Geology Reference
In-Depth Information
circum-Pacifi c belt are the southernmost active volcanoes at
Mount Erebus in Antarctica and a large caldera at Deception
Island that erupted most recently during 1970.
The second area of active volcanism is the
Mediterranean
belt
(Figure 5.15). About 20% of all active volcanism takes place
in this belt, where the famous Italian volcanoes such as Mounts
Etna and Vesuvius and the Greek volcano Santorini are found
(see Geo-Focus on pages 124 and 125). Mount Etna has issued
lava fl ows 190 times since 1500
B
.
C
., when activity was fi rst re-
corded. A particularly violent eruption of Santorini in 1390
B
.
C
.
might be the basis for the myth about the lost continent of At-
lantis, and, in
A
.
D
. 79, an eruption of Mount Vesuvius destroyed
Pompeii and other nearby cities (see the Introduction).
Nearly all the remaining active volcanoes are at or near
mid-oceanic ridges or the extensions of these ridges onto land
(Figure 5.15). These include the East Pacifi c Rise and the lon-
gest of all mid-oceanic ridges, the Mid-Atlantic Ridge. The
latter is located near the center of the Atlantic Ocean basin, ac-
counting for the volcanism in Iceland and elsewhere. It contin-
ues around the southern tip of Africa, where it connects with
the Indian Ridge. Branches of the Indian Ridge extend into the
Red Sea and East Africa, where such volcanoes as Kilamanjaro
in Tanzania, Nyiragongo in Zaire, and Erta Ale in Ethiopia
with its continuously active lava lake are found.
By the time you read this chapter, several more volca-
noes in Alaska will have erupted as the Pacifi c Plate moves
relentlessly northward only to be subducted at the Aleutian
Trench. The Alaska Volcanoes Observatory in Anchor-
age, Alaska, continues to monitor these volcanoes and issue
warnings about potential eruptions.
The
Cascade Range
(
Figure 5.16) stretches from Las-
sen Peak in northern California north through Oregon and
Washington to Meager Mountain in British Columbia,
Canada, which erupted 2350 years ago. Most of the large vol-
canoes in the range are composite volcanoes, but Lassen Peak
in California is the world's largest lava dome. Actually, it is a
rather small volcano that developed 27,000 years ago on the
fl ank of a much larger, deeply eroded composite volcano. It
erupted from 1914 to 1917, but has since been quiet except
for ongoing hydrothermal activity (Figure 5.16b).
Two large shield volcanoes lie just to the east of the
main Cascade Range volcanoes—Medicine Lake Volcano in
California and Newberry Volcano in Oregon. Distinctive features
at Newberry Volcano are a 1600-year-old obsidian fl ow and casts
of trees that formed when lava fl owed around them and solidifi ed.
Cinder cones are common throughout the range (Figure 5.9d).
What was once a nearly symmetrical composite volcano
changed markedly on May 6, 1980, when Mount St. Helens in
Washington erupted explosively, killing 63 people and level-
ing some 600 km
2
of forest (Figure 5.16c). A huge lateral blast
caused much of the damage and fatalities, but snow and ice on
the volcano melted and pyroclastic materials displaced water
in lakes and rivers, causing lahars and extensive fl ooding.
Mount St. Helens's renewed activity, beginning in late
September 2004, has resulted in dome growth and small
steam and ash explosions. Scientists at the Cascades Volcano
Observatory in Vancouver, Washington, issued a low-level
alert for an eruption and continue to monitor the volcano.
Several of the Cascade Range volcanoes will almost certainly
erupt again, but the most dangerous is probably Mount Rain-
ier in Washington. Rather than lava flows or even a colossal
explosion, the greatest danger from Mount Rainier is volcanic
mudfl ows or huge debris fl ows. Of the 60 large fl ows that have
occurred during the last 100,000 years, the largest, consisting of
4 km
3
of debris, covered an area now occupied by more than
120,000 people. Indeed, in August 2001, a sizable debris fl ow took
place on the south side of the mountain, but it caused no injuries
or fatalities. No one knows when the next fl ow will take place, but
at least one community has taken the threat seriously enough to
formulate an emergency evacuation plan. Unfortunately, the resi-
dents would have only 1 or 2 hours to carry out the plan.
◗
Part of the circum-Pacifi c belt includes volcanoes in the Pacifi c
Northwest, as well as those in Alaska. Both of these areas of
volcanism are at convergent plate boundaries. Of the 80 or so
potentially active volcanoes in Alaska, at least half have erupted
since 1760. The other active North American volcanoes are in
the Cascade Range in the Pacific Northwest where the Juan
de Fuca plate is subducted beneath North America. Many of
these volcanoes have been historically active, although since
1900, only Lassen Peak in California and Mount St. Helens in
Washington have erupted.
Many of the volcanoes in mainland Alaska and in the
Aleutian Islands (Figure 5.15) are composite volcanoes, some
with huge calderas. Mount Spurr has erupted explosively at
least 35 times during the last 5000 years, but its eruptions
pale by comparison with that of Novarupta in 1912. Its
defi ning event was the June 1912 eruption, the largest in the
world since the late 1800s. At least 15 km
3
of mostly pyro-
clastic materials erupted during about 60 hours.
When the eruption was over, 120 km
2
of land was bur-
ied beneath pyroclastic deposits as deep as 213 m. In fact,
the deposits fi lled the Valley of Ten Thousand Smokes—so
named because of the hundreds of fumaroles where gases
vented through the hot deposits for as long as 15 years fol-
lowing the eruption. Fortunately, the eruption took place
in a remote area so there were no injures or fatalities, but
enough ash, gases, and pumice were ejected that, for sev-
eral days, the sky was darkened over much of the Northern
Hemisphere.
AND PLUTONS
In Chapter 4, we discussed the origin and evolution of
magma and concluded that (1) mafi c magma is generated
beneath spreading ridges, and (2) intermediate and felsic
magma forms where an oceanic plate is subducted beneath
another oceanic plate or a continental plate. Accordingly,
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