I remember a novel about the Earth's landmass going from roughly a third of the total surface area to almost a half so that mankind had more room. I thought it was called "Cataclysm" and would have been written sometime in the 70's I believe. Trying to find the author's name and the official title.
Possibly the 1969 novel Cataclysm: The Day the World Died by Don Pendleton? The publication date would be about right. The story does include shifting landmasses e.g. with the ever popular California sliding into the ocean. I don't remember much else though I was reading this about the time I was the Mack Boland Executioner series so quite some time ago.
More info in the summary here.