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Chapter 5: Volcanoes
• How does a volcano erupt?
BrainPop: http://www.brainpop.com/science/earthsystem/volcanoes/
1: Volcanoes and Plate Tectonics
– Where Are Volcanoes Found on Earth’s Surface?
Volcano • A mountain that forms in
Earth’s crust when molten material, or magma, reaches the surface. – Magma: Molten mixture of
rock-forming substances, gases, and water from the mantle. (beneath the surface)
– Lava: Magma that reaches Earth’s surface. (on the surface)
Volcanoes and Plate Boundaries
• Volcanic belts form along the boundaries of Earth’s plates.
– Divergent boundaries
– Convergent boundaries
Divergent Boundaries
• Move apart
– Mid-ocean ridges
– Rift valleys
Converging Boundaries • Convergent (come together)
– Volcanoes can form where two oceanic plates collide:
• Older, denser plate sinks into the mantle creating a deep-ocean trench
• Due to the water entering the mantle, the mantle partially melts
• Magma rises up and breaks through the ocean floor creating a volcano
Ring of Fire
• Major belt of volcanoes
• Around the border of the Pacific Ocean
Island Arc
• String of islands (from the collision of two oceanic plates)
• Examples: Japan, New Zealand, Aleutians, and Caribbean islands
http://esminfo.prenhall.com/science/geoanimations/animations/35_VolcanicAct.html
Hot Spots
• Area where material from deep inside the Earth’s mantle rises through the crust and melts to form magma.
• Plates move over hot spots
• Example: Yellowstone National Park
– What Happens When a Volcano Erupts?
– What Are the Stages of Volcanic Activity?
INSIDE A VOLCANO Open to page 139 in your textbook.
Fill in Figure 1 as we go! • Magma chamber:
– The pocket beneath a volcano where magma collects.
Magma chamber
• Pipe:
– A long tube through which magma moves from the magma chamber to Earth’s surface.
Inside a Volcano
Pipe
• Vent:
– The opening through which molten rock and gas leave a volcano.
Inside a Volcano
Side vent
Central vent
Inside a Volcano
• Lava flow:
– The area covered by lava as it pours out of a volcano’s vent.
Lava flow
Inside a Volcano
• Crater:
– A bowl-shaped area that forms around a volcano’s central opening.
Crater
Inside a Volcano
Central vent
Crater Side vent
Lava flow Pipe
Magma chamber
– What Happens When a Volcano Erupts?
– What Are the Stages of Volcanic Activity?
A Volcanic Eruption
• When a volcano erupts, the force of the expanding gases pushes magma from the magma chamber through the pipe until it flows or explodes out of the vent. – Quiet
– Explosive
Quiet Eruptions • Magma is hot
• Low in silica (forms from elements oxygen and silicon)
• Thin magma
• Runny magma
• Example: Hawaiian Islands
Quiet Eruptions = Different Types of Lava
• Pahoehoe: from fast-moving, hot lava that is thin and runny; looks like ropelike coils
• Aa: from lava that is cooler and thicker (slower); rough surface consisting of jagged lava chunks
Explosive Eruptions • High in silica
• Thick
• Sticky
• Gases build up pressure (explosive)
• Example: Mount St. Helens
Chunks Thrown in the Air
• Smallest: ash (dust-sized)
• Medium: cinders (pebble-sized)
• Large: bombs (size of golf ball to size of car)
Volcano Hazards • Lava: set fire and bury
• Pyroclastic flow: mixture of hot gases, ash (small specks of dust), cinders, and bombs (large chunks) that flow down the sides of a volcano (occur during an explosive eruption)
• Landslide: mud, melted snow and rock
Stages of Volcanic Activity • Active: Live; is
erupting or has shown signs that it may erupt in the near future
• Dormant: Sleeping; expected to erupt in the future
• Extinct: Dead; Unlikely to ever erupt again
Warnings of a Possible Eruption
• Tiltmeter: detects slight surface changes in elevation and tilt caused by magma moving underground
• Monitor gases escaping from the volcano
• Monitor the many small earthquakes that occur around a volcano before an eruption
• Rising temperatures in underground water
1. Which has more silica – an explosive or a quiet volcano?
-Explosive
2. What are golf-ball to car-sized particles
shooting out of a volcano called?
-Bombs
3. A volcano that may erupt sometime in the future is known as _______.
-Dormant
3: Volcanic Landforms
– What Landforms Do Lava and Ash Create?
– What Landforms Does Magma Create?
What Landforms Do Lava and Ash Create?
• Calderas
• Shield Volcanoes
• Cinder Cone Volcanoes
• Composite Volcanoes
• Lava Plateaus
Caldera
• The hole left when a volcano collapses (a lake can form, filling the hole)
http://dli.taftcollege.edu/streams/Geography/Animations/Caldera.html
Cinder Cone Volcanoes
• Steep, cone-shaped hill or small mountain
• High silica (thick, sticky)
• Explosive
• Example: Paricutin (Mexico, 1943)
Composite Volcanoes
• Tall, cone-shaped mountains in which layers of lava alternate with layers of ash
• Silica content varies (lava flows and explosive eruptions)
• Example: Mt. Fuji (Japan) and Mount St. Helens (Washington)
Shield Volcanoes
• Gentle sloping mountain from lava flow
• Hot spot volcanoes are usually shield volcanoes
• Example: Mauna Loa (Hawaii)
Lava Plateaus • Lava flows out of long cracks
• Over time runny lava builds up a high, level area (called a plateau)
• Example: Columbia Plateau (Washington, Oregon, and Idaho)
Landforms Composite (Stratovolcano) Cinder Cone
Shield Lava Plateau
What Landforms Does Magma Create?
• Volcanic necks
• Dikes
• Sills
• Dome mountains
• Batholiths
Volcanic Neck
• Formed when magma hardened in an ancient volcano’s pipe.
• Later, soft rock around the pipe wore away, exposing the harder rock inside
Dikes and Sills
• Dike: Magma that forces itself vertically across rock layers and hardens
• Sill: Magma that squeezes between horizontal rock layers hardens to form a sill.
Dome Mountains • Forms when uplift pushes a large body of
hardened magma toward the surface
• Example: Black Hills (South Dakota)
Batholiths
• A mass of rock formed when a large body of magma cools inside the crust.
Half-Dome in Yosemite Natl. Park
The Big Question
• How does a volcano erupt?