VOLCANOES     I. VOLCANOES--surface expression of subsurface igneous activity A. Crater--circular depression at top B. Vent--opening within crater through which lava is expelled C. Slopes--sides, composed of lava and pyroclastic debris D. Magma chamber--houses liquid magma E. Pyroclastic--("fire broken") fragments of country rock and lava ejected in an eruption 4 types of pyroclastic debris, based on size < 2mm dust ~ 2mm ash between 2 mm & 2.5 cm cinders > 2.5cm bombs II. CALDERA--roughly circular depression formed by collapse or explosion of a volcano A. Mechanisms of caldera formation 1. Removal of magma from magma chamber and subsequent collapse e.g. San Juan Mountains, Colorado 2. Explosion of magma chamber e.g. Crater Lake, Oregon, Mt. Mazama exploded 6,600 years ago III. CLASSIFICATION OF VOLCANOES--based on activity A. Active--some historical record of eruption e.g. Kilauea, Hawaii B. Dormant (sleeping)--no eruption in historical time but capable of renewed activity e.g. Mt. St. Helens, prior to May 18, 1980 C. Extinct--no eruption in historical time, shows signs of erosion, and no activity IV. TYPES OF ERUPTIONS A. Quiet--nonexplosive, generally mafic lavas e.g. Hawaii B. Explosive--deadly, intermediate and felsic lavas e.g. Mt. St. Helens V. SOME ERUPTIONS ARE EXPLOSIVE AND SOME NOT A. Deals with the viscosity of the magma Viscosity--the measure of how easily a fluid flows High viscosity = pasty (e.g., molasses) Low viscosity = watery (e.g., water) B. High viscosity magmas clog the piping system. Gases (mostly steam) build up in the magma chamber and explode VI. CONTROLS OF VISCOSITY OF MAGMA A. Its chemical composition B. BRS--more differentiated (more felsic) magmas are more silica rich C. More silica--more tetrahedra trying to link up, forming more complex structures (chains, sheets, and frameworks) D. Viscosity increases with increasing linkage of tetrahedra VII. COMPOSITION VS ERUPTIVE STYLE A. Mafic--quiet and free flowing B. Intermediate--explosive with periods of quieter lava flows C. Felsic--totally explosive VIII. TWO TYPES OF LAVA A. Pahoehoe (pa-hoy-hoy)--ropey texture B. Aa (ah-ah)--angular and jagged, cindery blocks IX. THREE TYPES OF VOLCANOES A. Shield 1. Largest type of volcano 2. Very shallow slopes (2o at base, ~10o at top) 3. Basaltic lavas 4. Almost exclusively lava 5. Example Hawaii B. Composite 1. Large volcanoes (typically ~8,000 feet high) 2. Classic volcano shape, concave upward slopes (15o at base, ~30o at top) 3. Mostly intermediate compositions 4. Lava flows mixed with pyroclastic material 5. Example Mt. St. Helens Nuees ardentes (fiery cloud)--deadly, super-heated pyroclastic flow e.g. Pompeii > 800oC Glows Hovercraft effect Clocked at 100 miles per hour C. Cinder cone 1. Smallest type of volcano 2. Very steep slopes (30o - 40o) 3. Exclusively pyroclastic material 4. Generally subsidiary structures on the flanks of larger volcanoes 5. Example Craters of the Moon, Idaho Angle of repose (i.e., critical slope)--maximum slope where unconsolidated material becomes stable X. FISSURE ERUPTIONS--lava spews out from a fracture in the earth A. No volcanic cone, therefore, not a type of volcano B. Very low-viscosity, basaltic lavas (water like), called flood lavas C. Form basalt plateaus D. Example Columbia River plateau 1. Covers 163,000 Km2 eastern Washington, Oregon, and Idaho 2. In places ~6 Km thick 4. > 1x106 Km3 5. Formed between 17 and 5 m.y. ago E. Example Deccan Traps Plateau, India 1. ~66 m.y. ago 2. hypothesized to have killed off the Dinosaurs XI. DEATH & DESTRUCTION A. Mt. Vesuvius, Bay of Naples, Italy 1. August 24, AD 79 2. Inundated Pompeii and Herculanium 3. Nuees ardentes 4. Preserved a snapshot of ancient Roman culture B. Mt. Pele, St. Pierre, Martinique, 1902 1. Nuees ardente 2. Killed 28,000 residents 3. Only 2 survived, a prisoner in an underground cell and a guy who was quite burned up C. Krakatoa, South Pacific, between Java & Sumatra, 1883 1. "Greatest explosion on record" 2. Indonesia is Earth's most tectonically active region 3. Within 3 months, 3 volcanoes formed from fissures on ocean floor, each ~800 m (2,400 ft) high 4. At 10:20 AM, August 26, they all were blown into oblivion 5. Pressure wave circled the globe twice 6. Heard 5,000 km away 7. "Bishops ring", stunted crops causing famine 8. 36,500 people died, mainly due to a tsunami D. Mt. St. Helens, May 18, 1980 1. "Best documented eruption in history" 2. Lateral blast 3. 60 people died, David Johnson