Time and Geology

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Time and Geology

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Geol-102

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Environmental Geology (Geol-102) is a 3-unit college transfer course that deals with the geological aspects of human interaction with the earth and satisfies the general education requirement in physical sciences for most baccalaureate programs in North America.

Time and Geology

Dating of the geological past is based
on these basic principles:

the principle of uniformitarianism,
that the “present is the key to the past” (i.e., the geological processes now are as they have always been); and

the principle of superposition of strata,
i.e., the younger formations in an undisturbed succession of layers overlie the older ones.

The resulting measures of
geological time can be

relative, or

absolute

Click on this image on the right to browse the online edition of USGS publication "Geological Time"

Measuring relative time implies

determining the order in which a given succession of
geological events occurred, even if their precise dates are unavailable;

using the principles of

	original horizontality,
	superposition of strata and
	cross-cutting relationship;

and

correlation of the spatially separated rock
formations or geological events using the evidences of

	physical continuity,
	lithostratigraphic similarity, and
	fossil assemblages (i.e., the principle of faunal and floral succession).

An exercise from the web site

http://www.mhhe.com/earthsci/geology/plummer/student/olc/chap08chact.mhtml

In the geological section shown above,
the relative ages of the formations are as follows (starting with the youngest): 24, 23, 21, 20, 11, 10, 6, 5, 4, 3, 2 and 1.

The absolute time is measured by
radiometric dating, a method that

uses the “half life” (i.e., the time taken by a radioactive
parent isotope to decay to one-half of its initial quantity)
of radioactive parent isotopes tabulated here; and thus

estimates the age of a rock from the relative abundance
of these parent and daughter isotopes in it.

Note: Ma = Million Years, Ga = Billion Years

Radioactive Parent Isotope	Radiogenic Daughter Isotope	Half-life
Potassium-40	Argon-40	1250	Ma
Uranium-235	Lead-207	713	Ma
Uranium-238	Lead-206	4500	Ma
Rubidium-87	Strontium-87	49	Ga

Clearly, if Zoraster Granite in the Grand Canyon is ~1250 Ma old then, because ~10% of K⁴⁰ decays to the exclusively radiogenic Ar⁴⁰ and ~90% to the Ca⁴⁰ isotope that can also have other parentage, we should find 10 nanograms of K⁴⁰ for each nanogram of Ar⁴⁰ in it. Likewise, if the earth is indeed ~4.5 Ga old and these half-life values are valid, then we should find as much Pb²⁰⁶ as U²³⁸ and nearly 32 times as much U²³⁸ as U²³⁵. The observed data corroborate these expectations.

We do need to examine if our time scales of interest are not too long to be physically verifiable, particularly as establishing such long half-life values itself seems so hard to accomplish. What makes this possible is a simple trick called logarithms. Mathematically, for instance, if N is the number of nuclides of a radioisotope at any time t, and N₀ is its initial quantity, then

N = N₀ exp (-T/t_½) where t_½ denotes the half-life.

Thus, T = t_½ Ln (N/N₀), the age we need to estimate.

With t_½ = 1.25 billion years for the K⁴⁰ - Ar⁴⁰ decay series, the Table alongside thus shows that we can expect to find 0.056 nanogram of Ar⁴⁰ for each gram of K⁴⁰ one year after the onset of the process, or 0.28 nanogram of Ar⁴⁰ for each gram of K⁴⁰ five years after the onset of the process and 0.555 nanogram of Ar⁴⁰ for each gram of K⁴⁰ ten years after the onset of the process. With today’s highly accurate mass spectrometers, these numbers are easy to establish and experimentally verifiable.

Time since the onset of the process		K-40 (grams)	Ar-40 (nanograms)
0	year	1.0000000000	0.0000
1	year	0.9999999992	0.0555
2	years	0.9999999984	0.1109
3	years	0.9999999976	0.1664
4	years	0.9999999968	0.2218
5	years	0.9999999960	0.2773
6	years	0.9999999952	0.3327
7	years	0.9999999944	0.3882
8	years	0.9999999936	0.4436
9	years	0.9999999928	0.4991
10	years	0.9999999920	0.5545

Geological time and Evolution

Source: http://www.talkorigins.org/faqs/faq-age-of-earth.html#dal01

Estimating the Earth's Age

Lead-ratio (or the Pb²⁰⁷-Pb²⁰⁶ isochron) method provides the most direct means to estimate the Earth's age. As Pb²⁰⁷ comes from radioactive decay of U²³⁵ and Pb²⁰⁶ from that of U²³⁸, the plot of these two lead isotopes (relative to the non-radiogenic Pb²⁰⁴ or Pb²⁰⁸) should be linear if the solar system formed from a common pool of matter that was uniformly distributed in terms of the Pb-isotope ratios. The older the sample, the higher the uranium-to-lead ratio and Pb²⁰⁶/Pb²⁰⁴ and Pb²⁰⁷/Pb²⁰⁴ values will be. The slope of the straight-line fit to 5 meteorite and one terrestrial data thus yields an estimate of ~4.55 Ga. This is also the result from dating of meteorite samples by Rb-Sr and other radiometric methods.

