Geologic Time Scale

Units of Time

•Geologists have divided the history of Earth into time units based upon fossils contained within the rocks.

•Units are part of the geologic time scale – a record of Earth’s history from its origin to the present

•Allows scientists to correlate important geologic events

How do we divide time into different units?•Oldest division of time is at the bottom of

the geologic time scale•As you move up the scale, each division is

younger, just as the rock layers generally get younger as you move up.

•Time scale is divided into units called eons, eras, periods, and epochs

Eon

•Longest time unit measured in billions of years.

•Based upon the abundance of certain fossils

•There are 3 eons▫Phanerozoic – Present▫Proterozoic – Ended 2.5 billion years ago▫Archean – Ended 4.6 billion years ago

Era•Divides time into hundreds of millions of

years•Marked by major, striking, and worldwide

changes in the types of fossils present•The Phanerozoic Eon is broken into 3 eras

▫Cenozoic – recent life▫Mesozoic – middle life

End marked by large extinction (dinosaurs)▫Paleozoic – old life

End marked by largest extinction event in Earth’s history

Periods

•Divides time into tens of millions of years•Defined by the life forms that were

abundant or became extinct, or by the geologic region in which rocks of the age were first observed

•Several periods are of special significance

•Cambrian Period – 540 M.Y.B.P.▫Trilobites, brachiopods, other marine

invertebrates•Permian Period – 290 M.Y.B.P.

▫Reptiles evolve•Jurassic Period – 208 M.Y.B.P.

▫Dinosaurs dominant, first birds appear•Cretaceous Period – 146 M.Y.B.P.

▫Angiosperms appear▫Dinosaurs become extinct

Epochs

•Smaller divisions of time usually measured in millions of years to tens of millions of years

•Holocene Epoch – 0.01 M.Y.B.P.▫Homo sapiens evolved▫Most recent ice ages▫Grand Canyon forms

Relative-Age Dating of Rocks•The relative age of something is its age in

comparison to the ages of other things•Geologists determine the relative ages of

rocks and other structures by examining their places in a sequence.

•Relative age determination doesn’t tell you anything about the age of the rock layers in actual years.

•Determination of relative age is easy if the rocks haven’t been faulted or turned upside down

Uniformitarianism

•In 1770 James Hutton began observing and attempting to explain Earth’s landscapes

•Came up with the idea of Uniformitarianism▫States that the processes occurring today

have been occurring since Earth formed. Only the rate, intensity, and scale with which

they occur have changed

Geologic Principles• Original Horizontality

▫Sedimentary rocks are deposited in horizontal or nearly horizontal layers We can assume that the oldest rocks are at the

bottom and that each layer going upward is younger• Superposition –

▫ In an undisturbed rock sequence the oldest rocks are at the bottom and younger ones are on top

• Cross-cutting relationships▫An intrusion or fault is younger than the rock it

cuts across

Unconformities

•Most rock sequences are incomplete—layers are missing. These gaps in rock sequences are called unconformities.

•Unconformities develop when agents of erosion such as running water or glaciers remove rock layers by washing or scraping them away.

Disconformity

•Suppose you’re looking at a stack of sedimentary rock layers

• If you look closely, you might find an old surface of erosion.

•This records a time when rocks were exposed and eroded.

•Even though all rock layers are parallel, the rock record still has a gap.

•This type of unconformity is called a Disconformity.