Chapter 8

Volcanoes

8.1 Volcanic Activity

Lesson Objectives

• Explain how volcanoes form.


• Describe places where volcanoes occur.


• Describe what volcanic hot spots are and where they occur.

Introduction

Everybody has heard of volcanoes—the angry Earth spewing up its wrath, the sudden explo-
sion of a distant mountain top, and lava that slithers ominously toward villages. Volcanoes
are fantastic displays of the power of the Earth. But what actually is a volcano? How and
where are they formed? Why do some places have a history of volcanic activity? Volcanoes
explain a key piece of the Earth’s geologic puzzle (Figure8.1).

How Volcanoes Form

You have already learned about tectonic plates. Beneath the Earth’s surface, powerful forces
are at work. These forces move lithospheric plates and produce huge chambers ofmagma,
molten rock beneath the Earth’s crust. Like water that bursts from a tiny hole in a water
pipe, the liquid magma seeks cracks orfissuresin the Earth’s crust through which it could
flow. This is a volcano — an opening in the earth’s crust through which magma or gases
can erupt onto the surface.

When molten rock escapes from beneath the Earth’s surface, it changes from magma tolava,