Chapter 18 Notes
Volcanism
Volcanoes
• The location of volcanoes on Earth is not
random
Volcanoes at Convergent Boundaries
• Most volcanoes on land are at subduction
zones, characterized by explosive eruptions
– Circum-Pacific belt (Ring of Fire; ex: Pinatubo, St.
Helens)
– Mediterranean Belt (ex: Vesuvius, Etna)
Volcanoes at Divergent Boundaries
• About 2/3 of the Earth’s volcanic activity
occurs underwater at divergent boundaries
• Mid-ocean ridges
• Non-explosive, produces large amounts of lava
Hot Spot Volcanoes
• Some volcanoes form over stationary magma
plumes (hot spots), not at plate boundaries
• As a plate moves over the hot spot, a chain of
islands is formed
Flood Basalts
• Hot spots beneath
continental crust can
form flood basalts
Anatomy of a Volcano
• Magma travels
from the magma
chamber, through
a conduit and lava
emerges through a
vent
• Craters form
around the vent
Volcano Comparison
• Appearance of a
volcano depends on the
type of eruptions and
the type of material
forming the volcano
• Shield: non-explosive
• Cinder: small, steep
• Composite: explosive
Eruptions
• Characteristics of an eruption are determined
by:
– Temperature
– Pressure
– Magma composition (silica content)
– Dissolved gases (think about opening a can of pop
that has been shaken)
– Viscosity (determined by temperature and
composition)
Types of Magma: Basaltic
• Silica content: low
(less than 50%)
• Viscosity: low
• Eruptions: quiet,
very frequent
• Example: Kilauea,
Hawaii
Types of Magma: Andesitic
• Silica content: 50 to
60%
• Viscosity:
intermediate
• Eruptions:
intermediate
explosivity
• Examples: Tambora,
Indonesia; Colima,
Mexico
Types of Magma: Rhyolitic
• Silica content:
high (more
than 60%)
• Viscosity: high,
lots of
dissolved gas
• Eruptions:
explosive, very
infrequent
• Example:
Yellowstone
Explosive Eruptions
• Tephra: solid fragments of rock ejected from a
volcanic vent
– Ash (less than 2mm)
– Bombs
– Blocks
• Pyroclastic flows: clouds of tephra mixed with
hot gases
Pyroclastic flow
Intrusive Activity
• Plutons form from volcanic activity below the
Earth’s surface
– Batholiths: large, irregular shape
– Stocks: small irregular shape
– Laccoliths: round top, flat bottom
– Sills: parallel to existing rock
– Dikes: cut across existing rock
Plutons