Volcanoes

Chapter 7

Volcanoes and Plate Tectonics• Volcano – weak spot in the crust where a

mountain forms when layers of lava and volcanic ash erupt and build up and push through

• Magma - less dense than the rock around it so it is very slowly forced upward toward Earth’s surface made up of gases, water molten rock forming stuff

• Lava – magma that reaches the earths surface

Where Volcanoes Occur?• Volcanic Belts form along boundaries of Earth’s

crust, one major is the Pacific Ring of Fire

• 1. Divergent Boundary – plates moving apart – seafloor spreading

• Example: N. Atlantic Ocean. The North American plate is moving away from Eurasia and African plates.

• Iceland formed by the separation of these plates at the rift

• 2. Convergent Boundary – 2 plates collide, rumple up and cause mountains instead of volcano

• Juan de Fuca Plate is converging with the N. American plate

• Convergent Plates (cont)• Magma that is forced upward forms volcanoes of the

Cascade Mountains• Pacific Ring of Fire – area around the Pacific Plate

where there are earthquakes and volcano’s

• Hot Spots – middle of a plate, area’s in Earth’s mantle that are hotter than neighboring areas

• Magma forms and rises towards the crust

• Example: Hawaiian islands, plates move and form islands when it gets over the hot spot

Types of Eruptions – depend on the pressure build up and material

• 2 factors:• 1. Water Vapor and other gases trapped within the

magma, • if they escape easily – quiet eruptions• Gas trapped under high pressure eventually escapes –

explosive eruptions• 2. Magma is either basaltic or granitic

• Basaltic magma – contains less silica, is very fluid and produces quiet, non-explosive eruptions magma runs down the side like in Hawaii

• Granitic magma – thick and contains lots of silica and has a high water content, it gets trapped in vents, causing pressure to build up so when it erupts the gases expand rapidly carrying pieces of lava

3 Forms of Volcanoes• 1. Shield Volcano – Quiet eruptions that spread

out basaltic lava in broad, flat layers Hawaiian Islands (pg 399)

• 2. Cinder Cone Volcano – Explosive eruptions that throw lava into the air forms steep sided consolidated volcano ( pg 400) Paricutin• Tephra – various sizes of this lava after it has

cooled • Ash – fine• Bombs – large heavy rocks

• 3. Composite Volcano – formed at convergent plate boundaries above subduction zone, this type of volcano alternates between quiet and explosive tephra layer alternating with a lava layer Mt. St. Helens (pg 400)

• Subduction Zone – area where an ocean plate descends into the upper mantle

Volcano Features• Magma that cools underground forms igneus rock (most

magma does not reach the surface)• Balholith – largest igneous rock bodies, hundreds of km

wide and thick Yosemete has exposed one• Dike – magma squeezed into a vertical crack and

hardened• Sill – magma squeezed into a horizontal crack• Laccolith – dome of rock• Volcanic Neck – cone of the volcano erodes and leaves

the igneous rock Ship Rock New Mexico• Caldera – formed after eruption when the top collapses

down into magma chamber Crater Lake

Vent – opening in the surface which magma reaches the earths surface through (1000 yrs)Crater – opening at the top of a volcano’s vent

Geothermal Energy from Volcanoes• Geothermal energy – utilizing the heat from

magma and converting it to electricity by heating water which produces steam that turns the turbines that run the generators to make electricity (fig 15-5)

• Geothermal energy used in 20 countries and US• Hawaii wants to heat by geothermal energy of

Kilauea• Many people are against this because some of the

rain forest must be cleared for roads, drilling etc, they say it will cut down cultural trees