Chapter 7: Volcanoes

New Vocabulary

Lava- magma (or hot, liquid rock) that reaches the surface

Pyroclasts- hot rock fragments (from the Greek word “pyro” meaning fire and “clast” meaning broken)

Pyroclastic flows- mixture of gases and pyroclastic debris. It is so dense that it hugs the ground

What is a volcano?

A volcano is a hill or mountain formed when lava or other molten rock reaches the surface. However, very fluid lava may reach the surface and harden into a horizontal layer.

Videos

Volcano 101

Geological Journey(start at about 28:00 mark)

Why is volcanic activity important to geology?

1. Landforms are created

2. Landforms are destroyed (less common)

3. Provides clues about the interior of the Earth

How does volcanic activity affect humans?

1. Growth of landforms (ex. Hawaii)

• Produces new rock & fertile soil

• Tourist attraction

• Hawaii is built from a seamount (or magma plume); taller than Mt. Everest

2. Geothermal energy

3. Effects on weather

• In 1991, Mt. Pinatubo dropped the average temperature by 0.5°C for several years because of fine volcanic ash

• The 1815 eruption of Tambora in Indonesia caused a very cold summer in 1816 (snow and frost in New England in the summer)

4. Volcanic catastrophes• Eruption of Mt. Vesuvius destroyed Pompeii

(much death caused by 5-8 m of hot ash)• Krakatoa (Indonesia) exploded and caused a

tsunami that killed 34,000• Mt. St. Helen’s in Washington State, USA• Volcanoes can kill because of pyroclastic

flows, famine (because of crop destruction), pyroclastic fragments and ash (roof collapses)

What determines the severity of a volcanic event?

There are two things that determine the severity of a volcanic event:

1. Amount of gas in the lava or magma2. The ease or difficulty with which the gas

escapes. This is determined by the viscosity (or “thickness”) of the lava.

*Overall, the more viscous the lava and the greater the volume of gas trying to escape, the more violent the eruption

What determines the severity of a volcanic event?

Low viscosity (more like liquid):• Less silica (ex. basaltic rock)• Less violent eruptions• Ex. Hawaii

High viscosity (thick):• More silica (ex. rhyolite)• More violent eruptions • Ex. Mount St. Helens

What kinds of rocks do volcanoes produce?

All volcanic rocks are extrusive and most have fine-grained crystals because of quick cooling.Common volcanic rocks include the following:

• Rhyolite• Andesite• Dacite• Basalt• Obsidian (black volcanic glass)• Pumice (frothy glass; floats in water; good for pedicures)• Rocks with holes (like Swiss cheese)• Volcanic bombs

What are the characteristics of different types of volcanoes?

First of all, some new volcano lingo:

• Vent: opening through which an eruption takes place

• Crater: basin-like depression over a vent at the top of a volcano

• Caldera: a volcanic depression much larger than the original crater and having a diameter of at least 1km

Vent Caldera (Crater Lake, USA)

Crater

Three major types of volcanoes:

Shield Volcano:• Broad, gentle slopes• Lava spreads widely and thinly• Made of solidified layers of lava flows• Ex. Hawaiian Islands

Pyroclastic cones:• Made of pyroclastic fragments (chunks of material)• Steep slopes• Less common; tends to wear away quickly• Less than 500m high

Composite volcanoes:• Made of alternating layers of pyroclastic fragments and

solidified magma flows• Steeper than shield volcanoes, but not as steep as

pyroclastic cones• Built over many years; can be old and very large• Mostly found around the “Ring of Fire” and the

Mediterranean Belt• Ex. Mount St. Helens; Mt. Vesuvius; Mt. Etna

Mt. Vesuvius

Video

Flashcard Quiz

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