GEOLOGIC TIMESCALE

GEOLOGIC TIMESCALE

 

It should be noted that neither Cuvier or Smith was an evolutionist, and the and the principles of relative dating do not require evolutionary theory as part of the framework.                                          —Bruce MacFadden 1992:123

When geologists and paleontologists talk about the age or time of fossils or rock formations, they use words use words like Pliocene, Upper Pleistocene, Devonian or Jurassic.  This can be really annoying.  I mean what is a Pliocene?  Jurassic has something to do with dinosaurs (actually T. rex was Cretaceous). That is unless you are like me. These words have a meaning to me, like how long ago and for how long of a time.  Pliocene and Pleistocene are periods of time the geologists and paleontologists use. If you understand something of the meaning this blog will be more understandable to you. So we need to talk about how geologists and paleontologists measure time and name it. Geologists use two different time scales:  relative and absolute.  I am going to discuss relative time here.  Absolute time, and how it is determined, will be the subject of another post.

We measure time by clock and calendar. Minutes and hours make up days and days form weeks, months and years. We’re all familiar with this system and generally number our years based on the date of the birth of Jesus (the A.D., anno Domini, and B.C., before Christ, that are sometimes used with the year number). But there are other ways of numbering years. The Muslims use the date of the Hegira (July 16 622) as the year one. Other cultures have used other methods, e.g. the years since the accession of king, emperor, or pharaoh. There has been a trend to recognize the Christian bias of the common numbering system and use C.E. (Common Era) and B.C.E. (Before the Common Era).

Whichever year numbering system is used, they only work for events we have a record of and can determine how many calendar years have elapsed between them and now. (The calculation of the year Jesus was born was wrong so the Christian calendar is off by several years). Even at that, records may be inexact and no precise determination can be made. Then we have “dates” for events like: “in the 11th century B.C.” or “sometime before 960 A.D.” For prehistory (by definition before the written record) we have no way of measuring time by consulting a clock or calendar. We must use other means of determining time.

Whether you are aware of it or not, you use two forms of time: absolute and relative. An absolute date is one like your birth date or the Battle of Hastings on Saturday October 14, 1066 A.D. (or C.E. if you prefer). The Magna Carta was signed after the Norman Conquest because the date of the signing is after 1066, and if you look up the date (June 15, 1215) you can figure out just how many years elapsed between the two events (149 years). An absolute date can be considered as a date fixed in time at a precise point. It is possible to determine how many years (or minutes or hours if the time is figures that precisely) have passed since the event occurred.

Relative time is not a date or time interval that is known with exactitude. Before, after, long ago, recently, or short time before are terms that may be used to describe when an event occurred or how much time has elapsed relative to some other event of date. The Magna Carta was signed after the Norman Conquest. A long time elapsed between the building of the pyramids and the landing on the moon; and Columbus rediscovered the Americas between the two events. Relative time does not use calendar dates. Without knowing when William the Conqueror invaded England or when King John signed the Magna Carta we can determine the chronological sequence of the two events simply by knowing that King John was a descendant of William the Conqueror. Anything King John did must be later in time than anything William did. However we don’t know how long after,even if we know how many generations are between the two (William was the great-grandfather of John). You can estimate how much time might have elapsed between the two, but you won’t know exactly how much time or be sure how accurate the estimate is. We use relative time a lot, as in: “it was late last summer before Labor Day.”

If you looked at the rocks, particularly if they formed a cliff, you would notice the the rocks at the bottom had to have been present before the rocks above were deposited and therefore the lower rocks had to be older.passage of time. The sequence of rocks represented the passage of time, just how much time they had no idea.   Some thought that each strata of rock was the result of a single depositional event, say a flood, and might represent a few days or months. Others thought that the strata represented continuous small-scale deposition over hundreds, thousands, or tens of thousands of years.

When they examined the fossils contained in the rocks they discovered the fossils of animals, mostly unknown in the present world. Using the sequence of fossils, “index fossils,” and the stratigraphic sequence of rocks, geologists were able to correlate rocks across Europe and eventually most of the world. Without knowing how old a rock was or how long it took to deposit the rock (how much time the thickness of the rock represented) they could say that one rock strata in one place was the same age as, older than, or younger than the rock strata in another place. The rock units represented an interval of time and they could determine the relative age of the units.

