Geologic Time: Concepts and Principles

Some history:

            There has been several opinions about the absolute age of the Earth based throughout the human history. We can learn a couple of them:

1. James Ussher archbishop of Ireland: (1581-1656)

He announced that the earth has been created on October 23, 4004 B.C. His calculation was based on recorded history and genealogies described in Genesis. The very young age of  Earth than was reproduced in the bible, so this thinking was general before the XVIII. th century.

2. During the eighteenth and ninetieth centuries

Several attempts were made to determine the age of the earth based on scientific evidence.

    a. Georges Buffon (1700’s)

He thought that the earth formed as a molten ball of iron  gradually cooling down. He heated iron balls and cooled them down. Using his experience he than calculated that Earth has to be at least 96000 years old.

     b. Other Geologists of this time taught that if they new the erosion rate of  different rocks, and had idea of the thickness of missing sequences, that it would be possible to calculate the age of the earth. However because of the uncertainties of the erosion rate of different rocks and the thickness of  missing formations the calculated ages were very different from less than a  million to over a billion years.

    c. Some geologists tried the calculation based of the age of the ocean. They thought that the ocean formed together with the earth.

        John Joly 1899: Thought that the ocean originally was fresh and the salt got into it from eroding rocks. He calculated the earth to be  90 million years old. (his calculation is still much younger because he did not think  about recycling of the salt into the ocean.

During the development of the geological knowledge there were different ideas, and some of them became fundamental geologic principles:

Fundamental geologic principles

Nicholas Steno (1638-1686) Danish anatomist

He observed sediment transport and deposition during stream flooding near Florence:

He established three major principle which are essential even today in geology:

1. Principle of superposition

    Sediment is depositing in order having the oldest layer on the bottom, and younger ones on top

2. Original horizontality

     Sediment is depositing under gravity, therefore the sedimentary layers have to be horizontal.

3. Principle of lateral continuity

      sediments extends laterally in all direction until it terminates against the edge of the depositional basin. (Grand Canyon)

Early thoughts about the earth history:

Neptunism:

Werner 1787; german professor of mineralogy. He was a good mineralogist, but he is the one to remember as the father of the worst idea about the history of the earth. He thought that there was an universal ocean in which most known rocks were formed (he thought that granite, basalt and everything was precipitated from this ocean).

Catastrophism:

Cuvier (1769-1832) French zoologist

He explained the physical and biological history of the earth as resulting from a series of sudden widespread catastrophes. Each catastrophe would account for significant and rapid changes in the earth.  Following the catastrophes new organisms were created or migrated from somewhere else.

He thought of six major catastrophes.

Uniformitarianism

James Hutton: 1726-1797 (father of historical geology)

He did very detailed studies and observations on different rock sequences, and present day processes. These studies are the basis of two important geologic principles:

More fundamental geologic principles

4.The principle of cross-cutting relationship:

This states that igneous intrusions or faults must be younger than the rock it intrudes or displaces.

5.Principle of uniformitarianism:

After observing the processes of wave action, erosion by running water, he started to believe, that “ the past history of our globe must be explained  by what happening today. In other words  The presence is the key to the past!

 He by realizing the real slow geologic processes concluded that the earth was very old. Hutton was not a good publicist. So neptunism and catastrophism were the leading thoughts longer....

Hutton’s ideas were published by Charles Lyell in 1830.

He thought that the earth is in steady-state meaning that the geologic processes operated in the past with the same rate as today. He claimed that conditions for the earth as a whole had remained essentially constant and unchanged through the time. He also figured out a phenomena which stand as a basic principle:

6.Principle of inclusions

this states, that fragments within a larger rock masses has to be older than the rock mass in which they are enclosed.

Geologists today assume that principles and laws of nature are constant but the rates and intensities of geologic processes have varied through time

GEOLOGIC TIME

You probably would think of time as minutes, seconds, or years, months, probably thousand years, and of course not in millions, or billions of years.

We geologists have an other measure of time. We have to think in millions of years.

Probably the best way to get closer to geologic time is, to compare it with one calendar year:

New year’s day as representing 5 billion years ago;

We would figure, that January through October (more than three forth of the earth history) is exceedingly obscure.

The first recognizable fossil animals appeared in mid-November, extinction of dinosaurs occurred on Christmas day, and Homo Sapiens appeared at 11 p.m. on Dec.31.

All of the human history occurred in the last few second of new years eve.

Probably some of you have already picked up some rocks and Wondered about its age. Even possible that someone already answered to your question talking about hundreds of millions of years. Some of you possible were curious how can someone know the age of a certain rock just by looking.

The science which permit us geologists to describe the age of different rocks is called

Geochronology.

This science started about 400 years ago when Nicholas Steno described how the position of strata in superpositional sequence could be used to show the relative age of the layers.

There are two different time scale.

1. Relative ages

2. absolute age

We still use both of them

Early geologist of course used relative ages, because they did not have an exact way of knowing how many time units would be represented in the completed geologic time scale, nor could they know which fossils would be useful in correlation, consequently even the relative  time scale grew as a puzzle in an unsystematic manner. The geologic units were named as they were discovered, and studied. Most of the time the names were borrowed from local geography.

