Geologic Time Scale

A record of the life forms and geologic events in Earth’s history. Scientists developed the geologic time scale by studying rock layers and index fossils worldwide. Index fossils - a fossil known to have lived in a particular geologic age that can be used to date the rock layer in which it is found. For example, if you find a fossil from an unknown era near a fossil from a known time, you can assume that the two species were from about the same time. Scientists placed Earth’s rocks in order by relative age. Later, radioactive dating helped determine the absolute age of the divisions in geologic time scale. Geologic Time Scale

Relative AgeAbsolute Age I was bornI was born on August 2, 1999 I took my first stepsI took my first step on July 12, 2000 I started KindergartenI started Kindergarten on August 20, 2004 I started the Eighth GradeI started the Eighth Grade on August 27,2012 Today’s DateToday’s Date is September 17, 2012 Let’s Review: Relative Age vs. Absolute Age

Divisions of Geologic Time As scientists studied the fossil record, they found major changes in life forms at certain times. They used these changes to mark where one unit of geologic time ends and the next begins. Divisions of the geologic time scale depend on events in the history of life on Earth.

Geologic time begins with a long span of time called Precambrian Time. Covers 88% of earth’s history. It ended 544 million years ago. Time between Precambrian and now is divided into 3 eras—Paleozoic, Mesozoic and Cenozoic. Eras are subdivided into periods. Names of periods come from places around the world where geologists first described the rocks and fossils of that period. Periods are divided into Epochs. Epochs are the shortest division of time.

Pangaea the “Super-continent” Alfred Wegener hypothesized that the continents had moved! (early 1900’s) They were once joined together in a single landmass. (300 MYA) Over tens of millions of years Pangaea began breaking apart and eventually towards were the sit today. This idea became known as Continental Drift. Pangaea – “all lands”

Evidences of Pangaea Landforms – Fit together like puzzle pieces – Mountain ranges from Africa and South America line up. Fossils – Fossils from the same fern like plant found in Africa, South America, Australia, India, and Antarctica. Climate – Fossils of tropical plants found in the Arctic – According to Wegener, Earth’s climate has not changed, the positions of the continents have!

Evidence worksheet Top portion as a group Answer questions on bottom for homework.

Eurasia North America South America India Antarctica Australia Africa Evidences for Pangaea Map