Geologic Time

History Archbishop James Ussher 1581-1656 Studied writings and the bible Calculated age of the Earth Earth ~4000 years old Created around 6 pm on Sunday, October 22, 4004 bc

History Catastrophism Changes happen quickly

History James Hutton (1726–1797) Father of modern geology Watched erosion and sediments in a stream—took a long time to happen Earth is much older Uniformitarianism –  Present is the key to the Past Things that are happening now also happened in the past.

History Most modern geologists Combination of both Earth is very old Changes happen very slowly  Formation of layers Weathering and erosion Some catastrophic events occur quickly Earthquakes, volcanoes, meteors, etc.

Law of Superposition Bottom = older Top = younger

Law of Horizontality Layers form horizontally If not horizontal, assume they have been tipped.

Law of Cross Cutting Relationships If a feature cuts across another feature, then that feature must be younger.

Law of Inclusions An included rock is older than the rock around it.

Sedimentary Structures Stratification - bedded or layered  a. Different kinds of stratification (layers look different) are different environments of sedimentation. 

Unconformities Break in the Geologic Record Sometimes means missing information Angular Unconformity Non-conformity Dis-conformity (miss)

Angular Unconformity Sedimentary layers deposited horizontally, tilted upward, eroded back to level, more layers deposited on top. Lower layer meets upper layer at an angle

Development of an angular unconfomity

Angular Unconformity

Nonconformity Igneous/Metamorphic rocks form, pushed to surface, erosion, sedimentary rocks deposited.

Disconformity Sedimentary layers deposited horizontally, deposition stops, erosion, deposition resumes. Loss of layers = loss of time

Fossil Succession Fossil organisms succeed one another in a defined pattern. We know which came first, etc.

Geologic Time There are two ways of dating geological materials. Relative Age – This rock is older or younger than that rock. (you are older or younger than a sibling) Absolute Age –this rock is 3.2 million years old

Relative Ages Law of Superposition Law of Original Horizontality Fossil Succession Crosscutting Relationships

The geologic time scale – Earth’s “calendar” of history. Subdivides geologic history into units. Originally created using relative dates. Divisions are made when a major event started or ended Structure of the geologic time scale Eon – The greatest expanse of time (1000s of Ma). Era – subdivision of an eon (100s of Ma) Periods  Epoch