Geological Time
Scale
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.
geological time scale
Список литературы
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
boundary.]
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.]
Список литературы
Для
подготовки данной работы были использованы материалы с сайта http://www.geo.ucalgary.ca/