I'm not trying to be pedantic here, OK?
Most people think Rocha Negra is the entire cliff and cliff-top. It isn't. It means black rock, and refers to a volcanic lava stream from an eruption at Monchique a long, long time ago.
The bulk of that hill is a softish stone. The cliff face often crumbles down in intense periods of rain. Rocha Negra is made of much harder stuff and weathers any storm.
The top, the part that Kate ran to, has the trig point on it. Close up, it is enormous, so one assumes it has foundations. The top is also extensively cultivated. And for further proof the top is not volcanic, look at Mark Harrison's statement. An issue with his search was that between Madeleine's disappearance and the dog searches, a large old building had been removed.
You can see that building being removed on Google Earth, and it was indeed large. I am trying to find its history, simply because it interests me. However, a large building needs large foundations, and you don't do that on volcanic rock. It was replaced by a large Boa Vista golf clubhouse.
I am not suprised Kate got it wrong, because most people do.