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Difficulty: MediumComparing and Contrasting Models

### Models of the Cryogenian Glaciations

During the Cryogenian period (approximately 720 to 635 million years ago), Earth experienced widespread, severe glaciations. Geologists have proposed two models to explain the extent of the ice coverage.

Model 1 (Hard Snowball Earth)
This model asserts that glaciers reached the equator, and the entire surface of Earth's oceans was completely frozen over with ice sheets up to 1 kilometer thick. This global ice cover decoupled the oceans from the atmosphere, halting the hydrological cycle (evaporation and precipitation) and severely restricting photosynthesis due to the lack of sunlight penetrating the thick ice. According to Model 1, the glaciation ended only when carbon dioxide (CO2CO_2) released by volcanic outgassing accumulated in the atmosphere to extremely high levels (approximately 350 times modern levels), creating a massive greenhouse effect that rapidly melted the ice.

Model 2 (Slushball Earth)
This model asserts that while glaciers were widespread at high and middle latitudes, Earth was not completely frozen. A band of open water, or extremely thin sea ice, persisted around the equator. As a result, the hydrological cycle continued to function, albeit at a reduced rate. Marine photosynthetic microorganisms survived in these open equatorial waters. According to Model 2, the accumulation of CO2CO_2 from volcanic outgassing was also responsible for ending the glaciation, but the required concentration of CO2CO_2 to initiate melting was significantly lower because the albedo (reflectivity) of open water is much lower than that of solid ice.

Based on the passage, Model 1 and Model 2 agree on which of the following aspects of the Cryogenian glaciations?

  1. Volcanic outgassing was the primary driver that led to the termination of the glaciations.Answer
  2. B
    The thickness of the ice sheets covering the equatorial regions of Earth's oceans.
  3. C
    The complete suspension of the global hydrological cycle during the glacial periods.
  4. D
    The specific concentration of atmospheric carbon dioxide needed to initiate global melting.

Answer

Volcanic outgassing was the primary driver that led to the termination of the glaciations.
The correct answer states that volcanic outgassing drove the termination of the glaciations. This is correct because Model 1 asserts that the glaciation ended when carbon dioxide released by volcanic outgassing accumulated in the atmosphere, and Model 2 similarly states that the accumulation of carbon dioxide from volcanic outgassing was responsible for ending the glaciation.

Step-by-Step Solution

1
Analyze Model 1's proposed mechanism for terminating the glaciation.
Model 1 states that the glaciation ended when carbon dioxide (CO2CO_2) released by volcanic outgassing accumulated to extremely high levels to create a greenhouse effect.
To identify the cause of termination according to Model 1.
2
Analyze Model 2's proposed mechanism for terminating the glaciation.
Model 2 states that the accumulation of carbon dioxide (CO2CO_2) from volcanic outgassing was responsible for ending the glaciation.
To identify the cause of termination according to Model 2.
3
Compare the findings from both models to identify a point of agreement, and rule out points of disagreement.
Both models identify volcanic outgassing as the source of carbon dioxide that caused the warming to melt the glaciers. They disagree on equatorial ice thickness, whether the hydrological cycle halted, and the amount of carbon dioxide required.
To determine which option represents a shared premise rather than a difference.

Key Concept

Identifying points of agreement and disagreement between two scientific models describing the same phenomenon.
Estimated Time:1m 30s
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Topics

Comparing and Contrasting Models

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