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Oldest formation exposed on the surface smell some of these gases around volcanic vents and hot springs.
Hydrogen sulfide smells like rotten eggs or sewer gas. The sulfur
Youngest formation
smells like a wooden match that has just been struck.
The gases dissolved in magma play a major role in forcing
magma out of the ground. As magma nears the surface, it comes
under less pressure, and this releases some of the dissolved gases
from the magma. The gases help push the magma out of the
ground. This process is similar to releasing the pressure on a can
of warm soda, which releases dissolved carbon dioxide.
Magma works its way upward from its source below to Earth’s
surface, here to erupt into a lava flow or a volcano. A volcano is a
hill or mountain formed by the extrusion of lava or rock fragments
from magma below. Some lavas have a lower viscosity than others,
A
are more fluid, and flow out over the land rather than forming a
volcano. Such lava flows can accumulate into a plateau of basalt,
the rock that the lava formed as it cooled and solidified. The
Columbia Plateau of the states of Washington, Idaho, and Oregon
is made up of layer after layer of basalt that accumulated from lava
flows. Individual flows of lava formed basalt layers up to 100 m
(about 330 ft) thick, covering an area of hundreds of square kilo-
meters. In places, the Columbia Plateau is up to 3 km (about 2 mi)
thick from the accumulation of many individual lava flows.
The hill or mountain of a volcano is formed by ejected
material that is deposited in a conical shape. The materials are
deposited around a central vent, an opening through which
an eruption takes place. The crater of a volcano is a basinlike
depression over a vent at the summit of the cone. Figure 19.20
is an aerial view of Mount St. Helens, looking down into the
B crater at a volcanic dome that formed as magma periodically
welled upward into the floor of the crater. This photo was taken
FIGURE 19.18 (A) A sketch of an eroded structural dome
where all the rock layers dip away from the center. (B) A photo of several years after Mount St. Helens erupted in May 1980. The
a dome named Little Sundance Mountain (Wyoming), showing the volcano was quiet between that time and late 2004, when thou-
more resistant sedimentary layers that dip away from the center. sands of small earthquakes preceded the renewed growth of the
lava dome inside the crater. According to the U.S. Geological
Survey, the new dome grew to more than 152 m (500 ft) above
the old dome before the volcano entered a quiet state.
Mount Garibaldi, Mount Rainier, Mount Hood, Mount
Baker, and about ten other volcanic cones of the Cascade Range
in Washington and Oregon could erupt next year, in the next
decade, or in the next century. History and plate movement (see
Figure 19.25) make it very likely that any one of the volcanic
cones of the Cascade Range will indeed erupt again.
Volcanic materials do not always come out from a central
vent. In a flank eruption, lava pours from a vent on the side of
Original sharply bounded
fault blocks softened by a volcano. The crater on Mount St. Helens was formed by a
erosion and sedimentation flank explosion. Magma was working its way toward the sum-
mit of Mount St. Helens, along with the localized earthquakes
Sediments
that usually accompany the movement of magma beneath the
surface. One of the earthquakes was fairly strong and caused
a landslide, removing the rock layers from over the slope. This
reduced the pressure on the magma, and gases were suddenly
released in a huge explosion, which ripped away the north flank
of the volcano. The exploding gases continued to propel volca-
nic ash into the atmosphere for the next 30 hours.
There are three major types of volcanoes: (1) shield, (2) cin-
FIGURE 19.19 Fault block mountains are weathered and
eroded as they are elevated, resulting in a rounded shape and der cone, and (3) composite. The shield volcanoes are broad, gen-
sedimentation rather than sharply edged fault blocks. tly sloping cones constructed of solidified lava flows. The lava

490 CHAPTER 19 Building Earth’s Surface 19-14