A Little Bit of History… The history of geologic time begins with the human interest in mining. Interests in rock units began to flourish with commercial mining in the 1500s and 1600s People began to identify strata (rock layers) by the types of fossils that were found within them. Two scientists were credited with contributing the most research to this area -- Nicolaus Steno and James Hutton.

Nicolaus Steno AAAAdapted two geologic principles 1. L aw of Superposition : sedimentary rocks are found with the oldest on the bottom and youngest on top.E.x. Paint layers 2. P rinciple of Original Horizontality: If left undisturbed rock layers will stay in horizontal order

James Hutton 18th-century Scottish physician and farmer physician and farmer  Studied his farm land for geologic changes *The Present is the Key to the Past *The Present is the Key to the Past  Principle of Uniformitarianism: geologic processes from the past are the same as the current geologic processes. E.x. What happened then happens now

Relative Age: gives the age of a rock layer based on the ages of layers around it (younger or older) What sort of events can change the order of rock layers?

Law of Crosscutting Relationships  Fault or body of rock is younger than any other body of rock that it cuts through.  Igneous intrusions that may be uplifted to the surface would also be younger than the rock they push through Unconformities- breaks in geologic time where rock layers erode away or when sediment isn’t deposited

Paleontology  The study of fossils.  Fossils: remains of plants  and animals that lived in a previous geologic time  Organisms have changed throughout the geologic past.  Fossils provide the clues behind environmental changes of the past. What kinds of rocks are fossils found in?

Fossilization  Organisms with hard parts are typically preserved.  Those without hard parts are fossilized by quick burial. How are fossils formed?  Mummification  Amber  Tar seeps  Freezing  Petrification

Mummification Drying of organisms; found in deserts.

Amber Hardened tree sap, insects get stuck in sap and sap hardens

La Brea Tar Pits Thick petroleum Animals get stuck in tar and it preserves them

Freezing Animals preserved because bacteria cannot survive in cold climate to decay the bodies

Petrification When organic material is replaced with minerals

Types of fossils  Trace Fossils -Evidence that an animal existed. Ex. footprints

Types of Fossils continued…  Molds: animal remains dissolve away, but the shape of the animal remains.  Cast: when a mold fills in with sediment and becomes hardened.

Imprints- carbonized imprints Coprolites fossilized animal droppings (learn eating habits)

Gastroliths- stones found in dinosaur stomachs to help them digest food (are generally round and polished)

Index fossil: Unique fossil that lived during a specific time Trilobites lived 750 million years ago

Specific index fossils for every time period

The Geologic Time Scale is a type of calendar geologists use to put events in their proper order

Calculating Earth's Age  Earth’s age was estimated by measuring the radioactive decay of uranium to lead.  With improved techniques and evidence from tree rings and glaciers, the age of Earth is estimated to be about 4.5 – 4.6 billion years old.

Geologic Column By applying relative ages to sedimentary rocks we began to look at fossils to learn to date them. By applying relative ages to sedimentary rocks we began to look at fossils to learn to date them. Not one place contains a complete record of geologic time. Not one place contains a complete record of geologic time. Geologic Column- ordered arrangement of rock layers based on relative ages of the rock Geologic Column- ordered arrangement of rock layers based on relative ages of the rock Once geologists have dated a lot of the rock layers, they can fill in the gaps. Once geologists have dated a lot of the rock layers, they can fill in the gaps.

Divisions of Geologic Time MYA- millions of years ago MYA- millions of years ago BYA- billions of years ago BYA- billions of years ago Geologic time is typically divided by clear changes. Ex. extinctions and dominant fossils

Divisions of Geologic Time cont… Eons – largest unit of geologic time (4 total eons). Eons – largest unit of geologic time (4 total eons). We call the first three eons the Precambrian time which has little fossil evidence. We call the first three eons the Precambrian time which has little fossil evidence.

Microbe-rich stromatolite reefs, like this one in Shark Bay, Australia, were among the first sources of oxygen on Earth. They flourished throughout the planet's shallow waters 3.5 billion years ago, but are extremely rare today.

Continued… Era- Unit of time that includes two or more periods. Era- Unit of time that includes two or more periods. There are 3 eras that the Phanerozoic Eon can be divided into: There are 3 eras that the Phanerozoic Eon can be divided into: 1. Paleozoic*Zoic = “animal life” 2. Mesozoic 3. Cenozoic. Different fossils are represented in each of these eras: marine life, birds and reptiles and mammals. Different fossils are represented in each of these eras: marine life, birds and reptiles and mammals.

Paleozoic: explosion of diverse life  Beginning –Oceans  Middle to the End –Forests and jungles –Land animals evolved

Devonian Period: Age of Fishes

Mesozoic: Age of Reptiles Jurassic Period: Dinosaurs ruled the Earth

Cenozoic: Age of Mammals

Eras are divided into shorter time units called Periods. Eras are divided into shorter time units called Periods. Each period is characterized by specific fossils and is named for the location they were discovered. Each period is characterized by specific fossils and is named for the location they were discovered. Epochs-when the fossil record is so complete we can keep dividing periods into epochs. Epochs-when the fossil record is so complete we can keep dividing periods into epochs. We are currently here