Geology Reference
In-Depth Information
EXERCISE 4
Volcanoes and Volcanic Hazards
INTRODUCTION
The objective of this exercise is to investigate the
different types of volcanoes, volcanic products, and
volcanic hazards that exist in the United States.
Even though you may not live in an area with active
volcanoes, given appropriate wind directions and
large enough eruptions, all citizens of the United
States live downwind from volcanoes (Wright and
Pierson, 1991).
There are two major types of volcanoes that typi-
cally present hazards in the United States: basaltic
shield volcanoes,
as found in Hawaii; and
composite
volcanoes
(also known as stratovolcanoes), as found
in the Cascade Mountains of Washington, Oregon,
and California, and in Alaska. It is unusual for there
to be more than a few years without an eruption from
a Hawaiian volcano (Tilling, Heliker, and Wright, 1987),
and the eruption of Pu'u O'o has had more than 50
eruptive episodes over nearly a quarter century. In
the Cascade Range, Lassen Peak erupted in
1914-15, and Mount St. Helens had a major eruption
in 1980, which was followed by smaller eruptions
into 1986. After nearly a 20-year pause, Mount St.
Helens rejuvenated in 2004 with an ongoing series of
dome-building eruptions. Mount St. Helens is
explored in more detail in the next exercise.
Shield volcanoes are relatively flat and composite
volcanoes are often conical. The shapes are related to
the viscosity of the magma and the eruptive process.
Viscosity is a measure of how easily a substance flows;
it depends on chemical composition, temperature, and
gas content of the magma. A low-viscosity substance
will be very fluid, like water, while a high-viscosity sub-
stance will be very thick, like molasses or honey.
Most magmas are composed primarily of silica,
with lesser amounts of other elements. Basaltic magma
has comparatively low silica (approximately 50 percent
Si0
2
), andesitic magma has an intermediate silica con-
tent (approximately 60 percent Si0
2
), dacitic magma
has a slightly higher silica content (approximately 65
percent Si0
2
), and rhyolitic (also know as silicic)
magma has silica concentrations that range up to
about 75 percent Si0
2
. The relationships among
magma composition, magma viscosity, volcano form,
and typical eruption products are given in Table 4.1.
In addition to shield and composite volcanoes,
there are several other types of volcanoes that erupt
less frequently and have potential impacts that range
from minor to catastrophic. These include both
relatively small volcanic domes and large calderas
(volcanoes that collapse as a result of an eruption).
Pyroclastic flow eruptions take place typically in
volcanoes with dacitic or rhyolitic magma, and can
occur from the collapse of small domes or erupt from
large calderas, and are composed of volcanic ash
(tephra) fragments that are transported in a hot,
gas-rich cloud. Pyroclastic flows are the most devas-
tating type of volcanic eruption, as they can travel at
speeds of over 100 km per hour with temperatures
above 500°C. Exceptionally large but fortunately rare
eruptions known as "supervolcanoes" can have
worldwide impacts.
Dormant volcanoes can also be hazardous.
Landslides and debris flows can be triggered on
steep slopes by earthquakes or rocks that have been
weakened by chemical actions of hot water and
steam can suddenly slide without warning. Weakened
ground can suddenly collapse, and volcanoes, even
when not erupting, can give off gases.
Figure 4.1 is a simplified sketch of a composite
volcano and its associated hazardous phenomena.
Table 4.2 describes volcanic products and their
hazards in greater detail.
The first part of this exercise explores types of
volcanoes using data presented in Tables 4.1 and 4.2,
Figure 4.1, and this introduction. Then we look at
volcanic activity associated with Yellowstone National
Park, which is a large rhyolitic volcanic center; Mount
Rainier, which is typical of volcanoes of the andesitic
volcanic centers of the Cascade Range; and the
active volcanoes of Hawaii, which have basaltic
composition.
52
