Volcanoes and Other Igneous Activity 1 56 Volcanoes

Volcanoes and Other Igneous Activity 1 / 56

Volcanoes: Content Standards Addressed 6 th 1 Plate tectonics accounts for important features of Earth's surface and major geologic events. As a basis for understanding this concept: 1 a Students know evidence of plate tectonics is derived from the fit of the continents; the location of earthquakes, volcanoes, and mid-ocean ridges; and the distribution of fossils, rock types, and ancient climatic zones 1 d Students know that earthquakes are sudden motions along breaks in the crust called faults and that volcanoes and fissures are locations where magma reaches the surface. 1 e Students know major geologic events, such as earthquakes, volcanic eruptions, and mountain building, result from plate motions 1 f Students know how to explain major features of California geology (including mountains, faults, volcanoes) in terms of plate tectonics. 2 / 56

Origin of magma • Magma originates when essentially solid rock, located in the crust and upper mantle, melts 3 / 56

How Magma Rises 4 / 56

Where do volcanoes form? • Volcanoes form at: – Hot Spots (10% of all volcanic activity) 5 / 56

Where do volcanoes form? • Volcanoes form at: – Hot Spots (10%) – Spreading Centers (80% of all volcanic activity) 6 / 56

Where do volcanoes form? • Volcanoes form at: – Hot Spots (10%) – Spreading Centers (80%) – Convergent Plate Boundaries (10% of all volcanic activity) • Ocean–Continental • Ocean – Ocean 7 / 56

Plate tectonics and igneous activity • Intraplate igneous activity – Activity within a rigid plate – Plumes of hot mantle material rise – Form localized volcanic regions called hot spots • the Hawaiian Islands & the Columbia River Plateau 8 / 56

Plate tectonics and igneous activity • Divergent plate boundaries – Produces the greatest volume of volcanic rock • • Lithosphere pulls apart Less pressure on underlying rocks Partial melting occurs Large quantities of fluid basaltic magma are produced 9 / 56

Plate tectonics and igneous activity • Convergent plate boundaries – Descending plate partially melts & magma slowly rises upward – Rising magma can form • Volcanic island arcs in an ocean (Aleutian Islands) • Continental volcanic arcs (Andes Mountains) 10 / 56

Tectonic Settings and Volcanic Activity 11 / 56

Plate Boundary Features

Volcanic Eruptions • Factors that determine the violence of an eruption – Temperature of the magma – Composition of the magma – Dissolved gases in the magma • The above three factors actually control the viscosity of a given magma which in turn controls the nature of an eruption 13 / 56

Viscosity, Temperature and Water Content of Magmas Rock Type Si. O 2 content Basalt Andesite Rhyolite 45 -55% 55 -65% 65 -75% 800 – 1, 000 ºC 600 -900 ºC Magma 1, 000 – 1, 250ºC temperature Viscosity Low increasing High Gas escape from magma Easy increasing Difficult Eruptive style Peaceful increasing Explosive 14 / 56

Plate-Tectonic Setting of Volcanoes Revisited • Why more volcanic activity at spreading centers? – Low Si. O 2 content – High temperature – Low pressure as plates pull apart • Fluid basaltic lavas generally produce quiet eruptions • Why less volcanic activity at subduction zones? – High Si. O 2 content – Lower temperatures – Higher pressures • Highly viscous lavas produce more explosive eruptions 15 / 56

Volcanic Explosivity Index Plume Height VEI Description Volume Classification How often Example 0 non-explosive < 100 m 1000 s m 3 Hawaiian daily Kilauea 1 gentle 100 -1000 m 10, 000 s m 3 Haw/Strombolian daily Stromboli 2 explosive 1 -5 km 1, 000 s m 3 Strom/Vulcanian weekly Galeras, 1992 3 severe 3 -15 km 10, 000 s m 3 Vulcanian yearly Ruiz, 1985 4 cataclysmic 10 -25 km 100, 000 s m 3 Vulc/Plinian 10's of years Galunggung, 1982 5 paroxysmal >25 km 1 km 3 Plinian 100's of years St. Helens, 1981 6 colossal >25 km 10 s km 3 Plin/Ultra-Plinian 100's of years Krakatau, 1883 7 super-colossal >25 km 100 s km 3 Ultra-Plinian 1000's of years Tambora, 1815 8 mega-colossal >25 km 1, 000 s km 3 Ultra-Plinian 10, 000's of years Yellowstone, 2 Ma 16 / 56 Source: http: //volcano. und. edu/vwdocs/eruption_scale. html

Volcano: Any landform that releases lava, gas or ashes or has done so in the past Refers to both the VENT and the CONE Many shapes and sizes due to magma chemistry and origin 17 / 56

Volcano Types 18 / 56

Volcanoes can be classified into 6 major types • Volcanoes can be classified into 6 major types Based on their size, shape, and origin 19 / 56

Features Of The Two General Categories Of Volcanoes Central Vent Volcanoes ü ü central vent summit crater flank eruptions fissure eruptions Large-scale Volcanic Terrains ü no central vent ü network of source material ü generally massive 20 / 56

