Geologic Time Scale

Measuring Time• The Earth has existed

for 4.6 billion years• The geologic time scale

divides all those years into sections

• Like how your science book is divided into Units, then Chapters, then Sections, then Pages

• Each section is unique because it shows a specific change in life or in the Earth’s surface

Giving Time a Name• The largest sections are called “eons”• “Eons” are divided into “eras” (the 2nd largest

section)• “Eras” are divided into

“periods”• Then “periods” are

divided into “epochs”• Usually the oldest

sections are shown on the bottom and the most recent sections are shown on the top

How is Time Divided?• Major changes in Earth’s history mark the boundaries

between the sections• Most sections have been divided because a major

organism developed or went extinct.

Precambrian Time

Pre- Cambrian Time

• Oldest and Longest Time Period

• Almost 90% of Earth’s History

• From beginning of Earth (4.6 billion years ago) to 540 Million years ago

• Oldest rocks are the deepest rocks

• Ores are deposited (iron, silver, gold)

Paleozoic Era• Began 542 million years ago and

ended 251 million years ago• Divided into 6 periods• Many new life forms appeared

during the 1st period, the Cambrian Period = “Cambrian Explosion”

• For the first time on Earth, organisms had hard parts (shells, exoskeletons)

• Some organisms that were alive during this period still exist today (ferns and salamanders) but they were giants!

Phanerzoic Eon, Paleozoic Era

• Paleo- (Latin) ancient; -zoic (Latin) animals/ life

• Hard shelled animals appear• Pangea formed• Plants begin to grow on land• “Age of Fish” large fish populate oceans• Amphibians (frogs) go to land• Insects appear

Permian Extinction• The largest mass extinction

happened 251 million years ago

• Marked the end of the Paleozoic Era and the start of the Mesozoic Era

• Scientists are not sure what caused this mass extinction (maybe climate change & volcanoes)

• 90% of ocean life and 78% of land life died

• Reptiles and amphibians survived!

Paleozoic Con’t

• Pennsylvanian Period- very large swamps which later become coal

• Appalachian Mountains Form

Mesozoic Era

Mesozoic Era• Began 251 million years ago• Surviving reptiles and amphibians evolved into many

other species (like dinosaurs) = “Age of Reptiles”

Mesozoic Era

• Meso- “middle” -zoic “animals/life”

• Breakup of Pangea• Age of Reptiles-

dinosaurs and reptiles rule planet

• Rocky Mountains form

Cretaceous-Tertiary Extinction• Another mass extinction

happened 65 million years ago

• Marked the end of the Mesozoic Era and the beginning of the Cenozoic Era

• All of the dinosaurs and half of the other animals & plants went extinct

• Scientists think an asteroid hit Earth, the dust clouds blocked out the sun, plants died, then herbivores, then carnivores.

Mass Extinction

• Dinosaurs are killed by meteorite

• Animals and plants change

• 90% of all species that have ever lived are now extinct

Cenozoic Era

Cenozoic Era• Began 65 million years ago and continues today• Scientists know the most about this Era because the

fossils are in the top rock layers and are easier to find

• Land & climate has changed a lot

• What big change will happen that will end this era & start the next?

Cenozoic Era

• Ceno- “new”; -zoic “life”

• Last 45 million years

• Age of Mammals- Humans evolve

• Ice Ages

• Flowering Plants

• Colorado Plateau and Sierra Nevada Mountains formed