Volcanoes. Volcano Magma -weak spot in crust where magma comes up -molten mixture of rock-forming substance, gases, water from the mantle -called lava.

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