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Chapter 5 Volcanoes and Other Igneous Activity The nature of volcanic eruptions • Characteristics of a magma determine the “violence” or explosiveness of a volcanic eruption • Composition • Temperature • Dissolved gases • The above three factors actually control the viscosity of a given magma The nature of volcanic eruptions • Viscosity is a measure of a material’s resistance to flow • 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 (e.g., mafic lava such as basalt) The nature of volcanic eruptions • 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 – Basaltic lavas = mild eruptions – Rhyolitic or andesitic lavas = explosive eruptions Materials extruded from a volcano • Lava flows • Basaltic lavas exhibit fluid behavior • Types of basaltic flows – Pahoehoe lava (resembles a twisted or ropey texture) – Aa lava (rough, jagged blocky texture) • Dissolved gases • 1% - 6% by weight • Mainly H2O and CO2 A pahoehoe lava flow Aa lava flow Materials extruded from a volcano • Pyroclastic materials – “fire fragments” • Types of pyroclastic debris – Ash and dust - fine, glassy fragments – Pumice - porous rock from “frothy” lava – Cinders - pea-sized material – Lapilli - walnut-sized material – Particles larger than lapilli » Blocks - hardened or cooled lava » Bombs - ejected as hot lava • Eruption of Kilauea Volcano in Hawaii A volcanic bomb Bomb is approximately 10 cm long Volcanoes • General features • Opening at the summit of a volcano – Crater - summit depression < 1 km diameter – Caldera - summit depression > 1 km diameter produced by collapse following a massive eruption • Vent – surface opening connected to the magma chamber • Fumarole – emit only gases and smoke Volcanoes • Types of volcanoes • Shield volcano – Broad, slightly domed-shaped – Generally cover large areas – Produced by mild eruptions of large volumes of basaltic lava – Example = Mauna Loa on Hawaii Anatomy of a shield volcano • JDR Life Goal #38. Climb Mauna Kea (13,796 ft) shield volcano in Hawaii. Highest mountain on the planet from base to crest (almost 30,000 ft). Completed with my son Jonathan in May 2000. This shows profile of Mauna Loa shield volcano, as seen from summit of Mauna Kea (benchmark at lower right). Profiles of volcanic landforms Volcanoes • Cinder cones – Built from ejected lava (mainly cindersized) fragments – Steep slope angle – Small size – Frequently occur in groups Cinder cone volcano The Paricutin volcano in Mexico erupted in a corn field in 1943, burying the entire town. Volcanoes • Composite cone (stratovolcano) – Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens) – Large, classic-shaped volcano (1000’s of ft. high and several miles wide at base) – Composed of interbedded lava flows and pyroclastic debris – Most violent type of activity (e.g., Mt. Vesuvius) Anatomy of a composite volcano Seamounts over hot spots Volcanoes over hot spots have spawned several prominent chains of islands in the Pacific Ocean, as shown here. These volcanoes become increasing younger preceding southerly, then southeasterly. Most of their mass lies unseen, below the current sea level. The Hawaiian Islands As the Pacific Plate moves WNW at a rate of about 6 mm/yr, magma rises through the thin oceanic crust, causing volcanoes. The unrelenting erosion of waves tends to plane off the islands, unless protected by coral reefs The most active area is currently the Kilauea Rift along the southeastern coast of the island of Hawaii. Here large slump blocks produce tensile scarps, which allows molten lava to flow up to the surface more easily, loading the head of the slumps. Loihi is the next island forming. The Pali Escarpment The Pali Escarpment across Molokai and Oahu is a gigantic landslide headscarp, formed when the northern side of those islands detached itself and slid into the ocean, likely on a layer of altered volcanic ash Bathymetry of the detachments Bathymetry surveys off the north coasts of Oahu and Molokai, showing enormous debris fields Massive subaqueous landslide debris fields extending around the Hawaiian Islands, initially identified by J. G. Moore of the U.S. Geological Survey in the n1980s, which working on the 200 miles economic exclusions zone around the islands. Cascade Volcanoes • There are 13 potentially active volcanoes in the Cascade Range of the northwestern United States Mt. St. Helens – prior to the 1980 eruption Mt. St. Helens after the 1980 eruption • Precursory stages of the Mt St Helens eruption • A seismically-induced landslide reduced lateral and vertical confinement, triggering the May 18, 1980 eruption of Mt. St Helens • Cross sections thru Mt St Helens showing deep rotational slide blocks which slid off the peak • Atmospheric impacts of the May 18, 1980 Mt St Helens eruption • The Mt St Helens eruption shot volcanic ash to an altitude of 60,000 feet, into the stratosphere • Relative distribution of blown down trees and lahar debris flows in the Toutle River Valley on the north side of Mt St Helens The St Helens blast flattened Douglas fir trees over an area of 400 square kilometers • Lahars, debris flows, and debris chocking of rivers caused by the Mt. St. Helens eruption Volcanoes • Nuée ardente – A deadly pyroclastic flow » Fiery pyroclastic flow made of hot gases infused with ash and other debris » Also known as glowing avalanches » Move down the slopes of a volcano at speeds up to 200 km per hour • Lahar – volcanic mudflow » Mixture of volcanic debris and water » Move down stream valleys and volcanic slopes, often with destructive results A nueé ardente on Mt. St. Helens after the May 1980 eruption • Viscous lava flow passing through the village of Goma in the Congo during the eruption of Mt. Nyiragongo in January 2002 Other volcanic landforms • Caldera • Steep-walled depressions at the summit • Generally > 1 km in diameter • Produced by collapse • Pyroclastic flow • • • • Felsic and intermediate magmas Consists of ash, pumice, and other debris Material ejected at high velocities Example = Yellowstone plateau Formation of Crater Lake, Oregon Other volcanic landforms • Fissure eruptions and lava plateaus • Fluid basaltic lava extruded from crustal fractures called fissures • Example = Columbia River Plateau • Lava domes • Bulbous mass of congealed lava • Associated with explosive eruptions of gas-rich magma A lava dome Other volcanic landforms • Volcanic pipes and necks • Pipes - short conduits that connect a magma chamber to the surface • Volcanic necks (e.g., Ship Rock, New Mexico) - resistant vents left standing after erosion has removed the volcanic cone Formation of a volcanic neck The most famous volcanic neck in the United States is Shiprock, New Mexico JDR Life Goal #54: Climb Shiprock, New Mexico, 1700’ climb, to elevation 7,178 ft, in Four Corners area . Completed in June 1973 (after climb, discovered that climbing had been outlawed in 1970). Intrusive igneous activity • Most magma is emplaced at depth in the Earth • Once cooled and solidified, is called a pluton • Nature of plutons • Shape - tabular (sheetlike) vs. massive • Orientation with respect to the host (surrounding) rock – Concordant vs. discordant Intrusive igneous activity • Types of intrusive igneous features • Dike – a tabular, discordant pluton • 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 Igneous Structures • Relationships between volcanism and intrusive igneous activity. Cross cutting intrusions are called dikes, while those emplaced parallel to structure are called sills. • Diabase dike cutting thru preCambrian age Hakatai Shale at Hance Rapid in the Grand Canyon. JDR Life Goal# 40. Row a rubber raft down the Colorado River through the rapids of Grand Canyon from Glen Canyon Dam to Lake Mead . Completed in June-July 1978. Rowed it again in 1983, twice in 1984, and 1985. A sill in the Salt River Canyon, Arizona Intrusive igneous activity • Intrusive igneous features continued • Batholith – Largest intrusive body – Surface exposure > 100+ km2 (smaller bodies are termed stocks) – Frequently form the cores of mountains Classic Igneous Batholith • When batholiths are uplifted and exposed, they are usually resistant strata that form the roots of mountain ranges or eroded highlands. Batholiths of western North America Plate tectonics and igneous activity • Global distribution of igneous activity is not random • Most volcanoes are located within or near ocean basins • Basaltic rocks = oceanic and continental settings • Granitic rocks = continental settings Distribution of some of the world’s major volcanoes Plate tectonics and igneous activity • Igneous activity at plate margins • Spreading centers – Greatest volume of volcanic rock is produced along the oceanic ridge system – Mechanism of spreading » Decompression melting of the mantle occurs as the lithosphere is pulled apart » Large quantities of basaltic magma are produced Volatiles driven from subducting crust lower the melting temperature of these rocks, and they rise by density contrast. Plate tectonics and igneous activity • Subduction zones – Occur in conjunction with deep oceanic trenches – Partially melting of descending plate and upper mantle – Rising magma can form either » An island arc if in the ocean » A volcanic arc if on a continental margin – Associated with the Pacific Ocean Basin » Region around the margin is known as the “Ring of Fire” » Majority of world’s explosive volcanoes • A. Rising mantle plume; B. Rapid decompression melting producing flood basalts; and C. Rising plume tail produced by linear seafloor volcanic chain Plate tectonics and igneous activity • Intraplate volcanism • Occurs within a tectonic plate • Associated with mantle plumes • Localized volcanic regions in the overriding plate are called a hot spot – Produces basaltic magma sources in oceanic crust (e.g., Hawaii and Iceland) – Produces granitic magma sources in continental crust (e.g., Yellowstone Park) • Global distribution of flood basalt provinces and associated hot spots in the Earth’s crust. Some of these formed in failed continental rifts, like Siberia and the Keweenawan Rift in the USA Volcanoes and climate • The basic premise • Explosive eruptions emit huge quantities of gases (SO2) and finegrained debris • A portion of the incoming solar radiation is reflected and filtered out • Past examples of volcanism affecting climate • Mount Tambora, Indonesia – 1815 • Krakatau, Indonesia – 1883 Volcanoes and climate • Modern examples • Mount St. Helens, Washington - 1980 • El Chichón, Mexico - 1815 • Mount Pinatubo, Phillippines - 1991 Pinatubo Eruption June 1991 • Map showing areal distribution of pyroclastic flows of June 1991 and destructive lahars that ensued in September 1991, killing many more people than the eruption • Sulfer dioxide emissions of large volcanic eruptions from 1979-91, in thousands and millions of tons • Impact of SO2 emissions on global climate, caused by formation of H2SO4 aerosol, deflecting radiant energy from the Sun into the Stratosphere