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