Another thing you can do is make a mask of the sides of the island by subtracting the center from the whole, and use that to mask some very big fake stones as child of a surface shader (before or after a compute terrain, see what looks good), so you get some sort of rocky outcrops.
Center would be the image map taking through a color adjust, where you increase the black point, the whole island would the image map taken through a color adjust with white point decreased. Pull the fake stones through a transform shader set to world (final position).