Understanding Geologic Time

How old is it? How do we know?

• Absolute dating – process of assigning a precise numerical age to an organism, object or event

• Relative dating – process of placing objects or events in sequence (first, second, third…)

Uniformitarianism

• James Hutton proposed that laws of nature operate today as they did in the past (take along time)

– Mountain building

– Erosion

– Sediment deposition and lithification to rocks

Principles of Relative Dating

• Original horizontality – sediments are deposited in layers, oldest on the bottom.

• Overlapping features – (cross cutting) if a rock or fault cuts across a rock layer it must be younger than what it cuts across.

• Unconformities – gaps in the rock record where erosion destroys “time” or deposition of new rock does not occur.

Sequence One

Sequence Two

Fossils

• Remains or traces of organisms found in the rock record (usually SEDIMENTARY rocks)

– Correlation – matching fossils/rocks in one area with those in another (if B follows A in all places A is younger than B)

– Index fossil – evidence of organisms that lived over a large area but for a short period of time

How has animal life changed?

How has plant life changed?

Geologic Time Scale

• History of Earth as evidenced by fossils found in the rock record, boundaries determined by:

– Catastrophic geological events

– Major environmental changes

– Extinction and explosion of new life (change in fossils)

Absolute Dating

• Some minerals in IGNEOUS rocks are radioactive and decay in predictable ways (half life)

• Comparison of isotopes for the ‘parent’ and ‘daughter’ atoms provide a numerical “age”

Historical Extinction Events

Has a new Geologic epoch already begun?

Think about why scientists name a new time period.

Epoch

Anthropocence – Age of Man?

Anthropocene?