Geologic Time-Reading the Rock Record

Chapter 29

Relative dating   Rock layers are deposited on

top of rock layers.

  Provides evidence of rock ages

  Grand Canyon

Some layers reveal climate cycles, when the land was submerged beneath shallow seas, gradual uplift, etc..

Relative dating- ordering of rocks in sequence by their comparative ages.   Millions of years after the top layer was deposited the

Colorado River cut it way through and created the Grand Canyon

Relative dating principles 1.  Original horizontality- layers of

sediment are deposited as an even horizontal surface. Layers that are tilted & folded must have been moved into that position by disturbances.

2.  Superposition- undeformed (horizontal) sequence of sedimentary rocks, each layer deposited on top of preexisting layers below. Like layers of newspapers in a recycling bin.

Relative dating principles (cont.)

3.  Cross Cutting- An igneous intrusion or fault cuts through preexisting rock and is younger than the rock which it cuts through.

Relative Dating Principles (cont)

Inclusion- Pieces of one rock contained within another. Any inclusion is older than the rock containing it.

Relative Dating Principles (cont) Lateral Continuity- Sedimentary layers are deposited in all directions over a large area unless some sort of obstruction limits their deposition. Faulting, folding, and erosion can separate originally continuous layers into isolated outcrops.

Relative Dating principles (cont.) Faunal succession- fossil organisms follow one another in a definite, irreversible time sequence. Fossil communities change through time as some species become extinct and new ones appear. Such changes are reflected in the rock record. Fossils provide a key tool for recognizing the relative age of rocks.

  Fossils in the rock could be used to identify other rocks of the same age in other regions of the Earth.

Gaps in the Rock Record   Sediment is always being transported and deposited

  Deposition is ongoing, so are the processes of weathering and erosion, folding and faulting, and crustal uplift.

  This creates time gaps in the rock record.

  These gaps, are called unconformities and are found by observing the relationship of layers and fossils

Radiometric Dating (1/2 Life)

  Relative dating tells us which part of Earth’s crust are older or younger but not the actual age of a rock.

Radiometric dating- process used to determine the actual age of a rock by measuring the ratio between radioactive isotopes and their decaying products.

Half life- is the amount of time it takes for half of the material to decay.