Geology Reference
In-Depth Information
evidence suggest that the asteroid Vesta may be basaltic
and some meteorites (including those likely to be from
Vesta) appear to be fragments of bodies that experienced
basaltic volcanism.
3.3.1 Volcanic eruptions
Among the earliest recordings of volcanic eruptions are
the writings of Pliny the Elder, a Greek philosopher who
witnessed the eruption of Vesuvius in A.D. 79. Through
the centuries, an understanding of volcanic processes was
gained from similar observations of active volcanoes. This
understanding led to a classification scheme based on
speci
c volcanoes, which became type localities for the
various classes of eruptions. Today, it is recognized that
many of the categories of eruptions are gradational with
one another or are variations of very similar processes.
Nonetheless, modern literature still carries some of the
original classi
cation categories, such as Strombolian for
the classic volcano in the Mediterranean. In the simplest
classi
cation, eruptions are referred to as explosive
(driven by the release of gasses in the magma as it reaches
the surface) or effusive (magma released to the surface
as liquid lava
flows). Explosive eruptions produce pyro-
clastic (
“
re-broken
”
) materials, such as ash and cinders.
Most modern classi
cations for the styles of volcanism
are based on the characteristics of the products erupted
and on the degree of eruption explosivity, as given
in
Table 3.1
.
Figure 3.7. Cinder cones form by mild explosions (Strombolian
eruptions) that eject solid material (pyroclastic fragments) on
ballistic trajectories; the pyroclastic materials accumulate at the
angle of repose (~34°) to form cones, as seen here in Lassen National
Park, California.
Rheology refers to the
flow properties of the lavas, of
which the viscosity is of primary importance. Viscosity is
a function of many variables, including temperature, com-
position, degree of crystallization, and gas content. High
temperatures lead to low-viscosity (
Fig. 3.8
), or runny,
liquids (e.g., hot syrup), while low temperatures lead to
high-viscosity liquids (e.g., cold syrup). Composition
refers to the silica content, in which silica-rich minerals
(such as quartz) form complex molecular structures that
increase the viscosity and retard
flow, while ma
c miner-
als (such as olivine) tend to have simple molecular struc-
tures and lower the viscosity. As lava cools, crystallization
takes place and the entrained solids also increase the
viscosity. Gas content includes not only the amount of
volatiles present, but also their state. Generally, volatiles
enhance
flow, leading to lower viscosities, but, as the
gasses come out of solution, the entrained bubbles retard
flow and increase the viscosity. Rapid release of gasses is
responsible for
3.3.2 Volcanic morphology
The forms of volcanoes and related terrains result from
complex, often interrelated parameters. Planetary volca-
nologist Whitford-Stark (
1982
) noted that these factors
fall into three groups (
Table 3.2
): planetary variables,
magma properties controlling rheology, and intrinsic
properties of eruptions.
Planetary variables include those factors that are char-
acteristic for the particular body. For example, the height
of an explosive eruption is governed by such consider-
ations as the gravitational acceleration and the presence
or absence of an atmosphere. In turn, these factors in
u-
ence the shape of the resulting volcano; in an airless,
low-gravity environment such as the Moon, pyroclastic
deposits would be widespread, in contrast to on Earth,
where the ejection distance would be retarded by the
atmosphere and higher gravity, leading to the formation
of cinder cones (
Fig. 3.7
).
explosive eruptions, much like
in a can of soda that is shaken (releasing the gasses),
which then spews when it is opened. The most violent
eruptions on Earth involve explosions of silica-rich mag-
mas of the sort that occurred in southern California
760,000 years ago. This eruption formed the Bishop
“
driving
”
