Earth and its Geological time scale.:For RAS/RTS Mains Updated syllabus

Over the past 150 years detailed studies of rocks throughout the world based on stratigraphic correlation have allowed geologists to correlate rock units and break them into time units. The result is the geologic column (on next page), which breaks relative geologic time into units of known relative age.

Note that the geologic column was established and fairly well known before geologists had a means of determining numeric ages. Thus, in the geologic column shown below, the numeric ages in the far right-hand column were not known until recently.

Large divisions are Eons – Oldest to Youngest are

  • Hadean (very few rocks of this age are known, thus they are deeply buried if still present at all.
  • Archean (Ancient Rocks)
  • Proterozoic (Proto means early, zoic is life – so this means early life)
    These three units above are often referred to as the Precambrian.

Phanerozoic (means visible life)
The Eons are divided into Eras (only Phanerozoic Eras are shown in the chart). These include, from oldest to youngest:

Paleozoic (means ancient life)
Mesozoic (means middle life, also called the age of dinosaurs)
Cenozoic (means recent life, also called the age of mammals).
The Eras are divided into Periods. The Periods are often named after specific localities.

The Paleozoic Era has the following Periods:

Cambrian
Ordovician (first vertebrate organisms – fish)
Silurian (first land plants)
Devonian (first amphibians)
Carboniferous (in the U.S. this is further divided into:
Mississippian and Pennsylvanian (first reptiles)
Permian
The Mesozoic Era has the following Periods:

Triassic (first dinosaurs)
Jurassic
Cretaceous (first mammals. ended with extinction of dinosaurs).
The Cenozoic Era has the following Periods:

Tertiary
Quaternary
Further subdivisions of Periods are called Epochs. Only Epochs of the Cenozoic Era are shown in the Chart.

Note that for this course, you need to know the Eons, Eras, and Periods in age order. You will not be asked about the Epochs (at least for now). Also, you will not be asked to give the numeric ages for the above (at least for now).
Numeric Ages

Although geologists can easily establish relative ages of rocks based on the principles of stratigraphy, knowing how much time a geologic Eon, Era, Period, or Epoch represents is a more difficult problem without having knowledge of numeric ages of rocks. In the early years of geology, many attempts were made to establish some measure of numeric time.

Age of Earth was estimated on the basis of how long it would take the oceans to obtain their present salt content. This assumes that we know the rate at which the salts (Na, Cl, Ca, and CO3 ions) are input into the oceans by rivers, and assumes that we know the rate at which these salts are removed by chemical precipitation. Calculations in 1889 gave estimate for the age of the Earth of 90 million years.

Age of Earth was estimated from time required to cool from an initially molten state. Assumptions included, the initial temperature of the Earth when it formed, the present temperature throughout the interior of the Earth, and that there are no internal sources of heat. Calculations gave estimate of 100 million years for the age of the Earth.
In 1896 radioactivity was discovered, and it was soon learned that radioactive decay occurs at a constant rate throughout time. With this discovery, Radiometric dating techniques became possible, and gave us a means of measuring numeric age.

Radiometric Dating
Radiometric dating relies on the fact that there are different types of isotopes.

Radioactive Isotopes – isotopes (parent isotopes) that spontaneously decay at a constant rate to another isotope.
Radiogenic Isotopes – isotopes that are formed by radioactive decay (daughter isotopes).
The rate at which radioactive isotopes decay is often stated as the half-life of the isotope (t1/2). The half-life is the amount of time it takes for one half of the initial amount of the parent, radioactive isotope, to decay to the daughter isotope. Thus, if we start out with 1 gram of the parent isotope, after the passage of 1 half-life there will be 0.5 gram of the parent isotope left.

After the passage of two half-lives only 0.25 gram will remain, and after 3 half lives only 0.125 will remain etc.
Some examples of isotope systems used to date geologic materials. Note that with the exception of 14C, all techniques can only be used to date igneous rocks. Some elements occur in such small concentration or have such long half lives, that they cannot be used to date young rocks, so any given isotope system can only be used if the material available is suitable for that method.

