chapter 18 notes volcanism. volcanoes the location of volcanoes on earth is not random
TRANSCRIPT
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
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
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