Relative Dating of Rocks

Relative Dating and the Law of Superposition

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

Youngest layer of rock

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!!!!!

Dating Rock FeaturesAn igneous intrusion in

younger than the rocks it cuts through

An igneous extrusion in younger than the rocks it

cuts through

“A” is younger than B, C, and D

“D” is younger than E and F

Dating Rock Features

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”.

The event of deformation of strata is younger than the strata

itself.

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.

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.

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.

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