Ch. 13 Section 2Ch. 13 Section 2

Relative Ages of Rocks Relative Ages of Rocks

Principle of SuperpositionPrinciple of Superposition

The The principleprinciple of of superpositionsuperposition states states that in that in undisturbedundisturbed layers of rock, layers of rock, the the oldestoldest rocks are on the bottom rocks are on the bottom and the and the youngestyoungest rocks are at the rocks are at the top.top.

This assumes that This assumes that sedimentarysedimentary rock rock layers are horizontal and have not layers are horizontal and have not been disturbed. Rock layers can get been disturbed. Rock layers can get turned by things like turned by things like tectonic forcestectonic forces, , so scientists have to use other clues.so scientists have to use other clues.

Relative AgesRelative Ages

The Relative Age of something is its The Relative Age of something is its age in comparison to the ages of age in comparison to the ages of other things.other things.

Relative age Relative age does notdoes not tell you the tell you the age in actual age in actual yearsyears only that a layer only that a layer is younger than the layer is younger than the layer belowbelow it, or it, or older than the layer older than the layer aboveabove it. it.

Principal of Cross-Cutting RelationsPrincipal of Cross-Cutting Relations

The principal of The principal of cross-cuttingcross-cutting states states that it rock layers are disturbed by that it rock layers are disturbed by things like things like faultsfaults (breaks in the (breaks in the earth’s surface), or igneous earth’s surface), or igneous intrusionsintrusions, then you know that the , then you know that the rock layers are rock layers are olderolder than the fault or than the fault or intrusion.intrusion.

FossilsFossils can also help determine can also help determine relative ages of rocks.relative ages of rocks.

UnconformitiesUnconformities

Most rock layers are Most rock layers are incompleteincomplete.. Gaps in the rock sequences are Gaps in the rock sequences are

called called unconformitiesunconformities.. These develop when things like These develop when things like

running waterrunning water or or glaciersglaciers wash or wash or scrape rock layers away.scrape rock layers away.

There are There are 33 types. types.

1. Angular Unconformities1. Angular Unconformities

Horizontal rock layers are Horizontal rock layers are tiltedtilted and and uplifteduplifted..

ErosionErosion and and weatheringweathering wear down wear down the tilted rock layers.the tilted rock layers.

New sediment layers are New sediment layers are depositeddeposited horizontally on top of the older tilted horizontally on top of the older tilted and eroded layers.and eroded layers.

2. Disconformity2. Disconformity

A layer of horizontal rock was A layer of horizontal rock was exposedexposed and and erodederoded..

Younger rocks were formed by Younger rocks were formed by deposited deposited sedimentsediment over the old over the old erosion site.erosion site.

The rock layers are still The rock layers are still horizontalhorizontal, , but layers are missing.but layers are missing.

3. Nonconformity3. Nonconformity

MetamorphicMetamorphic or or IgneousIgneous rocks are rocks are uplifted and eroded.uplifted and eroded.

SedimentarySedimentary rocks are then rocks are then deposited over the eroded rock deposited over the eroded rock layers.layers.

Matching up the rock layersMatching up the rock layers

Geologists match up layers or Geologists match up layers or rocks rocks over long distances.over long distances.

To do this, they can sometimes walk To do this, they can sometimes walk along the layers to prove that they along the layers to prove that they are are continuouscontinuous..

Or, if the same type or Or, if the same type or fossilsfossils are are found in the same rock layer in 2 found in the same rock layer in 2 different places, it’s possible the different places, it’s possible the rocks are the same age.rocks are the same age.