Volcanoes Chapter 29.1

Unit Ten: Earth Science

29.1 Volcanoes29.2 The Surface of Earth29.3 Rocks and Minerals

Chapter 29 Formation of Rocks

29.1 VolcanoesThe eruption of Mount

St. Helens in 1980 reduced the height of this mountain from 2,932 meters (9,677 feet) to 2,535 meters (8,364 feet).

Early in the morning of May 18, 1980, an earthquake triggered a landslide that caused the bulge to eject magma, water, and gases.

Mount St. Helens

29.1 Volcanoes*Solid rock melts and becomes magma

under certain conditions that lower the melting point of the material.

*At subduction zones, water is the key for solid rock to melt and become magma.

29.1 Volcanoes*Most volcanic

activity is found at the edges of tectonic plates, namely at divergent and convergent plate boundaries, but does not occur at transform plate boundaries.

Shield Volcanoes:*Low viscosity,

fast-flowing lava is associated with shield volcanoes.

Because this lava easily flows down hill, shield volcanoes are gently sloped and flattened.

Composite Volcanoes*High viscosity

lava is associated with stratovolcanoes (also called composite volcanoes).

These volcanoes range in height from 500 to 10,000 meters high.

Cinder Cone Volcanoes*Cinder cone

volcanoes are steep stacks of loose pyroclasts (clumps and particles of lava).

Cinder cones are rarely higher than 300 meters.

Lava Facts:*Lava Explosive eruptions occur when the

lava has a lot of water and dissolved gases.Gentle eruptions are associated with fast-

flowing lava from oceanic crust. viscosity also determines how explosive an eruption will be.

Where volcanoes formVolcanoes also form when an oceanic plate slides

under another oceanic plate.

Hydrothermal Vents*Hydrothermal vents are deep sea,

chimney-like structures that occur along midocean ridges.

GemstonesSome gemstones are also associated with

volcanic activity. For example, diamonds form at high

temperatures deep underground when carbon crystallizes inside rocks called kimberlites.

Geothermal Energy*Geothermal energy

is the useful product of volcanic activity.

When steam from magma collects below ground, it can be tapped just like water in a well.

*The pressurized steam can be used to generate electricity.

29.1 Volcanoes

*Volcanic activity results in the formation of two kinds of igneous rocks:extrusiveintrusive

A batholith is a large underground rock that formed when a mass of magma cooled underground.

Key Questions:1. Why do some volcanoes

erupt explosively?2. Which volcanoes have a

slow flow of lava down the slopes?

3. Where do volcanoes occur?

4. What ocean does the Ring of Fire surround?