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Geological Time Scale
8 January 2001. (c) 1996 Andrew MacRae. This file may be freely
used for non-commercial purposes provided its original source
is indicated. Please contact [email]
the author for other arrangements.
Few discussions in geology can occur without reference
to geologic time. Geologic time is often dicussed in two forms:
- Relative time ("chronostratic") — subdivisions
of the Earth's geology in a specific order based upon relative
age relationships (most commonly, vertical/stratigraphic
position). These subdivisions are given names, most of which
can be recognized globally, usually on the basis of fossils.
- Absolute time ("chronometric") — numerical
ages in "millions of years" or some other measurement. These
are most commonly obtained via radiometric dating methods
performed on appropriate rock types.
Think of relative time as physical subdivisions of the rock
found in the Earth's stratigraphy, and absolute time as the
measurements taken upon those to determine the actual time
which has expired. Absolute time measurements can be used
to calibrate the relative time scale, producing an integrated
geologic or "geochronologic" time scale.
It is important to realize that with new information about
subdivision or correlation of relative time, or new measurements
of absolute time, the dates applied to the time scale can
and do change. Revisions to the relative time scale have occurred
since the late 1700s. The numerically calibrated geologic
time scale has been continuously refined since approximately
the 1930s (e.g., Holmes, 1937), although the amount of change
with each revision has become smaller over the decades (see
fig. 1.5 and 1.6 of Harland et al.) and a few numerical estimates
were available previously (but often for the duration of the
entire scale rather than its individual subdivisions).
In addition, like any good scientific measurement, every
dated boundary has an uncertainty associated with it, expressed
as "+- X millions of years". These can not be included in
the diagram for practical reasons, but can be found in Harland
et al., 1990, along with a detailed description of the history
of earlier-proposed time scales and the terminology, methodology
and data involved in constructing this geological time scale.
Because of continual refinement, none of the values depicted
in this diagram should be considered definitive, eventhough
some have not changed significantly in a long time and are
very well constrained (e.g., the Cretaceous/Tertiary boundary
has been at 65+-1 Ma for decades, and has been tested innumerable
times, with almost all dates somewhere between 64 and 66 million
years). The overall duration and relative length of these
large geologic intervals is unlikely to change much, but the
precise numbers may "wiggle" a bit as a result of new data.
This gelogical time scale is based upon Harland et al., 1990,
but with the Precambrian/Cambrian boundary modified according
to the most recently-published radiometric dates on that interval,
revising the boundary from 570+-15 million years to 543+-1
million years ago (Grotzinger et al., 1995). Other changes
have been proposed since 1990 (e.g., revision of the Cretaceous
by Obradovich, 1993), but are not incorporated because they
are relatively small.
The time scale is depicted in its traditional form with oldest
at the bottom and youngest at the top — the present
day is at the zero mark. Geologic time is finely subdivided
through most of the Phanerozoic (see Harland et al., 1990
for details), but most of the finer subdivisions (e.g., epochs)
are commonly referred to by non-specialists only in the Tertiary.
Because of the vast difference in scale, the younger intervals
have been successively expanded to the right to show some
of these finer subdivisions.
Blatt, H.; Berry, W.B.N.; and Brande, S., 1991. Principles
of Stratigraphic Analysis. Blackwell Scientific Publications:
Boston, p.1-512. ISBN 0-86542-069-6 [Chapter 4 provides an
introduction to geologic time. This is a good starting point
to get the basic principles.]
Grotzinger, J.P.; Bowring, S.A.; Saylor, B.Z.; and Kaufman,
A.J., 1995 (Oct.27). Biostratigraphic and geochronologic
constraints on early animal evolution. Science, v.270,
p.598-604. [The most recent revision of the age of the Precambrian/Cambrian
Harland, W.B.; Armstrong, R.L.; Cox, A.V.; Craig, L.E.; Smith,
A.G.; and Smith, D.G., 1990. A geologic time scale,
1989 edition. Cambridge University Press: Cambridge, p.1-263.
ISBN 0-521-38765-5 [One of the more recent compilations of
the entire geologic time scale.]
Holmes, A., 1937. The Age of the Earth (new edition,
revised). Nelson:London, p.1-263. [One of the earlier attempts
at an integrated geochronologic time scale.]
Obradovich, J.D., 1993. A Cretaceous time scale.
In: Caldwell, W.G.E. and Kauffman, E.G. (eds.), Evolution
of the Western Interior Basin. Geological Association of Canada,
Special Paper 39, p.379-396. [Proposes revisions to the Cretaceous
time scale at the resolution of stages (finer divisions than
shown on diagram above) and sub-stages.]
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This file may be freely used for non-commercial purposes provided
its original source is indicated. Please contact the author
for other arrangements. Source: http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html