The Geologic Time Scale

1. The Geologic Time Scale Geologic time is divided into Eons. • Precambrian Eon • Archean Eon • Proterozoic Eon • Phanerozoic Eon

2. The Geologic Time Scale Eons are further divided into Eras. • Archean Eon • Eoarchean Era • Paleoarchean Era • Mesoarchean Era • Neoarchean Era • Proterozoic Eon • Paleoproterozoic Era • Mesoproterozoic Era • Neoproterozoic Era • Phanerozoic Eon • Paleozoic Era • Mesozoic Era • Cenozoic Era

3. The Geologic Time Scale The Phanerozoic Eon is further divided into Periods. • Phanerozoic Eon • Paleozoic Era • Cambrian Period • Ordovician Period • Silurian Period • Devonian Period • Carboniferous Mississippian Period • Carboniferous Pennsylvanian Period • Permian Period • Mesozoic Era • Triassic Period • Jurassic Period • Cretaceous Period • Cenozoic Era • Tertiary Period • Quaternary Period

4. The Geologic Time Scale The Cenozoic Era is further divided into Epochs. • Cenozoic Era • Tertiary Period • Paleocene Epoch • Eocene Epoch • Oligocene Epoch • Miocene Epoch • Pliocene Epoch • Quaternary Period • Pleistocene Epoch • Holocene Epoch

5. Early Evolution of Earth Formation of Earth’s Layered Structure • As Earth formed, the decay of radioactive elements and heat from high-velocity impacts caused the temperature to increase. • Iron and nickel began to melt and sink toward the center • Lighter rocky components floated outward, toward the surface • Gaseous material escaped from Earth’s interior to produce the primitive atmosphere

6. The Precambrian Eon The Precambrian Eon, which is divided into the Archean and the Proterozoic Eons, spans about 88% of Earth’s history. Much of Earth’s stable continental crust was created during this time. Partial melting of the mantle formed volcanic island arcs and ocean plateaus.

7. The Precambrian Eon These crustal fragments collided and accreted to form larger crustal provinces. Larger crustal areas were assembled into larger blocks called cratons. Cratons form the core of modern continents.

8. Formation of Continental Crust According to one model, the growth of large continental masses were accomplished through the collision and accretion of various types of terrains, including volcanic arcs and oceanic plateaus.

9. The Extent of Crustal Materials Remaining From the Archean and Proterozoic Eons

10. Map showing the major geological provinces of North America and their ages in billions of years (Ga). It appears that North America was assembled from crustal blocks that were joined by processes very similar to modern plate tectonics. These ancient collisions produced mountainous belts that include remnant island arcs trapped by colliding continental fragments.

11. The Precambrian Eon The Supercontinents The supercontinents were large landmasses that consisted of all, or nearly all, of the existing continents. Pangaea was the most recent supercontinent, but perhaps an even larger one, Rodinia, preceded it. Splitting and reassembling of supercontinents have generated most of Earth’s major mountain belts. Supercontinents have also profoundly affected Earth’s climate over time.

12. Possible configuration of the supercontinent Rodinia Figure 12.15

13. The Precambrian Eon It was during the Precambrian Eon that life first began on the Earth. Although the Precambrian Eon contains some 88% of Earth's history, its fossil record is poor because organism were soft-bodied during this time, resulting in little remaining evidence.

14. The Precambrian Eon The majority of Precambrian fossils are stromatolites that are often heavily metamorphosed or deeply buried. However, preserved cells have been discovered at selective sites, such as the 2.0 Ba Gunflint Formation Jasper stromatolites from Gunflint Formation near Mackies, northern Ontario.

15. The Precambrian Eon The earliest life forms were prokaryotes that evolved in the seas, possibly as early as 3.8 Ga. • The first primitive prokaryotic single-celled organisms appeared in the oceans in the form of bacteria (eubacteria or Achaea).

16. The Precambrian Eon Earth’s first living organisms were probably chemotrophs existing in an anoxic world and producing H2S or CO2.

17. The Precambrian Eon Nearly 3.5 Ga, photosynthezing cyanobacteria began releasing oxygen into the atmosphere as a by-product of the process of photosynthesis.

18. The Precambrian Eon The first multi-cellular organisms appeared toward the end of the Precambrian Eon, sometime prior to 542 Ma.

19. The Precambrian Eon When the eukaryotes (single-celled organisms with a nucleus) evolved through endosymbiosis is disputed, with claims as early as 3.4 Ga, but with less equivocal fossils dating from 1.8 to .8 Ga.

20. The Precambrian Eon With the eukaryotes comes sexual reproduction, enabling genetic diversity and the concomitant ability to adapt to and survive environmental changes.