As the science of geology developed in the last half of the 18th century and early 19th century, geologists developed a time scale.  People realized that the same type of rocks did not always underlie the soil at the surface every where. different rocks were often exposed in different places.  One of the first attempts to explain why there were different rocks and why they changed depending on their location in the rock column was based on the story of Genesis in the Christian Bible.  It was proposed that the rocks had settled out of the primordial ocean, probably on the third day when God created dry land.  The hypothesis was that the rocks had been deposited in the order of their density, heavy rock like basalt and granite had been first, followed by others such as limestone, sandstone, shale, etc.  This scheme did not last long as it became obvious that in places sandstone and shales were inter-bedded in multiple repeating sequences.  The rock types which should have been deposited all over the earth at the same time, in the same sequence, were in fact in deposited in different order in different places.  In France, there was even an area where basalt, which should have been at the bottom of all the rocks, was on top and even looked like it had flowed across the land and even filled existing river valleys. The rock strata indicated that the rocks had formed and been deposited in some other fashion.

Several schemes were developed to organize the rock sequences and the time the represented, so that geologists could hold intelligent discussions that didn’t bog down over describing time in cumbersome terms (e.g. Formation X is about as old as Formation Q and both are older, really, really older than Formation J but not as old as Formation Z). Stratigraphic sequences, chosen from areas where they were well defined and with distinctive fossils, were used as “type formations” to represent a specific time interval. Most of these were given a name generally based on a Latinized name for the region or former inhabitants of the area: the Cambrian from Cambria, the Latin name for Wales,  Devonian for Devonshire, England, Silurian for the Siluries, an ancient Celtic tribe, or Permian for the Perm Mountains in Russia. So, now all you needed to say was that Formation X was early Devonian and everyone could figure out how old it was relative to all the other formations.GSA Geologic Time Scale

The largest geologic time unit is the Era, Eras are divided into Periods, and Periods into Epochs. There four Eras. The Eras are divided into from two (the Cenozoic Era) to five Periods (the Paleozoic Era). The Periods are the commonly used time units except in the Cenozoic Era where the Epochs (seven) are the commonly used time units.  Almost all fossils of mammals occurred within the Cenozoic, so much so that it is also called the Age of Mammals. The appearance of humans occurred in the Cenozoic, actually very near the end and extending into the following Quaternary and Holocene.

Attempts were made to estimate how much time was actually represented by the rocks and how old they were. Figures of 10, 15, or 50 million years were put forth. But it wasn’t until the discovery of radioactivity and radioisotopes that the guess work was replaced by more concrete data and absolute dates could be calculated for the relative time units. I will discuss some aspects of radioisotope dating in a further post on absolute dating methods, but for now, all that matters here is that the radioisotope dates confirmed the sequences determined by both the stratagraphic position and fossil correlations.

Because fossils were used to correlate rock units and to determine the relative age of the units, only those rock units that contained fossils could be dated. The age of non-fossiliferous rocks could only be estimated based on what fossiliferous rocks they were above, below, or between. Underneath all the fossiliferous rocks were entire stratigraphic sequences of rocks which had no fossils because they were believed to have been deposited before there was life on earth (at least before there were life forms with hard parts that would fossilize). These strata (often known as basement rocks) represented an unknown amount of time and, it was believed, vast amount of time (since the Cambrian rocks that overlaid these basement rocks were believed to be the oldest rocks fossiliferous rocks, the rocks below them were called Precambrian).

You may have noticed, although more likely you didn’t, that I have never mentioned the word “evolution ” or “evolved.”  Nothing that I have talked about here requires that there be evolution and as mentioned in the epigraph at the beginning of this post, the early practitioners of relative dating did not believe in evolution.  In fact, relative dating does not even require that fossils be the petrified remains of once living organisms (animal or plant). Only that most fossils have restricted range in time.  They appear in some rocks for a time, disappear, being replaced by other fossils and do not reappear again. I qualified this statement to say most fossils because there are some organisms that have appeared in the fossil record, then disappeared, only to be discovered alive and well.  The coelacanth fish is an example.  These organisms are often called “living fossil.”  The horseshoe crab is another.

 

 

7 thoughts on “GEOLOGIC TIMESCALE

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