The Rock and Fossil Record II. Relative Dating: Which Came First?

A. The Principle of Superposition 1. Relative Dating a.any method of determining whether an event or object is older or younger than other events 2. Superposition a.principle that states younger rocks lie above older rocks in undisturbed sequences b.rock sequences can be disturbed by forces within Earth B. The Geologic Column 1.geologists collect data from around the world to create geologic column 2.geologic column – the ideal sequence of rock layers that contains all known fossils and rock formations on Earth 3.geologic column is used to interpret puzzling rock sequences

C. Disturbed Rock Layers 1. geologists find layers cut across other layers 2. they use relationships between layers and features to assign ages 3. they know layers must be older for features to cut through them 4. events that disturb rock layers a. folding and tilting are events that disturb rock layers. b. these events are always younger than the layers they affect

D. Gaps in the Record - Unconformities 1. unconformities – the surface that represents a missing part of the geologic column 2.geologists must question if the layer was there and then destroyed, or if it never existed 3. nondeposition, the stoppage of deposition is one factor 4. erosion can also create unconformities

E. Types of Unconformities 1. Disconformities –a. most common –b. found where part of a sequence is missing 1. caused when uplift occurs, the top is eroded, and then deposition occurs again 2. the sediment will bury the old eroded surface

2. Nonconformities –a. found in horizontal sedimentary rock layers that lie on top of eroded surface of older intrusive igneous or metamorphic rock –b. igneous or metamorphic rock is uplifted –c. they are eroded –d. deposition causes the erosion surface to be buried –e. represents millions of years of lost layers

3. Angular Unconformities –a. found between horizontal layers of sedimentary rock and layers of rock that have been tilted or folded –b. tilted or folded layers were eroded before horizontal layers formed –c. represents millions of years of lost layers F. Rock Layer Puzzles –1. geologists often see layers that were affected by more than one event –2. they must piece events together to piece together events on Earth