Chapter 12 Volcanoes
Section 1 Outline
Volcanoes and Moving Plates
I.
What Are Volcanoes? A volcano is an opening in Earth’s crust that erupts
gases, ash, and lava. A volcanic mountain forms when layers of lava, ash, and
other material build up around these openings.
A. Most Active Volcanoes: Earth has more than 600 active volcanoes.
1. Kilauea, located in Hawaii, is the world’s most active volcanoslow moving lava, began erupting continuously since 1983.
2. Iceland is famous for its active volcanoes. Sits on divergent
platesThis island country is known as the land of fire and ice.
3. Soufriere Hills in Montserrat: This eruption was one of the largest
recent volcanic eruptions near North America. It erupted in 1995
4.
II. Effects of Eruptions:
A. Lava flows destroy everything in its path, falling ash can collapse buildings,
block roads, and can cause lung disease in people and animals.
B. Pyroclastic flows are fast-moving currents of hot gas, and rock, which can
travel away from a volcano at speeds of up to 700 km/h. These flows will
burn anything in their path.
C. Human and Environmental Impacts
a. Soufriere Eruption-largest eruptioni. Acid rain forms when sulfurous gases from volcanoes mix with
water vapor in the atmosphere. Acid rain can kill vegetation and
get into lakes and streams, poisoning fish and other wildlife.
III. How Do Volcanoes Form? Heat and pressure deep within Earth melt rock.
When melted rock breaks through Earth’s surface, a volcano forms.
A. Why Magma Rises Toward Earth’s Surface
1. Magma is less dense than the solid rock surrounding it, so it is forced
toward Earth’s surface.
2. When magma reaches Earth’s surface and flows out through an opening,
the opening is called a vent.
3. As lava flows out of the vent, it cools forming layers of igneous rock.
The steep walled depression around a volcano’s vent is called a crater.
a. Magma that reaches Earth’s surface is called lava.
b. Before magma reaches Earth’s surface, it collects underground in an area
called a magma chamber.
IV. Where do volcanoes occur?
A. Divergent Plate Boundaries. When plates separate, they form, long, deep
cracks called rifts. Lava flows from these rifts and builds up on the ocean
floor. If the rift eruptions rise above sea level, islands form.
1. The Mid-Atlantic Ridge is a divergent boundary.
2. Iceland has volcanic activity because it is on the Mid-Atlantic
Ridge.
3. Surtsey is a new volcanic island near Iceland.
B. Convergent Plate Boundaries: Volcanoes form at convergent boundaries where
plates are moving together. These form where one plate slides under another plate,
which is called a subduction zone.
1. The Andes Mountains in South America formed where an oceanic plate
is subducting under a continental plate.
2. Soufriere Hills in Montserrat is on a subduction zone.
C. Hot Spots: Hot spots are areas deep underneath Earth’s crust that are unusually
hot. Hot spots do not form at plate boundaries, but instead form in areas that have
localized melting in between the mantle and core. The hot spots remain stationary,
while the plates move over the hot spots.
1. The Hawaiian Islands formed as the Pacific Plate moved over a hot spot.
2. The Hawaiian Islands have formed in a line as the Pacific Plate continues to
move. As the plate moves, the older islands become dormant. Hawaii, the largest
of the Hawaiian Islands, continues to have active volcanoes because it is still
sitting over the hot spot.