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Difficulty: MediumIdentifying Hypotheses and Beliefs

Grand Canyon Formation Models

The Grand Canyon in Arizona is one of the most prominent geological features on Earth. Geologists have debated how and when the canyon was carved. Two models describe the potential mechanism and timeline of its formation.

*Model 1 (Catastrophic Flooding)*
The Grand Canyon was carved rapidly, likely within a few weeks to months, approximately 15,00015,000 years ago. Near the end of the last glacial period, large prehistoric lakes (Lake Hopi and Lake Bidahochi) were trapped behind natural sediment dams on the Colorado Plateau. As climate warming caused rapid ice melt, these lakes overflowed, breaching the dams. The sudden, catastrophic release of millions of acre-feet of water cut deeply through the soft sedimentary rock layers, creating the canyon in a brief geological event.

*Model 2 (Incremental Erosion)*
The Grand Canyon was carved slowly and steadily over 55 to 66 million years by the Colorado River. The process began when the Colorado Plateau underwent gradual tectonic uplift, increasing the river's gradient and flow velocity. Over millions of years, the river's water, carrying abrasive sediments like sand and gravel, acted as a slow saw, cutting downward through the rising plateau. Weathering, landslides, and tributary erosion slowly widened the canyon walls to their current dimensions.

According to Model 1, which of the following events was directly responsible for initiating the carving of the Grand Canyon?

  1. The sudden breach of natural sediment dams holding back prehistoric lakesAnswer
  2. B
    The gradual tectonic uplift of the Colorado Plateau over millions of years
  3. C
    The slow abrasion of sand and gravel in a low-gradient river system
  4. D
    A rapid cooling of the climate that formed massive glaciers on the plateau

Answer

The sudden breach of natural sediment dams holding back prehistoric lakes
The correct answer is correct because Model 1 explicitly states that the carving was initiated when prehistoric lakes overflowed and breached their natural sediment dams, releasing a sudden, catastrophic volume of water that rapidly cut through the rock layers.

Step-by-Step Solution

1
Identify the model in question and locate its description in the passage.
Model 1 proposes a catastrophic flooding mechanism that occurred approximately 15,00015,000 years ago.
This focuses the search on the specific details and hypotheses of Model 1.
2
Analyze Model 1's description for the event that initiated the carving process.
Model 1 states that prehistoric lakes overflowed and breached their natural sediment dams, releasing water catastrophically.
This identifies the specific cause-and-effect relationship proposed by Model 1's hypothesis.
3
Compare the identified cause with the given choices to find the match.
The option describing the sudden breach of sediment dams matches the initiating event of Model 1.
This selects the correct answer based on the model's explicit hypothesis.

Key Concept

Identifying the core hypotheses, mechanisms, and initiating events of competing scientific models.
Estimated Time:1m 30s
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Topics

Identifying Hypotheses and Beliefs

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