The Mid-Atlantic Ridge is slowly pushing the Americas away from Europe and Africa. At roughly what average rate does the seafloor spread there?
- About 2.5 millimeters per year
- About 2.5 centimeters per year
- About 25 centimeters per year
- About 2.5 meters per year
Why B? And why not the others?
Correct answer: B. About 2.5 centimeters per year
The U.S. Geological Survey puts the average spreading rate along the Mid-Atlantic Ridge at about 2.5 centimeters per year — roughly the pace at which human fingernails grow. The payoff is what patience does with that crawl: 2.5 centimeters a year adds up to 25 kilometers every million years, enough to have opened the entire Atlantic Ocean basin over geologic time. The figure of 2.5 millimeters per year sounds suitably geological but is ten times too slow, while 25 centimeters per year overshoots even the fastest-spreading ridges in the Pacific, which manage only around 15 centimeters. A rate of 2.5 meters per year would visibly redraw the map within a single human lifetime — dramatic, but wrong by two orders of magnitude.
Source: USGS, This Dynamic Earth: 'Understanding plate motions' (pubs.usgs.gov/gip/dynamic/understanding.html)