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VOLCANOES
21.2
What is a volcano?
• Volcano: opening in the Earth’s crust where
magma erupts onto the surface
• Magma: molten rock underground
• Lava: magma that has reached Earth’s
surface
How Does A Volcano Form?

Under certain conditions, small amounts of
mantle rock can melt, forming liquid
magma. The magma rises upward through
the crust, erupting at the surface as a
volcano.
Structure of a Volcano
3 Factors That Determine the
Viscosity of Magma:
1. Temperature – the higher the
temperature, the lower the viscosity
 2. Water Content – the more water, the
lower the viscosity
 3. Silica Content – the higher the silica
content, the higher the viscosity. Silica acts
like glue.

TYPES OF LAVA


Felsic
- high silica content
- light colored
- slow moving
- explosive eruptions
Mafic
- low silica content
- dark colored
- fast flowing lava
- quiet eruptions
LAVA FRAGMENTS

Tephra: solid fragments of lava
- ash: less than 2mm diameter
- lapilli: 2 – 64mm diameter
- bombs: 64mm or higher diameter
Ash
Lapilli
Bombs
OTHER LAVA FRAGMENTS

aa – broken, sharp
blocks of lava

Pahoehoe – smooth,
wrinkled lava
VOLCANO ZONES
Pacific Ring of Fire
- zone around the Pacific Ocean
- most active area of volcanoes in World
 Mid-Ocean Ridges
- zone along divergent plates
 Eurasian Belt
- zone along south border of Eurasian plate

Hot Spots
A hot spot is a region where hot rock extends
deep within the mantle to the surface.
 Example: Hawaii

3 TYPES OF VOLCANOES
1. CINDER CONE
Steep sides
 Smallest, most abundant
 Active for a short time, then become
dormant

2. COMPOSITE
Consists of alternating layers of ash,
cinders, and lava
 Steep sides
 Thick magma
 Lot of gases

3. SHIELD
Consists of layers of mafic lava
 Gently sloping sides, wide base

FAMOUS ERUPTIONS
Vesuvius, Italy – 79 A.D.
 Krakatau, Indonesia – 1883
 Mount St.Helens, U.S. – 1980
 Mt. Pinatubo, Phillipines – 1992
 Eyjafjallajökull volcano in Iceland – April,
2010

Mt. Vesuvius
Mt. Krakatau
1883
Mt. St.Helens
1980
BEFORE
AFTER
Mt.Pinatubo
1992
Mt.Kilaeau
Present
VOLCANOES IN OUTER
SPACE
The Moon – no volcanoes
 Mars
- Olympus Mons
- 3 times higher than Mt.Everest
 Io
- Moon of Jupiter
- volcanic activity
