Today:Today: Chapter 6 Chapter 6 VolcanismVolcanism

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 isWhat determines if an eruption is

3 primary factors:

magma’s composition

magma’s temperature

amount of gas in magma

magma viscosity

Its ability to move

more silica more viscous (chains)

hotter magma less viscous

Can increase fluidity

- violent?

- “gentle”?

- violent?

- “gentle”?

1. The nature of volcanic eruptions2. The nature of volcanic eruptions2. The nature of volcanic eruptions

Importance of gas in eruptions

Gas can provide the force to violently

hurl molten rock & ash from volcano

1. The nature of volcanic eruptions2. The nature of volcanic eruptions2. The nature of volcanic eruptions

•Fluid basaltic lavas generally produce quiet eruptions

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

eruptions

•Fluid basaltic lavas generally produce quiet eruptions

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

eruptions

1. The nature of volcanic eruptions2. The nature of volcanic eruptions2. The nature of volcanic eruptions

1. The nature of volcanic eruptions2. The nature of volcanic eruptions2. The nature of volcanic eruptions

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

Fig. 6.18

2. Materials extruded during an eruption3. Materials extruded during an eruption

Volcanoes erupt:Volcanoes erupt:

lava

gas

pyroclastic materials

broken rockashdust

broken rockashdust

2. Materials extruded during an eruption3. Materials extruded during an eruption lava gas pyroclastic stuff

Basaltic (low Si content)

(Hawaiian type)

pahoehoe flowflows in thin, broad sheets, ~10-300 meters/hr, smooth twisted or ropey texture & wrinkled outer skin with molten flow beneath

2. Materials extruded during an eruption3. Materials extruded during an eruption lava gas pyroclastic stuff

Basaltic (low Si content)

thicker & slower flows~5-50 meters/hr, rough & jagged surface

aa flow

2. Materials extruded during an eruption3. Materials extruded during an eruption lava gas pyroclastic stuff

Magmas contain dissolved gases, held in by pressure

Reduce pressure gases escape(decompression)

Gaseous portion 1-6% by weight, mostly water vapor, some CO2

2. Materials extruded during an eruption3. Materials extruded during an eruption 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

2. Materials extruded during an eruption3. Materials extruded during an eruption lava gas pyroclastic stuff

Gas can blow pulverized rock, glass, ash,

magma blobs into the air from a volcano

PyroclasticPyroclastic 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

Bomb is approximately 10 cm long

3. Materials extruded during an eruption lava gas pyroclastic stuff

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

4. Types of Volcanoes

See Fig. 6.9

Key terms: magma chambermagma chamber

vent/pipevent/pipe

calderacaldera

flank eruptionflank eruption

4. Types of Volcanoes

Types of volcanoes

Shield volcano

Cinder Cone

Composite Cone (Stratovolcano)

4. Types of Volcanoes

4. Types of Volcanoes

Broad, large area

Large volumes of basaltic lava

Mauna Loa on Hawaii

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

4. Types of Volcanoes

Sunset crater, AZ

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

4. Types of Volcanoes

Composite Volcano

Mt. St. Helens – Mt. St. Helens – a typical composite volcanoa 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

4. Types of Volcanoes

a.a. caldera caldera

Fig. 6.10

Today, Crater Lake

occupies a

caldera 10km wide

5. Other Volcanic Landforms

crater greater than 1 km wide

from collapse of summit Also:

Yellowstone!

Fig. 6.13

5. Other Volcanic Landforms

b. Basaltic fissure eruption

Fluid basaltic lava extruded from crustal fractures called fissures

Ocean ridges; Columbia River Plateau

5. Other Volcanic Landforms

Fig. 6.2

Flood basalt

of the Columbia River

Plateau formed during

extensive volcanic

activity 17 mio yr ago

Columbia River Basalts

c. lava dome

5. Other Volcanic Landforms

Bulbous mass of congealed lava

usually form after explosive eruption of gas-rich magma