1. Volcanoes
11.1
What Causes Volcanoes?

2. Volcanoes and You Volcano – opening in Earth’s surface
Often forms a mountain from layers of lava and volcanic ash
Most are dormant, not currently active
More than 600 currently active
Mount St. Helens
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erupts
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3. Eruptions of the Century
Active Volcanoes
Kilaua volcano in Hawaii
Iceland known for active volcanoes
52n
Eruptions of the Century
Mount Katmai, June 6, 1912 on the Alaska Peninsula
30 times greater volume than Mount St. Helens
Mount Pinatubo, June 1991, Philippines
Killed nearly 900 people
Millions of tons of sulfur dioxide and ash into Earth’s upper atmosphere
Mount Unzen, June 1991, Japan
44 people died

4. How do Volcanoes Form?
Heat and pressure cause rock to melt and form magma.
Magma Forced Upward
Magma is less dense than rock, so it is pressed upward.
Vent – opening through which magma flows out
As lava flows out, it cools quickly and becomes sold, forming layers of igneous rock
Crater – steep-walled depression around a volcano’s vent

5. Where do Volcanoes Occur?
Volcanoes are directly related to Plate movement and hot spots.
Plates Moving Apart
Iceland has volcanic activity because it sits on top of the Mid-Atlantic Ridge.
Mid-Atlantic Ridge – plates moving apart forming rifts
Magma flows from rifts and is cooled by seawater
As more lava flows, it builds up
It can form islands such as Iceland

6. Plates Moving Together
Soufriere Hills volcano on the island of Montserrat
Formed because North American and South American Plates are being forced under the less dense Caribbean plate
Magma forms because the forced under plates get hot enough to partially melt
Magma is forced up forming the volcanoes of the Lesser Antilles
Hot Spots
Hawaii, not at a plate boundary
Some areas of mantle are hotter than others
Hypothesis is that hot spot magma forms at the boundary between the mantle and the outer core
Magma rises carrying heat from Earth’s core

7. Hawaii formed when volcanoes rise above the water.
Islands in a straight line because plate is moving over the stationary hot spot.
Kauai is the oldest island.
As plate move, Kauai moved away from hot spot and became dormant.
Islands formed over about 5 million years.

8. Chapter 11
Types of Volcanoes
11.2

9. Styles of Eruption Trapped Gases Magma Composition
Water vapor and carbon dioxide are trapped in magma by the pressure of surrounding magma and rock
As magma nears surface, there is less pressure.
Gas escapes easily during quiet eruptions
Gas that builds up to high pressures eventually causes explosive eruptions
Magma Composition
Low in silica means fluid lava, and quiet eruptions
Kilauea
Lava pours from vents, down the sides of a volcano
Trapped gasses can escape easily

10. Forms of Volcanoes Magma Water Content
Silica-rich magma produces explosive eruptions
Sometimes forms where plates are moving together
Silica-rich magma is thick, and gases get trapped
When an explosive eruption occurs, gases expand rapidly, often carrying pieces of lava in the explosion
Magma Water Content
High water content causes explosive eruptions
Forms of Volcanoes
Depends on type of explosion and type of lava

11. Shield Volcano Quiet eruptions Silica-poor lava Flat layers
Shield Volcano – broad volcano with gently sloping sides
Hawaiian Islands

12. Cinder Cone Volcano Explosive eruptions
Tephra – rock or solidified lava dropped from the air
Varies in size
Cinder Cone Volcano – steep- sided volcano formed by loosely packed tephra

13. Composite Volcano Varies in explosions
Composite Volcano (Stratovolcano)– volcano formed from cycle of lava and tephra
Mostly found where plates come together
Mount St. Helens

14. Volcanoes
Igneous Rock Features
11.3

15. Intrusive Features
Igneous rock – rock formed from fast cooling lava at or near Earth’s surface (extrusive igneous rock) and from slow-cooling magma deep inside Earth (intrusive igneous rock)
Batholiths
Batholiths – largest intrusive igneous rock bodies
Hundreds of kilometers wide and long, and several kilometers thick
Can be exposed by erosion
Granite domes of Yosemite National Park are the remains of a huge batholith that stretches across much of the length of California

16. Other Features Dikes and Sills
Magma can squeeze into cracks in a rock below the surface
Dike – magma squeezed into a vertical crack
Sill – magma squeezed into a horizontal crack
Run a few meters to hundreds of meters long
Other Features
Volcanic Neck – solid igneous core left after cone erodes away
Caldera – large opening made when the top of a volcano collapses after an eruption
Igneous Features Exposed
Process of erosion wear down rock at the surface exposing features like batholiths, dikes, and sills.