Today: Chapter 6 Volcanism Movie: Eruption of Mauna Loa and Kilauea on Hawaii Nature of volcanic eruptions 3) Materials extruded during eruptions 4) Types of volcanoes 5) Other volcanic landforms

2. The nature of volcanic eruptions - violent? - “gentle”? What determines if an eruption is 3 primary factors: magma’s composition magma’s temperature amount of gas in magma magma viscosity more silica more viscous (chains) hotter magma less viscous Its ability to move Can increase fluidity

2. The nature of volcanic eruptions Importance of gas in eruptions Gas can provide the force to violently hurl molten rock & ash from volcano

Fluid basaltic lavas generally produce quiet eruptions 2. The nature of volcanic eruptions 1. The nature of volcanic eruptions Fluid basaltic lavas generally produce quiet eruptions Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions

2. The nature of volcanic eruptions 80% of all volcanoes are found along convergent plate boundaries. Fig. 6.18

broken rock ash dust 3. Materials extruded during an eruption Volcanoes erupt: lava gas pyroclastic materials broken rock ash dust

Basaltic (low Si content) lava gas pyroclastic stuff 3. Materials extruded during an eruption 2. Materials extruded during an eruption Basaltic (low Si content) pahoehoe flow flows in thin, broad sheets, ~10-300 meters/hr, smooth twisted or ropey texture & wrinkled outer skin with molten flow beneath (Hawaiian type)

Basaltic (low Si content) lava gas pyroclastic stuff 3. Materials extruded during an eruption 2. Materials extruded during an eruption Basaltic (low Si content) aa flow thicker & slower flows ~5-50 meters/hr, rough & jagged surface

mostly water vapor, some CO2 lava gas pyroclastic stuff 3. Materials extruded during an eruption 2. Materials extruded during an eruption Magmas contain dissolved gases, held in by pressure Reduce pressure gases escape (decompression) Gaseous portion 1-6% by weight, mostly water vapor, some CO2

lava gas pyroclastic stuff 3. Materials extruded during an eruption 2. Materials extruded during an eruption Magmas contain dissolved gases, held in by pressure Phreatic explosion Hot, gas-charged magma encounters water and causes a superheated steam explosion

Types of pyroclastic debris Ash and dust - fine, glassy fragments lava gas pyroclastic stuff 3. Materials extruded during an eruption 2. Materials extruded during an eruption Gas can blow pulverized rock, glass, ash, magma blobs into the air from a volcano Pyroclastic materials– “Fire fragments” Types of pyroclastic debris Ash and dust - fine, glassy fragments Volcanic bombs - ejected as blobs of lava, solidify in air

A volcanic bomb 3. Materials extruded during an eruption lava gas pyroclastic stuff 3. Materials extruded during an eruption A volcanic bomb Bomb is approximately 10 cm long

Opening at the summit of a volcano 4. Types of Volcanoes Opening at the summit of a volcano Crater - Depression at the summit < 1 km Caldera - Summit depression > 1 km, produced by collapse following a massive eruption Vent – opening connected to the magma chamber via a pipe See Fig. 6.9

4. Types of Volcanoes vent/pipe caldera flank eruption Key terms: magma chamber vent/pipe caldera flank eruption

Types of volcanoes Shield volcano Cinder Cone Composite Cone (Stratovolcano)

4. Types of Volcanoes Broad, large area Large volumes of basaltic lava Mauna Loa on Hawaii

4. Types of Volcanoes Built from ejected lava Steep slope angle Rather small size Frequently occur in groups

Sunset Crater – a cinder cone near Flagstaff, Arizona 4. Types of Volcanoes Sunset Crater – a cinder cone near Flagstaff, Arizona

4. Types of Volcanoes Sunset crater, AZ

Composite Volcano 4. Types of Volcanoes Associated with convergent plate boundaries (e.g., Fujiyama, Mt. St. Helens) Classic-shaped volcano (1000’s of ft. high & several miles wide at base) Composed of interbedded lava flows and layers of pyroclastic debris

Mt. St. Helens – a typical composite volcano 4. Types of Volcanoes Mt. St. Helens – a typical composite volcano

Mt. St. Helens following the 1980 eruption 4. Types of Volcanoes Mt. St. Helens following the 1980 eruption

A size comparison of the three types of volcanoes

5. Other Volcanic Landforms a. caldera crater greater than 1 km wide from collapse of summit Also: Yellowstone! Today, Crater Lake occupies a caldera 10km wide Fig. 6.10

b. Basaltic fissure eruption 5. Other Volcanic Landforms b. Basaltic fissure eruption Fluid basaltic lava extruded from crustal fractures called fissures Ocean ridges; Columbia River Plateau Fig. 6.13

Columbia River Basalts 5. Other Volcanic Landforms Columbia River Basalts Flood basalt of the Columbia River Plateau formed during extensive volcanic activity 17 mio yr ago Fig. 6.2

5. Other Volcanic Landforms c. lava dome Bulbous mass of congealed lava usually form after explosive eruption of gas-rich magma