Today: Chapter 6

Volcanism

1) Movie: Eruption of Mauna Loa and Kilauea on Hawaii 2) Nature of volcanic eruptions 3) Materials extruded during eruptions 4) Types of volcanoes 5) Other volcanic landforms

What determines if an eruption is

- violent?

- “gentle”?

3 primary factors:

 magma’s composition

more silica more viscous (chains)

 magma’s temperature

hotter magma less viscous

 amount of gas

Can increase fluidity

in magma magma

viscosity

Its ability to move

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

•

Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions

80% of all volcanoes are found along convergent plate boundaries.

Fig. 6.18

Volcanoes erupt:

 lava  gas  pyroclastic materials

broken rock ash dust

   lava gas pyroclastic stuff 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)

   lava gas pyroclastic stuff Basaltic (low Si content) aa flow thicker & slower flows ~5-50 meters/hr,

rough & jagged surface

   lava gas pyroclastic stuff

Magmas contain dissolved gases, held in by pressure

Reduce pressure Gaseous portion gases escape (decompression) 1-6% by weight,

mostly water vapor

, some CO 2

   lava gas pyroclastic stuff

Magmas contain dissolved gases, held in by pressure

Phreatic explosion Hot, gas-charged magma encounters water and causes a superheated steam explosion

   lava gas pyroclastic stuff

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

3. Materials extruded during an eruption

   lava gas pyroclastic stuff

A volcanic bomb

Bomb is approximately 10 cm long

4. Types of Volcanoes Opening at the summit of a volcano Crater 1 km - Depression at the summit < 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

Key terms:

magma chamber vent/pipe caldera flank eruption

4. Types of Volcanoes

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

4. Types of Volcanoes

Sunset Crater – a cinder cone near Flagstaff, Arizona

Sunset crater, AZ

4. Types of Volcanoes

4. Types of Volcanoes Composite Volcano 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

4. Types of Volcanoes

Mt. St. Helens – a typical composite volcano

4. Types of Volcanoes

Mt. St. Helens following the 1980 eruption

4. Types of Volcanoes

A size comparison of the three types of volcanoes

5. Other Volcanic Landforms a.

caldera

Today, Crater Lake occupies a caldera 10km wide 

crater greater than 1 km wide



from collapse of summit

Also: Yellowstone!

Fig. 6.10

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

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