Term
| Describe the growth of Laurentia during the Proterozoic. |
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Definition
| Laurentia was a continent that consisted of modern-day North America and Greenland. After the suturing together of Archean terrains to form a large craton, Laurentia continued to grow by the accretion of terrains to the east. A huge rifting event threatened to split the craton, but for unknown reasons the rifting failed and the craton remained intact. Finally, the mountain building event known as the Grenville orogeny stabilized a large volume of sediments that had accumulated along the margin of eastern North America. |
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Term
| What prompted the rifting event that threatened to split up the North American craton? |
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Definition
| Rifting occurs when the mantle becomes super-heated and sends up plumes under the crust. The lithosphere stretches and breaks, and graben forms, into which blocks of continent fall. |
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Term
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Definition
| A supercontinent with Laurentia at its centre. Rodinia formed at the time of the Grenville orogeny, and was fully assembled about 1 billion years ago. It split apart 800 million years ago, creating the Pacific Ocean to the west of Laurentia. |
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Term
| What is the term "snowball earth" used to describe? |
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Definition
| Twice during Neoproterozoic time continental glaciers spread throughout the world, even to tropical latitudes. The global extent of these glaciers has given rise to a "snowball earth" theory, suggesting that the planet was entirely covered by glacial ice. |
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Term
| What is albedo, and what role did it play in the two major ice ages of the Proterozoic? |
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Definition
| Albedo is the amount of radiation from the Sun that the Earth reflects back rather than absorbing as heat. At the time of both major ice ages supercontinents were clustered at high latitudes, where glaciers are likely to form; because ice has a high albedo, a positive feedback loop was created that cooled global temperatures further and allowed more glaciers to form. |
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Term
| What evidence supports the "snowball earth" theory? |
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Definition
| The return of banded iron formations during these ice ages indicates oxygen depletion, suggesting the possibility that a layer of ice separated oceans from the atmosphere. |
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Term
| What evidence opposed the "snowball earth" theory? |
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Definition
| The theory cannot account for the survival of acritarchs in the oceans during times of widespread glaciation. Additionally, the Arabian Peninsula displays pulses of glaciation into a marine basin, which could not have occurred if the entire planet was frozen over. |
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Term
| Why did stromatolites flourish during the Proterozoic? |
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Definition
| Continents increased in size; this included an increase in the breadth of continental shelves, the ideal habitat for stromatolites. Without any predators, stromatolites were able to dominate the seas. |
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Term
| How did advanced eukaryotic cells develop? |
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Definition
| Eukaryotic cells arose from the union of two prokaryotic cells, one inside the other. The bigger cell engulfed but failed to digest another, which became a mitochondrion and allowed the cell to derive energy from its food by respiration. Evidence of this is the presence of DNA and RNA within mitochondria that differ from the genetic material of the rest of the cell. |
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Term
| What distinguishes the Ediacara Fauna as animals rather than plants? |
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Definition
| Some are preserved in deep-water sediments below the photic zone, which means they could not have sustained themselves through photosynthesis. |
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Term
| Why have so many soft-bodied creatures from the Proterozoic been preserved? |
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Definition
1) Lack of predators/scavengers
2)Protective algal mats ("elephant skin")
3) Lack of grazers |
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