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Geologic Time Scale. Chapter 9, Section 3. Measuring Time. The 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 Earth’s surface. Giving Time a Name.
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Geologic Time Scale Chapter 9, Section 3
Measuring Time • The 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 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.
Extinction • Extinction = 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 Extinction • Mass 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)
Precambrian Time • Time from the formation of the Earth (4.6 billion years ago) to 542 million years ago • The Earth was very different then • At first, there was no oxygen or living things • Then the first organisms appeared in the oceans 3.6 billion years ago • They were called Prokaryotes
Prokaryotes • Only one cell and no nucleus • One type, Cyanobacteria, were the first organisms on Earth • Through photosynthesis, they released oxygen into the air • As oxygen began to build up, the ozone was formed • The ozone protects the Earth against the sun’s harmful rays (deadly to land organisms) • Now organisms could live on land too!
Eukaryotes • After another 1 billion years, more complex organisms developed • They were called Eukaryotes • Many cells with a nucleus • Evolved into organisms on Earth today
Paleozoic Era • Began 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 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!
Mesozoic Era • Began 251 million years ago • Surviving reptiles and amphibians evolved into many other species (like dinosaurs) = “Age of Reptiles”
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.
K-T Boundary- Boundary between Mesozoic and Cretaceous
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?
