An introduction

Geologic Time

Which elephant is older????

Oldest

Youngest

These two questions are related to relative time versus absolute ages.Relative time: Which elephant is oldest? Youngest?Absolute age: How old is the oldest elephant? Youngest?

oldest

youngest

Born in 1970

Born in 1999

Rocks Record Earth’s HistoryRocks record geological events and

changing life forms of the past

We have learned that Earth is much older than anyone had previously imagined and that its surface and interior have been changed by the same geological processes that continue today.

The same two questions are frequently asked in the

geosciences. Relative time: Which rock unit is oldest? Youngest

Absolute age: How old is the oldest rock unit? Youngest?

older

younger

975 Ma

375 Ma

I made these dates up!

Three terms Strata-layers of rockStratigraphy- science of studying,

describing, and correlating strataStratigraphic Column- An

ordered sequence of strata.

Discovering Earth’s History

Uniformitarianism means that the forces and processes that we observe today have been at work for a very long time.

Law of SuperpositionIn an undisturbed rock sequence, younger rocks are found on top of older rocks.

UnconformityAn unconformity represents a

long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed.

Principle of Cross-Cutting Relationships

The principle of cross-cutting relationships states that when a fault cuts through rock layers, or when magma intrudes other rocks and crystallizes, we can assume that the fault or intrusion is younger than the rocks affected.

Principle of Cross-Cutting Relationships

Another Example

Oldest to Youngest 1,2,3,4,5