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105_04-igneous_part_1 - Igneous Rocks and Intrusive Igneous...

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Unformatted text preview: Igneous Rocks and Intrusive Igneous Activity Igneous Rocks and Intrusive Igneous Activity Liquid rock! Chapter 4 The Rock Cycle The Properties and Behavior of Magma and Lava Magma Magma is molten rock material below the surface – Lower density causes magma to move upward towards the surface – Magma at the surface is lava The Properties and Behavior of Magma and Lava Extrusive Extrusive or volcanic rocks – Lava flows – Pyroclastic material The Properties and Behavior of Magma and Lava Plutonic Plutonic or intrusive rocks – Plutons 1 Composition of magma Defined Defined by silica content – Felsic (Feldspar + Silica) (Feldspar Silica) SilicaSilica-rich magma; abundant potassium, aluminum > 65% silica 65% How hot is magma and lava? – Lava: ~1,000° to 1,200°C ~1,000° 1,200° – (Temp of 1350 °C has been recorded) – No direct measurements of magma have been taken – Rock is a poor conductor of heat; lava lava flows and plutons may retain heat for thousands or millions of years light dark – Intermediate Compositions Compositions between felsic and mafic 53 53 – 65% silica – Mafic (Magnesium, ferrum = iron) (Magnesium, SilicaSilica-poor magma 45-52% silica; abundant calcium, iron, magnesium 45- – Ultramafic < 45% silica; mostly olivine 45% How hot is magma and lava? Viscosity Resistance to flow Direct Direct measurements come from low-risk lowvolcanoes such as the mafic lavas of the Hawaiian Island volcanoes Viscosity Composition Composition – High silica (Felsic) = ↑ viscosity – Low silica (Mafic) = ↓ viscosity Viscosity Mafic magma: low viscosity, hot, gases escape magma: low easily - magma flow (Hawaii) Heat Heat – ↑ temperature = ↓ viscosity Dissolved Dissolved gas content – ↑ gas content = ↓ viscosity Felsic magma: high viscosity, colder, thick, magma: high pasty flow, pressure builds up - explosive (St. Helens) 2 Why do rocks melt? Heat Heat Pressure Pressure Water Water Heat Heat Why do rocks melt? – Geothermal gradient – temperature increases with depth below the surface (avg = 3° C / 100 meters) 3° – Mantle plumes (“hot spots”) Pressure Pressure Water Water Why do rocks melt? Heat Heat Pressure Pressure – Melting point of a mineral increases with increasing pressure. Why do rocks melt? Heat Heat Pressure Pressure Water Water – Presence of water at depth helps to break bonds within minerals, thereby lowering the mineral’s melting point Water Water Bowen’s Reaction Series – Shows us the sequence in which minerals crystallize in a cooling magma – Discontinuous branch & Continuous branch Where do rocks melt? The The Origin of Magma at Divergent Boundaries – (a) Melting temperature rises with increasing pressure – (b) Melting temperature decreases when water is present 3 Where do rocks melt? The The Origin of Magma at Divergent Boundaries – Melting is initiated by a pressure decrease at spreading ridges – Presence of water further decreases melting temperature Where do rocks melt? Convergent Convergent Boundaries and the Origin of Magma – Partial melting of a mafic crust forms intermediate and felsic magmas – Melting of sediments and contamination with continental crust rocks changes the magma composition to a more felsic magma Where do rocks melt? Convergent Convergent Boundaries and the Origin of Magma Evolution of magmas with different compositions Crystal Crystal Settling Evolution of magmas with different compositions Assimilation Assimilation – incorporation of pre-existing country rocks into prea magma Evolution of magmas with different compositions Mixing Mixing of magmas – two magmas of different composition merge 4 ...
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