History of the Earth: Geologic Time and Rock Stratification

Stratified Layers in Rocks

• The primary reason for the occurrence of stratified layers in rocks is due to the deposition of sediments over time, which leads to the formation of distinct layers with different characteristics.

Determining the Age of Fossils

• Scientists determine the relative age of fossils in stratified rocks by analyzing the position of the fossil within the rock layer, with fossils found in lower layers being older than those found in upper layers.
• The principle of superposition is used to determine the relative age of fossils, which states that older rocks are buried beneath younger rocks.

Index Fossils

• Index fossils are significant in defining subdivisions of the geologic time scale because they are found in specific rock layers and are characteristic of a particular time period.
• The presence of an index fossil in a rock layer indicates the age of the layer, allowing scientists to correlate the layer with other layers of similar age.

Relative and Absolute Dating

• Relative dating determines the age of stratified rocks relative to each other, without determining their exact age.
• Absolute dating, on the other hand, determines the exact age of rocks in years.

Stratification of Rocks

• The stratification of rocks primarily indicates the sequence of geological events over time, with each layer representing a specific period in the Earth's history.

Showing Relative Ages of Rock Layers

• Layers of stratified rocks are used to show the relative ages of different rock layers by analyzing the position of the layers and the characteristics of the rocks within each layer.

Relative Age Determination

• The method used to determine the age of rocks by comparing them with rocks in other layers without figuring their exact date is called relative dating.

Law of Stratigraphy

• The law of superposition states that in the sequence of sedimentary rocks, those below the bed of rocks are older and those above are younger.

Unconformity

• A hiatus unconformity represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed.

Hiatus Unconformity

• The name given to the long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed is a hiatus.