Geological Time and the Evolution of Life

Life and the Geological Scale of Time

It is not clear whether life intrinsically evolved on the earth or, having originated elsewhere, proliferated on the earth after the first oceans appeared ~4 Ga ago. Based on the earliest evidence of life, the 3.7-4 Ga old stromatolites, the first 500-1000 Ma of earth’s history appears to have been altogether barren.

Based on the fossil evidence, we divide the geological time into the following:

Phanerozoic: the most recent 570 Ma of earth's history with a well-preserved record of life, comprising

Paleozoic: the 245-570 Ma era of primitive life-forms (vertebrate life evolved in the early part of this era, with Devonian as the age of fish);

Mesozoic: the 65-245 Ma era that began with the evolution of dinosaurs as also mammals but was dominated by the dinosaurs; and

Cenozoic: the most recent 65 Ma of earth's history that began with the extinction of dinosaurs and has been dominated by the mammals.

Precambrian: the period since the earth's evolution, ~4.5 Ga ago, until the dawn of the Phanerozoic (circa 570 Ma), and therefore named Precambrian, it is divided into:

Proterozoic: an era of primitive life-forms that is usually divided into the

late (570-1250 Ma before the present)

middle (1.25-1.9 Ga before the present); and

early (1.9-2.5 Ga before the present); and

Azoic or Archean: the earliest 2-2.5 Ga of earth's history.

Gradualism, Punctuated Equilibrium and Mass Extinctions:

Evolution of life over the geological times has followed three strands:

	evolution of new species, e.g., the end-Permian appearance of dinosaurs and mammals,
	extinction of some existing species, e.g., the end-Cretaceous extinction of dinosaurs, and
	proliferation of some existing species, e.g., the Cenozoic domination of mammals.

Darwinian evolutionary model sought gradual morphological changes, leading to the evolution of new species, as would result from adaptation to the environmental change. But, compared to this ‘gradualism’, the observed fossil record displays sudden appearance of new species following periods of prolonged morphological statis. The Eldredge-Gould model of ‘punctuated equilibrium’ (i.e., new species appear suddenly when, under environmental stress, portions of the gene pool of some existing species undergo rapid speciation) overcomes this problem.

See, for instance, “Punctuated Equilibrium at Twenty: A Paleontological Perspective” by Donald Prothero (Skeptic vol. 1, no. 3, Fall 1992, pp. 38-47): http://www.skeptic.com/01.3.prothero-punc-eq.html and “Score One for Punk Eek: The fitful evolution of bacteria supports a controversial theory” by John Horgan (Scientific American, July 21, 1996): http://www.sciam.com/article.cfm?chanID=sa004&articleID=000DFABC-A1BF-1C76-9B81809EC588EF21

Want to visit the Grand Canyon?

Click on this profile of the Colorado plateau, shown on the left, to learn about the geology of the Grand Canyon of Colorado river.

Some Other Websites or Links of Interest:

	Dinosaurs: Facts and Fiction (http://pubs.usgs.gov/gip/dinosaurs/) Few subjects in the Earth sciences are as fascinating to the public as dinosaurs. The study of dinosaurs stretches our imaginations, gives us new perspectives on time and space, and invites us to discover worlds very different from our modern Earth.
	Fossils, Rocks, and Time (http://pubs.usgs.gov/gip/fossils/) Fossils are the recognizable remains of past life on Earth and are fundamental to the geologic time scale. To tell the age of most layered rocks, scientists study the fossils these rocks contain. Fossils provide important evidence to help determine what happened in Earth history and when it happened.
	Geologic Time (http://pubs.usgs.gov/gip/geotime/) The Earth is very old -- 4.5 billion years or more -- according to recent estimates. This vast span of time, called geologic time by earth scientists, is difficult to comprehend in the familiar time units of months and years, or even centuries. How then do scientists reckon geologic time, and why do they believe the Earth is so old? A great part of the secret of the Earth's age is locked up in its rocks, and our centuries-old search for the key led to the beginning and nourished the growth of geologic science.
	Our Changing Continents (http://pubs.usgs.gov/gip/continents/) Where were the land areas and oceans of the North American Continent 1 million years ago, compared to their present locations? Was North America always about the same size and shape that it is today? To answer these questions, geologists must interpret the clues they find preserved in the rocks.