Volcanoes and Volcanism - The Changing Earth: Exploring Geology and Evolution - page 108

a The Mount Vesuvius region on the shore of the Bay of Naples in Italy. Vesuvius erupted in A . D . 79
and destroyed the cities of Pompeii, Herculaneum, and Stabiae.

Nola

CAMPI PHLEGRAEI

ITALY

Pozzuoli Naples

VESUVIUS

Misenum
CAPE
MISENO

Herculaneum

Pompeii

PROCIDA

ISCHIA

Bay of Naples

Stabiae

N

CAPRI

0 5 10

km

b The excavated ruins of Pompeii are a popular tourist
attraction.

◗ Figure 5.1 The 79 A . D . Eruption of Mount Vesuvius

c Body casts of some of the volcano's victims in Pompeii.

the solar system. Triton, one of Neptune's moons, probably
has active volcanoes, and Jupiter's moon Io is by far the most
volcanically active body in the solar system. Many of its hun-
dred or so volcanoes are erupting at any given time.

continues, and one cannot help noticing the rotten-egg odor
of hydrogen sulfi de gas ( Figure 5.2).

When magma rises toward the surface, the pressure is
reduced and the contained gases begin to expand. In highly
viscous, felsic magma, expansion is inhibited and gas pressure
increases. Eventually, the pressure may become great enough
to cause an explosion and produce pyroclastic materials
such as volcanic ash. In contrast, low-viscosity mafi c magma
allows gases to expand and escape easily. Accordingly, mafi c
magma generally erupts rather quietly.
Most volcanic gases quickly dissipate in the atmosphere
and pose little danger to humans, but on several occasions,
they have caused fatalities. In 1783, toxic gases, probably

◗

Volcanic Gases
Volcanic gases from present-day volcanoes are 50% to 80%
water vapor, with lesser amounts of carbon dioxide, nitrogen,
sulfur gases, especially sulfur dioxide and hydrogen sulfi de,
and very small amounts of carbon monoxide, hydrogen,
and chlorine. In areas of recent volcanism, such as Las-
sen Volcanic National Park in California, emission of gases