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The Earth's rivers constantly carry dissolved salts into its oceans. Clearly, therefore, by taking the resulting increase in salt levels in the oceans over the past hundred years and then determining how many centuries of such increases it would have taken the ocean to reach current salt levels from a hypothetical initial salt-free state, the maximum age of the Earth's oceans can be accurately estimated.
Which of the following is the assumption on which the argument depends?
| Text from Passage | Analysis |
| The Earth's rivers constantly carry dissolved salts into its oceans. |
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| Clearly, therefore, by taking the resulting increase in salt levels in the oceans over the past hundred years and then determining how many centuries of such increases it would have taken the ocean to reach current salt levels from a hypothetical initial salt-free state, the maximum age of the Earth's oceans can be accurately estimated. |
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The argument starts with a basic scientific fact about rivers carrying salt to oceans, then jumps to conclude that we can use recent salt level changes to calculate how old the oceans are by working backward to when they were salt-free.
The maximum age of Earth's oceans can be accurately estimated by measuring recent salt increases and calculating how long it would take to reach current salt levels from zero.
The argument uses the premise that rivers constantly add salt to oceans to support the conclusion that we can reverse-calculate ocean age from recent salt data. However, this logic assumes the rate of salt increase has been constant over time, which is the key assumption the argument depends on.
Assumption - We need to find what the argument must assume to be true for the conclusion to work. The argument claims we can accurately estimate ocean age by measuring recent salt increases and extrapolating backward.
The argument makes a precise quantitative claim about 'accurately estimating' ocean age using a specific calculation method based on salt level increases over the past hundred years.
To find assumptions, we need to identify ways this conclusion could fall apart while keeping the facts intact. The facts are: rivers do carry salt to oceans, and we can measure recent salt increases. But what must be true for this backward extrapolation method to work accurately? We should look for conditions about consistency, other factors, and the validity of the calculation approach.