Historical Geology 102
Geologic Time


What is time?
The measured period during which a action, process or condition exists or continues
A continuum which lacks spatial dimensions and in which events succeed one another from past through present to future
Units of timekeeping:
1 year:

Geologic Time Scale

Based on the same time units
Generally greater periods of time

Geologic Time Scale

Geologic Time Scale - An arbitrary chronologic arrangement or sequence of geologic events.
Arbitrary because certain geologic and evolutionary events and features were used to divide the scale into smaller units - these smaller units are not of equal duration.
6 major divisions or time units - Eon, Era, Period, Epoch, Ages, Chrons
Eon is the largest time unit, with each subsequent unit being of a subdivision of the preceding unit and covering a smaller time duration.

Additional information about the Geologic Time Scale can be found at: Geologic Time Scale

Dating Techniques

Two ways in which geologists look at time
Relative Dating
Absolute Dating

Relative Dating

Measure of the sequence of events without knowing the exact date at which the event occurred
What happened first?
What happened next?

Principles of Relative Dating


The order in which rocks are placed or accumulated in beds one above another.
The oldest rocks are on the bottom.
The youngest rocks are on the top.
Superposition Example

Original Horizontality

Based on the assumption that sedimentary rocks were originally deposited horizontally.
Original Horizontality Example

Lateral Continuity

A water-laid stratum, at the time it was formed, must continue laterally in all directions until it thins out as the result of non-deposition or until it abuts against the edge of the original basin of deposition.
Lateral Continuity Example 1

Cross-Cutting Relationships

Any feature that cuts across another rock or structure is younger than what it cuts.


Rock facies which contain inclusions of another rock are younger than the inclusion.
Inclusions Example 1
Inclusions Example 2
Inclusions Example 3


Unconformities are gaps in the rock record.
A piece of time is missing.
Primarily produced by erosion.
Disconformity Example 1
Disconformity Example 2
Nonconformity Example 1
Nonconformity Example 2
Angular Unconformity:
Angular Unconformity Example 1
Angular Unconformity Example 2
Angular Unconformity Example 3

Biologic Succession

Fossils found in one rock layer will differ from layers above and below.
By determining the sequence of fossils in one location, it is possible to correlate rocks from another location if they contain the same fossils.


Theory of Evolution - Natural Selection
Individuals within a species with favorable adaptations or mutations will have the best chance of survival.
Those that survive will transmit those favorable traits to the next generation.
"Survival of the Fittest"
Survival of the most suitably adapted.


Understanding the present is the key to unlocking the past.
Processes occurring today also occurred in the past.

Principles of Relative Dating

Additional information about the Principles of Relative Dating Techniques can be found at: The Science of Historical Geology

Absolute Dating

Measure of the actual date at which an event occurred
Example: Mt. St. Helens erupted on May 18, 1980
Observation of an event

Radiometric Dating

Absolute dating technique that is based on the fact that certain elements are radioactive
Radioactivity is the result of the breakdown of atomic particles
As a result energy is released in the form of radiation

Radioactive Decay

Alpha Decay
Alpha particle - α - 2 neutrons and 2 protons
238U → 234Th + α
Beta Decay
Beta particle - β
234Th → 234Pa + β
- neutron in the nucleus splits into a proton and electron
- the electron is emitted as a beta particle
Gamma Radiation

Parent - Daughter Relation

Parent isotope is the original atom before decay.
Daughter isotope is the atom after decay.
The rate at which decay occurs can then be used to determine the age of a sample.


The half-life decay rate is the time it take for half of the parent isotope in a sample to decay to the daughter isotope.
By measuring the amount of parent material and the amount of daughter material, and by knowing the half-life, the age of the sample can be determined.

Useful Radio Isotopes

Parent-Daughter Isotope Half-Life Dating Range
 Uranium-Lead  238U → 206Pb + α + β  4.5 by  10 my - 4.6 by
 Uranium-Lead  235U → 207Pb + α + β  704 my  10 my - 4.6 by
 Thorium-Lead  232Th → 208Pb + α + β  14 by  10 my - 4.6 by
 Potassium-Argon  40K → 40Ar + 40Ca + β    1.3 by  50,000 - 4.6 by
 Rubidium-Strontium   87Rb → 87Sr + β  47 by  10 my - 4.6 by
 Carbon-Nitrogen  14C → 14N + β  5730 yrs    100 - 50,000 yrs