Why are the Marshall Islands made up mostly of low coral atolls?

The Marshall Islands are composed of low-lying coral atolls because they are essentially coral caps situated on the rims of submerged volcanoes that rise from the ocean floor.1

According to the subsidence theory, these formations represent the final stage of a process where coral reefs grow upward around a sinking volcanic island. As the volcanic cone subsides—or as sea levels rise—the reef continues to grow toward the surface, eventually leaving behind a ring-shaped reef that encloses a central lagoon. Because these islands are built upon these submerged volcanic foundations, they remain low-lying, with none of the atolls in the Marshall group rising more than 20 feet (6 meters) above high tide.231