What the Earth Giveth, the Earth Taketh Away.

Seafloor spreading centers and volcanoes create new land every day; seafloor subduction trenches gobble it back up. So who is winning – the land or the sea?

Q: Hi my question is: If you were to add up the length of all the convergent and divergent plate boundaries, would they approximately be equal?

- Julienne Y

A: The mid-oceanic ridge system - a divergent tectonic plate boundary - is the longest mountain chain in the world, extending through all global oceans (including the Sea of Cortez and the Red Sea, but not the Mediterranean Sea). All these divergent boundaries together are estimated to be about 80,000 kilometers in length.

There are estimated to be about 50,000 km of convergent plate margins, mostly around the Pacific Ocean (the so-called “Ring of Fire”). This total includes oceanic (subduction) trench systems, but also land features like the Himalayas and the Alps.

In principle, one would think the different boundaries would average out to be the same, but this doesn't incorporate either fractal behavior nor does it incorporate actual geography (and spherical geometry). From basic fractal theory we know that a 5 kilometer endpoint-to-endpoint segment of any boundary can be equal to or substantially longer than 5 kilometers depending on its rugosity (irregularity). Also, in a simplest topological model, you could have an outer rim of divergent seafloor spreading, and an inner rim of trenches and plate convergence. This may help explain why the latter (trenches) would necessarily be smaller than the former (seafloor spreading centers) in our modern Earth. By the way: this modern 50,000km/80,000 km ratio may have been very different - substantially reversed - when the Pangaea supercontinent was just starting to break up about 500 million years ago, because the divergent margins were inside the proto-continent, and most convergent boundaries would have had to be outside. 

Note that I’ve discussed only the lengths of convergent and divergent tectonic boundaries here. The calculation of volumes of materials “created” or “consumed” at these boundaries is far more difficult. This requires making a rather daunting number of assumptions, in lieu of actual data that are very hard to come by.