Physical Geology Laboratory

Laboratory 8 - Dating of Rocks, Fossils, and Geologic Events


Terms:


absolute dating

fossil

original horizontality


angular disconformity

fossil succession

radiometric dating


cross-cutting relationship

half-life

relative dating


disconformity

inclusion

superposition


eon

lateral continuity

unconformity


era

nonconformity

uniformitarianism

Objectives:


1.

Learn and be able to apply techniques for relative age dating of Earth materials and events


2.

Use fossils to date some rock bodies and infer some of Earth's history.


3.

Learn and be able to apply techniques for absolute age dating of Earth materials and events.

Concepts:

Relative Time

Relative Time is the order of events - what geologic event came first, what came second, what came third, etc. In determining relative time you are not concerned with the exact time at which an event occurred only when it occurred in relation to other events.

Superposition

If layers have not been disturbed than the younger layers must be on top. Layers near the rim of the Grand Canyon must have been deposited later than those lower down.

Original Horizontality

Sedimentary layers are deposited horizontally or nearly horizontal. If they are NOT horizontal, then, there must have been some sort of disturbance.


Lateral Continuity

When deposited, sedimentary layers continues, unbroken, laterally until they pinch out to nothing or reach the side of their basin.


All three of the above principles are illustrated in this picture of a sedimentary basin.

Cross-cutting Relationships

A fault or dike that cuts across a rock must be younger than that rock. That is, the rock had to already be there for the fault or dike to cut across it.


Inclusions

For a piece of a rock to be included in another rock, then the first rock already had to exist in the location. In this example, for the granite to be included in the sedimentary rock above it, it already had to be in place. Weathering and erosion created fragments that could be included in the sedimentary layer that was later deposited.


Unconformities

Unconformities represent missing time in the rock layer. If the rock record is a recording of geologic events on Earth, then they represent a period for which the recorder was stopped (deposition did not occur) or it was erased (erosion). There are three types of unconformity.

angular unconformity

younger sediment rests on the eroded surface of tilted or folded older rocks.

disconformity

younger sediment rests upon an unconformity above parallel sedimentary beds.



nonconformity

younger sediment resets upon an unconformity above unstratified igneous or metamorphic rock.

Absolute Dating

The Standard Geologic Time Scale

Assignment:


See Lab Assignments Page