Discovering Earth’s History Earth is very old (4.6 billion years old) and has changed over geologic time The rock record provides evidence of geological events and life forms of the past Processes that happened in the past are still happening today which is known as uniformitarianism

Chapter 12.1Relative Age Dating Animation Relative-age dating tells the sequence of events without exact dates. This is done by comparing one event or rock layer to another. Stratigraphy is the study of rock layers. Chapter 12.1Relative Age Dating Animation CAN YOU TELL WHAT EVENTS HAPPENED IN WHAT ORDER?

LAWS OF GEOLOGIC HISTORY LAW OF SUPERPOSITION States that in an undistributed rock sequence, the oldest rocks are at the bottom and as you travel up the rock sequence the layers get younger

LAW OF SUPERPOSITION D YOUNGEST C B A OLDEST

LAW OF HORIZONTALITY States that layers of sedimentary rocks are always laid down horizontally (flat) Tectonic forces (mountain building, uplifting, erosion) may tilt or bend them in the future

LAW OF HORIZONTALITY F YOUNGEST E D C B OLDEST A

LAW OF CROSS-CUTTING States that anything cutting across a rock layer is younger. The rock layer has to be there first. Cross-cuts can be igneous intrusions or faults (breaks or cracks in the rock layers)

LAW OF CROSS-CUTTING B A D E C

A D C B E

LAW OF INCLUSIONS States those pieces of rock (that have been eroded from bottom layers) are ALWAYS older than the layer of rock they are found in.

LAW OF UNCONFORMITIES Sometimes deposition is interrupted by erosion events called unconformities. A. deposition ends B. erosion removes previously formed rocks C. deposition resumes Therefore missing rock layers show missing time Usually shown on rock sequence maps by drawing a wavy line

The 3 types of unconformities: A. Disconformity: horizontal sedimentary layers are interrupted by erosion then more sedimentary layers are on top B. Nonconformity: Igneous rocks are on the bottom with sedimentary rocks on top C. Angular: on the bottom are tilted sedimentary layers with horizontal sedimentary layers on top Unconformities

AMAZING EARTH SCIENCE FACTS 1. Oldest rocks are on the___________. 2. Any event that crosses another layer is _________ than the layer it crosses (faults, folds, intrusions) 3. Unconformity—ancient erosional surface – represents _________________________which means missing time 4. Fossils, Superposition and Cross-cutting are used to determine________________. 5. Relative ages are placing events in ____________ without assigning exact numerical ages. bottom younger missing rock layers relative ages sequence