1. Chapter 6: Volcanoes

2. Introduction to Volcanoes
Bill Nye Episode 74: Volcanoes
10 Most Active Volcanoes
What is a Volcano?

3. Where Are Volcanoes Found on Earth’s Surface?
Have you ever witnessed a volcanic eruption?
Molten material rises high into the atmosphere.
The ground is covered in volcanic ash.
Magma is the molten mixture of rock-forming substances, water and gas from the mantle.
Lava is magma that reaches the surface.

4. Volcanoes and Plate Boundaries
Volcanoes form a regular pattern on Earth.
Volcanic belts form along the boundaries of Earth’s plates.
They occur where the Earth DIVERGES.
They occur where the Earth CONVERGES
The Ring of Fire is a major belt of volcanoes in the Pacific Ocean.
North America
South America
Philippines
Japan

5. Diverging Boundaries Volcanoes form along mid-ocean ridges.
Underwater
Lava pours out of the rift valley
Builds new mountains
Can be found on land.
Great Rift Valley

6. Converging Boundaries
One plate subducts under the other
The more dense plate sinks into the mantle and creates a deep-ocean trench
Water in the sinking plate leaves the crust, causing the rock to melt
Can form chains of islands called island arcs
Japan
Caribbean
Aleutians
New Zeland
Also occur where ocean plate is below a continental plate
Andes Mountains
Plate collisions produce volcanoes
Mount St. Helens
Mount Rainier

7. Hot Spots Some volcanoes are results of “hot spots” in the mantle.
This is a place deep within the Earth where the mantle rises through the crust and melts to form magma.
A volcano is formed from this when the magma erupts through the crust and reaches the surface.
Hot spots stay in the same place for millions of years while plates move over them.

8. 6.2 Volcanic Eruptions
What are pipes and vents used for in your home, school and other buildings?

9. What Happens When a Volcano Erupts?
Think about magma.
Magma is less dense than the material around it, so when it reaches the surface a volcano can form.

10. Inside a volcano
Magma Chamber: a pocket where magma collects beneath the surface
Pipe: tube that extends from Earth’s crust up through the top of a volcano
Vent: an opening where rock and gas leave a volcano
Central and side
Lava Flow: the spread of lava as it pours out a vent
Crater: a bowl-shaped area that may form at the top of a volcano

11. A Volcanic Eruption
During an eruption magma rises to the surface, pressure on mamga decreases.
As pressure falls the gas bubbles are released from the magma
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.
The type of explosion depends on the silica content and whether the magma is thin and runny or thick and sticky.

12. Magma is hot and low in silica. Gases in magma bubble out gently
Quiet
Magma is hot and low in silica.
Gases in magma bubble out gently
Lava oozes quietly from the vent and can flow for many kilometers.
Forms rock:
Pahoehoe
Fast moving and thinner
Solid mass of rope-like coils Aa
Cooler and thicker
Rough surface with jagged chunks
Formed Hawaiian Islands
degrees Celsius
Explosive
Magma is high in silica.
Trapped gases build up pressure until they explode.
Erupting gases and steam push the magma out with incredible force.
Like plugging the cork on a bottle
Forms volcanic ash, cinders and bombs

13. Volcano Hazards
Both types of eruptions can cause damage far from a crater’s rim.
During an explosive eruption a volcano can put out gases, hot rock and ash that flow down the side of a volcano. This is called a PYROCLASTIC FLOW.
Landslides can also form.
Tsunamis are also a hazard that can be caused if a volcano that is an island arc collapses.

14. Stages of Volcanic Activity
Activity may last 10 to 10 million years!
Geologists often use the terms active, dormant, or extinct to describe a volcano’s stage of activity.
Dormant: sleeping, may wake in the future to become active
Extinct: dead, unlikely to erupt ever again
Hot Spot Volcanoes may become extinct after they drift away from the hot spot!
We are sometimes able to predict when an explosion will occur
Changes in land around a volcano gives us these clues.
Tiltmeters detect changes in surface elevation
Small earthquakes can be monitored
Gases escaping from a volcano can be monitored
Rising temperatures in underground water may signal magma is near the surface

15. 6.3 Volcanic Landforms
Do all volcanoes look the same?

16. What Landforms Do Lava and Ash Create?
Volcanic eruptions create landforms made of lava, ash and other materials.
Shield volcanoes
Cinder cone volcanoes
Composite volcanoes
Lava plateaus
Caldera
Huge holes left by the collapse of a volcano

17. Cinder Cone Volcanoes Forms from high silica volcanoes
Ash cinders and bombs build up around the vent in a steep, cone-shaped hill or small mountain
Paricutin in Mexico

18. Composite Volcanoes Varying amounts of silica content in eruptions
Tall cone-shaped mountain where layers of lava alternate with layers of ash
Mount Fuji in Japan
Mount St. Helens in Washington
More than 4800 meters high!

19. Shield Volcanoes Wide and gently sloping
Formed from thin layers of lava hardening on previous layers
Typically formed from hot spot volcanoes on the ocean floor.
Hawaii
Mauna Loa is over meters above the ocean floor!

20. Lava Plateaus Formed when lava flows along cracks in an area.
Moves a larger area and then cools and hardens
Repeated floods of lava over millions of years
The Columbia Plateau in Washington State, Oregon and Idaho

21. What Landform does Magma Create?
Volcanic Necks
Forms when magma hardens in an old volcano’s pipe and the rock around the pipe wore away
Dikes and Sills
Dikes form when magma forces itself across rock layers
Sills form when magma squeezes between horizontal rock
Dome Mountains
Formed when uplift pushes layers of rock to move upward into a dome
Black Hills Mountains in South Dakota
Batholiths
A mass of rock created when magma cools inside the crust
Form the core of many mountain ranges
Water and ice allow the batholith to look like a mountain