rock would you get at Hawaii? Mafic or felsic?  Things that come out of a volcano: Lava - Most of the lava are basaltic in composition - Flow rate 10-300meters / hour Types: 1. Aa lava: The outer crust has sharp edges. 2. Pahoehoe (ropy) lava: “on which one can walk”. - Pahoehoe is extremely slow in its progression - Pahoehoe could transform into aa lava once it starts cooling. http://www.youtube.com/watch?v=xExdEXOaA9A&feature=relat ed http://dsc.discovery.com/videos/understanding-volcanoes-lava- flow.html 3. Pillow lava: When lava forms underwater http://video.yahoo.com/watch/185214  Things that come out of a volcano: Gases Main volatile components: - Water - CO2 - SO2 Implications: 1. Atmosphere 2. Structure of the volcano - Once the magma reaches the surface, buoyant force from the magma body cracks the rock open. - Blasts of high pressure gases expand the cracks and develop a passageway to the surface (Remember Pegmatite?) - Hot gases and rock fragments erode the walls of the passageway producing a conduit.  Things that come out of a volcano: Pyroclastic material - Pyro = fire, Clast = fragment - Ejected particles from a volcano - Size range dust to boulders - Explosive eruptions often generates volcanic ash and tuff - Depending on the size of the particle, it could be called: Lapilli (little rock) Cinders Bombs Blocks - Depending on the texture and composition, it could be called 1. Pumice (Felsic): very light 2. Scoria (Mafic)  Main types of volcanic landforms 1. Shield volcanoes 2. Cinder cones 3. Composite cones / Stratovolcano  Shield volcanoes -Looks like warrior’s shield - Produced by accumulation of basaltic lavas. - Most have grown up from ocean floor to form islands Iceland or seamounts. - Example: Hawaiian chain, Iceland  Cinder cone - Built from the ejected particles from the volcano - Most abundant type of volcano - Example: Mount Etna, Italy Mount Etna  Composite cone - Mostly located in “Ring of fire”. - Most destructive type. - Large, nearly symmetrical structure - Composed of both lava & pyroclastic deposits. -Product of gas-rich andesitic magma. -Example: Vesuvius, Mount St. Helens. This type of volcano is also called a stratovolcano --- it has stratified layers of lava and pyroclastic deposits.  Vesuvius: a case study Eruption of Vesuvius: 24th August, A. D. 79 Location: Pompeii, Italy Death: more than 2000 -Entombed bodies under pumice and ash -Detailed picture of ancient Roman life -Chronology of the event ~ steam discharge ~ eruptive cloud of pumice and ash and pumice shower ~ blast of searing hot ash & gas Pliny (the younger) provides a first-hand account of the eruption of Mount Vesuvius. http://vids.myspace.com/index.cfm?fuseaction=vids.individual&videoid=47313543  Victims of Pompeii http://www.youtube.com/watch ?v=Y3RlcP8yUPo