Coral Reefs by Charles Darwin [PDF]

Information: Coral Reefs

• Author: Charles Darwin
• Publication Date: 1842
• Main Characters:
  • Subsidence Theory: Darwin's central proposal that coral atolls form as the ocean floor gradually sinks, causing coral to grow upward and eventually create ring-shaped islands.
  • Fringing Reefs: The first stage of reef development in Darwin's model. These reefs grow directly along the shoreline of a volcanic island.
  • Barrier Reefs: The second stage, where continued subsidence creates a lagoon between the reef and the island, separating them by a channel of deeper water.
  • Atolls: The final stage. Once the central island sinks entirely below the surface, only the ring of coral reef remains, encircling a shallow lagoon.
  • Coral Growth Limits: Darwin examines the biological constraints on reef-building corals, including maximum depth, water temperature, and exposure to open ocean waves.
  • Volcanic Island Subsidence: The geological mechanism driving reef formation. Darwin argues that vast regions of the Pacific floor are slowly sinking, taking islands down with them.
• Brief Summary: Coral Reefs presents Darwin's theory that the three main types of coral formations (fringing reefs, barrier reefs, and atolls) represent successive stages of development caused by the gradual subsidence of the ocean floor. As volcanic islands slowly sink, the coral continues growing upward, eventually forming ring-shaped atolls. Darwin supports this theory with extensive observations from the Beagle voyage, including detailed descriptions of reef structures in the Pacific and Indian Oceans, measurements of coral growth, and geological evidence of land subsidence.
• Thematic Analysis: The central theme is the relationship between geological processes and biological growth. Darwin demonstrates how slow, large-scale earth movements (subsidence) interact with the steady biological activity of coral organisms to produce the reef formations observed worldwide. The book also explores the limits of coral growth, examining factors like water temperature, depth, and wave exposure. A secondary theme is scientific methodology itself. Darwin builds his case systematically, presenting competing theories, evaluating evidence, and showing why his subsidence model explains observations that other theories cannot.
• Historical Context: Darwin wrote Coral Reefs during 1835-1842, making it part of the geological trilogy he produced from his Beagle voyage observations (alongside Volcanic Islands and Geological Observations on South America). The book entered a lively scientific debate. Charles Lyell had proposed that atolls formed on the rims of submerged volcanic craters, while others suggested they grew on submarine banks. Darwin's subsidence theory was well received and largely replaced earlier explanations. The second edition (1874) addressed new research by James Dwight Dana and Joseph Jukes. Deep drilling at Enewetak Atoll in 1952 confirmed Darwin's core prediction: coral deposits extend hundreds of meters deep atop volcanic rock, exactly as his subsidence model predicted.