The Nature of Volcanic Eruptions 1 Factors determining the violence or explosiveness of a volcanic eruption a Composition of the magma b Temp of the magma c Dissolved gases in the magma 2 These three factors control the viscosity of a given magma which in turn controls the 3 Viscosity is a measure of a material s resistance to flow higher viscosity materials flow nature of an eruption with greater difficulty 4 Factors affecting viscosity a Temp hotter magmas are less viscous b Composition silica content i Higher silica content higher viscosity felsic lava such as rhyolite c Lower silica content lower viscosity or more fluid like behavior e g mafic lava such as basalt d Dissolved gases magma e In summary eruptions i Gas content affects magma mobility ii Gases expand within a magma as it nears the Earth s surface as a result of decreasing pressure iii The violence of an eruption is related to how easily gases escape from i Fluid basaltic lavas generally produce quiet eruptions ii Highly viscous lavas rhyolite or andesite produce more explosive Materials Extruded During and Eruption 1 Lava flows a Basaltic lavas are much more fluid i Pahoehoe lava resembles a twisted or ropey texture ii Aa lava rough jagged blocky texture Dissolved Gases 1 One to six percent of a magma by weight 2 Mainly water vapor and carbon dioxide Pyroclastic Materials 1 Fire fragments a Ash and dust fine glassy fragments b Pumice porous rock from frothy lava c Lapilli walnut sized material d Cinders pea sized material e Blocks hardened or cooled lava f Bombs ejected as hot lava Anatomy of a Volcano 1 General features a Opening at the summit of a volcano i Crater steep walled depression at the summit generally less than 1 km in diameter ii Caldera a summit depression typically greater than 1 km in diameter produced by collapse following a massive eruption b Vent opening connected to the magma chamber via a pipe Volcanoes 1 Types of volcanoes a Shield volcano i Broad slightly dome shaped ii Composed primarily of basaltic lava iii Generally cover larger areas iv Produced by mild eruptions of large volumes of lava v Mauna Loa on Hawaii is a good example i Built from ejected lava mainly cinder sized fragments ii Steep slope angle iii Rather small size iv Frequently occur in groups b Cinder cone c Composite cone stratovolcano i Most are located adjacent to the Pacific Ocean e g Fujiyama Mt St Helens wide at base ii Large classic shaped volcano thousands of feet high and several miles iii Composed of interbedded lava flows and layers of pyroclastic debris 2 Composite cones a Most violent type of activity Mt Vesuvius b Often produce a pyroclastic flows i Fiery pyroclastic flow made of hot gases infused with ash and other debris ii Move down the slopes of a volcano at speeds up to 200 km hour c May produce a lahar which is a volcanic mudflow Other Volcanic Landforms 1 Calderas a Steep walled depressions at the summit b Size generally exceeds 1 km in diameter 2 Pyroclastic flows a Associated with felsic and intermediate magma b Consist of ash pumice and other fragmental debris c Material is propelled from the vent at a high speed for example Yellowstone a Fluid basaltic lava extruded from crustal fractures called fissures for example plateau 3 Fissure eruptions and lava plateaus Columbia River Plateau 4 Lava domes a Bulbous masses of congealed lava b Most are associated with explosive eruptions of gas rich magma 5 Volcanic pipes and necks a Pipes are short conduits that connect a magma chamber to the surface b Volcanic necks Ship Rock NM are resistant vents left standing after erosion has removed the volcanic cone Plate Tectonics and Igneous Activity 1 Global distribution of igneous activity is not random a Most volcanoes are located within or near ocean basins b Basaltic rocks are common in both oceanic and continental settings whereas granitic rocks are rarely found in the oceans i The greatest volume of volcanic rock is produced along the oceanic ridge 2 Igneous activity among plate margins a Spreading centers system ii Mechanism of spreading 1 Lithosphere pulls apart 2 Less pressure on underlying rocks 3 Results in partial melting of mantle 4 Large quantities of basaltic magma are produced 3 Igneous activity along plate margins a Subduction zones i Occur in conjunction with deep oceanic trenches ii Descending plate partially melts iii Magma slowly moves upward iv Rising magma can form either 1 An island arc if in the ocean 2 A volcanic arc if on a continental margin v Associated with the Pacific Ocean Basin 1 Region around the margin is known as the Ring of Fire 2 Most of the world s explosive volcanoes are found here 4 Intraplate volcanism a Activity within a tectonic plate b Associated with plumes of heat in the mantle c Forms localized volcanic regions in the overriding plate called a hot spot i Produces basaltic magma sources in oceanic crust Hawaii and Iceland ii Produces granitic magma sources in continental crust Yellowstone Park