Geologic Time ScaleChapter 9, Section 3
Measuring TimeThe Earth has existed for 4.6 billion years
The geologic time scale divides all those years into sections
Each section shows a specific change in life or in the Earths surface
Giving Time a NameThe largest sections are called eonsEons are divided into eras (the 2nd largest section)Eras are divided into periodsThen periods are divided into epochsUsually 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 Earths history mark the boundaries between the sectionsMost sections have been divided because a major organism developed or went extinct.
ExtinctionExtinction = the death of every member of a species
It can happen because the environment changed, and some organisms could not survive in the new environment.
Mass ExtinctionMass extinction = the extinction of many species at the same time
Can you think of an example of a mass extinction?
It can be caused by gradual changes in climate or ocean currents (Uniformitarianism)Or by catastrophic events, like an asteroid hitting the earth and blocking out the sun (Catastrophism)
Lets Talk About the Eras
Precambrian TimeTime from the formation of the Earth (4.6 billion years ago) to 542 million years agoThe Earth was very different thenAt first, there was no oxygen or living thingsThen the first organisms appeared in the oceans 3.6 billion years ago
They were called Prokaryotes
ProkaryotesOnly one cell and no nucleusOne type, Cyanobacteria, were the first organisms on EarthThrough photosynthesis, they released oxygen into the airAs oxygen began to build up, the ozone was formedThe ozone protects the Earth against the suns harmful rays (deadly to land organisms)Now organisms could live on land too!
EukaryotesAfter another 1 billion years, more complex organisms developedThey were called EukaryotesMany cells with a nucleusEvolved into organisms on Earth today
Paleozoic EraBegan 542 million years ago and ended 251 million years ago
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!
Permian ExtinctionThe 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!
Mesozoic EraBegan 251 million years agoSurviving reptiles and amphibians evolved into many other species (like dinosaurs) = Age of Reptiles
Cretaceous-Tertiary ExtinctionAnother mass extinction happened 65 million years ago Marked the end of the Mesozoic Era and the beginning of the Cenozoic EraAll of the dinosaurs and half of the other animals & plants went extinctScientists think an asteroid hit Earth, the dust clouds blocked out the sun, plants died, then herbivores, then carnivores.
Cenozoic EraBegan 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?