Geologic Time: Concepts and Principles - The Changing Earth: Exploring Geology and Evolution - page 465

These surfaces encompass long periods of geologic time
for which there is no geologic record at that location.
Three types of unconformities are recognized. A discon-
formity separates younger from older sedimentary strata
that are parallel to each other. An angular unconformity
is an erosional surface on tilted or folded rocks, over
which younger sedimentary rocks were deposited. A
nonconformity is an erosional surface cut into igneous
or metamorphic rocks and overlain by younger sedimen-
tary rocks.
Correlation is the demonstration of time equivalency of
rock units in different areas. Similarity of rock type, posi-
tion within a rock sequence, key beds, and fossil assem-
blages can all be used to correlate rock units.
Radioactivity was discovered during the late 19th cen-
tury, and soon afterward, radiometric dating techniques
enabled geologists to determine absolute ages for rock
units and geologic events.
Absolute dates for rocks are usually obtained by deter-
mining how many half-lives of a radioactive parent
element have elapsed since the sample originally crystal-
lized. A half-life is the time it takes for one-half of the
original unstable radioactive parent element to decay to a
new, more stable daughter element.

The most accurate radiometric dates are obtained from
long-lived radioactive isotope pairs in igneous rocks. The
most reliable dates are those obtained by using at least
two different radioactive decay series in the same rock.
Carbon-14 dating can be used only for organic matter such
as wood, bones, and shells and is effective back to approxi-
mately 70,000 years ago. Unlike the long-lived isotopic
pairs, the carbon-14 dating technique determines age by
the ratio of radioactive carbon-14 to stable carbon-12.
The geologic time scale was developed primarily during
the 19th century through the efforts of many people. It
was originally a relative geologic time scale, but with the
discovery of radioactivity and the development of radio-
metric dating methods, absolute age dates were added at
the beginning of the 20th century. Since then, refi nement
of the time-unit boundaries has continued.
Stratigraphic terminology includes two fundamentally
different kinds of units: those based on content and those
related to geologic time.
To reconstruct past climate changes and link them to pos-
sible causes, geologists must have a geologic calendar that
is as precise and accurate as possible. Thus, they must be
able to date geologic events and the onset and duration of
climate changes as precisely as possible.

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Important Terms

absolute dating (p. 436)
angular unconformity (p. 441)
biostratigraphic unit (p. 459)
carbon-14 dating technique (p. 455)
correlation (p. 446)
disconformity (p. 441)
fi ssion track dating (p. 455)
guide fossil (p. 447)

half-life (p. 452)
lithostratigraphic unit (p. 459)
nonconformity (p. 441)
principle of cross-cutting relationships
(p. 438)
principle of fossil succession (p. 441)
principle of inclusions (p. 441)
principle of lateral continuity (p. 438)

principle of original horizontality (p. 438)
principle of superposition (p. 438)
radioactive decay (p. 451)
relative dating (p. 436)
time-stratigraphic unit (p. 462)
time unit (p. 462)
tree-ring dating (p. 458)
unconformity (p. 441)

Review Questions

1. Because of the heat and pressure exerted during
metamorphism, daughter atoms were driven out of a
mineral being analyzed for a radiometric date. The
date obtained from this mineral will therefore be
_______ its actual age of formation.
a. _____ younger than;
b. _____ older than;
c. _____ the same as;
d. _____ it can't be determined;
e. _____ none of these.
2. Placing geologic events in sequential or chronological order
as determined by their position in the geologic record is
a. _____ absolute dating;
b. _____ correlation;

c. _____ historical dating;
d. _____ relative dating;
e. _____ uniformitarianism.
3. If a radioactive element has a half-life of 16 million
years, what fraction of the original amount of parent
material will remain after 96 million years?
a. _____ 1 / 2 ;
b. _____ 1 / 16 ;
c. _____ 1 / 32 ;
d. _____ 1 / 4 ;
e. _____ 1 / 64 .