Volcanoes
• Volcano
• Magma
• -weak spot in crust where magma comes up
• -molten mixture of rock-forming substance, gases, water from the mantle
• -called lava on surface• -lava forms solid rock
when cooled
• Volcanic belt
• Ring of Fire
• -forms along plate boundaries
• -magma can reach surface when plates diverge/converge and fracture crust
• -major volcanic belt• -rims (circles) Pacific
Ocean
• Island Arc
•
• -string of volcanic islands
• -formed from subduction at converging oceanic plates
• -ex. Japan, Caribbean Islands
• Hot Spots • -where magma melts thru crust
• -found in middle of plates and along boundaries
• -volcanoes can form above hot spot
• -ex. Hawaiian Islands
Properties of Magma
• Viscosity
• Viscosity of Magma
• -resistance of a liquid to flow
• -greater viscosity, slower it flows
• -depends on silica content and temperature
• Silica Content • -made of oxygen and silicon
• -one of the most abundant materials in crust
• -more silica = higher viscosity (sticky, flows slow)
• -less silica = low viscosity (flows quickly)
• Temperature • -viscosity increases as temp decreases
• -hotter magma flows faster
• -cooler flows slower
Volcanic Eruptions
• Magma reaches the surface
• -materials in asthenosphere under great pressure
• -liquid magma less dense than solid material around it
• -flows upward until opening in rock allows to magma to reach surface
• Inside a Volcano • Magma Chamber
• Pipe
• Vent
• -pocket that collects magma
• -tube that connects chamber to surface
• -opening thru which molten rock and gas leave
• -central vent at top• -sometimes, side vents
on sides
• Lava Flow
• Crater
• -area covered by lava as pours out of vent
• -bowl-shaped area that may form at top around central vent
• Eruption • -gases in magma under lots of pressure
• -gases expand and form bubbles, as magma rises to surface
• -when erupts, force of gas pushes magma from chamber thru pipe until it flows or explodes out of vent
• Quiet Eruptions • -low silica; low viscosity; flows easily
• -oozes out quietly and flows for many km
• -sets fire to and buries everything in path
• -produces both pahoehoe & aa
• -ex. Hawaiian islands
• Explosive Eruptions • -high in silica; high viscosity; thick/sticky
• -builds up in pipe; plugs it like a cork
• -trapped gases build up pressure until they explode
• -breaks lava into fast cooling pieces
• -volcanic ash: smallest; fine, rocky particles
• -cinders: pebble-sized• -bombs: large pieces;
baseball-sized to size of car
• -ex. Mount St. Helens
• Pyroclastic Flow
• Life Cycle
• -explosive eruption hurls out mixture of hot gases, ash, cinders, bombs
• -active: live; erupting or showing signs of erupting in near future
• -dormant: sleeping; expected to awaken in future and become active
• -extinct: dead; unlikely to erupt again
• Shield Volcanoes • -wide, gently sloping mountain made of layers of lava from quiet eruptions
• -ex. Hawaiian islands
• Cinder Cone Volcano • -steep, cone-shaped hill or small mountain made of volcanic ash, cinders, bombs piled up
• -ex. Paricutin
• Composite Volcanoes • -tall, cone-shaped mountain in which layers of lava alternate with layers of ash/other materials
• -ex. Mount St. Helens
• Calderas • -huge hole left by a collapsing volcano
• -enormous eruptions empty the vent and chamber, leaving it hollow with no support
• -sometimes fill with rain water
• -ex. Crater Lake in Oregon