Fossils, rocks, and time


What is a rock? A purely descriptive definition is that a rock is -A naturally occurring aggregate of minerals and other solid material. - Usually, there are several minerals in the aggregate, though some rocks may have only one. The other materials may include natural glasses, organic material (lignite, coal, or petroleum), or fossils.

Geologists usually think of rocks in a second important way, however. Please memorize this and recite it like a mantra:

* A rock is a record of the environment in which it formed. *

The three major rock types:

The Rock Cycle: Consider the three basic rock types and how they form:

The material that makes up any rock might have a complex history.

Geologists describe this range of possible histories as the Rock Cycle. As the schematic shows, it actually encompasses many possible cycles.

The three major rock types revisited:

Why do we care?

Because rocks are the record of the environments in which they form. We particularly care about sedimentary rocks, because sedimentary rocks form where life lives. Thus, for us, our primary interest is in sedimentary rocks, the rocks that preserve the record of the history of life.. Thus, in them we find:

Geologic Time:

The science of Geology didn't take shape until the discovery of geologic time (deep time).

Fossils are found in rocks. What was a shark tooth doing inside a rock?

Illustration from Prodromus - NASA Earth Observatory

This led Steno to study how the rocks (which we today recognize as sedimentary) formed. He recognized them to be composed of lithified remains of sediment deposited in layers (or strata) and proposed a set of several principles of stratigraphy by which one could distinguish younger and older sediments. His results were published in De solido intra solidum naturaliter contento dissertationis prodromus (Preliminary discourse to a dissertation on a solid body naturally contained within a solid, mercifully referred to simply as Prodromus) in 1668.
The two most significant principles are:

Using these, it began to be possible to say what order the separate rock layers had formed in, provided they could be seen in association. More importantly, Steno was the first to grasp that rocks could be formed from fragments of preexisting rocks that had been transported as sediments and that different rocks formed at different times. Of course, Steno thought that all of this deposition had occurred during the 40 days and nights of Noah's flood.


Prologue: Remember, Steno, a traditionalist, still thought that all of this deposition took place during the flood. In the next stage of the discovery of geologic time, natural historians took up the question of whether the Earth has a long or short history.

Proposed "Uniformitarianism" in his paper The Theory of the Earth 1795. Simply paraphrased, uniformitarianism holds that

"The present is the key to the past."

Charles Lyell -

This meant that the processes acting on the Earth's surface today were the same ones that had been acting upon it for all of history. This was a momentous and liberating idea because it meant that geologists could reconstruct the history of the Earth by observing processes in the modern world. They didn't have to invoke catastrophes in which the normal rules don't apply. Alas Hutton was a poor writer or popularizer, so his ideal got little attention until it was taken up by Charles Lyell (1797 - 1875) who popularized then in the three volumes of the Principles of Geology (1830-1833). The first proper textbook of geology.

"You will at once perceive," continued Professor Ichthyosaurus, "that the skull before
us belonged to some of the lower order of animals, the teeth are very insignificant the
power of the jaws trifling, and altogether it seems wonderful how the creature
could have procured food."
Note: Although modern textbooks treat uniformitarianism as something unambiguusly good, it has it's downsides:

Green River Shale Formation in Desolation Canyon, UT,
from ENSI (Evolution & the Nature of Science Institutes)
Each meter of sediment (roughly the height of cows in foreground) equals ~7000 years.
Nevertheless, in the 19th century, the effects were mostly liberating. The biggest revelation was simply that the Earth had to be millions, not thousands of years old.

Relative Dating: Thus the notion of Geological time was born. Now the problem was how to measure it. There are now two complimentary approaches:

During the 19th century, however, geologists could only establish the relative ages of rock units. The means to do so had already been provided by Steno and Hutton, but only provided rocks could be seen in direct association.

Once again, the study of fossils provided the stimulus for a major innovation:

William Smith - Wikipedia
Faunal succession:
  • In 1796 William Smith, a British civil engineer, added a fourth principle: Faunal succession, noting that different groups of fossil organisms were preserved in different rock units.

    By this means it became possible to say that one rock was the same age as another rock halfway around the world.

    The Geologic Time Scale: Using Steno's, Hutton's, and Smith's principles, Geologists gradually developed, a standardized a Geologic Time scale developed. Link to it in simplified form or in its full glory. It's major features:

    Sir William Thomson, Lord Kelvin -

    Absolute Dating

    Early attempts: Initially, three lines of evidence were pursued:

    The discovery of radioactivity: Ironically, radioactive decay, which frustrated Kelvin's purpose, ended up providing the true key to the absolute dating of rocks.

    Radiometric dating:

    Thus, sedimentary and metamorphic rocks can't be radiometrically dated.

    Although only igneous rocks can be radiometrically dated, ages of other rock types can be constrained by the ages of igneous rocks with which they are interbedded.