1. Relative Dating of Rocks

2. Relative Dating and the Law of Superposition
Youngest layer of rock
Relative dating is determining the age of a rock or geologic event when comparing it to other rocks or geologic events.
The Law of Superposition states that
“…in an area of undisturbed strata, the bottom layer would be the oldest (since it was the first to form). Each layer above the bottom layer would be progressively younger. The top layer of exposed strata would be the youngest (since it was the last to form).”
Oldest layer of rock

3. Exceptions to the Law of Superposition
Much of the rock strata observed on Earth’s surface has been impacted by the crustal dynamics of the Earth.
“How can a scientist determine the relative age of rock layers under these conditions?”
Relative Dating of undisturbed rock? EASY!!!!!
Relative Dating of disturbed rock? MORE DIFFICULT!!!!!

4. Dating Rock Features
An igneous intrusion in younger than the rocks it cuts through
“A” is younger than B, C, and D
An igneous extrusion in younger than the rocks it cuts through
“D” is younger than
E and F

5. Dating Rock Features
An unconformity is a buried weathered or erosional surface. An unconformity is younger than the disturbed rock layer.
Layer J was deposited and lithified first. Layer J was weathered and/or eroded (unconformity). Lastly, Layer K was deposited and lithified.
An inclusion is a portion of rock that is within an igneous intrusion. For various reasons, the portion of rock did not melt and would be older than the intrusion it is within.
The rock fragments with intrusion “M” are older than “M”.
Layers Q, O, N. M and L were originally deposited as horizontal layers. These layers were then tilted. The tilting of the strata is younger than the formation of the strata.
The event of deformation of strata is younger than the strata itself.

6. Relative Dating – Sample #1
Determine the relative age of the rock and geologic events from the strata seen below. Arrange the strata from youngest to oldest.

7. Relative Dating – Sample #2
Determine the relative age of the rock and geologic events from the strata seen below. Arrange the strata from youngest to oldest.