Geologic Time: Concepts and Principles - The Changing Earth: Exploring Geology and Evolution - page 464

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Figure 17.29 Stalagmites and Climate Change ( continued )

c The layers of a stalagmite can
be dated by measuring the U 234 /
Th 230 ratio, and the O 18 /O 16 ratio
determined for the pore water trapped
in each layer. Thus, a detailed record
of climatic change for the area can
be determined by correlating the
climate of the area as determined by
the O 18 /O 16 ratio to the time period
determined by the U 234 /Th 230 ratio.

The layers in a core or slice
of a stalagmite are dated
radiometrically.

1

2 The pore water is analyzed

in each layer for O 18 /O 16
and species of plants (from
the pollen).

3 A record of climatic change is put

together for the area of the caves.

Warmer,
drier

Present

Cooler,
wetter

(today)

Time

Geo-Recap

Chapter Summary

Time is defi ned by the methods used to measure it. Rela-
tive dating places geologic events in sequential order as
determined from their position in the geologic record.
Absolute dating provides specifi c dates for geologic rock
units or events, expressed in years before the present.
During the 18th and 19th centuries, attempts were made
to determine Earth's age based on scientifi c evidence
rather than revelation. Although some attempts were
ingenious, they yielded a variety of ages that now are
known to be much too young.
James Hutton, considered by many to be the father of
modern geology, thought that present-day processes oper-
ating over long periods of time could explain all of the
geologic features of Earth. His observations were instru-
mental in establishing the principle of uniformitarianism

and the fact that Earth was much older than earlier
scientists thought.
Uniformitarianism, as articulated by Charles Lyell,
soon became the guiding principle of geology. It holds
that the laws of nature have been constant through
time and that the same processes operating today have
operated in the past, although not necessarily at the
same rates.
Besides uniformitarianism, the principles of superpo-
sition, original horizontally, lateral continuity, cross-
cutting relationships, inclusions, and fossil succession are
basic for determining relative geologic ages and for inter-
preting Earth history.
An unconformity is a surface of erosion, nondeposi-
tion, or both separating younger strata from older strata.

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