Volcanoes

Chapter 18

Magma• minerals, dissolved Mixture of molten

rock, suspended gases BELOW Earth’s surface• Rocks melt in asthenosphere (really hot)• Why isn’t the whole rest of the mantle

liquid too?• Pressure- pressure increases with depth,

increased pressure raises the melting point of rock

• Water- tiny water droplets trapped in rock raise melting point

Types of Magma

• Basaltic- like basalt; dark in color; forms when rocks in asthenosphere melt; low viscosity → flows quickly & rises to surface quickly. Small amounts of silica; Quiet eruptions; Hawaii

Types of Magma Con’t

• Andesitic- found on continental margins (subduction)→ made from oceanic crust that travels through continental crust; 60% silica; high silica → intermediate eruptions; Mount St. Helens

Types of magma

• Rhyolitic- forms when molten material rises through thick continental crust and mixes with silica and water; high viscosity → violent eruptions; Yellowstone

• Viscosity- hotter temperature → lower viscosity• Higher silica → Higher viscosity

Volcano Anatomy

• Vent- hole where lava comes out• Crater- cone-like depression at top of

volcano• Magma Chamber- place under volcano

where magma is stored• Neck- Tube where lava travels from

chamber to surface• Caldera- large depression where top of

volcano collapsed- Crater Lake, OR

Types of Volcanoes

• Shield- broad, gently sloping sides and circular base; form from layers of basaltic lava flows; non-explosive (quiet) eruptions- Hawaii

Types of Volcanoes

• Cinder Cone- Ash falls back and piles up near vent; steep sides; small in size; high water and silica content; lots of gases → more explosive

Types of Volcanoes

• Composite Cone- layers of ash alternate with lava; large amounts of silica, water and gases; much larger than cinder cones; violently explosive; Mount St. Helens

Volcanic Material

• Tephra- Rock fragments thrown into air by eruption- Classified by size (ash, blocks, bombs)

• Pyroclastic Flows- Clouds of gas, ash, tephra traveling at fast speeds (90mph) and at high temps (370 degrees)

Volcanoes on the web

• http://www.avo.alaska.edu/• http://vulcan.wr.usgs.gov/• http://hvo.wr.usgs.gov/• http://lvo.wr.usgs.gov/• http://volcanoes.usgs.gov/yvo/

Intrusive Activity

• Plutons- very large collections of cooled magma• Intrusive Igneous Rock Formations- (“Inside

lava rock”)• Cut across older rocks

• Batholiths- largest pluton, millions of years to form, common in interiors of mountain chains; in N America usually granite; up to 680 miles long

Intrusive Activity

• Stocks- smaller irregularly shaped pluton

• Laccolith- magma forced up into parallel rock layers close to surface → push some rocks upward → mushroom shaped pluton; small in size (7 miles); Black Hills, SD

Intrusive Activity

• Sills- parallel to rock layers → like window sill

• Dike- pluton that cuts across the grain• Volcanic Neck- Volcano eroded away

leaving the hardened neck exposed (Devils Tower, WY)

• Plutons are important in mountain building• CA, West Coast, Adirondacks• Yosemite National Park- exposed granite

batholith

Hot Spots

• Unusually hot regions of Earth’s crust where plumes of magma rise up• Plumes move vertically, not horizontal• Hawaii- Kilauea is over hot spot now,

but Loihi is forming off Hawaii as Pacific Plate moves NW