Discovering Earth’s History

Two Methods to Date Rock Layers Absolute dating

Relative Dating By studying the layers of rock exposed at the surface, geologists can infer the order in which the layers formed. Places rocks in chronological order Can determine the sequence of events by comparing rocks, but cannot tell us how long ago the events occurred. Follows different “rules” to interpret what happened to the exposed rocks

The Rules to Follow Law of Superposition - older rocks are found under younger rocks Principle of Original Horizontality – sediment is deposited in flat, horizontal layers Intrusions – pieces of rock that are contained within another rock layer

Rules Continued Principle of Cross-cutting Relationships – A fault or intrusion must be younger than any other layer that it cuts through Unconformities – A break in the rock record. 3 Types Disconformity Nonconformity Angular Unconformity

Disconformity Nonconformity Only with Sedimentary Rock - Erosion occurs before new layers are formed Difficult to see since rock layers are very similar Erosion occurs to metamorphic or igneous rocks and then younger sedimentary layers are formed Nonconformity

Angular Unconformity New layers of sedimentary rock are deposited over older layers that have been tilted or folded

Practice – Which type of Unconformity?

Online Interactives http://www.wwnorton.com/college/geo/animations/geologic_history.htm   http://sciencelearn.org.nz/Contexts/Dating-the-Past/Sci-Media/Animations-and-Interactives/Relative-rock-layers

Practice

Correlation Scientists compare rock layers at different locations to get a more complete picture of the rock record The rock layers they compare formed at the same time and by the same processes. Usually fossils are also used to help when areas being compared are on different continents