Volcanoes Form as Volcanoes Form as Molten Rock EruptsMolten Rock Erupts

Pages 146-153 (section B)Pages 146-153 (section B)

What is a Volcano?What is a Volcano?

• Volcano- an opening in the Earth’s crust through which molten rock, rock fragments, and hot gasses erupt.– A mountain built up from erupted material is also

called a volcano.

What happens when a volcano What happens when a volcano erupts?erupts?

• Volcanoes may erupt violently or gently

• Even if no magma reaches the surfaces volcanoes can be dangerous– Hot rocks can fly out starting fires– Rocks can land on roofs in large numbers

causing the roofs to collapse

MagmaMagma• Magma contains silica and gases

• Magma HIGH in silica is resistant to flowing– Causes violent eruptions

• Magma LOW in silica flows easily

• Magma rises as long as it is less dense than the surrounding rock.

• When magma erupts it is called lava

VolcanoesVolcanoes

VolcanoesVolcanoes

Rock FragmentsRock Fragments• Fragments form as:

– Escaping gas bubbles pop, tearing magma apart– Larger pieces of lava are thrown into the air, cooling

and hardening during flight– Rocks of all sizes rip loose from volcanoes’ walls

during eruptions

Rock FragmentsRock Fragments• Tiny rock fragments form volcanic ash, sized

between dust fragments and grain of rice.– Stay in the air a long time, but travel far distances

• Cinders are larger than ash, smaller than bombs

• Bombs are the largest size rock fragments– Fall the fastest

Ash Cinder

Bomb

Volcanic GasesVolcanic Gases

• What looks like smoke coming from a volcano is actually a mixture of ash and gases.– The main gases in magma are water vapor and

carbon dioxide

Pyroclastic FlowPyroclastic Flow• Volcanic gases can mix with rock fragments

and stay near the ground.

• Pyroclastic flow- a dense cloud of superhot gases and rock fragments.– Can reach temperatures of 800 degrees C (1500 degrees F)– Can reach speeds of 160 km/hr (100 mi/hr)– Pyroclastic flows are the most dangerous type of volcanic

eruption.

Most volcanoes form along plate Most volcanoes form along plate boundariesboundaries

• Most of Earth’s volcanic activity takes place under water- along plate boundaries

• Volcanoes can also occur along hotspots away from plate boundaries- this is less common

Types of VolcanoesTypes of Volcanoes

• Shield Volcano– shaped like a broad, flat dome.

• Built up by man eruptions of lava that is relatively low in silica.

Types of VolcanoesTypes of Volcanoes• Cinder Cone- a steep, cone-shaped hill

formed by the eruption of cinders and other rock fragments that pile up around a single crater.

• Form as gas-rich magma erupts- often on the side of another volcano

• Ten to hundreds of meters tall

Types of VolcanoesTypes of Volcanoes• Composite Volcano- a cone shaped volcano

that is built up of layers of lava and layers of rocks fragments.

• Steep at the top flattens out at the bottom

• Tends to have very violent eruptions

CalderaCaldera• Caldera- a huge crater formed by the

collapse of a volcano when magma rapidly erupts from underneath it.

Active VolcanoActive Volcano

• A volcano that currently vents lava or gases etc, or has the capability of doing so

Dormant VolcanoDormant Volcano

• A volcano that has not erupted for a considerable length of time, but is still capable of erupting

Warning Signs of Volcanic ActivityWarning Signs of Volcanic Activity• Temperature of rocks in the volcanic cone

increases

• The cone may change shape- swell or bulge just before eruption

• Small earthquakes and tremors in the area around the volcano

• Emission or increase of gas from the volcano

• Often animals may act strangely before the eruption

Scientist Monitor VolcanoesScientist Monitor Volcanoes

• Scientist cannot prevent volcanoes from erupting

• The signs of an eruption can allow people to leave an area so the loss of human life can be minimized

• http://www.snagfilms.com/films/title/volcano_natures_inferno/