Volcano Notes
volcano
vent
fissure eruption
hot spot
7.1 Volcanoes and Plate Boundaries
1. How do volcanoes form?
• When rocks become hot enough and
conditions are right, they can melt.
• Magma is less dense than surrounding rock.
• Magma rises towards Earth’s surface
because of the buoyant force pushing up.
How a Volcano Works
7.1 Volcanoes and Plate Boundaries
2. Convergent Plate Boundaries
• Volcanoes tend to erupt more violently.
• Volcanic arcs—form when an oceanic plate
subducts beneath a continental plate
• Island arcs—form when two oceanic plates
move toward each other
7.1 Volcanoes and Plate Boundaries
3. Divergent Plate Boundaries
• Eruptions can occur in either the ocean,
as a mid-ocean ridge, or on a continent,
as a continental rift.
• Magma reaches Earth’s surface as
fissure eruptions.
7.1 Volcanoes and Plate Boundaries
4. Volcanoes Away from Plate
Boundaries
• Not all volcanoes occur along plate
boundaries.
• Hot spots are localized areas of high heat
in Earth’s interior.
• Hawaiian Islands
7.2 Volcanic Eruptions and Features
5. Types of Volcanoes
• Shield Volcanoes
– Huge, gently sloping volcanic landform
– Mainly composed
of basaltic lava
– Hawaiian Islands
are an example
7.2 Volcanic Eruptions and Features
Types of Volcanoes (cont.)
• Cinder Cone Volcanoes
– Cone shaped
– Mainly composed
of solid fragments
called tephra
– Typically
produce
explosive
eruptions
7.2 Volcanic Eruptions and Features
Types of Volcanoes (cont.)
• Composite Volcanoes
– Forms tall, majestic mountains
– Mainly composed
of alternating layers
of lava and tephra
– Typically produce
alternating quiet
and explosive
eruptions
7.2 Volcanic Eruptions and Features
6. Volcanoes
in California
• A variety of active
volcanoes occur
along the western
coast of the
United States.
• Form at convergent
plate boundary as
part of a volcanic arc
7.3 Hazards of Volcanic Eruptions
7. Volcanic Ash
• Volcanic ash is a very fine grained
tephra.
• Composed of tiny, sharp mineral and
glasslike particles
• Affects humans and wildlife habitats
by burying plants and animals and their
food sources and contaminating the
water supply.
7.3 Hazards of Volcanic Eruptions
8. Lahars
• A lahar is a rapidly flowing mixture of
volcanic debris and water.
• Some large lahars begin as landslides.
• A fast-moving lahar provides little time for
warning.
7.3 Hazards of Volcanic Eruptions
9. Gas Emissions
• Gas released from volcanic eruptions can
be a silent hazard to humans and wildlife.
– Sulfur dioxide and hydrogen sulfide
gases, when mixed with water, can form
sulfuric acid precipitation.
7.3 Hazards of Volcanic Eruptions
10. Pyroclastic Flows
• A fast-moving body of hot gases and solids
• Can burn and destroy all forms of life
in its path
• Typically produced during explosive
eruptions
• Tend to follow valleys
7.3 Hazards of Volcanic Eruptions
11. Lava Flows
• Occur from either an explosive or quiet
volcanic eruption
• Destroy human and wildlife habitats by
starting fires and releasing smoke
• Move slowly enough that humans can be
warned of possible dangers