Volcanoes and Other Igneous Activity 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 actually control the viscosity of a given magma, which in turn, controls the nature of an eruption The Nature of Volcanic Eruptions
Viscosity is a measure of a materials resistance to flow (e.g., higher viscosity materials flow with great difficulty) Factors affecting viscosity temperaturehotter magmas are less viscous compositionsilica (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) The Nature of Volcanic Eruptions dissolved gases - gases expand within a magma as it nears Earths surface due to decreasing pressure
- the violence of an eruption is related to how easily gases escape from magma fluid basaltic lavas generally produce quiet eruptions highly viscous lavas (rhyolite or andesite) produce more explosive eruptions
A Pahoehoe Lava Flow A Typical AA Flow Materials Extruded from a Volcano Pyroclastic materialsfire fragments and types of debris
ash and dustfine, glassy fragments pumiceporous rock from frothy lava lapilli (lapillus)walnut-sized material cinderspea-sized material particles larger than lapilli blockshardened or cooled lava lava bombsejected as hot lava
Volcanic Ash and Lapilli A Volcanic Bomb Volcanoes General features openings at the summit of a volcano
cratera steep-walled depression at the summit; generally less than 1 km ( mile) in diameter calderaa summit depression typically greater than 1 km in diameter and produced by a collapse following a massive eruption ventan opening connected to the magma chamber via a pipe
Types of Volcanoes Shield volcano broad, slightly dome-shaped composed primarily of basaltic lava generally covers large areas produced by mild eruptions of large volumes of lava Mauna Loa in Hawaii
Mauna LoaA Shield Volcano Types of Volcanoes Cinder cone built from ejected lava (mainly cinder-sized) fragments steep slope angle
rather small size frequently occur in groups Lava Butte - Oregon Lava Butte A Cinder Cone Volcano Types of Volcanoes
Composite cone (Stratovolcano) most are located adjacent to the Pacific Ocean (e.g., Mount Fujiyama and 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
Mount Fujiyama, Japan A Composite Volcano A Composite Volcano A Composite Volcano most violent type of activity (Mount Vesuvius)
often produce nue ardentes fiery pyroclastic flows made of hot gases infused with ash and other debris move down the slopes of a volcano at speeds up to 200 km (140 miles) per hour may produce a lahar (volcanic mudflow) A Pyroclastic Flow
A Size Comparison of the Three Types of Volcanoes Other Volcanic Landforms Calderas steep-walled depressions at the summit size generally exceeds 1 km in diameter
Crater Lake, Oregon Other Volcanic Landforms Fissure eruptions and lava plateaus fluid basaltic lava extruded from crustal fractures called fissures (Columbia River plateau, Idaho,
Washington, Oregon) Other Volcanic Landforms Lava domes bulbous mass of congealed lava most are associated with explosive
eruptions of gas-rich magma Mount St. Helen, Washington Other Volcanic Landforms Volcanic pipes and necks
volcanic necks are resistant vents left standing after erosion has removed the volcanic cone volcanic pipes are short conduits that connect a magma chamber to the surface. Shiprock, New Mexico Plate Tectonics and Igneous
Activity Global distribution of igneous activity is not random most volcanoes are located within or near ocean basins (at subduction zones) basaltic rocks are common in both oceanic and continental settings; whereas granitic rocks are rarely found in the oceans
Distribution of Some of the Worlds Major Volcanoes Plate Tectonics and Igneous Activity Igneous activity along plate margins
spreading centers the greatest volume of volcanic rock is produced along the oceanic ridge system mechanism of spreading lithosphere pulls apart less pressure on underlying rocks results in partial melting of mantle large quantities of basaltic magma are produced
Plate Tectonics and Igneous Activity subduction zones occur in conjunction with deep oceanic trenches descending plate partially melts magma slowly moves upward 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 where most of the worlds explosive volcanoes are found Plate Tectonics and Igneous
Activity intraplate volcanism activity within a tectonic plate associated with plumes of heat in the mantle form localized volcanic regions in the overriding plate 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 National Park) Intraplate Volcanism
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