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Intrusive Igneous Rocks Enter question text... 1.Test Enteryour answer text... clicker 100% Igneous Rocks • Magma is molten rock below the Earth’s surface. • Lava is magma on the Earth’s surface. • Igneous rocks form from cooling of: (1) magma = intrusive OR (2) lava = extrusive Magma Formation • Magma collects in pools just beneath the surface • Why does magma rise? – Melt is less dense than surrounding solid rock! • Animation: Density & magma movement 1 Magma Formation • Rocks melt at 650°-1000° C • Exact temperature is controlled by: – Pressure – Dissolved gases (water vapor) – Types of minerals present Magma Formation • Sources of heat: 1. Heat from below: Geothermal gradient • 30°/km 2. Mantle plumes Where does magma form? 1. At mantle plume 2. In subduction zone 3. Beneath mid-ocean ridge 4. Beneath continental rift *mostly at plate margins (#2,3,4)! 2 Characteristics of magma • Consists of three components: – Liquids (melt) composed of mobile ions – Solids (“crystals”) – Volatiles (gases) dissolved in the melt… • water vapor (H2O) • carbon dioxide (CO2) • sulfur dioxide (SO2) Characteristics of magma • Cooling of magma results in systematic arrangement of ions into orderly patterns. – What do we call these solids? • Silicate minerals crystallize in a predictable order!!! (Bowen’s Reaction Series) 3 Bowen’s Reaction Series Discontinuous Branch Continuous Branch Lessons from Bowen’s Reaction Series • Variety of igneous rocks is produced by large variety of magma compositions • Separation of early-formed ferromagnesian minerals from a magma body increases the silica content of the remaining magma • Minerals melt in the reverse order of that in which they crystallize from a magma Igneous Activity and Plate Tectonics • Igneous activity occurs primarily at or near tectonic plate boundaries! • Divergent boundaries mafic composition – From partial melting of the asthenosphere 4 Igneous Activity and Plate Tectonics • Convergent boundaries intermediate composition – From partial melting of basaltic oceanic crust and overlying continental crust Igneous Activity and Plate Tectonics • Adjacent to Convergent boundaries felsic composition – From partial melting of the granitic continental crust Igneous Activity and Plate Tectonics • Intraplate volcanism – Mantle plumes produce localized hotspots when they produce magmas that rise through the crust – Ex. Hawaii: mafic Yellowstone: felsic 5 Intrusive Bodies • Named based on: – Depth of formation – Size – Shape – Relationship to country rock Insert new Fig. 3.11 here Shallow Intrusive Bodies • Form <2 km beneath Earth’s surface • Small scale • Likely internal “plumbing” of volcanoes • Chill and solidify quickly aphanitic rocks Insert new Fig. 3.11 here Volcanic neck: Magma solidifies in throat of volcano Ship Rock, NM 6 • Devil’s Tower, Wyoming. Shallow Intrusive Bodies • More common tabular structures form when magma fills cracks. • Dikes – vertically oriented. • Sills – horizontally oriented. Dike 7 Sill Dike Sill Canary Islands, Basalt Cliffs Volcano Dike Canary Islands, El Teide 8 Dike Canary Islands, El Teide Deep Intrusive Bodies • Pluton – body of magma or igneous rock that crystallized slowly at depth • At the surface they are divided into: – Stock (<100 km2) – Batholith (>100 km2) 9 Sierra Nevada batholith Lake Tahoe 10 Yosemite National Park Mt. Rushmore Enter question text... 1. Enter answer text... Extrusive Igneous Rocks 11 Igneous Review • Magma is molten rock below the Earth’s surface. • Lava is magma on the Earth’s surface. • Igneous rocks form from cooling of (1) magma = intrusive OR (2) lava = extrusive Lava flow Lava flow 12 Ashfall Pyroclastic flow Extrusive Igneous Rocks • Can be observed directly – Temperature measured – Solidification of rock observed • Compare with similar rocks where volcanism is no longer active to interpret ancient volcanic activity 13 Extrusive Textures • Size of grains • Aphanitic – grains <1mm – Fast cooling • Porphyritic – large crystals in fine matrix. – Slow then fast cooling Extrusive Rock Composition • Most extrusive rocks are aphanitic so composition can be determined by color Felsic Quartz K-feldspar Na plagioclase feldspar Mafic Amphibole Pyroxene Ca plagioclase feldspar 14 Rhyolite Basalt Andesite Special Extrusive Textures • Glassy – composed of glass – Instant cooling: Ions frozen in place – No minerals formed – More likely to form in viscous, felsic magma Obsidian Special Extrusive Textures • Vesicular – air bubbles (vesicles) – Released pressure causes dissolved gases to come out of solution – As lava solidifies air bubbles are trapped Vesicular basalt 15 Pumice Scoria • More viscous magmas trap more gas – form a froth • Very low density Special Extrusive Textures • Fragmental – consist of pyroclasts – Angular rock fragments formed by a volcanic explosion – Lava ejected into air • Lava bombs – cool rapidly in flight 16 • Volcanic breccia – composed of large fragments • Tuff – composed of ash (>2 mm) Volcanic Eruptions • Provide information on the Earth’s interior • Vary in nature and violence – Magma composition Pyroclastic flow Lava flow 17 Kilauea, Hawaii Mt. St. Helens, Washington Type of explosion controlled by… 1. Viscosity (“stickiness”)- a fluid’s resistance to flow • Silica content • Lava temperature 2. Amount of dissolved gases • More dissolved gases more fluid lava 18 Gases • Most gas released during an eruption is water vapor – Also carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrochloric acid • Expanding, hot gases propel debris into the atmosphere • Origin of atmosphere and oceans Explosiveness • Mafic: low violence, lava flows, divergent margins, intraplate zones • Intermediate: moderate violence, lava flows and pyroclastic explosions, subduction zones • Felsic: extremely violent, pyroclastic explosions, intraplate zones Volcano Anatomy • Ejected volcanic material forms the typical conical shape • Vent – opening • Crater – depression over vent • Caldera – enlarged crater – At least 1 km diameter – Formed by explosion of summit or collapse into magma chamber 19 Caldera Crater • Crater Lake, Oregon – caldera. • Animation. Caldera Examples Haleakala NP, Hawaii Yellowstone NP, Wyoming 20 21 • Extent of ash and pyroclastic debris from Yellowstone eruptions • Yellowstone tuff • 100s of meters thick Volcano Anatomy • Volcanic domes – steep sided, dome or spine shaped masses of volcanic rock solidifying above a vent. – Felsic or intermediate volcanoes – high viscosity. – “Corks” vent – traps gas and magma under pressure. 22 Volcanic domes, eastern California 23 Types of Volcanoes Shield Volcano • Broad, gently sloping • Made of solidified lava flows • Low viscosity lava spread widely and thinly • Occur within oceanic plates or at mid ocean ridges • Ex. Islands of Hawaii Animation 24 Caldera • Mauna Loa, Hawaii. • Most lava flows underground in lava tubes • Lava makes it to the sea • Builds islands outward 25 Shield Volcanoes • 2 Types of Lava – Pahoehoe: ropy – A’a: rough and jagged Cinder Cone • Constructed of pyroclastic fragments • Small size • Steep slopes – material falls close to vent • Mafic and intermediate • Animation • Short life span. • Easily eroded. 26 Composite Volcano • Alternating layers of lava flows and pyroclastic debris • Intermediate steepness • Long life spans • 100s to 1000s years inactivity Animation Composite Volcano: Guatemala 27 Composite Volcano • Present at subduction zones • Intermediate magma – andesite • Variable temperature – Hotter: lava flows • Variable gas concentration – More gas: pyroclastic explosion Composite Volcanoes • Most common type of volcano at convergent plate boundaries – Ex. Pacific Ring of Fire, Mediterranean Other Eruption Types • Flood eruptions – Very fluid (basalts) – Extremely large in volume – Create extensive lava plateaus Columbia river flood basalts Pillow basalts 28 Siberian Traps Siberian Traps • 2 million km2 • 5000+ m thick • 250 Ma • Largest mass extinction • 90% of all species 29