Volcano in the Ocean

Three men aboard a little boat see a big ship from somewhere unknown. Their country has been isolated for 260 years, since Yellowstone erupted in a near-extinction event. In a panic, they hurry home to tell their islands that someone is on the ocean again.The GR (The Gethsemane Republic) is an oil and steel society, and they don't believe that the islanders invented their silicate society. They figure they invented it, and they want it.The islanders will not give it to them. Over ten years and many visits, the book ends with the islanders and the GR planning to exchange people to learn more about one another. The islanders with a view towards having peace, the GR with a view to taking over the islands and getting the technology.