Volcanism
Mt Pelee, Martinque Mt St Helens, WA 1981 Mauna Loa, Hawaii Notice the different shapes, some active, some not, some have lots of lava, some lots of ash, some on land, some islands Mauna Loa, Hawaii
Mt Fuji, Japan Thera, (Santorini) Greece Mt Etna, Italy Mt Vesuvius, Italy
What causes Volcanism? Volcanism occurs when rock melts Melting due to Increased temp Decreased pressure Liquid rock is less dense than solid rock…rises toward surface
Convergent Subduction Plate Boundary Volcanism Divergent Plate Boundary Volcanism Along plate boundaries & intra-plate areas Ring of Fire Mid-Ocean Ridge Hot Spots Volcanic Arcs Intra-Plate Volcanism (hotspots)
Convergent Subduction Volcanism Plates are colliding with each other and subducting. lithosphere is being destroyed Due to increased temps Sometimes on coasts, sometimes ocean-ocean plates=islands “ring of fire” Divergent Volcanism Plates are ripping apart Less pressure leads to melting asthenosphere New lithosphere is being created Intraplate Volcanism In middle of plates Hot spot in mantle plume rises towards the surface due to density difference Both land and water
Nature of Eruption: quiet vs. explosive Conditions that cause pressure to build up cause explosive eruptions amount of gas viscosity of lava (more viscous, more explosive) What affects viscosity and gas content? Magma’s composition (felsic=more gassy and more viscous) Temperature (cold=more viscous)
Felsic (granitic) vs. Mafic (basaltic) Quartz, feldspar, mica granite and sedimentary rocks (continental crust) Pyroxene, amphibole, olivine Basalt (oceanic crust)
Eruptive materials Lava Pyroclastics/tephra Gases (1-6%) Pahoe Hoe, Aa Blocks, Bombs, Lapilli/cinders, ash Gases (1-6%) 70% water 15% Carbon Dioxide 5 % nitrogen 5 % sulfur Chlorine, hydrogen, argon
Shield Volcano Shield Volcanco Low angle slopes of 1-10 Largest volcano Composed primarily of basalt lava flows from single or multiple vents Hot, low viscosity magma Gentle/quiet eruption Generally on ocean floor Hawaii, Iceland 10
Composite Cone/Stratovolcano Volcanoes on continents over ocean-land subduction zones Built up by alternating layers of lava and pyroclastics Violent/Explosive eruptions Steeper slopes 10-25 taller than cinder cones. Cascades (Mt. St. Helens), Andes Built over tens to hundreds of thousands of years Andesitic (or Granitic) composition magma 11
Cinder Cones Cinder Cones: formed by gas-rich lava of any composition (usually basaltic). Built of pyroclastics (tephra/cinders)with lots of gas holes Generally short lived eruptions - weeks to a few years until the magma is degassed, then it solidifies in the pipe and flows form from the base Smallest volcanic features have large craters with steep slopes of 30-40 Paricutin, Mexico, cinder cone soon after its birth in 1943 in a Mexican cornfield. 12
Lava Domes Very viscous magma Piles up in dome shape Entire structure builds up pressure and either cools as dome or explodes and creates a caldera when magma chamber emptied Yellowstone National Park
Magma Composition Compostion Silica Content VIscosity Gas Content Eruption Volcanic Cone Basaltic (Mafic) Less than 50% Lowest Least 1-2% Quiet Shield Basalt plateau Cinder cone Andesitic 60% Middle Middle 3-4% Explosive Composite/stratovolcano Granitic (felsic) (rhyolitic) 70% Highest Most 4-6% Volcanic dome