Geologic Time

Laboratory II Geologic Time Objective: One of the most difficult things to appreciate in any study of the geological sciences is the vast stretches of time associated with Earth history. In this lab you will be constructing a proportional time scale that we will use throughout the rest of the semester. Drawing a time scale will help you begin to grasp geologic time at a deeper level. It surprises most students to learn that the geological time scale used conventionally was devised without any real sense for the age of the Earth. Certainly, the early scientific geologists (like James Hutton 1726−1797) knew that the Earth was older than the 6000 years calculated by Irish archbishop James Ussher (based on a Biblical analysis Ussher concluded that the Earth was created on 23 October 4004 BCE). But they had no real sense for how old. The first scientific estimate of Earth’s age was published in 1899. Physicist Lord Kelvin calculated that the Earth was about 25 million years old based on the rate at which it must have cooled from a molten state1. Instead of being based directly on age, the geologic time scale was based on the observation, originally Hutton’s, that there was a distinct stratigraphic succession of fossil assemblages that never repeated. Hutton observed, and delighted in the game, that one could predict which rock units were associated with one another by their fossil assemblages. This idea was not all that hair−brained even in his day. The French anatomist, Georges Cuvier, was beginning to convince his peers that some species existed only in the fossil record and thus they must have become extinct at some time in the past. The idea of species origination was still some way in the future, but geologists embraced that fossil assemblages varied stratigraphically. In addition to forming the basis of the geologic time scale we use today, we can reason in reverse and age−date rocks by comparing their fossil assemblages with those of the time standards. Look for this in an upcoming lab. In this class, our primary geological time scale reference will be that compiled by the Geological Society of America during the 1980−1990 Decade of North American Geology (DNAG). Subsequent to its publication in 1989, the DNAG time scale has been updated once (in 1999) and will be updated about once a decade into the future. The DNAG time scale includes several important features with which you should become familiar. The time scale (last page of this handout) is presented graphically in four columns, one each for (left to right) Cenozoic, Mesozoic, Paleozoic, and Precambrian (both Proterozoic and Archean). If you look at the left− most heading on each of these columns (Age, Ma) you will note that the scale on each is different. For the Cenozoic each tick represents 1 Ma (Ma = mega annum = million years), for the Mesozoic 2 Ma, for the Paleozoic 5 Ma, and for the Precambrian 50 Ma. Thus, although this time scale give you all the basics, it doesn’t graphically convey the relative lengths of time represented by each of the Eras. The next column to the right on the DNAG time scale describe magnetic polarity. An important tool for temporal correlation in the geologic record is that pattern of polarity reversals in the Earth’s magnetic field. Although one reverse polarity interval looks pretty much like another, the pattern of normal and reverse intervals in a stratigraphic sequence, coupled with other data, can allow for detailed temporal correlation. On the DNAG time scale, normal (black) and reversed (white) polarities are noted. To refer to these normal−reversed couplets, two 1 Kelvin turned out to be wrong (way wrong) primarily because he didn’t understand radioactivity and did not know that the Earth’s core was producing heat by radioactive decay. 1 Arens −GEO 390 numbering systems exist. The first (Anom.) is based on the sequence of marine magnetic anomalies first discovered by looking at the horizontal pattern of magnetic stripes on oceanic crust. The second system (Chron.) refers to a couplet of normal and reversed magnetic polarity as a polarity chron. Both systems are number in sequence beginning in the present and working backward. Because the best magnetic record is found in oceanic crust derived from spreading centers, the scale extends only to the mid Jurassic, the oldest known sea floor. You will note that some polarity chrons contain more than one black and one white stripe. These record couplets of subchrons within the more inclusive chron. In general, polarity chrons are usually on the order of 100 Ka (Ka = kilo annum = 1000 years) to millions of years in duration. Subchrons are shorter. Like just about everything else, the subdivisions of the geologic time scale are hierarchical. The most inclusive subdivision is the Eon (Archean, Proterozoic, Phanerozoic). Eons