Unraveling Geological Time

I. How do we take the geometry we see in the rock record and unravel its history and the history of life?

There are four basic fields that help us along:

€ STRATIGRAPHY:

€ BIOSTRATIGRAPHY:

€ GEOCHRONOLOGY:

These principles go back to the 17th century.

II. Most important is the Dane Nils Stensen - AKA ‹ Nicolaus Steno: 1638-1686

Glossopetrae or "tongue stones" were found scattered along some layers in the mountains around Tuscany. They were in fact found imbedded within stone. Steno did a detailed description of a sharks head and reasoned that the Glossopetrae were infact the teeth of sharks that has lived when the mountains were under water.

This lead him to his second work which was of great power and dealt with, the question of how did a solid body (such as the sharks tooth) get within another solid body (i.e. rock)?

"The prodromus of Nicolaus Steno's dissertation concerning a solid body enclosed by a process of nature within a solid"

Steno postulated 4 principles of stratigraphy

€ 1. Principle of superposition of strata

€ 2. Principle of initial horizontality of strata

€ 3. Principle of stratal continuity

€ 4. Principle of cross cutting relationships

ALL ABOUT INFERRING TIME FROM GEOMETRY

Big effect in Great Britian where the next major advance was made.

III. James Hutton

Lived from 1726-1797: during time when natural phenomena were being explained by natural causes.

Observed unconformities, and believed in cycles.

Proposed the Rock Cycle:

EXPLAINED EARTH'S NATURAL PHENOMENA BY NATURAL CAUSES WHICH HE COULD SEE OPERATING TODAY
Led to what we call:

THE PRINCIPLE OF UNIFORMITARIANISM - REVELED IN THE CONCEPT OF DEEP TIME

He viewed the Earth and Earth history as having, "no vestige of a beginning, - no prospect of an end."

IV. William Smith 1769-1839

Age of canal building - Smith was a canal engineer

Discovered that different rock formations contain distinct fossil assemblages and that even isolated areas of rocks could be identified in their sequence by the contained fossils.

Made map of England published in 1814-1815

Strata identified by organized fossils 1816-1819

€ Principle of Biotic Succession

Allowed rocks in disparate areas to be ordered.

V. Principles, evidence, and results brilliantly summarized by Charles Lyell (1797-1875) in "Principles of Geology" 1830; went through 11 editions

Very strong uniformitarian

Felt that magnitude of processes had not changes

Earth must be very old

All the big chnages in Earth History, all due to the sum of small effects

VI. Charles Darwin (1811-1882)

Applied Lyellian Uniformitarianism to organisms

Insensibly graded series - evolution by natural selection

Origin of species - 1859

VAST AMOUNTS OF TIME NEEDED

EXPLAINED WHY PRINCIPLE OF BIOTIC SUCCESSION WORKS

Not the kinds of fossils present, that shells vs. teeth, but rather the specific species of plants or animals present

For example: Ammonites - Restricted to Late Paleozoic and Mesozoic

One particular species Psiloceras planorbis is known only from rocks of early Jurassic age.

€ CONSTANT TESTING AND RETESTING OF CONSISTENCY

VII. BUT HOW TO MEASURE GEOLOGICAL TIME IN YEARS?

€ You could use sedimentation rates.

Determine the amount of sediment being delivered to the sea by rivers. Calculate how this one year's worth of sediment would be spread over the ocean floors and use that rate (thickness of 1 year's sediment / 1 year). Then use this to convert the thickness of rock strata from thickness to years.

[ thickness / ((thickness of 1 year's sediment) / 1 year)) ]

What do you think some of the problems with this would be? (e-mail me some answers!)

€ You could do a similar caculation with the saltiness of the ocean. (e-mail me some suggestion on how to do this!)

VIII. William Thomson (Lord Kelvin) - 1824-1880's

Sun is hot now, how long will it remain so:

1855 - 20-25 my for the right temperature for organisms to exist.

Also similar estimates for age of Earth from cooling from a molten ball.

Most geologists and especially paleontologists suffer from "physics envy" and are susceptible to bludgeoning by math and physics wielded by a strong physicist such as Kelvin.

In Kelvin's 1868 address on geological time he did just that and kept a lot of geologists from following the idea that the Earth was very very ancient.

IX. But in 1899, the geologist T. C. Chamberlin gave an address in which he said,

"Is present knowledge relevant to the behavior of matter under such extraordinary conditions as obtain in the interior of the sun sufficiently exhaustive to warrant the assertion that no unrecognized sources of heat reside there? What the internal composition of the atoms may be is as yet an open question. Is it not improbable that they are complex organizations and the seats of enormous energies? ...No cautious chemist would probably venture to assert that the component atomecules, to use a convenient phrase, may not have the energies of rotation, revolution, position and be otherwise comparable to those of a planetary system. Nor would he probably be prepared to affirm or deny that the extraordinary conditions which reside in the center of the sun may not set free a portion of this energy.

X. THE KEY TO MEASURING GEOLOGICAL TIME IN YEARS TURNED OUT TO BE RADIOMETRIC DATING OF ROCKS AND MINERALS

1896 - Henri Becquerel discovers radioactivity -acts like x-rays discovered by Röntgen and darkens photgraphic plate.

1902 - Ernest Rutherford and Fredrick Soddy framed atomic disintegration theory:

"the atoms of radioactive bodies are unstable and a certain fixed proportion of them become unstable every second and break up with explosive violence, accompanied in general by the expulsion of an alpha- or beta-particle. The residue of the atom, in consequence the loss of an a-particle, is lighter than before and becomes the atom of a new substance quite distinct in chemical and physical properties from its parent."

(alpha-particles are helium atoms and beta-particles are electrons)

1904 - Rutherford described how heat of Earth and Sun could be accounted for by radioactive decay

1905 - Bertram Boltwood, Yale U-Pb, and John William Strutt (with Ra-He) dated various "old rocks and got ages of 400 to 2000 million years

1911- 1927 - Holmes, U-Pb, U-He methods produced the first calibrated geological time scale.

XI. RADIOMETRIC DATING OR ROCKS AND MINERALS - overall statement

1. When a crystal of a mineral forms it starts with some amount of a radioactive element.

2. As the "parent" radioactive element decays, the "daughter" element stays locked within the crystal.

3. No "extra" of the daughter-type element can be present.

4. The decay rates of the element must be known.

5. Then the ratio of daughter to parent can be used to calculate the age of the formation of the crystal.

XII. This has allowed fantastically detailed knowledge about the past both in terms of relative order and in terms of absolute time.

I know its difficult to remember, but here are some acronyms that will help you.