Geology Reference
In-Depth Information
Key bed
volcanic ash
◗
Figure 17.13
Correlating Rock Units In areas of adequate exposure, rock units can be traced
laterally, even if occasional gaps exist, and correlated on the basis of similarity in rock type and
position in a sequence. Rocks can also be correlated by a key bed—in this case, volcanic ash.
can be processed for a variety of microfossils that aid in deter-
mining the geologic age of the rock and the environment of
deposition.
Geophysical instruments may be lowered down the drill
hole to record such rock properties as electrical resistivity and
radioactivity, thus providing a record or
well log
of the rocks
penetrated. Cores, well cuttings, and well logs are all extremely
useful in making subsurface correlations (
Marie Curie that radioactive decay produces heat meant that
geologists fi nally had a mechanism for explaining Earth's in-
ternal heat that did not rely on residual cooling from a mol-
ten origin. Furthermore, geologists now had a powerful tool
to date geologic events accurately and to verify the long time
periods postulated by Hutton and Lyell.
As discussed in Chapter 3, all matter is made up of chemical
elements, each composed of extremely small particles called
atoms
. The
nucleus
of an atom is composed of
protons
(positively
charged particles) and
neutrons
(neutral particles), with
electrons
(negatively charged particles) encircling it (see Figure 3.2). The
number of protons defines an element's
atomic number
and
helps determine its properties and characteristics.
The combined number of protons and neutrons in an
atom is its
atomic mass number
. However, not all atoms of
the same element have the same number of neutrons in their
nuclei. These variable forms of the same element are called
isotopes
(see Figure 3.4). Most isotopes are stable, but some
are unstable and spontaneously decay to a more stable form.
It is the decay rate of unstable isotopes that geologists mea-
sure to determine the absolute ages of rocks.
Figure 17.17).
Subsurface rock units may also be detected and traced
by the study of seismic profi les. Energy pulses, such as those
from explosions, travel through rocks at a velocity deter-
mined by rock density, and some of this energy is refl ected
from various horizons (contacts between contrasting lay-
ers) back to the surface, where it is recorded (see Figure 9.3).
Seismic stratigraphy is particularly useful in tracing units in
areas such as the continental shelves, where it is very expen-
sive to drill holes and other techniques have limited use.
◗
Although most of the isotopes of the 92 naturally occurring
elements are stable, some are radioactive and spontaneously
decay to other more stable isotopes of elements, releasing
energy in the process. The discovery in 1903 by Pierre and
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