http://www.iflscience.com/environment/rocks-formed-plastic-found-beach

Rocks Formed From Plastic Found On Beach

June 5, 2014 | by Lisa Winter

Every single piece of synthetic plastic that has ever been manufactured
since 1950 is still in existence today, as each one takes thousands of
years to break down. A recent study has indicated that plastic litter
can become fused with rocks and other materials to form a new material:
plastiglomerate. This material could very well become a permanent part
of the geological record, forever marking humanity?s impact on the
world. The results of the report were published in the journal of the
Geological Society of America, GSA Today.

The material has been discovered before, but it wasn?t until it had
washed up on Hawaii?s Kamilo Beach, notorious for its litter, that it
was studied and identified. The discovery of the Hawaii sample was made
by Captain Charles Moore of the research vessel of Alguita as well as
lead author Patricia Corcoran. It is believed that human campfires
melted plastic on the beach, causing it to fuse to shells, coral, and sand.