Volcanism. Mauna Loa, Hawaii Mt Pelee, Martinque Mt St Helens, WA 1981.
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Transcript of Volcanism. Mauna Loa, Hawaii Mt Pelee, Martinque Mt St Helens, WA 1981.
VolcanismVolcanism
Mauna Loa, Hawaii
Mt Pelee, Martinque
Mt St Helens, WA 1981
Mt Vesuvius, Italy Mt Etna, Italy
Thera, (Santorini) Greece
Mt Fuji, Japan
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
• Along plate boundaries & intra-plate areas– Ring of Fire
– Mid-Ocean Ridge
– Hot Spots
– Volcanic Arcs
Divergent Plate Divergent Plate Boundary VolcanismBoundary Volcanism
Convergent Subduction Plate Convergent Subduction Plate Boundary VolcanismBoundary Volcanism
Intra-Plate Intra-Plate Volcanism (hotspots)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. Nature of Eruption: quiet vs. explosiveexplosive
• 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 materialsEruptive materials• Lava
– Pahoe Hoe, Aa
• Pyroclastics/tephra– 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 1-10Largest volcano
Composed primarily of basalt lava flows from single or multiple vents
Hot, low viscosity magmaGentle/quiet eruption
Generally on ocean floor
Hawaii, Iceland
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
Composite Cone/Stratovolcano
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
Cinder Cones
Paricutin, Mexico, cinder cone soon after its birth in 1943 in a Mexican cornfield.
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% Explosive Volcanic dome