Notes on Basic Geology
Notes created & information organization based on the book:
The Dynamic Earth - an introduction to physical geology"
Brian Skinner & Stephen C. Porter   (further book information here)
also look at for additional resource information
Geologic Time - Page 2

Time Measurement using Radiation

In 1896 the discovery of radioactivity changed the way we measure time.

Remember isotopes are atoms with different numbers of neutrons but the same number of protons. 12C, 13C, 14C are the three isotopes of carbon.

The process of changing the ratio of protons to neutrons in the decay process.

• beta emission

neutron ---> proton + electron-

Notice though, this changes the number of protons in the nucleus, so this kind of decay actually changes the atom to another one.

positron emission

proton ---> neutron +positron+

positron emission reduces the number of protons in the nucleus and increases the number of neutrons.

• electron capture

proton + electron- ---> neutron

Electron capture converts one proton in to a neutron.

• alpha particle emission

loss of (2 protons + 2 neutrons)

The loss of an alpha particle only occurs in atoms with high atomic number.

gamma ray emission
a high energy emission that happens in-concert with one of the above processes.
From these 5 types of radioactive decay we will look at 3 that are important to measurements of geologic time. (beta decay, electron capture and alpha decay.)
Rates of Decay

Decay rates are unaffected by chemical environment, or physical environment.

They are not influenced by any geologic process.

They proportion of atoms that decay during any unit of time are always the same.

The rate of radioactive decay is determined by the Half Life, which is the amount of time it takes for exactly one half of the original atoms to decay.

Assume we start with 1.000.000 atoms with a half life of 1 hr.

Time 0
Time 1 hr
Time 2 hr
Time 3 hr
Time 4 hr
Parent Atom
Daughter Atoms
Here is the key point === the number of parent and daughter atoms is always a constant and the starting number is equal to the sum of the remaining + decayed.
Potassium - Argon Dating

Because argon is a gas it is easily strapped in crystalline structure and because it is an "Inert Gas" it stays there without reacting.

40K + electron- ---> 40Ar .................40K ---> 40Ca + electron-

Potassium (K) always changes to Ar at a 12% ratio.

It has a half life of 1.3 BILLION years.

Because a rock will not rap any Ar gas as it cools, and the act of crystallizing also purifies the rock, any Ar measures in the future from the rock must have been put there by the radioactive decay process.

So if we find an igneous rock with a potassium mineral, we can measure the amount of potassium remaining and the amount of Argon present. From these two value we can calculate the time the rock crystallized.

It can be used for rocks as young as 20,000 years.

Carbon 14 dating

A unique dating system: 14C ---> 14N + beta-

1.) it has a short half life. (5370 years)

2.) the process of decay is reversible (14N + neutron (cosmic radiation) ---> 14C

What does this mean?

The rate of production of 14C and the rate of decay of 14C is nearly constant in the air because the two rates are fundamentally the same.

As a result we can measure the ratio of carbon isotopes in a previously "living sample" as there will no longer take up new carbon, then the ratio of isotopes will tell us the age because there will be no new 14C being added after death. It is good to measure times between 100 - 70,000 years.

In summary there are other radio isotopes also used to get different ages They include Isotopes of Uranium and Thorium (very old measurements), and Rubidium also very old measurements.