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