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Chapter 5 Volcanoes Magma Sources and Types • Magma sources tend to be 50 to 250 km deep into the crust and upper mantle • Temperatures increase as depth increases • Some of the internal heat is left over from the earth’s formation; more heat is generated by the decay of radioactive elements in the earth • Volcanoes are generated at: – Divergent Plate Boundaries – Convergent Plate Boundaries – “Hot Spots” Magma Sources and Types • Magma compositions vary in SiO2 , iron, magnesium, and volatile gases • Mafic magma – low in SiO2 (45-50 %) but high in iron, and magnesium • Felsic magma – high in SiO2 (up to 75 %) but low in iron, and magnesium • Intermediate magma – intermediate range of SiO2 (50-65 %), iron, and magnesium • Amount of volatile gases will affect explosive characteristics of eruptions The nature of volcanic eruptions • Factors determining the “violence” or explosiveness of a volcanic eruption • Composition of the magma • Temperature of the magma • Dissolved gases in the magma • The above three factors control the viscosity of a given magma, which in turn controls the nature of an eruption • Viscosity is a measure of a material’s resistance to flow (e.g., higher viscosity materials flow with greater difficulty) Factors affecting viscosity: –Temperature – hotter magmas are less viscous –Composition – silica (SiO2) content • Higher silica content = higher viscosity (e.g., felsic lava such as rhyolite) • Lower silica content = lower viscosity or more fluid-like behavior (e.g., mafic lava such as basalt Factors affecting viscosity: – Dissolved gases • Gas content affects magma mobility –Gases expand within a magma as it nears the Earth’s surface due to decreasing pressure –The violence of an eruption is related to how easily gases escape from magma In summary: • Fluid basaltic lavas generally produce quiet eruptions • Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions Materials extruded from a volcano • Lava flows • Basaltic lavas are much more fluid • Types of basaltic flows – Pahoehoe lava (resembles a twisted or ropey texture) » Lava tubes, pillow lavas, – Aa lava (rough, jagged blocky texture) • Dissolved gases( volatiles ) • One to six percent of a magma by weight • Mainly water vapor and carbon dioxide A typical aa flow Figure 4.7 A Pahoehoe flow Pyroclastic materials • “Fire fragments” Types of pyroclastic debris • Ash and dust – fine, glassy fragments • Pumice – porous rock from “frothy” lava • Lapilli – walnut-sized material • Cinders – pea-sized material • Particles larger than lapilli – Blocks – hardened or cooled lava – Bombs – ejected as hot lava – Stromboli! A volcanic bomb Figure 4.9 left Figures 5.9 A-D Volcanic Structures • Opening at the summit of a volcano – Crater – steep-walled depression at the summit, generally less than 1 kilometer in diameter – Caldera – a summit depression typically greater than 1 kilometer in diameter, produced by collapse following a massive eruption • Vent – opening connected to the magma chamber via a pipe Types of volcanoes •Shield volcano »Broad, slightly dome-shaped »Composed primarily of basaltic lava »Generally cover large areas »Produced by mild eruptions of large volumes of lava »Mauna Loa on Hawaii is a good example Figures 5.8 a – d YouTube - Kilauea Volcano Erupts - Dramatic Video Shield Volcano – Mauna Loa Types of volcanoes •Cinder cone »Built from ejected lava (mainly cindersized) fragments »Steep slope angle »Rather small size (less than 1000ft ) »Frequently occur in groups Sunset Crater A cinder cone near Flagstaff, Arizona Copyright © 2006 Pearson Prentice Hall, Inc. Paricutin • Observed by farmer, forming in a Mexican cornfield , 1943 • Grew to 130 ft in first day, Day 5 – 330 ft. • Covered a nearby village in 1944 (aa flow) • Ended in 9 years - Figures 5.10 A and B Composite cones (stratovolcanoes) »Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mount St. Helens) »Large, classic-shaped volcano (thousands of feet high and several miles wide at base) »Composed of interbedded lava flows and layers of pyroclastic debris Composite cones »Most violent type of activity (e.g., Mount Vesuvius) »Often produce a nuée ardente »Fiery pyroclastic flow made of hot gases infused with ash and other debris »Move down the slopes of a volcano at speeds up to 200 kilometers per hour »May produce a lahar, which is a volcanic mudflow Mount St. Helens • Mount St. Helens National Volcanic Monument • Mount St. Helens -- From the 1980 Eruption to 2000, Fact Sheet 036-00 Mt Shasta – composite with small parasitic cone on left Mt Vesuvius 79 A.D. – 2m of pumice in 24hrs buried 2000, then hot gases suffocated those still alive Mount Etna Erupts A composite volcano Figure 4.10 Figure 5.11 A-B Types and Locations of Volcanoes • Volcanic Domes – Composed of more viscous andesite or rhyolite • these lavas do not flow – Ooze out onto surface from a tube and pile up close to the vent – Compact, small, and steep sided – Various locations around Pacific Ring of Fire Figure 5.12 – domes formed in Mt. St. Helen A lava dome on Mount St. Helens Figure 4.25 A size comparison of the three types of volcanoes Figure 4.13 Other volcanic landforms • Calderas • Steep-walled depressions at the summit • Size generally exceeds 1 kilometer in diameter • produced by collapse following a massive eruption Deriba Caldera, western Sudan outer – 3mi diameter about 3500 yrs ago, inner formed later Formation of Crater Lake, Oregon Figure 4.22 • Volcanic necks (e.g., Ship Rock, New Mexico) are resistant vents left standing after erosion has removed the volcanic cone • Volcanic necks (e.g., Ship Rock, New Mexico) are resistant vents left standing after erosion has removed the volcanic cone Plutonic igneous activity •Most magma is emplaced at depth in the Earth •An underground igneous body, once cooled and solidified, is called a pluton •Classified by: •Shape »Tabular (sheetlike) »Massive And: •Orientation with respect to the host (surrounding) rock »Discordant – cuts across sedimentary rock units – ex. Dike – a tabular, discordant pluton »Concordant – parallel to sedimentary rock units –ex. Sill – a tabular, concordant pluton (e.g., Palisades Sill in New York) • Laccolith –Similar to a sill –Lens or mushroom-shaped mass –Arches overlying strata upward •Batholith »Largest intrusive body »Surface exposure of over 100 square kilometers (smaller bodies are termed stocks) »Frequently form the cores of mountains Figure 4.28 B A batholith exposed by erosion Figure 4.28 C A sill in the Salt River Canyon, Arizona Figure 4.30 Magma Sources and Types • Mafic magmas produce basalt lavas – Intrusive equivalent is gabbro • Intermediate magmas produce andesite lavas – Intrusive equivalent is diorite • Felsic magmas produce rhyolite lavas – Intrusive equivalent is granite Figure 5.2 Plate tectonics and igneous activity • Global distribution of igneous activity is not random • Most volcanoes are located within or near ocean basins • Basaltic rocks are common in both oceanic and continental settings • granitic rocks are rarely found in the oceans? Distribution of some of the world’s major volcanoes Figure 4.33 Magma at Divergent Plate Boundaries • Magma produced at a Divergent Plate Boundary is typically melted asthenosphere material • Asthenosphere is extremely rich in ferromagnesian (ultramafic) and a melt from it is mafic (or ultramafic) • Basalt is emplaced as new seafloor at the spreading ridge or a rift • Rift systems in continental crust may melt granitic crust and produce andesite or rhyolite lavas – A bimodal suite of extrusive igneous rocks characterize rift volcanoes Magma at Convergent Plate Boundaries • Magmatic activity at convergent boundaries is complex • The composition of the subducted plate determines the composition of the lava – Subducted continental crust may melt and produce rhyolite lava – Subducted oceanic crust may melt and produce basalt or andesite lava – Subduction of sediments derived from the top of the subducted slab may produce a variety of lavas Magma at Hot Spots • Magmas associated with a hot spot volcano in an ocean basin will produce a basalt lava • Magmas associated with a hot spot volcano under continental crust generally will produce a felsic lava (and often an explosive one) Volcanism on a tectonic plate moving over a hot spot Figure 5.7 some “hot spots” Locations of Volcanoes • Seafloor Spreading Ridges – Most voluminous volcanic activity – About 50,000 km of ridges around the world – Mostly under the oceans - except at Iceland – Generally, harmless mafic fissure eruptions • Continental fissure eruptions – Pour out of cracks in lithosphere – Result in large volume of “flood basalts” – Columbia Plateau (over 150,000 km2 and 1 km thick) – Other locations include India and Brazil Figure 5.4 – continental fissure eruption Figure 5.5a Columbia River flood basalts Figure 5.6 – recently active volcanoes Hazards Related to Volcanoes • Lava, the principal hazard? But not lifethreatening generally • Pyroclastics, more dangerous than lava flows • Pyroclastic Flows - Nuées Ardentes • Toxic Gases • Steam Explosions - Phreatic eruption – Krakatoa, 1883 • Secondary Effects; Climate and Atmospheric Chemistry Lava flow – Heimaey Is., Iceland, 1973 CVO Website - Heimaey 1973 Eruption, Iceland Pyroclastic s – blew off over 1300 ft , Mount St. Helen’s - 1980 Pyroclastic flows , nuée ardente on Mount St. Helens • 5.18 1902 Mount Pelee, Martinique almost 28,000 casualties of Nuee Ardente Lahars, a volcanic ash and water mudflow Armero, Colombia: 1985 Mount Pinatubo eruption, 1991, Angeles City, Philippines Figure 5.20 Soufriere Hills volcano A. ash cloud over Plymouth on Montserrat,, West Indies, 1996 B. by 2003 pyroclastic flows covered much of the mountain Toxic gases – • Vog – volcanic fog – Water vapor, carbon dioxide, sulfur and hydrochloric acid • CO2 – – Lake Nyos, Cameroon • Aug. 1986 -suffocated 1700 – Nearby Lake Monoun • 1984, 37 died 1990 – Mammoth Mountain, CA – tree kill, 1990. Secondary Effects Climate and Atmospheric Chemistry Figure 5.23 Mt Pinatubo, 12 June 1991 Mount Pinatubo The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines, Fact Sheet 113-97 Airborne sulfuric-acid mist formed from SO2 Figure 5.25 Effect of Pinatubo Predicting Volcanic Eruptions • Classification by activity – Active: erupted in recent history (300-500) – Dormant: no historic erupts but not badly eroded – Extinct: no historic eruptions and badly eroded • Not real reliable! – Vesuvius WAS considered extinct…until it erupted – Pinatubo WAS dormant, it had not erupted for 400 years Volcanic Explosivity Index • Volcanic Precursors – Seismic activity Seismic – harmonic tremors – different in character from earthquakes – Bulging, tilting or uplift – Monitoring gas emissions around volcano –SO2 Present and Future Volcanic Hazards in the United States • Hawaii: active or dormant volcanoes • Cascade Range: a series of volcanoes in the western United States and southwestern Canada resides above the Pacific Northwest subduction zone • The Aleutians: South-central Alaska and the Aleutian island chain sit above a subduction zone • Long Valley and Yellowstone Calderas Cascade Range volcanoes Figure 5.30 Mt Rainier - mud flow danger Figure 5.31 - 51 active Aleutian volcanoes monitored Redoubt Volcano, Alaska -December 15, 1989 - KLM 747, 4 engines failed, fell 13000 ft and did restart, landing safely with 231 passengers Redoubt Long Valley and Yellowstone- collapse calderas Yellowstone Has Bulged as Magma Pocket Swells Figure 5.34 Volcano Hazards Program Volcanoes and Eruptions