Divisions in the geologic time scale:

Eon (The largest division)

HADEAN

ARCHEAN (OLDEST KNOWN ROCKS)

PROTEROZOIC  (ARCHEAN AND PROTOZOIC TOGETHER WOULD BE CALLED   PRECAMBRIAN)

PHANEROZOIC

            AS A RESULT OF OBSERVATIONS, USING GEOLOGIC PRINCIPLES GEOLOGISTS HAVE DIVIDED THIS INTO THREE

MAJOR SUBDIVISION TERMED ERA:

            PALEOZOIC

            MESOZOIC

            CENOZOIC

ERAS ARE DIVIDED INTO PERIODS, AND EPOCHS.

THESE DIVISIONS OR GEOLOGIC AGES ARE RELATIVE AND ALL REPRESENT INTANGIBLE  INCREAMENT OF TIME.

THE ROCKS LAID DOWN DURING A SPECIFIC TIME INTERVAL (CHRONOLOGIC UNIT) ARE CALLED TIME-STRATIGRAPHIC UNITS.

FOR EXAMPLE THE ROCKS COMPRISING A GEOLOGIC SYSTEM WERE DEPOSITED DURING A GEOLOGIC PERIOD.

The rocks in the Cambrian System were formed during the Cambrian Period.

Absolute ages.

Kelvin 1846

Thought, that the earth was originally molten (temp. raise downward), and since it does not have internal heatsource it is gradually cooling down. (for the gradient he used three times the one we use today). He extrapolated the cooling rate backward, and got a very young age for the earth of 20-30 million years.

He did not know about radioactivity!!!!!

Radioactivity

We know, that chemical elements are made up atoms. In the nucleus there are protons and neutrons. If the number of protons are different, while the atomic number is same, it is called isotope.

Oxygen 16, 18; Carbon 12, 13. Some of the isotopes are so called stable isotopes, meaning that their ratio in the earth is permanent. Some of the isotopes are unstable, meaning, the they will decay into an other element.

In 1896 in Paris Henri Becquerel and his assistants Pierre Curie and Marie Curie figured out about a miracle radiation which was invisible for humans, but exposed a photographic film. Becquerel did not discover new elements, but the magic radiation. The new elements were discovered by Pierre and Marie Curie (Sklodovska), and they named them Radium, and Polonium.

The elements with unstable nuclei (radioactive elements;Parent isotopes)  undergo spontaneous change (radioactive decay), until permanent, stable configuration is reached, called daughter isotope. These process results in three types of emissions from the nucleus at a fixed average rate, characteristic of any particular isotope.

1. Alfa particle (Alfa decay)

Two protons, and two neutrons are emitted from the nucleus, resulting in a loss of atomic number by 2, and mass number by 4. Alfa particle is the same as a Helium nucleus.

2. Beta decay

A fast moving electron is emitted from a neutron in the nucleus, changing that neutron to a proton, therefore changing the atomic number by 1(larger), but with no atomic mass number change.

3. Gamma ray

Rearrangement of the nucleus to a more stable form. There is no change in the atomic nor mass number.

Sometimes there is only one step from unstable to stable isotope for ex.

Rb87 - Sr87 by Beta emission.

Other radioactive elements undergo several decay steps.

U235 decays to lead 207 by 7 alpha steps and six beta steps

U238 decays to lead 206 by eight alpha and six beta steps.

The rate of decay  is constant and characteristic for each element. We refer to the rate in term of half  lifes.

Half life of a radioactive element is the time it takes for one half of the atoms of the original unstable elements to decay into the stable isotope. Since the decay rate is constant, the half life also must be constant.

Half life of different elements are very different

the rate of decay of an elements is not linear, but logharitmic. 

After the formation more atom will decay. People used to make a curve with units of half life on it.

Age Calculation:

Age=ln(daughter/parent+1)*1/l

l=decay constant (can be measure for each radioactive isotope)

Daughter element (amount)

Parent element (amount)

We measure the amount of the different isotopes with mass spectrometer.

Several radioactive isotopes are abundant in rocks, therefore they are useful in geologic dating. They are mostly occur in igneous rocks. If we know the amounts of parent and daughter isotopes presently present in the rocks we can calculate the time elapsed since the rocks were formed.

Problems

The date these minerals will show actually is the so called blocking temperature, when the mineral becomes unchangeable, meaning no more radioactive element will came from outer source, and none will live the system without decay.

During metamorphism high temperature can be a problem. The clock can be restarted. 

The timing should be done based on the length of half lives, meaning, that carbon should not be used for Precambrian rocks.

Carbon 14 is the radioactive isotope of carbon which has two well known stable isotopes 12, 13.

The half life of C14 is 5730 years, so this technique is only good for samples not older than 70,000 years. C14 forms in the upper atmosphere and builds into the plants together with the stable plants. After the plant buries, there will not be change in its C14 content. C14 decays into N14.

FISSION-TRACK dating

There is an other effect in case of some elements during decay; The rapidly moving alpha particle will cause some damage in the crystal structure, which became visible under microscope After etching its surface with HF. The number of damages, are related to the original radioactive isotope content. Than the sample is put into neutron field and forced for the remained U to decay. After this the traces have to be count again, and compare with the first number.

Problem: high temp. damages would heal.