This is completely wrong. They are not. They are fairly rigid, but the edges get stretched or compacted depending upon the stresses. Imagine a demolition derby. A car is essentially rigid, but collisions will deform the edges extensively.
Continental plates are the same, but for extensional deformation too.
See the Himalayas, the Basin and Range province, the North Sea, or Iceland for examples. While I can admit that Rose's theories are original, their predictions fail tests consistently. You can discuss all the human-historical background you want, but the geologic components of Rose's theories are wrong . The Afar Triangle as the nether reaches of Eden and Babel. Kronos, v.1, p.12-46.