Precambrian Time: Vast and Puzzling

1. Precambrian Time: Vast and Puzzling Chapter 13, Section 1

2. Precambrian History • The Precambrian encompasses immense geological time, from Earth’s distant beginnings 4.56 billion years ago until the start of the Cambrian period, over 4 billion years later • The Precambrian comprises about 88% of the geologic time scale • Most Precambrian rocks do not contain fossils, making correlating rock layers difficult • Many rocks are metamorphosed and deformed, extremely eroded, and hidden by overlaying strata

4. Earth Forms • Scientists hypothesize that Earth formed as gravity pulled together dust, rock, and ice in space; gravity increased as Earth grew and began pulling in more materials • The high velocity impacts caused the planet to melt and divide into specific layers based on density • Over several hundred million years, the crust and mantle cooled and hardened, forming rock

5. Earth Forms

6. Earth’s Atmosphere Evolves • Earth’s original atmosphere was made up of gases similar to those released in volcanic eruptions today—water vapor, carbon dioxide, nitrogen, and several trace gases, but no oxygen • As Earth’s crust cooled, gases that had been dissolved in molten rock were gradually released • The water vapor condensed to form clouds, and great rain began • Torrential rains continued and slowly filled low areas, forming the oceans; this reduced the water vapor and carbon dioxide in the atmosphere, leaving behind a nitrogen rich atmosphere • Primitive organisms evolved that used photosynthesis and released oxygen • Slowly, the oxygen content increased, seen in the banded iron formations (which took the free oxygen out of the atmosphere) • Oxygen began to accumulate in the atmosphere about 2.5 billion years ago

7. Evidence of Early Oxygen

8. Precambrian Rocks • The lack of Precambrian rock illustrates the law of superposition, it is covered by much younger strata • Precambrian rocks do show through the surface where younger strata are extensively eroded, such as in the Grand Canyon and in some mountain ranges • Shield – a large relatively flat expanse of ancient metamorphic rock within the stable continental interior • Much of what we know about Precambrian rocks comes from ores mined from shields • The mining of iron, nickel, gold, and other metals has provided Precambrian rock samples for study

9. Remnants of Precambrian Rocks

10. Precambrian Fossils • Precambrian fossils are rather disappointing if you are expecting to see fascinating plants and animals, for these organisms had not evolved yet • The most common Precambrian fossils are stromatolites • Stromatolite – structure produced by algae trapping sediment and forming layered mounds of calcium carbonate • Remains of cyanobacteria have been found that extend the record of life back beyond 3.5 billion years • Many of these ancient fossils are preserved in chert—a hard, dense chemical sedimentary rock • Chert must be sliced very thin and studied under a high powered microscope to see the bacteria and algae fossils within it • Plant fossils date from the middle Precambrian, but animal fossils date to the late Precambrian (many are trace fossils)

11. Precambrian Fossils

12. Assignment • Read Chapter 13, Section 1 (pg. 364-368) • Do Section 13.1 Assessment #1-7 (pg. 368)