How Old is Old? Geologic Time Scales

Relative vs. Numerical Age Geologists strive to establish both the sequence of events that produced geologic features AND the date on which each event occurred…. Relative Age: the age of one geologic feature with respect to another

Numerical Age (absolute age): the age of a geologic feature given in years Geologists learned how to determine relative age long before they could determine numerical age!

Unconformities: Gaps in the Record!

Devonian “Old Red sandstone” Unconformity Ordovician sandstone and shale The gray sandstone had been deposited, turned into rock, and tilted BEFORE the red sandstone had been deposited!

Hutton deduced that the surface between the gray and red rock sequences represented a time interval during which new strata had not been deposited at Siccar Point and the older strata had been eroded away.

A surface – representing a period of non-deposition and possibly erosion, an unconformity! Rocks below the unconformity were tilted or folded before the unconformity developed.

Because of unconformities, no single location on Earth contains a complete record of Earth history! Does the Grand Canyon strata represent all Earth’s history?

The Geologic Column Geologists have pieced together a composite stratigraphic column – the geologic column. Remember: Strata – a succession of several layers or beds together

Each segment represents a specific interval of time The largest subdivisions break Earth history into the Hadean, Archean, Proterozoic, and Phanerozoic Eons. The Hadean eon is rarely shown because rocks do not preserve a record of it.

The suffix –zoic means life! Phanerozoic means visible life and Proterozoic means first life.

The Phanerozoic Eon is subdivided into eras. 1. Paleozoic – ancient life 2. Mesozoic – middle life 3. Cenozoic – recent life Each era is divided into periods and each period into epochs.

Naming the Column – between 1760 and 1845 Sometimes by location of fairly complete strata Example: Devonian Period crop out near Devon, England Sometimes by characteristic Example: Carboniferous – contain a lot of carbon!

Using Fossils! Back in the Industrial Revolution – coal was in demand! Excavations of canals for coal and iron were done and these excavations exposed bedrock, which previously had been covered by vegetation.

Engineers learned to recognize distinctive layers of sedimentary rock and to identify the fossil assemblage Fossil assemblage: a group of fossil species found in a specific sequence of sedimentary rock

Other observations included: Principle of fossil succession: In a stratigraphic sequence, different species of fossil organisms appear in a definite order Once a fossil species disappears in a sequence of strata, it never reappears higher in the sequence

This provided the geologic underpinning for the theory of evolution!

Fossils are also used to determine the relative age of the beds! Index fossils (guide fossils): used by geologists to associate the strata with the specific time interval

Simple bacteria and archaea appeared during the Archean Eon, but complex shell-less invertebrates did not evolve until the late Proterozoic.

The appearance of invertebrates with shells defines the Precambrian-Cambrian boundary! Sudden diversity of life over a relatively short period of time – the Cambrian explosion!

Paleozoic – first fish, land plants (Silurian), amphibians (Devonian), reptiles (Carboniferous) Mesozoic – first dinosaurs and small mammals (Triassic), birds (Cretaceous) Cenozoic – diversification of mammals!