There are two islands on opposite sides of a mid-ocean ridge in the Atlantic Ocean. During the last 8 million years, the distance between the islands has increased by 200 kilometers. Calculate...

 There are two islands on opposite sides of a mid-ocean ridge in the Atlantic Ocean. During the last 8 million years, the distance between the islands has increased by 200 kilometers. Calculate the rate at which the two plates are diverging

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sciftw | High School Teacher | (Level 1) Educator Emeritus

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To calculate the rate (speed) of an object, you can use the formula "speed = distance/time."  

The distance of the two islands has increased by 200 km.  It took them 8 million years to do that.  

200 km / 8,000,000 year = .000025 km/year

That number is next to incomprehensible, so it is best to convert the kilometers into a number that is much easier to visualize.  

.000025 km/year is the same as 2.5 cm/year or 25 mm/year.  

On average a human fingernail grows about 3 mm/month, which is equivalent to 36 mm/year.  In other words, the two tectonic plates in your question are moving apart at a rate that is slightly slower than the rate your fingernails are growing right now.  Which is why it took 8 million years to move only 200 km. 

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