Geologic Time and Relative Dating

Geologic Time Geologic time—beer goggles for geologists – Ex. geologically recent events Earth is 4.6 billion years old – The passage of this amount of time is difficult for humans to fathom – Let’s think about it in a different way…

Earth Forms: noon First single celled organism: 4:00 pm First Multi-celled Organisms: 9:00 pm First Dinosaurs: 11:00 pm How about the first Humans?30 seconds before midnight

Geologic Time Continued… Geologists have two ways of measuring geologic time – Relative age dating: Refers only to the order of events; does not assign a specific ages – Absolute age dating: Assigning an “exact” age to something

Relative Dating Using one or more of the following principles to determine the order in which things (rocks) have changed over time:

Principle of Original Horizontality Sediment is deposited in nearly horizontal layers

Folded Rock Example How in the world did this happen?

We start with horizontal layers… Time passes, rocks are deformed Time 1 Time 2 Erosion removes top of fold Time 3

Principle of Superposition In an undisturbed sequence of rock, the youngest rocks are always at the top and the oldest on the bottom – Ex. Cake! Oldest layers Youngest layers

Principle of Crosscutting Relationships A rock must exist before something can happen to it – Picture ex: Order of events? Here’s a geological example…

Layer A Layer B Layer C Layer D Layer E Layer F Layer G Layer H Layers A-H were deposited; dike c cuts Through layers A-H; dike B cuts dike C; Dike A cuts Dike B. Layer A is the oldest, Dike A is the youngest unit.

Principle of Inclusions An inclusion is a piece of one rock unit contained in another – Think about the game “Operation”

Evolution and Fossils Evolution in a nutshell: – Critters change with time—successive generations adapting to their environments Fossil = remains/traces of ancient critters preserved in rock

How Are Fossils Useful? Different animals and plants have lived at different times in the past – Ex: Dinosaurs, wicked-big insects Different fossils will appear and disappear in the same order that organisms evolved and died out—Law of Faunal Succession Rock correlation

Conformity and Unconformity Erosion may cause rock record to be incomplete – unconformity There are several types of unconformities:

Angular Unconformity: Horizontal Sedimentary rock layers over tilted rock layers Disconformity: Erosion surface Separating horizontal sedimentary Rock layers Nonconformity: Sedimentary rocks lie On top of igneous or metamorphic rocks