Geologists once thought that the molten rock known as lava was an underground remnant of Earth's earliest days, sporadically erupting through volcanoes, but they now know that it is continuously created by the heat of the radioactivity deep inside the planet.

A)was an underground remnant of Earth's earliest days, sporadically erupting

C)was an underground remnant of Earth's earliest days, which sporadically erupted

OA is A
and my question:
I know that 'which' must touch its noun which it modifies but in situations in which there are noun and an of-prepositional phrase like the one in C, ("remnant of Earth's earliest days, which sporadically erupted") isn't 'which' correctly modifying remnant?
Bests
Arash