Igneous Rock Names and Compositions EXTRUSIVE IGNEOUS • Composition • Intrusive • Volcanic Name Rock Name Rock Name ROCKS • Ultramafic • Peridotite • (Komatiite) • Mafic • Gabbro • Basalt Volcanoes and Volcanic Rocks • Intermediate • Diorite • Andesite • Silicic (felsic) • Granite • Rhyolite Magma and Lava • Magma is the term for any molten silicate material, whether below the surface or on top. • Volcanic rocks are erupted on the surface. – Volcanic rocks are fine-grained (<1 mm) • Lava is the term for a magma on the surface. Most Volcanoes are at Plate Boundaries Geologic Settings 1 Basaltic Eruptions Pahoehoe, Hawaii (Basalt) • Low Viscosity • High Temperature (1000 - 1200ºC) • Normally Quiescent Lava Flows – Aa - early, low temperature, blocky flow – Pahoehoe - later high temperature ropy flow – Pillow Lavas - underwater flow • Edifice – Shields – Cinder cones Aa, Hawaii (Basalt) Pillow Lava, Kauai (Basalt) Nyiragongo, Congo (Zaire) 2 Goma, Congo (Zaire) Skjaldbreidur Shield, (Basalt) Cinder Cone + Caldera, (Basalt) Cinder Cone, Pu’uo’o, Hawaii (Basalt) Basaltic Eruptions Most Volcanoes are at Plate Boundaries • Low Viscosity • High Temperature (1000 - 1200ºC) • Normally Quiescent Lava Flows – Aa - early, low temperature, blocky flow – Pahoehoe - later high temperature ropy flow – Pillow Lavas - underwater flow • Edifice – Shields – Cinder cones 3 Andesite Eruptions Geologic Settings • Higher viscosity (higher polymerization) • Lower temperature • More water, more explosive • Subduction zone volcanism • Flows – Viscous lava flows – Pyroclastic flows • Edifice – Composite Cones Andesite Flow, Mt Shasta Composite Cone, Mt. Fuji Composite Cone, Mt. Daisen Composite Cone, Mt. St Helens 4 Andesite Eruptions • Higher viscosity (higher polymerization) • Lower temperature • More water, more explosive • Subduction zone volcanism • Flows – Viscous lava flows – Pyroclastic flows • Edifice – Composite Cones 5 Rhyolite Eruptions Rhyolite Eruptions • Very high viscosity • Associated Phenomena • Low temperature (600 - 800ºC) – Hot Springs • Massive Pyroclastic eruptions – Geysers – Air fall (pumice) – Fumaroles – Ash Flow (Nuée Ardente) Tuff • Geologic Setting – Obsidian Flows – Continental Margins and Interiors • Edifice – Subduction Zones – Caldera (5 - 25 km) – Resurgent dome Geologic Settings Pumice = Glass foam Ash-Flow Tuff Unit Ash-Flow Tuff Non-Welded Top with Air- Fall Units Welded Core Vitrophyre Non-welded base 6 Silicic Tuff Units (Jemez, NM) Sulfur Fumarole, Nysseros Obsidian Flow Non-welded Air-Fall Units Caldera with Cinder Cone Crater Lake, OR Rhyolite Eruptions Geyser • Very high viscosity Old Faithful • Low temperature (600 - 800ºC) (Yellowstone) • Massive Pyroclastic eruptions – Air fall (pumice) – Ash Flow (Nuée Ardente) Tuff – Obsidian Flows • Edifice – Caldera (5 - 25 km) – Resurgent dome 7 Andesite Eruptions Basaltic Eruptions • Higher viscosity (higher polymerization) • Low Viscosity • Lower temperature • High Temperature (1000 - 1200ºC) • More water, more explosive • Normally Quiescent Lava Flows • Subduction zone volcanism – Aa - early, low temperature, blocky flow • Flows – Pahoehoe - later high temperature – Viscous lava flows ropy flow – Pyroclastic flows – Pillow Lavas - underwater flow • Edifice • Edifice – Composite Cones – Shields – Cinder cones Geologic Settings Most Volcanoes are at Plate Boundaries 8.