Determining Relative Age The relative age of the earth can be estimated by using the geological history available to us from rock outcroppings.

Rock Outcroppings Layers of rock that appear above the earth’s surface.

Layers of Rock called Strata act like pages in a history book.

Rock Types Igneous rocks and Metamorphic rocks are usually unstratified. Most commonly, Sedimentary layers are used to determine the relative age of rocks.

Law of Superposition Each layer builds on the previous one as sediments are deposited. The law of Superposition simply states that in un-deformed sedimentary rock the older layers are underneath the younger ones.

Along with the law of superposition is the Law of Horizontality Along with the law of superposition is the Law of Horizontality. This law states that as the layers are deposited, they lay horizontal as long as they are undisturbed.

Law of Original Lateral Continuity Layers of rock extend laterally in all directions. The layer of sediment will continue until either it thins to zero at another boundary or it extends to the edge of a basin.

Law of Original Lateral Continuity

Layers of sedimentary build up can be disturbed by geological processes. Volcanic intrusions or plate tectonics can disturb the perfectly horizontal sedimentary layers.

Law of Correlation Layers of strata can become separated over time due to geological processes. Geologists can use the Law of Correlation to match strata from different regions. The law states that strata from different locations can be matched in time based on similar characteristics.

A few keys or basis to matching strata Similar thickness and composition Identification of key minerals or rocks located in the strata Distinctive cycles of rock types Index fossils which lived at a certain period of time Radiometric dating

Law of Faunal Succession Fossils are often a key to matching layers. The Law of Faunal Succession states that older fossils are found in older rocks and thus can provide a time key to help fit the puzzle together.

Unconformity Movement of the earth’s crust or uplifting can cause breaks in the historical layering of strata. In general when there is a break in the geological time layer, it is called an Unconformity.

Nonconformity is when unstratified (igneous or metamorphic) rock lies on stratified rock and there is a break in the historical time line.

Angular unconformity is when layers of rock are folded or tilted and layers build up horizontally over this tilting of layers. The composition of the stratus layers will not be a uniform time line.

Sometimes layers are lifted above the surface but remain horizontal Sometimes layers are lifted above the surface but remain horizontal. Weathering may erase historical strata and then a new layer is formed. This is called disconformity. It is a large break in the time between layers.