The history of the Earth is told by the pieces of crust that survive the giant recycling machine of plate tectonics. The problem is that the crust is somewhat like a jigsaw puzzle that has just been dumped out of the box. The pieces may or may not all be there, and they are mixed up all over the place. It takes a clever puzzler to put it back together again. Fortunately, there are some rules that have been devised which help put the puzzle back together again. These are as follows:
Geologists work in two ways. They order the puzzle pieces in terms of relative ages, then they try to put absolute dates on key pieces. Not every rock can be dated in an absolute sense, and the relative sense of ordering cannot possibly give the overall absolute time framework, so the two approaches go hand in hand. First we will talk about setting up the relative framework, then I'll tell you how we put absolute numbers on the geologic ages.
The following rules may seem obvious to you, and they should be. Nonetheless, they are quite powerful and have served geologists well. Please note that they are not LAWS, but rules-of-thumb and sometimes are not valid.
In an undeformed stack of sedimentary rocks, the oldest rocks are on the bottom and the youngest rocks are on the top.
Sedimentary rocks are deposited in continuous sheets that may have extended some considerable distance. The same rock on the opposite sides of a stream bed can be tentatively identified as the same bed.
In general, sedimentary rocks are deposited in nearly horizontal layers. Sedimentary beds which are tilted have been deformed since deposition.
If sedimentary rocks meet at an angle, or are separated by an erosional surface, there is an unconformity, which constitutes a gap in the record. There is a puzzle peice missing. There are four kinds of unconformity: 1) an angular unconformity (shown below) whereby layers meet at an angle, 2) a nonconformity where two different types of rocks meet (for example sediments overlying igneous or metamorphic rocks), 3) a disconformity where an irregular surface indicating erosion occurs between two parallel beds and 4) a paraconformity which is a planar surface between two beds which suggests a period of non-deposition, but no erosion.
If a rock includes another rock (a xenolith for "strange rock"), the xenolith is older.
If a geologic unit cuts another geologic unit (for example a dike or sill), the one that cross-cuts is younger. This also goes for faults. The fault happened AFTER the sediments were laid down.
The physical laws governing the universe operate pretty much business as usual through out geologic history. Processes going on today are similar to those that went on in the past.
Fossils show a progressive devlopment through time, and similar fossils indicate similar age rocks the world over.
Sometimes things go wrong. Meteorites fall from the skies. Ice ages come and go. Catastrophes leave their mark and forever change the face of the Earth.
Using the above principles, it is possible to stack up the pieces of the puzzle into an ordered pile. This ordered sequence is known as the Geologic Time Scale:
Geologic history is broken into time spans with varying durations, from longest to shortest: eons, eras, periods, and epochs. The Phanerozoic eon represents the time during which there were multi-celled plants and animals sufficiently sophisticated to leave hard parts behind (phanerozoic means "apparent life"). The Phanerozoic eon is broken into the Paleozoic, the Mesozoic and Cenozoic eras. These in turn are broken into periods. The end of the Cretaceous period is marked by the demise of the dinosaurs (along with a large fraction of every other living thing) when a large meteorite hit the Earth some 65 million years ago. You should know the geologic time scale to the level of the period. There are many mnemonics to help you remember them. My favorite is a non-sense word: COS DE CARPT-JuCr PEOMP (the PEOMP part are the epochs of the Tertiary period).
I just mentioned a meteorite impact that occurred some 65 million years ago. Nothing in what I have told you so far can give you that precise an age. In fact, counting up all the unconformities and estimating rates of sedimentary deposition from the principles of uniformitarianism led early naturalists to guess that the Earth was many millions of years old - they never dreamed that it was BILLIONS of years old!
The method of providing absolute ages to the geologic time scale became possible when radioactivity was discovered at the end of the last century. In the first few lectures I mentioned that certain isotopes of certain elements were unstable and underwent radioactive decay. Radioactive "parents" decay to stable "children" according to the following curve:
The time scale is determined by the half-life or the time it takes for half the parent to decay away. The age of a particular sample is determined by comparing the ratio of parent to child, assuming there was no child in the sample to begin with, and none has been lost in the mean time. All radioactive elements decay in the same way, just some take a long time and some decay very rapidly. For a material to be useful to geologists, it has to have a half-life on the order of geologic processes and be around. Here is a list of commonly used isotopes and their half-lives:
1.25 billion yrs
48.8 billion yrs
14 billion years
704 million years
4.47 billion years
Because of the requirement that no child product be incorporated in the material to begin with, the minerals that are favored separate parent from child. Igneous rocks do this pretty well by excluding gases like argon and separating rubidium from strontium (these partition into different minerals during crystallization). Dating minerals in sediments generally will give you the age when the mineral formed - not the sedimentary rock, so geologists favor igneous rocks for dating purposes.
Most of the isotopes used for dating were made billions of years ago in a super-nova explosion, like the rest of the stuff we are made of. However, please observe the short half-life of for example, carbon-14. This could not possibly have survived from before the birth of the Earth and in fact is made in the upper atomosphere by bombardment of cosmic rays. Carbon is also different in that it is incorporated into organic material. It is used for dating things like trees, fires, cloth, soils, corals, etc. and is only good for the last 50,000 years or so.
Over the last few decades, geologists have found datable horizons which could be identified as a particular piece of the geologic time scale. For example, an volcanic ash bed found in association with a particular fossil horizon, can be dated. The age thus derived then applies to that fossil horizon everywhere. In this way, we have pulled ourselves up by our bootstraps and placed absolute ages on the geologic time scale.
The geologic time scale now serves as a means for discussing rates of processes, like sea floor spreading, or the opening and closing of great ocean basins. Rates of mountain uplift and erosion can be determined and rates of extinction of flora and fauna as well. This is important, so that we can place our current lives in context. How rapidly can climate change? (Frighteningly fast!). Is that rate of extinction currently observed "normal" (yes, if you consider what happened at the end of the Cretaceous! - and that was not a pretty sight!).
These are some excellent links: