"No vestige of a beginning, no prospect of an end."
-- James Hutton, 1788
In this lecture, we'll put a time scale of geological history, first invoking the notion of Relative Time, and then Absolute Time. The key to probing the past is to recognize that the rocks of the continents preserve aspects of their experience (rock formation, incorporation of relics of life existence--fossils, and chemical tracers that give the age of the rock or of its constituents). Geologists have managed to extract remarkable stories by systematic application of scientific method and deductive reasoning to this rock record.
But all of this is a new idea, relatively speaking, and most of human understanding of the rock history of the planet is less than 200 years old. Throughout the preceding two thousand years theological dogma was taken to give the history of the Earth, and there was even a learned study of the biblical record to assess the age of the planet and the history of life (this continues today in the fold of the Creation Research Institute). In the mid-1600s Archbishop Ussher produced a biblical estimate of the origin of the Earth of October 22, 4004 B.C. John Lightfoot pursued this approach in 1654, obtaining the estimate of 9 am (in Mesopotamia) on October 26, 4004 B.C.! Religious doctrine was so firmly established that it was deemed heretical to invoke any arguments contesting this basic timescale, but scientific method was emerging as valid in its own right, so it was still viable to consider the relative time implied by geological structures.
One of the key notions of geological time is the simple idea that processes that produce rock formations are such that usually younger rocks are formed on top of older rocks. This was argued by Steno, who lived from 1638-1686, and is the essence of the Law of Superposition, the notion that rocks are laid down on top of preexisting rocks. Closely related is the law of Original Horizontality, in which most rock formation processes at the Earth's surface are recognized to produce nearly horizontal layers. These ideas are simple, but require an acceptance that rocks are in fact still being created today, by processes of sediment deposition and compaction, or lava flows building layer upon layer. This was observable to a limited extent, but there was a major philosophical difficulty with embracing the notion of Earth as still changing around us (versus the mountains always having been there and always to be there, etc.). The major obstacle to human appreciation of the slow changes in rock structures is the time factor; erosion works very slowly to transport sediments to make new rocks, and the slow vertical motions that take those sediments down to higher pressures that pack them into rocks are not detectable by eye. Only in volcanic areas are the rates of rock production truly rapid enough to reveal the ongoing changes.
The breakthrough in understanding of the implications of the rock record came with the work of James Hutton in the 1780s. Key to his enlightenment was the observation that many rock formations are not now horizontal, but presumably were when they were created. The superposition of flat, younger layers on dipping older layers that were truncated had a vast implication: A huge deformation had occurred to the older rocks, taking them from their original positions, uplifting, deforming and eroding them, then lowering them so that new layers could be superimposed. The contact between the older rocks and the younger rocks spanned a great albeit uncertain gulf of time, and Hutton called the contact an Unconformity. This, combined with the observation that current rock eroding and forming processes act very gradually gave Hutton the conviction that this did not all take place in the last 6000 years, but great amounts of time had transpired. In 1788 Hutton published Theory of the Earth, which was the first true geology book. It was revised in 1795 and in 1802 Playfair published supporting figures of rock formations that illustrated Hutton's logic process (Illustrations of the Huttonian Theory of the Earth). The work of Hutton profoundly influenced Charles Lyell, his disciple, who published in 1833 the Principles of Geology, which made Hutton's approach much more accessible. Lyell's writing in turn strongly influenced Darwin.
In addition to laying out simple concepts of geology, the early 1800's saw the systematic tracking of layers and continuity of structures. This is the essence of Stratigraphy, the subarea of geology that deals with strata or layers that had a uniform origin, such as in a single lava flow or a single depositional unit involving an accumulation of sediments that was later lithified (turned to stone). But, while any layer may have obeyed the basic ideas of Horizontality (at least approximately) and Superposition, all geological formations are laterally finite, and there was a need for a key to recognizing the coincident formation of two separate strata at different locations. The solution to this puzzle came in the form of recognizing that fossils of lifeforms follow the same sequence in different environments. The idea emerged that life forms in the past have had finite durations of existence, and thus, when two different rock formations hold a common fossil type, the rocks are at least as similar in age as the total duration of a particular species existence. Many fossils appear to have a very restricted distribution in the rock record, and thus provide a relative time scale for the age of rocks.
