Fossils are found in rocks. What was a shark tooth doing inside a rock?
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 coud distinguish younger and older sediments.
His results were published in De Solido Intra Solidium Naturaliter Contento Dissertationis Prodromus (AKA Prodromus - Preface to a Dissertation on a Solid Naturally Contained within a Solid), 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: 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.
Hutton had his epiphany at Siccar Point. Which rocks are younger, the ones above or below?
"The present is the key to the past."
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. In some ways, Hutton and his intellectual successor Charles Lyell went too far.
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. (Scroll to the bottom of this link for and example.)
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:
Faunal succession: Our understanding of fossils has a varied history:
By this means it became possible to say that one rock was the same age as another rock halfway around the world.
However, you have to use the right fossils:
The Geologic Time Scale: Using Steno's and Smith's principles, Geologists gradually developed, a standardized a Geologic Time scale developed. It's major features:
Several early attempts were made to come up with absolute dates, based on things like thickness of sediments and ocean salinity. The attempt with the most influence - for good or ill - was based on .....
The discovery of radioactivity: Ironically, radioactive decay, which frustrated Kelvin's purpose, ended up providing the true key to the absolute dating of rocks.
Result is that oldest known Earth rocks are aprox 3.8 g.a. Oldest in Solar System 4.56 g.a.
|Radioactive isotope||Daughter substance||Half Life||Applicable range|
|Uranium 238||Lead 206||4.5 g.a.||10 m.y. to 4.6 g.a.|
|Potassium 40||Argon 40||1.3 g.a.||100,000 to 4.6 g.a.|
|Carbon 14*||Nitrogen 14||5,730||to 70,000|
* used in plant material. Note that the effective range of these dating systems is limited by the degree of error in measurement.
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.
The Earth generates a magnetic field that encompasses the entire planet. In the last fifty years, a new dating method has emerged that exploits two aspects of rocks' interactions with the Earth's magnetic field. It is, in essence a form of relative dating.
The result is that the paleomagnetic polarity of igneous rocks is either:
What kinds of rocks retain paleomagnetism:
Result: We can hang a numerical age on these rocks if their paleomagnetic "fingerprint" can be matched with that of a sequence of igneous rocks that can be radiometrically dated.
The utility of paleomagnetism:
|A clastic sedimentary rock is made of grains of preexisting sediment that are all older than the rock they make up. What if you want to know the age of the grains (and therefore the age of the preexisting rock they came from?) The microscopic crystal lattice of some minerals (notably apatite, zircon and monazite) is damaged by the particles that fly off when a radioactive nucleus decays. This damage can be seen as fission tracks. The density of fission tracks in a mineral grain are a good indicator of its age, thus fission track dating has emerged as a means of determining the age of individual crystal grains. The winner so far, a zircon grain from the Jack Hills of Western Australia dated to 4.4 billion years. (As compared to 3.8 billion for the oldest whole rocks.)|