Volcanism

Volcanism Eruptive Style Volcanic Materials Volcanoes Other Volcanic Landforms Plate Tectonics and Igneous Activity

Eruptive Style Explosive Effusive

Eruptive Style Why do volcanoes have different eruptive styles? Pressure vs. resistance

Eruptive Style A volcano is like a giant pop can that’s been shaken up Pressure builds up When pressure is released, material is pushed out

Eruptive Style Pressure comes from gases dissolved in magma within the volcano Mostly water vapor and carbon dioxide When magma rises toward vent, gases come out of solution and expand

Eruptive Style Resistance to this pressure comes from magma’s viscosity Viscosity = resistance to flow (“thickness” or “stickiness”) Higher viscosity = thicker, more resistant

Eruptive Style Explosive eruptions Effusive eruptions High-viscosity magma resists gas pressure Effusive eruptions Low-viscosity magma offers little resistance

Eruptive Style Why are some magmas more viscous than others? Temperature Silica content Silica = silicon and oxygen dissolved in magma

Eruptive Style Why are some magmas more viscous than others? Temperature Hotter = less viscous (runnier)

Eruptive Style Why are some magmas more viscous than others? Silica content More silica = more viscous (thicker)

Eruptive Style This volcano is fed by high-silica, low-temp magmas with high viscosity This one’s magma is low-silica and high-temp, and low viscosity

Eruptive Materials

Eruptive Materials Lava Aa lava flow Pahoehoe lava flow

Eruptive Materials Gases Water vapor Carbon dioxide Smaller amounts of other gases

Eruptive Materials Pyroclastics Ash and dust – fine, glassy fragments Lapilli – walnut-sized material Cinders – pea-sized material Blocks – hardened or cooled lava Bombs – ejected as hot lava

Eruptive Materials Pyroclastic flow Hot, fast-moving cloud of pyroclastic material

Eruptive Materials Lahar Volcanic mudflow Mixture of water, soil, and ash Triggered by melting of snow during eruption

Eruptive Materials Three Japanese lahars

Volcanoes

Volcanoes Shield volcanoes Largest type Dome-shaped Effusive eruptions Much lava, few pyroclastics

Volcanoes Mauna Kea, a Hawaiian shield volcano

Volcanoes Composite cones Smaller than shield volcanoes Classic “volcano shape” Explosive eruptions Much pyroclastic material, little lava

Volcanoes Mt. Fuji, a composite cone in Japan

A Composite Volcano Figure 4.11 Interbedded pyroclastic deposits and small lava flows Figure 4.11

Volcanoes Cinder cones Smallest type Loose pyroclastic materials

A Size Comparison of the Three Types of Volcanoes Figure 4.14

Other Volcanic Landforms

Other Volcanic Landforms Calderas Pits caused by magma chamber collapse Three types Hawaiian-type Crater Lake-type Yellowstone-type

Other Volcanic Landforms Hawaiian-type calderas On shield volcanoes Olympus Mons Kilauea

Other Volcanic Landforms Crater Lake-type calderas Catastrophic eruptions

Other Volcanic Landforms Yellowstone-type calderas Largest Caldera outline (diameter approx. 30 mi)

Other Volcanic Landforms Basalt plateaus Very large, flat areas covered with basalt Outpourings of low-viscosity lavas from fissure eruptions

Other Volcanic Landforms Fissure eruption in Hawaii Basalt plateau in Washington

Plate Tectonics & Igneous Activity

Plate Tectonics & Igneous Activity Most volcanism occurs along tectonic plate boundaries Divergent boundaries: decompression melting Subduction zones: hydration melting

Plate Tectonics & Igneous Activity The “Ring of Fire” is a chain of active volcanoes around the Pacific Ocean. It marks the boundaries of tectonic plates.

Plate Tectonics & Igneous Activity Hot spot (intraplate) volcanism Not near plate boundaries Fed by magma reservoirs beneath the crust

Plate Tectonics & Igneous Activity Hawaiian Islands were formed by hot spot volcanism North Oahu 3.7 my Molokai 1.9 my Kauai 5.1 my Maui 1.3 my Hawaii (Big Island) < 1 million yrs old

Plate Tectonics & Igneous Activity Hot spot animation

End of Chapter 6