Relative and absolute age dating

The nature of an unconformity can change with distance.

Notice how if we are only examining a small area in the figure above, we would determine a different type of unconformity at each location, yet the unconformity itself was caused by the same erosional event.

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Stratigraphic Superposition - Because of Earth's gravity, deposition of sediment will occur depositing older layers first followed by successively younger layers.

Thus, in a sequence of layers that have not been overturned by a later deformational event, the oldest layers will be on the bottom.

Nonconformities occur where rocks that formed deep in the Earth, such as intrusive igneous rocks or metamorphic rocks, are overlain by sedimentary rocks formed at the Earth's surface.

The nonconformity can only occur if all of the rocks overlying the metamorphic or intrusive igneous rocks have been removed by erosion.

We call such a break in the stratigraphic record a hiatus (a hiatus was identified in our trash pit example by the non-occurrence of the Ceramic Cups layer at the UNO site).

When we find evidence of a hiatus in the stratigraphic record we call it an unconformity.

one based on physical characteristics or material properties of the rocks - Rock Stratigraphic Units, and the other based on the time over which the material was formed - Time Stratigraphic Units.

Distinctive bodies of rocks that differ from the rocks above and below in the general characteristics. A bodies of rocks that were deposited during the same geologic time interval. In order for rock units to be correlated over wide areas, they must be determined to be equivalent.

Because the Earth's crust is continually changing, i.e due to uplift, subsidence, and deformation, erosion is acting in some places and deposition of sediment is occurring in other places.

When sediment is not being deposited, or when erosion is removing previously deposited sediment, there will not be a continuous record of sedimentation preserved in the rocks.

Age of Earth estimated from time required to cool from an initially molten state.

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