Parent

Daughter

t1/2

Useful Range

Type of Material

238U

206Pb

4.5 b.y

>10 million years

Igneous Rocks and Minerals

235U

207Pb

710 m.y

232Th

208Pb

14 b.y

40K

40Ar & 40Ca

1.3 b.y

>10,000 years

87Rb

87Sr

47 b.y

>10 million years

14C 14N

5,730 y

100 – 70,000 years

Organic Material

Example: Potassium – Argon (K-Ar) Dating

In nature there are three isotopes of potassium:

39K – non-radioactive (stable)
40K – radioactive with a half life of 1.3 billion years, 40K decays to 40Ar and 40Ca, only the K-Ar branch is used in dating.

41K – non-radioactive (stable)

K is an element that goes into many minerals, like feldspars and biotite. Ar, which is a noble gas, does not go into minerals when they first crystallize from a magma because Ar does not bond with any other atom.

When a K-bearing mineral crystallizes from a magma it will contain K, but will not contain Ar. With passage of time, the 40K decays to 40Ar, but the 40Ar is now trapped in the crystal structure where the 40K once was.

Thus, by measuring the amount of 40K and 40Ar now present in the mineral, we can determine how many half lives have passed since the igneous rock crystallized, and thus know the absolute age of the rock.
Example – Radiocarbon (14C) Dating
Radiocarbon dating is different than the other methods of dating because it cannot be used to directly date rocks, but can only be used to date organic material produced by once living organisms.

14C is continually being produced in the Earth’s upper atmosphere by bombardment of 14N by cosmic rays. Thus the ratio of 14C to 14N in the Earth’s atmosphere is constant.

Living organisms continually exchange Carbon and Nitrogen with the atmosphere by breathing, feeding, and photosynthesis. Thus, so long as the organism is alive, it will have the same ratio of 14C to 14N as the atmosphere.

When an organism dies, the 14C decays back to 14N, with a half-life of 5,730 years. Measuring the amount of 14C in this dead material thus enables the determination of the time elapsed since the organism died.

Radiocarbon dates are obtained from such things as bones, teeth, charcoal, fossilized wood, and shells.

Because of the short half-life of 14C, it is only used to date materials younger than about 70,000 years.
Other Numeric Age Methods

There are other means by which we can determine numeric age, although most of these methods are not capable of dating very old materials. Among the methods are:

Tree Ring Dating- based on annual growth rings produced by trees.

Fission Track Dating – based on counting scars left by nuclear decay products in minerals

The Magnetic time scale, based on reversals of the Earth’s magnetic field.
Absolute Dating and the Geologic Column
Using the methods of absolute dating, and cross-cutting relationships of igneous rocks, geologists have been able to establish the numeric ages for the geologic column. For example, imagine some cross section such as that shown below.

From the cross-cutting relationships and stratigraphy we can determine that:

The Oligocene rocks are younger than the 30 m.y old lava flow and older than the 20 m.y. old lava flow.
The Eocene rocks are younger than the 57 m.y. old dike and older than the 36 m.y. old dike that cuts through them.
The Paleocene rocks are older than both the 36 m.y. old dike and the 57 m.y. old dike (thus the Paleocene is older than 57 m.y.

By examining relationships like these all over the world, numeric age has been very precisely correlated with the Geologic Column. But, because the geologic column was established before radiometric dating techniques were available, note that the lengths of the different Periods and Epochs are variable.
The Age of the Earth

Theoretically we should be able to determine the age of the Earth by finding and dating the oldest rock that occurs. So far, the oldest rock found and dated has an age of 3.96 billion years. Individual zircon grains in sandstones have been dated to 4.1 to 4.0 billion years old. But, is this the age of the Earth? Probably not, because rocks exposed at the Earth’s surface are continually being eroded, and thus, it is unlikely that the oldest rock will ever be found. But, we do have clues about the age of the Earth from other sources:

Meteorites – These are pieces of planetary material that fall from outer space to the surface of the Earth. Most of these meteorites appear to have come from within our solar system and either represent material that never condensed to form a planet or was once in a planet that has since disintegrated. The ages of the most primitive meteorites all cluster around 4.6 billion years.

Moon Rocks – The only other planetary body in our solar system from which we have collected samples of are moon rocks (samples of Mars rocks have never been returned to Earth). The ages obtained on Moon rocks are all within the range between 4.0 and 4.6 billion years. Thus the solar system and the Earth must be at least 4.6 billion years old.