Coral reef

Lore & Background

As communities established themselves, the reefs grew upward, keeping pace with rising sea levels. Reefs that rose too slowly could become drowned, without sufficient light. Coral reefs are also found in the deep sea away from continental shelves, around oceanic islands and atolls, the majority of which are volcanic in origin, while others have tectonic origins where plate movements lifted the deep ocean floor. In The Structure and Distribution of Coral Reefs, Charles Darwin set out his theory of atoll formation, conceived during the voyage of the Beagle. He theorized that uplift and subsidence of Earth's oceanic crust formed the atolls, describing a sequence of three stages: a fringing reef forms around an extinct volcanic island as the island and ocean floor subside, becoming a barrier reef and ultimately an atoll reef. Darwin predicted that underneath each lagoon would be a bedrock base, the remains of the original volcano, and subsequent research supported this hypothesis. As sea level rise continued, water topped most of the continental islands, and corals overgrew the hills, forming cays and reefs.

Reader's Guide

Coral reefs are among the most biodiverse ecosystems on Earth, often called rainforests of the sea, yet they occupy less than 0.1% of the world's ocean area. They provide habitat for at least 25% of all marine species, including fish, mollusks, worms, crustaceans, echinoderms, sponges, tunicates, and other cnidarians. Threats include excess nutrients, rising ocean heat content and acidification, overfishing practices such as blast fishing and cyanide fishing, sunscreen use, and harmful land-use practices like runoff and seeps. Coral reefs deliver significant ecosystem services for tourism, fisheries, and shoreline protection. The two main variables determining reef geomorphology are the nature of the substrate and the history of sea level change relative to that substrate. In the geologic past, maximum reef development occurred in the Middle Cambrian, Devonian, Carboniferous, Late Cretaceous, and Neogene periods, with different reef-building organisms dominating at different times. Measurements of oxygen isotopic composition in coral skeletons can indicate changes in sea surface temperature and salinity, helping scientists infer paleoclimate. Darwin's identification of three classical reef formations—fringing reef, barrier reef, and atoll—has been expanded to include further types such as table reef and bank or platform reef.

Did You Know?

• Healthy tropical coral reefs grow vertically from 1 to 25 cm per year.