20,000 square miles (possible spoiler)

Dr Sorenson says a piece of the earth, 20,000 square miles will break off. That's not really a big piece, it's 2/3 the size of Ireland. I'm no mathematician but an area of 20,000 square miles means a circumference of about 500 miles. If the crack was traveling at 3MPH (as stated in the beginning before it sped up) that means it should have only taken 167 hours or less than 7 days to get back to the beginning.