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?