Geologic Time Notes By studying the characteristics of rocks and the fossils within them geologists can… interpret the environments in which the rocks were deposited reconstruct Earth’s history possibly predict events or conditions in the future.

Geologic Time Scale is a record of Earth’s history from its origin 4.6 billion years ago to the present This scale shows the similar ages the correlation of geologic events, environmental changes, and the development of life-forms that are preserved in the rock record.

Geologic Time Scale is Divided into: Eon: Largest Division (billions of years before present). ex: Archean, Phanerozoic Era: defined by the differences in life-forms found in rock and measured in hundreds of millions to billions of years, is the second-longest span of time on the geologic time scale. ex: Cenozoic, Paleozoic

Period: measured in tens of millions of years before present Period: measured in tens of millions of years before present. Defined by the life-forms abundance and extinction rate. Third largest span of time. ex: Triassic, Jurassic

Epochs Epochs: even smaller divisions of geologic time measured in millions of years to tens of millions of years

Uniformitarianism: states that processes that have been occurring today such as weathering and mountain building have been occurring since the Earth formed.

Stop here

How to Determine the Ages of the Rocks 1. Relative Dating: estimating the dates of rocks and fossils according to the other rocks and fossils around it. 2. Absolute Dating: Determining the age of rocks and fossils by doing tests to get the exact age.

Principle of Original Horizontality Sedimentary rocks are deposited in horizontal layers

Principle of Superposition: In a rock sequence, the oldest rocks are located at the bottom and the youngest are at the top.

Methods for Relative Dating: Cross-cutting relationships 2. Inclusions 3. Unconformity 4. Correlation

Cross-cutting relationships: states that an intrusion or a fault is younger than the rock it cuts across.

Unconformity: Gap in the rock record usually caused by an eroded surface becoming buried by the deposition of younger rocks.

Correlation: the matching of outcrops of one geographic region to another. Ex: If a sandstone layer in one area contains oil, it is possible the same layer in a different area also contains oil.

Absolute Dating Methods: Radioactive Decay constant break down of an element over time. Is not affected by any sort of physical changes. Radiometric Dating scientist use the decay rate of elements to determine how old things are Half-life time it takes for ½ of the original element to decay into another element Tree Rings Seasonal Climatic Changes Distinctive Sediment Layers.

Fossils: Evidence of once living material 1. Original Preservation:

2. Altered Hard Parts: When the Hard parts of an organism have been removed and replaced with minerals.

3.Molds and Casts: molds are the impression that organisms leave in rocks and Casts are when they are filled in with minerals.

4.Index Fossil: the remains of a plant or animal that can be used to correlate rock layers or to date particular rock layers. 5.Trace Fossils: Indirect evidence of plant and animal life. Ex: tracks