Volcanic eruptions Factors that determine the violence of an eruption Composition of the magma Temperature of the magma Dissolved gases in the magma.

Volcanic eruptions Factors that determine the violence of

an eruption • Composition of the magma • Temperature of the magma• Dissolved gases in the magma

Viscosity of magma • Viscosity is a measure of a material's

resistance to flow

Volcanic eruptions Viscosity of magma

• Factors affecting viscosity • Temperature (hotter magmas are less viscous) • Composition (silica content)

• High silica – high viscosity • Low silica – more fluid

• Dissolved gases (volatiles) • mostly H2O and CO2

• Gases expand near the surface

Volcanic eruptions Viscosity of magma

• Factors affecting viscosity• Dissolved gases (volatiles)

• Provide the force to extrude lava• Violence of an eruption is related to how

easily gases escape from magma • Easy escape from fluid magma • Viscous magma produces a more violent

eruption

A low silica lava flow

A high silica flow

Pyroclastics

• Pyroclastic material: fragments of rock formed during eruptions– Ash and dust: fine, glassy fragments– Lapilli: walnut sized rocks– Bombs: magma ejected into the air cools and

forms rounded rocks– Blocks: ejected as large rocks

Volcanoes General features

• Conduit, or pipe carries gas-rich magma to the surface

• Vent, the surface opening (connected to the magma chamber via a conduit)

• Crater• Steep-walled depression at the summit • Caldera (a summit depression greater than 1

km diameter)

Volcanoes Types of volcanoes

• Shield volcano• Broad, slightly domed (like a shield)• Primarily made of fluid lava • Generally large size

A shield volcano

Volcanoes Types of volcanoes

• Cinder cone • Built from ejected lava fragments • Steep slope angle • Relatively small size

Sunset Crater – a cinder cone near Flagstaff, Arizona

Volcanoes

Types of volcanoes • Composite cone (or stratovolcano)

• Most are adjacent to the Pacific Ocean (e.g., Mt. Rainier)

• Large size • Interbedded lavas and pyroclastics • Most violent type of activity

A composite volcano (stratovolcano)

Mt. St. Helens – a typical composite volcano

Mt. St. Helens after eruption

A size comparison volcanoes

Other volcanic landforms

Calderas • Steep walled depression at the summit • Formed by collapse • Nearly circular • Size exceeds one kilometer in diameter

Crater Lake in Oregon

Hot Spots

Locations of some of Earth’s major volcanoes

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Continental and island volcanic arcs

Super volcanoes

• “super eruptions:” – eject magma with a mass greater than 1015

kg, equivalent to a volume greater than 450 km3

• explosive eruptions of this magnitude have a volcanic explosive index (VEI) of 8 or above and produce > 1000 km3 of fragmental deposits

• A “super volcano” can be defined as a volcano that has produced at least one explosive super eruption.

Super eruptions require:

- ENORMOUS volume of highly viscous magma (silica content >60%) accumulating in shallow magma chambers, which are part of even bigger magma reservoirs

- High volatile content (mostly water vapor)

* Thick, relatively low-density crust, common in continents or old island arcs help to create such HUGE magma reservoirs.

Explosive eruptions can happen if (when?):

• The magma body becomes partially solid and the volatiles are forced into the remaining liquid, making the liquid saturated in gas

• Fresh batch of hot magma intrudes in the magma chamber

• The gas-rich liquid magma escapes from the semi-solid crystal mush and is stored beneath the roof of the magma chamber, and,

• Earthquakes and faulting fracture a magma chamber and/or “shakes up” the magma chamber