Precambrian Time: Vast and Puzzling Chapter 13, Section 1

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

Concept Check Why are specific events in Precambrian history difficult to determine? Most Precambrian rocks do not contain fossils, which makes correlating rock layers difficult. Many Precambrian rocks are metamorphosed and deformed, extremely eroded, or hidden by overlying strata.

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

Earth Forms

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 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

Evidence of Early Oxygen

Concept Check Why did the amount of oxygen in Earth’s atmosphere increase dramatically? The evolution of organisms that used photosynthesis caused the release of increasing amounts of oxygen into the atmosphere.

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

Remnants of Precambrian Rocks

Precambrian Fossils 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 Plant fossils date from the middle Precambrian, but animal fossils date to the late Precambrian (many are trace fossils)

Precambrian Fossils

Assignment Read Chapter 13, Section 1 (pg ) Do Section 13.1 Assessment #1-8 (pg. 368) Print Out “Application Lab 13 – Modeling the Geologic Time Scale”