• 7.4.c Students know that the rock cycle includes the formation of new sediment and rocks and that rocks are often found in layers, with the oldest generally on the bottom.

• Relative age- the age of a rock compared to the age of other rocks

• Absolute age- the number of years since the rock was formed

• Sedimentary rocks form when sediment is deposited in layers

The Position of Rock Layers

• Key Concept: According to the law of superposition, in horizontal sedimentary rock layers the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

• Scientists can’t determine absolute age using the law of superposition but they can get relate age

The Position of Rock Layers

• Extrusion- lava that hardens on the surface

• Intrusion- when magma cools and hardens beneath the surface

• Formation of Igneous Rock

• Fault- a break in the Earth’s crust

Determining Relative Age

Determining Relative AgeKey concept: To determine relative age, geologists also study extrusions and intrusions of igneous rock, faults, and gaps in the geologic record, and inclusions.

• Geologists use extrusions, intrusions, or faults to date rocks using the principle of cross-cutting

• Principle of cross-cutting states that when something cuts across a body of rock, it is younger than the rock it cuts across

• Geological record of sedimentary rock is not complete

• Deposition slowly builds layers but some layer erode away exposing older layers, deposition forms new on top of those older layers

Determining Relative Age

Determining Relative AgeUnconformity- the surface where new rock layers meet a much older rock surface

Determining Relative AgeUnconformity shows a gap in the geologic record because of erosion

Determining Relative AgeAn unconformity occurs where erosion wears away layers of sedimentary rock

Determining Relative AgeOther rock layers then form on top.

• Helps date rocks when there is an unconformity

• Definition- a piece of rock that is contained in another rock

• Starts as a solid rock piece that breaks off an existing rock and becomes part of another rock

• The rock containing an inclusion is younger than the rock the inclusion came from

Inclusions

Using Fossils to Date Rocks• Index fossils- a fossil

that is widely distributed and represents a type of organism that existed briefly

• Key concept: Index fossils are useful because they tell the relative ages of the rock layers in which they occur.

• Index fossils are used to match up rock layers at locations that may be far apart

Ammonites in layer C are index fossils because they:1.only occur in one layer (existed briefly) 2.are in different locations (widely distributed)