Coral and Coral Reef
Major Controls: Crustal Subsidence And Sea Level Change
In addition to the environmental requirements for coral growth described above, other factors play a role in coral reef character over long time intervals, that is, during geologic time spans. The two most important controls are both related to water depth—sea level change and crustal movement.
World-wide fluctuations in sea level can be caused by volume changes of fresh water in global reservoirs (lakes, groundwater, or glaciers) and by changes in the volume of ocean basins. If sea level rises while environmental conditions remain favorable for reef growth, coral reefs may grow upward rapidly enough to keep pace with rising sea level. If conditions are unfavorable, upward growth will be slow and light levels on the reef will slowly decrease as water depth increases, causing the reef to "drown." If sea level drops, the crest of the reef may be exposed and eroded, while deeper zones will "back-step" down the reef slope as the water depth decreases.
Crustal movements—uplift or subsidence of the coast—result from tectonic events such as mountain building and continental rifting or from changes in crustal loading due to volcanism, erosion, or deposition. Uplift is analogous to a sea-level drop and results in coral reefs back-stepping from the coast. Subsidence has the same effects on coral as a sea level rise, and the coral reef must either grow to keep up or else drown. A "keep up" reef grows at a rate sufficient to keep up with the relative sea level rise and remains at or near sea level; a "give up" reef has a growth rate that falls behind the rate of relative sea level rise, and "drowns."
Coral reefs occur in two distinct settings: oceanic settings and continental shelves. Deep water surrounds oceanic coral reefs, which generally lie hundreds of miles from continental shelves. These may be fringing reefs, barrier reefs or atolls. Charles Darwin, who started his scientific career as a geologist, developed a theory in the mid-nineteenth century about the origins of and relationships between fringing reefs, barrier reefs, and atolls in oceanic settings. Darwin visited Pacific atolls and also observed barrier and fringing reefs in the south Atlantic. He hypothesized that the first stage of development of an atoll is the creation of a fringing reef around a volcanic island. The volcano subsides under its own weight over millions of years, but the reef's upward growth keeps pace with subsidence and so it remains in shallow water, developing into a barrier reef. If a volcanic island becomes completely submerged, the coral reef itself may be the only thing at sea level, forming an atoll.
Darwin was essentially correct, since the primary factor determining what types of reefs are present in oceanic settings is usually an island's rate of subsidence. However this model is less applicable to shelf reefs, which usually experience less significant rates of subsidence. Continental shelves are typically fairly stable, so sea-level changes tend to exert more control than subsidence on reef morphology (form). Shelf reefs occur on the margins of continents. A carbonate shelf is a broad, flat, shallow margin where conditions favor reef development. Most shelf reefs develop either on benches or banks or at the continental shelf's seaward margin.
Bench reefs form on the outer edge of a submarine erosional terrace, or bench, produced by near-shore erosion during times of lower sea level. This bench provides a substrate for reef development. Generally, bench reefs form at water depths of 35 ft (10 m) or less. Bank-barrier reefs form on a shallow (less than 60 ft, 18 m) area of the shelf where, in the past, scattered corals trapped various coarse-grained skeletal material, forming a pile, or bank. Later, when water depths are suitable, corals use the bank as a substrate for reef development. Shelf-margin reefs are located at the outer edge of the shelf, beyond which water depths increase very rapidly.
When sea level rises or falls along a shelf, vast areas of land are flooded or exposed, respectively. If sea level drops on a shelf, bench or bank reefs cannot necessarily back-step—there may be no where to go, if the entire shelf becomes exposed. Shelf-margin reefs may back-step, however, sediments from erosion of the newly exposed shelf often overcome the reefs. Likewise, during rising sea level, water quality may be unfavorable to reefs due to coastal erosion of newly flooded land areas.
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