Chapter 12Chapter 12VolcanoesVolcanoes

Section 1: Volcanoes Section 1: Volcanoes and Earth’s Moving and Earth’s Moving

PlatesPlates

Volcano: opening Volcano: opening in Earth that in Earth that

erupts gases, ash, erupts gases, ash, and lava.and lava.

Volcanoes can kill people, destroy Volcanoes can kill people, destroy property, and disrupt the environment.property, and disrupt the environment.

Lava and pyroclastic flows can bury Lava and pyroclastic flows can bury cities and towns in their paths.cities and towns in their paths.

Sulfurous gases from volcanoes can Sulfurous gases from volcanoes can create acid rain, which can kill create acid rain, which can kill organisms and pollute water.organisms and pollute water.

Acid Rain Destruction

Volcanoes form when magma flows out of Volcanoes form when magma flows out of a surface opening called a vent; a steep-a surface opening called a vent; a steep-walled depression around the vent is called walled depression around the vent is called a crater.a crater.

Volcanoes often form where plates are Volcanoes often form where plates are moving together or moving apart.moving together or moving apart.

The Mid-The Mid-Atlantic Atlantic

Ridge is a Ridge is a divergent divergent

plate plate boundary boundary that forms that forms

rifts through rifts through which lava which lava can flow.can flow.

At convergent plate boundaries, At convergent plate boundaries, volcanoes tend to erupt more violently volcanoes tend to erupt more violently

than they do in other areas.than they do in other areas.

At the boundary between Earth’s mantle At the boundary between Earth’s mantle and core, unusually hot areas form hot and core, unusually hot areas form hot spots, such as at the Hawaiian Islands.spots, such as at the Hawaiian Islands.

Section 2: Types of VolcanoesSection 2: Types of Volcanoes

The amount of water vapor and other The amount of water vapor and other gases present is one factor that determines gases present is one factor that determines whether a volcanic eruption will be quiet or whether a volcanic eruption will be quiet or

explosive.explosive.

Gases can be trapped in magma by Gases can be trapped in magma by pressure of surrounding magma and rock; pressure of surrounding magma and rock;

eventually they cause an explosive eventually they cause an explosive eruption.eruption.

Magma at convergent plate boundaries can Magma at convergent plate boundaries can contain a lot of water vapor that can cause contain a lot of water vapor that can cause

explosive eruptions.explosive eruptions.

The composition of magma is a The composition of magma is a second factor affecting the nature of a second factor affecting the nature of a

volcano’s eruption.volcano’s eruption.

Low-silica magma, called basaltic, is Low-silica magma, called basaltic, is fluid and produces a quiet, fluid and produces a quiet,

nonexplosive eruption.nonexplosive eruption.

Pahoehoe lava runs down the side Pahoehoe lava runs down the side of a volcano.of a volcano.

Aa lava is a stiff, slow moving lava.Aa lava is a stiff, slow moving lava.

High-silica magma called granitic and High-silica magma called granitic and intermediate silica magma called intermediate silica magma called

andesitic produce explosive eruptions.andesitic produce explosive eruptions.

Three types of volcanoes form from Three types of volcanoes form from the three types of lava.the three types of lava.

As quiet eruptions of basaltic lava As quiet eruptions of basaltic lava spread out in flat layers, they form a spread out in flat layers, they form a

broad volcano with gently sloping sides broad volcano with gently sloping sides called a shield volcano.called a shield volcano.

As tephra (bits of rocks or solidified As tephra (bits of rocks or solidified lava) falls to the ground, it forms a lava) falls to the ground, it forms a steep-sided, loosely packed cinder steep-sided, loosely packed cinder

cone volcano.cone volcano.

Cinder ConeCinder Cone

A composite volcano forms from A composite volcano forms from alternating layers of quiet lava and alternating layers of quiet lava and

more explosive tephra.more explosive tephra.

Section 3: Igneous Rock FeaturesSection 3: Igneous Rock Features

Many intrusive igneous features form Many intrusive igneous features form underground and are later exposed.underground and are later exposed.

Batholiths: rock bodies formed when Batholiths: rock bodies formed when magma bodies that are being forced upward magma bodies that are being forced upward

from inside Earth cool slowly and solidify from inside Earth cool slowly and solidify before reaching the surface.before reaching the surface.

Dike –magma that hardens after Dike –magma that hardens after being forced into a crack cutting being forced into a crack cutting

across rock; across rock;

Sill-magma that hardens after being Sill-magma that hardens after being forced into a crack parallel to rock forced into a crack parallel to rock

layers.layers.

Volcanic neck forms when the cone Volcanic neck forms when the cone is eroded away, leaving the solid is eroded away, leaving the solid

igneous core.igneous core.

Caldera –large depression formed Caldera –large depression formed when the top of a volcano collapses.when the top of a volcano collapses.

Weathering and erosion wear down Weathering and erosion wear down surface rock and expose igneous surface rock and expose igneous

rock features.rock features.