Earth Science

10-2

Factors Affecting Eruptions

1. The Primary factors that determine whether a volcano erupts explosively or quietly include characteristics of the magma and the amount of dissolved gases in the magma. Mount Saint Helens before and after eruption

Viscosity

• Viscosityo Is a substance’s resistance to flowo Viscosity is strongly affected by temp. o If lava cools and begins to harden, its

Viscosity increases

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• Chemical composition– Chemical composition on magma has a more

important effect on the type of eruption– Because of high silica content, rhyolitic lavas

are very viscous and erupt explosively– Less silica has a calmer eruption

Dissolved Gases

• Vent– An opening to the surface– During explosive eruptions, the gases trapped

in magma provide the force to propel molten rock out of the vent.

– These gases are mostly water vapor and carbon dioxide

Volcanic Material

1. Lava Flows

a) Silica content and the temperature affect the characteristics of lava flows.

b) Temperature differences produce 2 types of basaltic lava: pahoehoe and aa

2. Pahoehoe a) Is hotter, fast moving

basaltic lava

b) Pahoehoe forms relatively smooth skin that wrinkles as the still-molten lava continues to flow

3. Aaa) Is cooler, slower-moving

basaltic lava

b) Aa forms a surface of rough, jagged blocks with sharp, spiny projections

4. Gasesa) Magmas contain

varied amounts of dissolved gases held under pressure in the molten rock

b) The gaseous portion of the magma is a small portion about 1 to 6%

5. Pyroclastic Materialsa) Particles produced in

volcanic eruptions are called pyroclastic materials

b) The fragments ejected range in size from very fine dust and volcanic ash to pieces that weigh several tons

Types of volcanoes

1. Anatomy of a volcanoa) Volcanic activity usually

begins when a fissure, or crack, develops in the crust

b) Volcano- repeated eruptions of lava or pyroclastic material eventually build up a mountain

c) Crater- steep-walled depression

• The three main volcanic types of volcanoes are shield volcanoes, cinder cones, and composite cones.

Volcano eruption types

Shield volcano Cinder conesComposite

cones

1. Shield volcanoesa) Are produced by the

accumulation of fluid basaltic lavas.

b) Shield volcanoes have a shape of a broad, slightly domed structure that resembles a warrior’s shield.

2. Cinder conesa) Ejected lava

fragments that harden in the air build a cinder cone

b) The shape of a cinder cone is determined by the steep-sided slope that forms as loose pyroclastic material builds up around the vent

3. Composite conesa) Is a large, nearly

symmetrical volcano mountain composed of layers of both lava and pyroclastic deposits

b) Most composite cones are located in a relatively narrow zone that rims the Pacific Ocean, the Ring of Fire

How many of these Volcanoes are located in the U.S.

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composite cinder shield

• Volcanic landforms include calderas, volcanic necks, and lava plateaus

• Long after eruptions have ended, these landforms can provide evidence of volcanic activity

1. Calderasa) A caldera is a depression in a volcanic

mountain.

b) Most calderas form in two ways:1) By the collapse of the top of a composite volcano

after an explosive eruption

2) From the collapse of the top of a shield volcano after the magma chamber drained

2. Volcanic necksa) Is a landform made of magma that hardened

in a volcanoes pipe and later was exposed by erosion

b) Weathering and erosion work constantly to wear away the volcanoes

c) When a volcanoes activity ends, magma remaining in the pipe hardens to form igneous rock

3. Lave plateaus a) Is a volcanic landform

produced by repeated eruptions of very fluid, basaltic lava

b) Instead of building a cone the lava spreads out over a wide area

c) The lava that forms the lava plateaus erupts through long cracks called fissures

Volcanic Hazards• Volcano hazards include lava flows, volcanic ash,

pyroclasitc flows, and mudflows.• Lahar

– Occurs when water-soaked volcanic ash and rock slide rapidly downhill

• Composite volcanoes are among the most dangerous. They can cause a lahar

• An explosive eruption can also create a pyroclastic flow• Volcanoes usually give some warnings that an eruption

is near• Usually most people settle near a volcano because of

the fertile land from the ash