10.1 THE NATURE OF VOLCANIC

ERUPTIONS

What causes a Volcano?

Volcanic activity often starts when a crack develops in the crust. Magma is then forced toward the surface.

Repeated eruptions of lava or pyroclastic materials eventually build a mountain which is called a volcano

Factors Affecting Eruptions The primary factors that determine

whether a volcano erupts violently or not include:Magma Composition Magma TemperatureAmount of dissolved gasses in the magma

Viscosity – is a substance’s resistance to flow

Ex- Syrup is thicker than water

Magma from a more explosive volcano can be 1000x more viscous than that of a “quieter” volcano

Higher Temperature = less viscous

Lower Temperature = more viscous (hardens) = mobility decreases

More Silica in magma = higher viscosity

Dissolved Gases Gases are mostly water vapor and CO2 As magma gets closer to the surface,

the pressure of the magma is reduced, which allows for the dissolved gases to be released

Volcanic Material Lava Flows

Depends on silica content○ High silica = harder to flow○ Low silica = more fluid like

Gases Pyroclastic Materials

Particles that are produced in a volcanic eruption

Fragments ejected during an eruption range in size from extremely fine dust & ash to pieces that weigh several tons

Types of Volcanoes

Shield Volcanoes Cinder Cones Composite Cones

Shield Volcanoes

Produced by the build up of fluid basaltic lava

Shape = broad & slightly domed Have “grown up” and from the sea floor

and have formed islands Examples: Hawaiian Islands & Iceland

Cinder Cones

Cinder cones are built from ejected lava fragments which harden in the air

Fragments range in size Typically a result of gas rich basaltic

magma Composed mostly of Pyroclastic

material Occasionally releases lava

Shape: simple & smaller; determined by the steep sided slopes

Usually the result of a single eruption that lasts only a few weeks and very rarely more than a few years

The eruption only occurs once due to the fact that the pipe connecting the vent to the magma chamber hardens

Composite Cones

Large & symmetrical Composed of layers of lava &

pyroclastic material Viscous lava (travel short distances) Beautiful & possibly the most dangerous Located in a narrow zone along the

Pacific Ocean “Ring of Fire”

Other Volcanic Landforms Calderas – a large depression in a

volcanoForm in 1 of 2 ways: collapse of the top of a

shield volcano OR collapse of the top of a composite volcano

Necks & PipesIn most volcanoes, magma travels through

pipes that connect the magma chamber to the surface

Lava PlateausThe greatest volume of material comes from

fissuresBasaltic lava flows from the fissures (which

covers a large area)

Resources

http://video.nationalgeographic.com/video/101-videos/volcanoes-101

http://discoverykids.com/games/volcano-explorer/

Sources

Google Prentice Hall Earth Science Book National Geographic Discovery