The southeast tradewinds of the Southern Hemisphere combine with the northeast tradewinds of the Northern Hemisphere to blow surface waters towards the equator, across the seas that circle S.E.A. Water moves east through the Straits of Malacca, between the Malay Peninsula and Western Indonesia, turning north between Kalimantan and Sulawesi. The currents on the north islands of S.E.A., in contrast, are complicated and poorly understood. Some examples of S.E.A. reefs are patch reefs, fringing reefs, barrier reefs, and atoll reefs. Patch reefs exist in shallow waters of S.E.A. on continental shelves. Fringing reefs hug the coastline of Insular S.E.A. Barrier reefs exist at the offshore continental margins.
Atoll reefs vary in type and size.
True atoll reefs rise out of the deep water and are made up of a ring of reef and low islands. Sometimes patch reefs exist with atoll reefs as secondary reefs. Atoll reefs range in size from as much as 500 sq. The theory on coral atoll formation was first described by Charles Darwin and is still widely accepted today. Coral atolls, according to Darwin, are built upon the remnants of a subsiding volcano and coral growth has kept pace with this subsidence over millions of years to produce the structure which we see today.