Question

The rate of spreading about a symmetric spreading center at the middle of a 4000 km wide sea is 40 mm/year. The spreading began $\underline{\hspace{1cm}}$ million years before present. [in integer]

Hint:

In mid-ocean ridge problems: halve the total ocean width, convert spreading rate to consistent units (km/yr), then divide distance by rate.

Answer 1

Step 1: Convert total width to half-width.
The sea is 4000 km wide. Since spreading is symmetric: \[ \text{Half-width} = \frac{4000}{2} = 2000 \, \text{km} \]

Step 2: Convert units of spreading rate.
\[ 40 \, \text{mm/year} = 0.04 \, \text{m/year} = 0.00004 \, \text{km/year} \]

Step 3: Time required for spreading.
\[ t = \frac{\text{Distance}}{\text{Rate}} = \frac{2000}{0.00004} = 50{,}000{,}000 \, \text{years} \]

Step 4: Express in million years.
\[ t = 50 \, \text{million years} \]

Final Answer:
\[ \boxed{50 \, \text{million years}} \]