Lecture 2: Unraveling Geological Time

Outcrop in Nova Scotia, Canada of the McCoy Brook Formation of Early Jurassic age in which a dinosaur skeleton was found (box)	Reconstruction by O. C. Marsh of type of dinosaur (Anchisaurus sp.) found on left.

You could make up stories.
But how to decide which is right?

First, we all agree that we are going to make a scientific study. What does that mean?

In Science we make up explanatory stories and try to make generalizations.

Science differs from all other human activities in that there is an external source of judgment:

* Correspondence to reality

* Scientific hypotheses have TESTABILITY.

Many equally valid interpretations are possible of this painting - and you may feel many equally valid emotions.

II. Now in the case of our little dinosaur, we can't start at the beginning - we go into a large body of shared knowledge - i.e. lots of hypotheses which have stood up over many tests - HELPS US TO EXTEND OUR PERCEPTION

The Grand Canyon

There are three basic fields: * STRATIGRAPHY  * BIOSTRATIGRAPHY * GEOCHRONOLOGY

These principles go back to the 17th century.

III. Most important is the Dane Nils Stensen - AKA ? Nicolaus Steno: 1638-1686

"The head of a shark dissected"

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 in fact the teeth of sharks that had 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

IV. Antoine-Laurent Lavoisier

Lavoisier was to become the greatest chemist of the 18th century, but before this, while his chemical interests were still growing, he worked with Jean-Etienne Guettard, a geologist mapping of the geology of France. Gould (1998), has recently pointed out that Lavoisier deduced the vertical succession of rock strata from the map distribution of rock-types and hence translated geometry of the present into a past sequence of events, represented by the succession of rock strata.

Marie-Anne Pierrette Paulze (1758-1836) and Antoine-Laurent Lavoisier (1743-1794). Paulze married Lavoisier at 14. Lavoisier was beheaded in 1794 after the French Revolution; Paulze later married the American physisist Benjamin Thompson (Count Rumford) and they published Lavoisier's Memoires des chemie.

V. James Hutton

Lived from 1726-1797: during a 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."

VI. 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 fossils they contained.

Made a 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.

VII. 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 changed. Earth must be very old. All the big changes in Earth History were all due to the sum of small effects.

Charles Lyell

VIII. 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 (e.g. 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

IX. BUT HOW TO MEASURE GEOLOGICAL TIME IN YEARS?

* You could use sedimentation rates:

X. 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.

XI. But in 1899, the geologist T. C. Chamberlain 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."

XII. 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 photographic plate.

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

XIII. 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.

XIV. 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.

NOW LETS LOOK AT OUR DINOSAUR AGAIN:

Outcrop in Nova Scotia, Canada of the McCoy Brook Formation of Early Jurassic age in which a dinosaur skeleton was found (box)	Drawing of the outcrop on left, emphasizing layering.

Layering in sandstone (a) tilted - must have been more extensive. Lava flow (b) all busted up and abutting sandstone which drapes over it. Sandstone must be younger than basalt.

Dinosaur in sands which look like dune sands, in detail - probably are fossil dunes produced in a desert.

Sands contain blocks of basalt arranged as talus cones - probably are fossil talus cones.

PRINCIPLE OF UNIFORMITARIANISM

Turns out to be a prosauropod dinosaur, probably Anchisaurus - known elsewhere in the world only in rocks of Late Triassic and Early Jurassic age of the Mesozoic -

In the lava flow itself:

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