Section 5 Geologic Time Scale

Geologists study all 4.6 billion years of the Earth’s history Grand Canyon is one of best places to see Earth’s history in rock layers Colorado River has eroded many layers of rock in the canyon for the past 6 million years Can see 2 billion years (almost half!) of Earth’s history there The rock and fossil record represent geologic time _Luca_Galuzzi_2007.jpg

Geologists use it to divide the Earth’s history into manageable parts. Time is divided into distinct intervals Eons – largest division Eras – 2 nd largest, includes 2 or more periods Periods – 3 rd largest Epochs – 4 th largest

Archaen Eon Earliest known rocks are formed on Earth 3.8 billion years ago

Paleozoic Era First era well represented by fossils Ended with largest mass extinction in Earth’s history Extinction (see vocab) Mass extinction – extinction of a large number of species in a short period of time 90% of species were killed in this event

Mesozoic Era “Age of Reptiles” Reptiles (dinosaurs) dominated Birds and small mammals begin to evolve Dinosaurs become extinct at end of era Possible cause = Global climate change

Cenozoic Era “Age of Mammals” Mammals (which evolved during last era) begin to flourish This era is still going on We are currently in this era!

Paleontologists believe there have been at least 5 mass extinction events. Climate change (global cooling) is thought to be the cause of these mass extinction events.

A record of Earth’s climate is frozen in glaciers and ice sheets. Scientists drill ice cores (cylinders of ice) to reconstruct Earth’s climate history. Dust, soot, ash, air bubbles, and chemicals from air are trapped in the ice. Shows scientists the composition of Earth’s atmosphere at a particular time in the past.