By studying the rocks and geologic features of an area, experts
can assess whether it is vulnerable to future volcanic eruptions.
Scientists have performed extensive studies at and near Yucca Mountain
to determine whether future volcanoes could possibly affect the
proposed repository for nuclear waste.
The earth’s crust is broken into plates that resemble a giant
jigsaw puzzle. Our planet has 16 major plates. These rigid plates
sit on a layer of molten rock. As the plates move about, they slide
past each other, push together, or pull apart.
Most volcanoes occur near the edges of continental plates. In areas
where plates are moving apart, magma can come to the surface and
erupt. When plates push together, one plate will sometimes slide
beneath the other. If the plunging plate goes deep enough, some
of its rock melts and forms magma that can move upward and erupt
at the earth’s surface.
The “Pacific Ring of Fire” is an example of where volcanoes
are occurring along the edges of continental plates. Currently,
more than half of the world’s active volcanoes encircle the
Pacific Ocean. In the United States, most of these volcanoes are
in the Aleutian Islands, the Alaska Peninsula, the Hawaiian Islands,
and the Cascade Range of the Pacific Northwest.
In addition, there are a few areas on the earth called “hot
spots,” where magma has melted through the plate. Because
it is lighter than the solid rock around it, the magma rises and
collects in magma chambers. If water is nearby, the magma heats
the water, which sometimes erupts at the surface in the form of
geysers. “Old Faithful” in Yellowstone National Park
is a geyser resulting from a current hot spot. Under certain conditions
in a hot spot, magma can move through vents and fissures and erupt
at the earth’s surface. (Once on the surface, it is usually
called lava.)
Some volcanic eruptions are explosive, and others are not. The
explosiveness of an eruption depends on the consistency of the magma.
If the magma is thin and runny, gases can easily escape. When this
type of magma erupts, it flows gently from cracks in the earth’s
surface. If the magma is thick and sticky, gases cannot escape,
and pressure builds until the gases explode in a violent eruption.
Volcanoes that are now extinct contributed significantly to the
formation of the earth’s crust. Over time, the cooling and
hardening of the earth’s crust has forever changed the conditions
under which these early volcanoes existed.
Smaller-scale volcanic activity will continue in vulnerable areas
such as hot spots or where continental plates meet.
Experts agree that future volcanism is more likely to occur within
or near areas where volcanoes have occurred in the past several
hundred thousand years. Using modern dating methods, experts can
determine when volcanoes occurred in a specific area.
Millions of years ago, a series of large explosive volcanic eruptions
occurred to the north of Yucca Mountain. These eruptions produced
dense clouds of volcanic ash and rock fragments, which melted or
compressed together to create layers of rock called tuff, forming
the mountains and hills of the region.
The large-scale volcanic eruptions that produced Yucca Mountain
ended about 12 million years ago. This explosive volcanism produced
almost all (more than 99 percent) of the volcanic material in the
Yucca Mountain region.
Several million years ago, a different type of eruption began in
the area. These eruptions were smaller and much less explosive.
These small eruptions were marked by lava and cinders seeping and
sputtering from cones or fissures. The last such small eruption
occurred about 80,000 years ago. The remaining volcanic material
(less than 1 percent) in the Yucca Mountain region is a result of
these smaller eruptions.
Yucca Mountain is not in an area where continental plates meet,
nor is it located near any volcanic hot spots. In fact, experts
consider the Yucca Mountain region one of the least active volcanic
fields in the western United States.
To assess the possibility of future volcanic activity in the Yucca
Mountain area, the U.S. Department of Energy relied upon careful
evaluation by some of the world’s foremost experts in such
fields as volcanology, geophysics, and geochemistry.
Their studies started with extensive analysis of the location,
age, and volume of past volcanic activity in the Yucca Mountain
area. Using the data from these studies, along with information
from studies of both modern and ancient volcanoes throughout the
world, the scientists performed a volcanic hazard analysis.1 This
analysis evaluated the likelihood of magma entering the potential
repository area. The analysis also evaluated the possibility of
magma intersecting the repository and erupting up through the mountain’s
surface.
A panel of independent external experts continues to provide the
DOE with ongoing advice about volcanic hazards. This will help ensure
that a sound technical basis is presented in future licensing interactions
with the U.S. Nuclear Regulatory Commission.
Using their extensive studies of the Yucca Mountain region, experts
estimate the chance of a volcanic event disrupting the proposed
repository to be about one in 63 million per year. This equals about
0.0000016 percent chance per year that a volcano will disrupt the
repository. Put another way, it means there is about a 99.9999984
percent chance per year that a volcanic event will not
disrupt the repository.
1. Probabilistic Volcanic Hazard Analysis for Yucca
Mountain, Nevada (U.S. Department of Energy, CRWMS M&0 1996)
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