Like Frith said, the island is volcanic, but volcanism has nothing to do with what's happening here. Also, why is this terrifying? I'd much rather see loose plastic agglomerated into 'rocks' than floating around at large in the ocean where it can harm animals. They're strangely attractive rocks as well, plus I'd imagine those little pores and bubbles provide various opportunities of some sort for small creatures.

"This is new and terrifying at the same time, because pollution has reached geology," said Fernanda Avelar Santos, a geologist at the Federal University of Parana.

He apparently got his diploma from the back of a cereal box. Pollution has 'reached geology' for the past several thousand years or so.