Geoscience Reference
In-Depth Information
volcanoes on the island, such as Mauna Loa (Figure 13.40b).
Mauna Loa is the largest volcano on Earth and is one of five that
collectively comprise the island (Figure 13.40c).
Evidence indicates that the Hawaiian hotspot has been
active for a long time and has a fascinating history related to
continental drift. Figure 13.41 shows a trail of islands and sub-
merged highlands called
seamounts
that extend to the northwest
from the island of Hawaii. This trail exists because the oceanic
crust in the Pacific basin has rotated over the Hawaiian hotspot
for the past 70 million years. Do you see the sharp turn in the
island/seamount chain? The features north of that point, begin-
ning with the
Emperor Seamount Chain
, must have formed
when the plate was moving generally to the north as it flowed
over the hotspot. Shortly after the Emperor Seamount Chain
formed, approximately 43 million years ago, the plate began
moving more toward the northwest. This movement has con-
tinued to the present day and explains why the state of Hawaii
includes a string of islands northwest of the island of Hawaii.
Each of these islands was formed when that particular part of
the Pacific plate was over the hotspot. With an eye to the future,
where would you expect the next island of the Hawaiian Islands
to develop—to the northwest of Hawaii or to the southeast?
Why?
Whereas the Hawaiian hotspot provides an excellent ex-
ample of a fluid system that lies within the middle of the ocean,
hotspots are also associated with explosive volcanic eruptions
that occur within the interior of continents. Such a continental
example is the Yellowstone hotspot, which lies beneath Yellow-
stone National Park in northwestern Wyoming. Similar to the
Hawaiian hotspot, the Yellowstone hotspot is a fixed zone of up-
welling magma. In this case, however, the hotspot lies beneath
the North American plate and the magma is highly viscous. Geo-
logical studies indicate that the apparent location of the Yellow-
stone hotspot has moved over time (Figure 13.42a), reflecting
the west/southwestern migration of the North American plate.
About 16 million years ago the hotspot was located in what
is now the southeastern corner of Oregon. Between 12 million
and about 2 million years ago, it migrated across what is now
southern Idaho.
The Yellowstone hotspot has been in its present location
for approximately the past 2 million years, with three major
eruptions occurring during this time at 2.2 million years ago,
1.3 million years ago, and about 630,000 years ago. Each of
these eruptions was cataclysmic, resulting in the formation of
a giant caldera (Figure 13.42b). The Yellowstone caldera—that
is, the crater from the most recent eruption—is over 50 km
(~32 mi) wide. As you can imagine, eruptions of this magni-
tude must have ejected enormous amounts of volcanic debris.
The first eruption in the area, which is the largest of the three,
ejected 2500 km
3
(~1550 mi
3
) of material. By comparison, only
1 km
3
to 2 km
3
(~0.6 mi
3
to 1.2 mi
3
) of material was ejected by
Mount St. Helens in 1980. Some more recent eruptions have oc-
curred at Yellowstone, such as a lava flow around 70,000 years
ago and a steam explosion about 14,000 years ago, but these
events were relatively small and isolated.
Imagine what the large Yellowstone eruptions must have
been like! If a similar eruption happened in the present time, it
would impact the United States in a truly catastrophic way. It
is conceivable that the eastern half of the country would
shut
down
as long as the eruption occurred and volcanic ash fell
across the landscape. The last major eruption of Yellowstone
about 600,000 years ago, for example, resulted in ash that ac-
cumulated to a depth of about as much ~30 cm (~1 ft) in what
is now Iowa, over 1200 km (~750 mi) away. A glimpse into the
nature of the potential impact was seen in April 2010 when an
erupting volcano in Iceland grounded air travel in Europe for
about a week due to high concentrations of ash in the atmo-
sphere. Given the truly gargantuan size of a potential Yellow-
stone eruption and the fact that it would likely last for weeks, it
would have devastating consequences for the nation's economy
and human health downwind. Global climate would also be
impacted because the extensive ash plume would reflect solar
Figure 13.41 Migration of the Hawaiian hot-
spot over time.
In addition to creating the mod-
ern Hawaiian Islands, the apparent migration of
the hotspot formed the Emperor Seamount Chain.
Note that the chain contains a sharp turn (arrow)
that reflects a shift in the drift of the Pacific plate.
