Age Relationships

I. Law of Superposition – the order of layering of rocks in relation to their age

A. The Law of Superposition is a basic method used to age and date Earth History

1. Most sedimentary rock are laid down in horizontal layers

2. The oldest layers are at the bottom and youngest beds are at the top

B. Many rock layers have been deformed by folding and faulting

1. These events make the Law of Superposition

II. Principles of Age Relationship

A. Fossil Correlation uses fossils to identify beds in a sedimentary sequence

1. Fossils are usually found only in sedimentary rocks

2. Fossils help ID the rocks

III. Relative Dating places events in a sequence of order but not how long ago it occurred

A. This method can be used with sedimentary rock, layered igneous rock , volcanic ash and lava flows

B. Relative ages can be determined by geologists

1. Construct a local history of geologic events from rock in a specific area

2. They then match this history with layered rocks from another area

3. This information then helps the geologists arrange the layers in order in which they were deposited

4. This is called a Geologic Column

IV. Rock Record is the history “recorded” in rock

A. Unconformities are surfaces that separate the rock layers

1. The missing layers represent erosion, non-deposition, or both

B. Angular Unconformities are tilted or folded rock layers

1. Angular Unconformities can indicate deposits, erosion and uplifting

2. Unconformities may also separate 2 layers of horizontal beds

3. Gaps that separate 2 horizontal series of beds are called disconformities

V. Fossil Clues and Evidence

A. Fossils are formed when the hard parts of an organism is rapidly covered with sediments

1. Organic remains are most often preserved in Marine Sediments, flood plains or lake deposits

2. Sometimes plants and soft bodied animals and tracks are preserved in sand and mud

3. Petrifaction is a type of fossil preservation

a. Original organic substances are replaced with hard minerals