For a fossil to be of greatest value for dating the corresponding life form should have had a relatively short duration of existence and the organism needs to have been widespread, preferably mobile. These two factors make for Index Fossils, which are the key dating agents. The fact that the number and types of taxa change upward in the rock record, with many well preserved index fossils provides the Relative Geological Time Scale. The geologists subdivided relative time into units with different fossil assemblages. With increasing age from the present the units are:
|
ERA |
PERIOD |
EPOCH |
|---|---|---|
|
Cenozoic |
Quaternary |
Holocene |
|
Pleistocene |
||
|
Tertiary |
Pliocene |
|
|
Miocene |
||
|
Oligocene |
||
|
Eocene |
||
|
Paleocene |
||
|
Mesozoic |
Cretaceous |
Late |
|
Early |
||
|
Jurassic |
Late |
|
|
Middle |
||
|
Early |
||
|
Triassic |
Late |
|
|
Middle |
||
|
Early |
||
|
Paleozoic |
Permian |
Late |
|
Early |
||
|
Pennsylvanian |
Late |
|
|
Middle |
||
|
Early |
||
|
Missippian |
Late |
|
|
Early |
||
|
Devonian |
Late |
|
|
Middle |
||
|
Early |
||
|
Silurian |
Late |
|
|
Middle |
||
|
Early |
||
|
Ordovician |
Late |
|
|
Middle |
||
|
Early |
||
|
Cambrian |
Late |
|
|
Middle |
||
|
Early |
||
|
Precambrian |
The flourishing of life that occurred about 600 million years ago leads to the great diversity that allows such a fine relative time scale from the Paleozoic to the Cenozoic. The Precambrian spans a much vaster time duration, but there is little in the way of fossil record to subdivide the time. So, how does this work. Basically, if you pick up a rock that has a particular index fossil in it, say a type of trilobite (a hard-shelled creature like a horse-shoe crab, but different), that will identify immediately that the rock is Cambrian in age. No younger rocks have trilobites, since all of these went extinct at the end of the Cambrian period. The particular species can sometimes place the rock in a specific epoch or yet finer subdivisions of time. Most good index fossils are ocean lifeforms, which are widespread and in rapidly evolving systems. Some creatures, such as crocodiles have been around for great spans of time and so do not provide good time resolution when their fossils are found.
Preservation of a life structure as a fossil is highly selective, and only some forms of life have been so preserved. Probably the vast majority, particularly of soft-bodied creatures like insects left no hard parts that could be silicified or calcified and preserved as rock fossils. Early paleontologists viewed the increasing diversity of fossils found in younger rocks as evidence for a cone of increasing diversity with time, like a upward branching tree with many dendritic extensions from some common root. More recent thought, and the discovery of a great diversity of different life-forms in late Precambrian rocks suggests that actually there was a vast variety of different life complexities, and many whole families failed entirely by extinction, with only one or two successful families persisting to today. This puts the particular flora and fauna found today as much more of a fortuitous selection than had originally been thought. Indeed, the whole pattern of evolution appears to have involved decimation to the same or a greater extent than diversification, at least after the first broad flourishing of complex oxygen utilizing organisms 600 million years ago.
But the Geological Relative Time Scale, as remarkable of a discovery as it is, does not provide the absolute ages that are so profound to an understanding of how evolution has occurred, and how the geological structures around us have developed. This requires absolute time. Prior to 100 years ago, the only real progress on this problem was following the Huttonian approach of appealing to present day processes to infer the age of structures. For example, if we measure the present day rates at which sediment is transported down rivers and deposited in a floodplane or delta, we can estimate how long it would take for steady accumulation of sediments (Sedimentation Rate) to pile up enough material to be turned into stone giving a layered sequence such as that, for example, exposed in the Grand Canyon. This led Hutton to suggest that at least 100's of millions of years were needed, if not an eternity. Especially given the huge gulfs of time not recorded by the rocks when they were uplifted and eroded to be visible today.
Even more direct measures include annual deposits, such as the fluctuating layers in lakes, called Varves or the growth rings of trees. Seasonal changes in sediment supply, growth, organic deposition, etc., allow one to directly count back in time, dating at least 10,000 years of Earth history, but this is but a tiny instant of the history of the planet. How to put it on a more reliable long-term absolute scale?