Comparison Of The Six Types Of Volcanoes 21 / 56

Shield volcano • Low silica, low gas magma originates in the mantle = fluid, basaltic lava • Broad, gentle slopes. • Phreatomagmatic eruptions occur when lava contacts water. 22 / 56

SHIELD VOLCANOES ARE A TYPE OF CENTRAL VENT VOLCANO. Mauna Loa 23 / 56

Cinder Cones • high lava fountains on the vents of shield volcanoes. üComposed of pyroclastic debris. üCinders formed by high gas content. 24 / 56

Sunset Crater, Flagstaff, Arizona 25 / 56

Mt. Fuji Aniakchak Caldera STRATOVOLCANOES AND RHYOLITE CALDERA COMPLEXES ARE CENTRAL VENT VOLCANOES. 26 / 56

Composite cone or stratovolcano – Alternating andesitic lava flows and layers of explosively ejected pyroclastics. – Intermediate magma = viscous lava that is difficult to erupt. – Explosive eruptions due to buildup of gases. – Often produce nuée ardente – May produce a lahar - volcanic mudflow 27 / 56

Mt. St. Helens 28 / 56

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Mt. St. Helens New Dome Rock Glacier Vent 1980 -1986 Dome http: //earthobservatory. nasa. gov/Newsroom/New. Images/images. php 3? img_id=16721 30 / 56

Mt. St. Helens, October 1, 2004 http: //www. nasa. gov/vision/earth/lookingatearth/mshelenslidar. html 31 / 56

Eruptions in the Cascades Ranges 32 / 56

Calderas • High-silica, high-gas magmas. • Massive explosions (most explosive of all types). • Collapse, producing an “inverse volcano”, or Caldera (Spanish for cauldron). 33 / 56

Calderas • Famous (or infamous) collapsed calderas: – Long Valley, California (Mammoth) – Crater Lake (Mount Mazama), Oregon – Yellowstone, Wyoming – Krakatau, Indonesia, 1883 – Santorini and the Lost Continent of Atlantis 34 / 56

How Calderas Form 35 / 56

Large-scale volcanic terrains lack a central vent 36 / 56

Monogenetic Fields San Francisco Volcanic Field • Poorly understood. • Multiple maar vents and cinder cones. • Erupt at different times. • Grow laterally. • Usually a single magma source 37 / 56

Monogenetic Fields • Large Igneous Provinces – Fed by massive mantle plumes – Caused by flood basalts – Discharge over Columbia River Basalts time through long fissures (cracks). – Create large plateaus. 38 / 56

The Columbia River basalts 39 / 56

Monogenetic Fields • Mid-ocean Ridges – develop at Spreading Centers – Basaltic flow creates global network of interconnected ridges submarine volcanism 40 / 56

Other volcanic landforms • Lava Domes – Bulbous mass of congealed lava – Most are associated with explosive eruptions of gas-rich magma – One is currently developing in Mt. St. Helens 41 / 56

Other volcanic landforms • Volcanic pipes and necks – Pipes are short conduits that connect a magma chamber to the surface – Volcanic necks (e. g. , Ship Rock, New Mexico) are resistant vents left standing after erosion has removed the volcanic cone 42 / 56

Formation of a volcanic neck 43 / 56

Damage is caused by: • Lava flows • Pyroclastic flows. • Lahars (mudflows). • Ash falls. • Volcanic bombs. • Massive lethal, violent explosions. • Earthquakes. • Landslides. • Open cracks and chasms. • Phreatomagmatic eruptions. • Climate change. Volcanic Hazards Threaten Human Communities 44 / 56

Lava flows Molten rock that has flowed out onto the Earth’s surface 45 / 56

Pahoehoe lava (braids or ropes) 46 / 56

Aa lava (rough, jagged) 47 / 56

Pillow basalts 48 / 56

Gases • One to 5 percent of magma by weight • Mainly water vapor and carbon dioxide 49 / 56

Pyroclastic materials Ash and dust – fine, glassy fragments Pumice – from "frothy" lava Cinders – "pea-sized" Lapilli – "walnut" size Particles larger than lapilli Blocks Bombs weblink 50 / 56

TYPES OF PYROCLASTIC DEBRIS ASH PUMICE VOLCANIC BOMBS LAPILLI VOLCANIC BLOCK WELDED TUFF 51 / 56

Nuée Ardentes Fiery pyroclastic flow made of hot gases infused with ash Flows down sides of a volcano at speeds up to 200 km (125 miles) per hour & 800º C Examples: – Mt. Vesuvius – Mt. Shasta – Mt. St. Helens Mayon Volcano, Philippines, in 1984 Source 52 / 56

Mount Unzen, Japan. Note the broad paths of the pyroclastic flows across flat cultivated land. source Lahars (mudflows) How to make a lahar: Add water + volcanic material = lahar Water sources: Melt glaciers Displace lakes Rain 53 / 56

Intrusive igneous structures 54 / 56

Intrusive Igneous Features 55 / 56

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