Relative Dating: Which Came First? Chapter 3 Section 2 p. 64 - 69 Rank the following pictures from oldest to youngest: Vocabulary: 1. relative dating 2. superposition 3. geologic column 4. unconformity

The Principle of Superposition p. 64 Geologists try to determine the order in which events have happened during Earth’s history. The rely on rocks and fossils to help them in their investigation. Determining whether an object or event is older or younger than other objects or events is called relative dating. Layers of sedimentary rock, like the ones shown here, are stacked on top of each other. As you move from top to bottom, the layers get older. The principle that states that the younger rocks lie above the older rocks in undisturbed sequences is called superposition.

Disturbing Forces p. 65 Not all rock sequences are arranged with the oldest layers on the bottom and youngest layers on top. Some rock sequences are disturbed by forces within the Earth. These forces can push other rocks into a sequence, tilt or fold rock layers, and break sequences into movable parts. Sometimes geologists even find rock sequences that are upside down! To make their job easier, geologists combine data from all the known undisturbed rock sequences around the world. Using this information, they create a geologic column, which is an ideal sequence of rock layers that contains all the known fossils and rock formations on Earth, arranged from oldest to youngest.

Constructing the Geologic Column

How Rock Layers Become Disturbed Geologists use the relationship between rock layers and the features that cut across them to assign relative ages to the features and the layers. They know that the features are younger than the rock layers because the rock layers had to be present before the features could cut across them. Faults and intrusions are examples of features that cut across rock layers. Folding and tilting are two types of events that also disturb rock layers.

Missing Evidence p. 67 Missing rock layers create breaks in rock-layer sequences called unconformities. An unconformity is a surface that represents a missing part of the geologic column. Unconformities also represent missing time – time that was not recorded in layers of rock. Most unconformities form by both erosion and nondeposition. Geologists place unconformities into three major categories: disconformities nonconformities angular unconformities

Disconformities Disconformities are found where part of a sequence of parallel rock layers is missing. A disconformity can form in the following way: A sequence of rick layers is uplifted. Younger layers at the top of the sequence are removed by erosion and the eroded material is deposited somewhere else. At some future time, deposition resumes, and the sediment buries in the old erosion surface. Disconformities represent thousands to many millions of years of missing time.

Nonconformities Nonconformities are found where horizontal sedimentary rock layers lie on top of an eroded surface of older intrusive igneous or metamorphic rock. When those rocks are raised to Earth’s surface, they are eroded. Deposition causes the erosion surface to be buried. Nonconformities represent millions of years of missing time.

Angular Unconformities Angular unconformities are found between horizontal layers of sedimentary rock and layers of rock that have been tilted or folded. The tilted or folded layers were eroded before horizontal layers formed above them. Angular unconformities represent millions of years of missing time.

Quiz Time! 1. What does the Principle of Superposition state? Oldest layers are on the bottom and the youngest layer is at the top. 2. Tell two ways that geologists use the geologic column. 1. – to interpret rock sequences 2. – to identify layers in puzzling rock sequences 3. What is an unconformity? A surface that represents a missing part of the geologic column

4. Name three types of unconformities. disconformity, nonconformity, and angular unconformity 5. Molten rock that squeezes into existing rock and cools is called a(n): A. fold B. fault C. intrusion D. unconformity C - intrusion