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- 1. Geologic Time Scale A BRIEFHISTORY OF EARTH SINCE 4.6 BILLION YEARS.
- 2. Measuring Time • TheGeological time scale is a record of the life forms and geological events in Earth’s history. • Scientists developed the time scale by studying rock layers and fossils world wide. • Radioactive dating helped determine the absolute divisions in the time scale.
- 3. Giving Time aName • The largest sections are called “eons”. • “Eons” are divided into “eras” (the 2nd largest section). • “Eras” are divided into “periods”. • ‘Periods’ are divided into ‘epochs”.
- 4. Eons • Largest, mostgeneral division of time. • 2 Eons • The eons are: Cryptozoic (Precambrian) eon and Phanerozoic eon. • Precambrian and Phanerozoic eon both consists of three eras as shown in the figure.
- 5. Eras • Each Eonis broken up into Eras. • Major eras in Earth’s history: • Archean (4600 mya - 2500 mya) • Proterozoic (2500 mya - 540 mya) • Paleozoic (540mya - 250mya) • Mesozoic (250 mya - 65.5 mya) • Cenozoic (65.5 mya – present) *mya refers to million years ago.
- 6. Periods • Each Erais divided into even more specific blocks of time called periods. • Various geologic events are associated with each period. • Each period is again classified into different epochs.
- 7. How is TimeDivided? • 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 in each section.
- 8. Cryptozoic eon (Precambriantime) • Lasted from 540 million years ago to 4600 million years ago. • Oldest and longest (covers almost 90% of earth’s history). • simple organisms- bacteria, algae, protozoa was born. • Oldest rocks that we know were found in this eon which dates to about 3.5 billion years old. • Divided into 2 eras: Proterozoic and Archean era.
- 9. Archean era • Lastedfrom 2500 million years ago – 4600 million years ago. • Earliest plants (marine algae) developed. • The first life bacteria came into existence (3800 million years ago). • The oldest rocks (3500 million years ago) were formed in this era.
- 10. Proterozoic era • Lastedfrom 540 million years ago to 2500 million years ago. • Marine invertebrates were probably common, few with shells. • Glaciations took place in this era, probably worldwide.
- 11. Phanerozoic eon • Beganfrom 540 million years ago and is still continuing today. • Divided into three eras: i) Paleozoic era (Past life) ii)Mesozoic era (Middle life) iii)Cenozoic era (Present life)
- 12. Palaeozoic Era • Began570 million years ago and ended 250 million years ago. • Divided into 6 periods (Permian, Carboniferous period, Devonian, Silurian, Ordovician, Cambrian). • For the first time on Earth, organisms had hard parts (shells, exoskeletons). • Evolution and development of pteridophytes, amphibians, reptiles, fishes, wing bearing insects, trilobites etc.
- 13. Mesozoic era • Lastedfrom 250 million years ago to 65 million years ago. • Popularly known as Time of Reptiles. • Divided into three periods: Triassic, Jurassic and Cretaceous. • Dinosaurs, mammals and gymnosperms appeared.
- 14. Cenozoic era • From65 million years ago to present days. • Divided into two periods: Tertiary and Quarterny. • Development of modern mammals, angiosperms and human beings.
- 15. Phanerozoic Eon inbrief:
- 16. IMPORTANT PERIODS INTHE HISTORY OF EARTH i) Carboniferous Period It lasted from 360 million years to 300 Million years. It is an important period of Paleozoic era. During this period, first reptiles and pteridophytes were formed and coal plants got spread. The dead bodies got buried in this period forming the coals that we use today.
- 17. ii) Permian It liedbetween 300 million years to 250 million years. It is also an important period of Palaeozoic era. Largest mass extinction happened in this period. Scientists are not sure what caused this mass extinction (maybe climate change & volcanoes). 90% of ocean life and 78% of land life died.
- 18. iii) Jurassic Jurassic isan important period of Mesozoic era. It lasted from 250 million years ago to 200 million years ago. During this period, first birds and mammals were formed. Gymnosperms were dominating plants. The dinosaurs were formed in Triassic period reached at their peak in this period.
- 19. iv) Cretaceous It isa period of Mesozoic era which lasted from 150 million years ago to 65.5 million years ago. It 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 in this period. Scientists think an asteroid hit Earth, the dust clouds blocked out the sun. As a result, plants died, then herbivores, then carnivores.
- 20. v)Quarternery period It isa period of Cenozoic era which has been lasting from 1.8 million years ago to present days. In this period, the humans evolved as the most intelligent creature of earth and took over. It is divided into 2 epochs namely: Pleistocene and Holocene.