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?