Geology Reference
In-Depth Information
In some areas of volcanism, volcanoes fail to develop at all.
For instance, during
fissure eruptions
, fluid lava pours out
and simply builds up rather fl at-lying areas, whereas huge
explosive eruptions might yield
pyroclastic sheet deposits
,
which, as their name implies, have a sheetlike geometry.
Rather then erupting from central vents, the lava fl ows making
up
basalt plateaus
issue from long cracks or fi ssures during
fi ssure eruptions
. The lava is so fl uid (has such low viscosity)
that is spreads out and covers vast areas. A good example is
the Columbia River basalt in eastern Washington and parts of
Oregon and Idaho. This huge accumulation of 17- to-6- million-
year-old overlapping lava flows covers about 164,000 km
2
(
Figure 5.14a and b), and has an aggregate thickness of more
than 1000 m. And some individual fl ows are enormous—the
Roza flow advanced along a front about 100 km wide and
covered 40,000 km
2
. Geologists have identifi ed 300 huge fl ows
here, one of which fl owed 600 km from its source.
Similar accumulations of vast, overlapping lava fl ows are
also found in the Snake River Plain in Idaho (Figure 5.14a
and c). These fl ows are 5.0 to 1.6 million years old, and they
represent a style of eruption between fi ssure eruptions and
◗
◗
Figure 5.13
The Yellowstone Tuff The walls of the Grand Canyon
of the Yellowstone River are made up of the hydrothermally altered
Yellowstone Tuff that partly fi lls the Yellowstone caldera.
WASHINGTON
b
About 20 lava fl ows of the Columbia River basalt are exposed
in the canyon of the Grand Ronde River in Washington.
OREGON
IDAHO
a
Relief map of the northwestern United States showing the
location of the Columbia River basalt and the Snake River Plain.
c
Basalt lava fl ows of the Snake River Plain near Twin Falls,
◗
Figure 5.14
Basalt Plateaus Basalt plateaus are vast areas of overlapping lava fl ows that issued
from long fi ssures. Fissure eruptions take place today in Iceland; however, in the past, they formed
basalt plateaus in various areas.
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