Shield Volcanoes • Low angle volcano constructed principally Basaltic Volcanoes of basalt lavas • Examples: Mauna Loa and Kilauea, Hawaii • Thin lava flows References: • Summit calderas Encyclopedia of Volcanoes, pp. 283-289 • May have lateral rift zones • May have lava lakes in the crater Skaldbreidur Mauna Kea Mount Etna, Italy Stratovolcanoes • A volcano constructed of alternating layers of lava and pyroclastic materials • Steep slope angles (~35°) • Examples: Fuji, Japan and Kliuchevskaya, Kamchatka • May exhibit persistent activity (Etna, Sicily) • Volumes of erupted lava proportional to repose time Stratovolcano structure Popocatepetl Mount Augustine Cones in Arc Settings • Tholeiitic or calc-alkaline compositions • Merge to andesites • Continuous feeding from depth into a shallow chamber • May be associated with fissures and monogenetic scoria cones • Cones may be truncated by edifice collapse – Example: Augustine, Alaska Facies at a stratovolcano Monogenetic Volcano Fields • Composed of volcanoes that erupt only once • Common edifice is a scoria cone • Tuff rings and small shields are also common • Example: Pinacate, Mexico • Vent systems are dikes or pipes (diatremes) Scoria Cones Scoria Cones • Simplest and commonest volcanic form • Characterized by three parameters Tuff Rings – Height, width, crater width • Standard initial slope of 30o Pillow Lavas • Conical shape • Occur in several environments Sunset Crater, Arizona Scoria Cone Section Diatremes Tuff Cones • Breccia pipes • Massive deposits • Kimberlite Diatreme • Thickly bedded • Contains diamonds • Palagonitized • Ultramafic magmas • Bedding up to 30o • Mixture of rocks Vulcano, Italy •Wet surges • Driven by deep CO2 Tuff Rings • Thinly-bedded Cerro Colorado, Mexico • Poorly-indurated • Beds less than 12o • Sandwave beds • Dry surges Basaltic Scoria Cone Fields SanSan Francisco Francisco Volcanic Field Volcanic Field • 10s to 1000s of cones • General elliptical shape • Aspect ratio of 2:1 to 5:1 • 10 to 70 km in length • Areas of extensional tectonics σ • Elongate perpendicular to 3 • Widespread in western USA • Pinacate example Small Fields Flood Basalt Fields • North rim of Grand • Extensive sheets of lava flows Canyon • Flood the landscape • Scoria cones aligned • Associate with crustal dilation along fault planes • May coincide with mass bioextinctions • Examples: Deccan, India and Columbia River Plateau, USA Flow Units • Compound lava flows • Overlapping pahoehoe flows • Interconnected lava shields • Lava ponds in depressions • Inward directed dips of large fields • Cut by numerous dike swarms+ Columnar Join Shasta Central Volcanoes • A volcano that erupts magmas of various compositions – Mafic to silicic • Many have a central caldera composed of silicic rocks and rhyolite domes • A shallow magma chamber is part of the eruptive system Volcano Interior • The eroded core of these volcanoes may expose a granite pluton • Feeding system may be a ring complex • These may be a host for economic mineral deposits Alteration Zones Silicic Silica Potassic Adularia Sericitic Mica Argillic Clays, no mica Chloritic Chlorite Propylitic Albite, epidote, Chlorite.