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