This issue, born of the debates between the catastrophists and the uniformitarians (as well as the parallel debate sparked by Darwin's application of similar reasoning to invoke long times required for organic evolution), brought in the interest of physicists. One of the most prominent of the day was William Thompson (Lord Kelvin), who provided a 'definitive' estimate of the age of the Earth in 1862. He gave a number of 20-400 million years, later revised to 20-40 million years. His calculation was simple, and seemingly irrefutable (especially by a bunch of rock-loving geologists who don't like math all that much; this is still true today to a large extent). Lord Kelvin simply assumed that the Earth is cooling with time (which it must be), from some initial temperature that was close to or at the melting temperature of rock. The atmosphere sets the surface temperature of the body, and then Kelvin used Fourier's law of Heat Conduction, to estimate the time that it would take for an initially uniform temperature molten rock Earth, to cool to have the present surface temperature and the observed geothermal gradient (the rate at which temperature increases with depth into the crust). The geothermal gradient was directly measured in mines and Kelvin used a number near 25-30 degrees/kilometer, which is accurate. He then solved for the minimum age of the Earth assuming cooling by conduction. While his estimate was still deemed heretical by those preferring a biblical origin, it was too short to sit well with geologists, but no refutation of the calculation came forth for many years.
There were two things wrong with Lord Kelvin's calculation, and this is an essential lesson to remember about science. If the assumptions that are made are wrong, the conclusions tend to be wrong. The assumption that the Earth has no way of sustaining heat as a function of time, and must be cooling from its initial hot temperature is wrong. But it took the discovery of radioactivity and the recognition that many radioactive materials exist in the rocks of the Earth to recognize that Kelvin had erred here. His other assumption, that the Earth cools just by conduction was also flawed, as in fact the primary method of cooling is by thermal convection which drives plate tectonics. But this was not to be fully accepted until 100 years after Kelvin's calculations.
The discovery of radioactivity was key, not only as an erroneous assumption in Kelvin's calculation (one which allowed for a far older Earth to still be as warm as it is today), but also because radioactivity itself is a process that allows us to date Earth materials. Radioactivity was discovered in the 1890's and theoretically outlined in 1902 by Rutherford. The principle idea is that some of the very largest atoms in the periodic chart, all built in the shockwaves of exploding stars, involve so large of a nucleus that they can go unstable spontaneously and shed off parts of the atom, decaying to a new element, which may itself decay, finally reaching a stable element that no longer undergoes spontaneous decay. This process of evolving from a Parent Atom to a Daughter Atom involves the loss of alpha, beta or gamma particles. Alpha particles are the equivalent of helium nuclei (two protons, two neutrons). Beta particles are electrons, and gamma particles are light photons. The likelihood of a particular atom spontaneously going unstable has an assignable probability, with the probability differing for different materials.
We characterize the decay process for a bunch of atoms of a radioactive parent, by determining the length of time that it takes for half of the atoms to decay. This is called the half life of the radioactive substance. After time equal to two half lives, only 1/4 = 1/2 x 1/2 of the parent material remains. After three half lives, only 1/8 = 1/2 x 1/2 x 1/2 remains, and so on. Essentially, the accretion of the Earth incorporated various abundances of radioactive materials into the planet (along with the daughter products from prior decay) and ever since then the radioactivity has reduced the number of parent atoms, while increasing the number of daughter atoms. Substances that decay very slowly provide good time estimates for the age of old rocks into which the radioactive materials got concentrated. Important decaying substances include:
|
U235 -> Pb207 |
704Million year half life |
|
U238 -> Pb206 |
4.47 Billion year |
|
K40 -> Ar40 |
1.25 Billion year |
|
Rb87 -> Sr 87 |
48.8 Billion year |
|
C14 -> N14 |
5573 years |
The decay of Uranium isotopes to lead provides important constraints on the age of the planet. Decay of carbon to nitrogen is valuable for dating organic substances such as charcoal and tree parts. A host of radioactive materials have been used, and strategies evolved to stably estimate how much parent/daughter material exists relative to the initial abundance in a given substance.
Earth Scientists can measure the rates of decay using particle detectors, but the most common approach is to actually count the number of atoms in a given rock sample using a Mass Spectrometer to separate the various radioactive isotopes. Essentially, the rock is vaporized by heat or a laser, and then the rock gas is passed by a magnet, which deflects the lighter elements more than the heavier ones, allowing their relative numbers to be measured. This allows many rock samples containing fossils to be dated, and thus we now have absolute times put on the Geological record. For example, the boundaries of the major eras are now known to be:
Cenozoic/Mesozoic = 65 Million Years
Mesozoic/Paleozoic = 225 Million Years
Paleozoic/Precambrian = 570 Million Years
From Lead isotope studies, the age of the Moon, primitive meteorites, and the Earth is given as 4.55 billion years. This is quite an increase from Ussher, Kelvin, and others, and begins to approach the nearly infinite time envisioned by Hutton. Nonetheless, it is a finite time, albeit vast, and the Earth system probably has a comparable span yet to go.
![[return icon]](../dinoheadt.jpg)
Return to the Earth Sciences 80a Lectures Page