are divided into Eras (Early/Middle/Late for the Archean and Proterozoic, Paleozoic, Mesozoic and Cenozoic for the Phanerozoic)2. Eras are divided into Periods (Cambrian, Ordovician, Silurian etc.), which are divided into Epochs (Early/Middle/Late for most of the Paleozoic and Mesozoic; Paleocene, Eocene, Oligocene, Miocene, Pliocene for the Cenozoic). The smallest subdivision of time is the Age. The DNAG time scale presents a series of internationally accepted Age names. However, there are still a lot of regional and continental Age systems out there and you will encounter many of these when you begin to read the paleontological literature. For example, the North American Land Mammal Ages are the most common Age scale used in terrestrial rocks of North America. The land mammal ages are defined by the first (but sometimes the last) appearance of a particular taxon. Since land mammals were marching to a different drummer than the marine invertebrates used to create the international ages of the DNAG time scale, the two age systems will correlate irregularly. There’s no real need to worry about this at the moment. However, be aware that there are many age systems out there and you will run into them sooner or later. To the right of the "Age" column in the DNAG time scale are a set of "Picks". These are radioisotopic3 age dates for the boundaries between the ages. These dates were developed from measurements taken at the stratotype section of each age. Like biological species and geologic formations, time intervals have type sections too. Ideally, the stratotype section is one where the transition of key fossils and rocks suitable for radiometric dating co−occur. Stratotype sections are identified by an international committee that takes applications for candidate stratotypes and evaluates each. Sometimes the decision is political, but the stratotype must have the key components against which all other sections, world wide, can be compared. At minimum, this includes the key fossils for the assemblage defining that age and rocks suitable for radioisotopic 2 A few words about writing geological ages. All of the Eon, Era, Period, Epoch and Age names are capitalized. For example, if you want to write about the interval between 16.4 Ma and 11.2 Ma, you will talk about the Middle Miocene. However, if you want to talk about the interval that includes the Aptian, Albian, Cenomanian and Turonian ages of the Cretaceous, you would say something like the mid Cretaceous, because there is no formal "Middle", only the Early Cretaceous and Late Cretaceous (there’s no logical "why", it’s a historical artifact). Also note that "early", "middle" and "late" refer to time intervals, while "lower", "middle" and "upper" refer to rock units. Thus, when you talk about the upper Permian, you are talking about a system of rocks of Late Permian age. 3 Radioisotopic dates are based on the known rates of decay of unstable radioactive isotopes. One would measure the ratio of parent and daughter (decay) product in a rock and use the known rate of decay to calculate age. Some of the most common isotopes used for this type of dating include uranium−238 (decays to lead−206 with a half life of 4.5 billion years), potassium−40 (decays to argon−40 with a half life of 1.3 billion years), rubidium−87 (decays to strontium−87 with a half life of 47 billion years), and carbon−14 (decays to nitrogen−14 with a half life of 5730 years). Needless to say, carbon−14 is not useful for very ancient objects. 2 Arens −GEO 390 age dating of the boundaries. Most importantly, note the difference between the "picks" column and the "age" column to the left. The "picks" column represents an actual age determination (thus each is presented with its uncertainty, the right−most column). The "age" column is simply an interpolation between those data points. Most of the time units in the DNAG time scale were named for the place where they were first described (although this may not be where their modern stratotype is located). For example, Cambrian fossils were first noted in Wales, which was called Cambria by the Romans. The Silurian and Ordovician were carved out of the originally much more inclusive Cambrian and were named for two Celt tribes that defied Roman rule in Cambria. The Devonian was named for Devon in England, while the Carboniferous got its name from its rich deposits of coal (carbón in French). The Permian was first recognized in the old Russian province of Perm. The Jurassic got its name from the Jura Valley in Switzerland, and the Cretaceous took its name from le terrain crétacé (chalky land in French) because of the abundance of chalk in rocks containing the typical fossil assemblage across England, Holland, Sweden, Denmark, Germany and Poland.

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