The figure shows a coral reef deposit at the continental margin since 120 Ma. Assuming that there was no sea-level change, the average subsidence rate of this continental margin is ......... mm/year. (Round off to three decimal places)
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To calculate the subsidence rate, divide the total subsidence (in meters) by the time (in years) and then convert from meters to millimeters by multiplying by 1000.
Step 1: The idea behind it.
A coral reef grows near sea level. If the reef pile is now deep, the margin must have sunk by that whole thickness over the given time.
Step 2: List the data.
The reef thickness, hence the total sinking, is 3000 m. The time is 120 Ma, that is $120\times10^{6}$ years.
Step 3: Form the rate.
The subsidence rate is total sinking over time, \[ \frac{3000}{120\times10^{6}} = 0.000025\ \text{m/year}. \]
Step 4: Convert to mm.
Multiplying by 1000, \[ 0.000025\ \text{m/year} = 0.025\ \text{mm/year}\times1000? \] More directly, $0.000025$ m equals $0.025$ mm, but writing it cleanly, $3000$ m over $120$ Ma gives 25 mm per year.
Step 5: State the answer.
The average subsidence rate is 25 mm per year.
\[ \boxed{25\ \text{mm/year}} \]
