Geologic Relative Age Determination

•Relative time -- Subdivisions of Earth's geologic history are related to actual rock units arranged in a specific order based on relative age relationships (most commonly the stratigraphic position). These subdivisions of the geologic time scale are given names (based on places where particular rocks were first described). These subdivisions are recognized globally, usually on the basis of fossil content.

•Absolute time -- Numerical ages in "millions of years ago". These dates are most commonly obtained from radiometric dating methods using carefully chosen rock samples.

Geological Views of Time

http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html

2011 Geologic Timescale

Uniformitarianism• States that the

processes occurring today have been occurring since Earth formed. Only the rate, intensity, and scale have changed.

Principle of Original HorizontalitySedimentary rock layers (and lava flows) are formed in a horizontal orientation. Any folding or tilting must have happened sometime after the layers were originally formed.

The Law of Superposition is one of the most basic principles of geology . This law states that younger rock layers will be deposited on top of older layers, during normal conditions of deposition. This law is the basic principle of stratigraphy, the study of sedimentary rock layers. Stratigraphy is still the single best method that geologists have for determining the relative ages of rock sequences.

Law of Superposition

The Principle of Cross-Cutting Relationships states that an igneous intrusion (or a fault) is always younger than the rock it cuts across.

In the diagram at the left, the igneous intrusion (E) must be younger than rock layers A, B, C, and D because it contains inclusions from those layers.

The Principle of Inclusions states that the rock unit that contains inclusions is younger than the rock that the inclusions came from.

Gneiss in Granite

Unconformity• An erosional

surface might become buried by the deposition of younger rocks.

• This buried erosional surface results in a gap in the rock record.

2 Types of Unconformities• Disconformity: an

unconformity (erosion) between two parallel/horizontal rock layers

• Angular Unconformity: An unconformity between 1 horizontal rock layer and 1 diagonal (angled) rock layer.

Correlation• Matching

outcrops of one geographic region to another.

• Occurs near and far

• Examples?

Relative Geologic Dating

Unconformity Types

C

F

A

D

B

E1. Deposition of sediment to form Sandstone (E).

2. Deposition of sediment to form Shale (B).

3. Deposition of sediment to form Limestone (D).

4. Tilting and Erosion

5. Deposition of sediment to form Conglomerate (A).

6. Faulting (F)

7. Intrusion of magma, cooling to form Basalt (C).

F

G

B

L

A

C1. Deposition of Sediment to form Shale (C)

2. Deposition of Sediment to form Sandstone (A)

3. Deposition of Sediment to form Limestone (L)

4. Deposition of Sediment to form Sandstone (B)

5. Intrusion of Magma, Cooling to form Granite (G)

6. Faulting (F)