Geologic Time

Dating rocks Relative dating Absolute dating Compare events and put them in order based on their sequence of formation, oldest to youngest. Absolute dating Using radioactive decay to determine the exact age of rocks

The Geologic Time Scale p. 182 Cenozoic Phanerozoic Mesozoic Precambrian Paleozoic

Geologic Time Scale Geologists have divided the earth’s history into various eras, periods and epochs using fossil evidence (which life forms lived when) The chart has been updated many times as more fossil evidence is located

Principles of relative dating Law of superposition In an undisturbed sequence of sedimentary rocks the oldest rocks are on the bottom, youngest on top

Law of superposition

Principles of relative dating Principle of original horizontality Layers of sediment are generally deposited in a horizontal, flat-lying position Rock layers that are flat have not been disturbed

Principles of relative dating Principle of cross-cutting relationships Younger features cut across older feature A fault or dike that cuts across sedimentary layers is younger than the layers. Igneous dikes cutting across metamorphic rock

Principles of relative dating Included Fragments An inclusion is a piece of rock that is enclosed within another rock (2 examples: xenolith and pebble in conglomerate) Rock containing the inclusion is younger, the inclusion is older

Principles of relative dating Unconformity An unconformity is a break in the rock record produced by erosion and/or nondeposition of rock layers

Sketch of an unconformity:

Geological events of an unconformity: 1. Sedimentation / deposition and lithification 3. Subsidence, water is covering 4. More sedimentation happened once covered with water 2. Deformation / folding and erosion

Unconformity in rocks: Angular unconformity at Siccar Point, Scotland