Continental rifts have a diverse suite of igneous rocks associated with them because melting occurs .

I'm pretty sure it's c. extremely gradual but not 100% sure though.

Evidence peice 1: millions of years ago existed the supercontinent pangea after millions of yearsnit broke into peices and formed smaller continents and islandsevidence peice 2: there is a massive fault line in san fransico and california that has the potential to cause massive earthquakes which supports the theory of fault linesevidence peice 3: mountains are said to be formed by earths tectonic plates crashing against each other for millions of years and eventually if forms a big point know as a mountain.evidence peice 4: the mariana trech is the deepest place (so far) discovered in the oceans of earth. it is so deep that you can literally see how earths plates are divided and or continents as well

That is the correct answer to the question, why is the southern portion of the ring of fire so complex?