Coral reefs are natural structures formed from deposits of the calcium carbonate secretions of coral, a marine animal that lives in colonies. In general, coral reefs are grouped into one of three categories, atolls, barrier reefs, and fringing reefs. Atolls are usually easily distinguished because they are modified horseshoe-shaped reefs that rise out of very deep water far from land and enclose a lagoon (a body of certain water surrounded by a coral reef). With few exceptions, atolls are found only in the Indo-Pacific area. Barrier reefs and fringing reefs, on the other hand, tend to grade into each other and are not readily separable. Some scientists would prefer to group them into a single category. Both types occur adjacent to a landmass, with a barrier reef being separated from the landmass by a greater distance and deeper water channel than the fringing reef. Fringing reefs and barrier reefs are common throughout the coral reef zones in all oceans.

Different types of reefs and reefs in different oceans may have diverse origins and histories. The greatest interest in the origin of reefs has centered on atolls. For many years, humans speculated as to how such reefs could develop in such deep water, miles from the nearest emergent land. This interest was heightened when it was discovered that reef corals cold not live deeper than 50-70 meters. This led to the development of several theories concerning the origin of atolls. Only one need be discussed here——Darwin’s subsidence theory——the fringe corals grow on the shores of newly formed volcanic islands that have pushed to the surface from deep water. These islands often begin to subside, and if the subsidence is not too fast, reef growth will keep up with the subsidence. The reef growth will then form a barrier reef and, ultimately, an atoll as the island disappears beneath the sea. When the island has disappeared, corals continue to grow on the outside and keep the reef at the surface. On the inside, where the island used to be, quiet water conditions and high sedimentation prevail. These conditions prevent continued vigorous coral growth, hence, a lagoon develops. This theory links all three reef types into evolutionary sequence, but is not an explanation for all fringing and barrier reef types.

Since the current surface features of atolls give no evidence of a volcanic base, in the years after the development of Darwin’s theory other explanations were offered, and the whole concept of the origin of atolls became embroiled in the controversy over the origin of coral reefs. If Darwin’s theory was correct, it must be assumed that drilling down through the current atoll reefs would yield layer after layer of reef limestone until, finally, volcanic rock would be encountered. The ability to drill to the base of atoll reefs and resolved the problem had to wait until the mid-twentieth century in 1953. Ladd and other geologists reported borings at Eriwetok atoll in the Marshall Islands that penetrated 1,283meters of reef limestone and then hit volcanic rock. This was the evidence that Darwin’s theory was substantially correct. The correctness of this theory has been strengthened by the discovery of flat-topped mountains or guyots that, at present, have their tops many hundreds or thousands of meters below the ocean surface, but have on their surface the remains of shallow water corals. Evidently, these mountains sank too fast for reef growth to keep above the ocean surface.

Although the subsidence theory links all three reef types in a successional sequence, not all barrier reefs and fringing reefs can be explained by this mechanism. Indeed, the reasons barrier and fringing reef types occur around continental margins and high non-volcanic islands are simply that these areas offer suitable environmental conditions for the growth of reefs and a suitable substrate (surface) on which to begin growth. The extensive reefs around the Indonesian Islands, the Philippines, New Guinea, Fiji and most of the Caribbean Islands are there because a suitable substrate in shallow water existed on which they could initiate growth. In none of these areas are large land areas subsiding, not will these reefs ultimately become atolls.

珊瑚礁是一种天然的结构，由珊瑚的碳酸钙分泌物沉积而成，珊瑚是一种聚集生活的海洋动物。 一般来说，珊瑚礁分为三类，环状珊瑚岛，堡礁和边缘珊瑚礁。环状珊瑚岛通常很容易区分，因为它们是经过改良的马蹄形珊瑚礁，这种珊瑚礁从远离陆地的深水中升起，并包围着一个泻湖（一个由珊瑚礁环绕的特定水体）。 除少数例外，环礁仅在印度 - 太平洋地区发现。 另一方面，堡礁和边缘礁往往彼此分级并且不容易分开。 一些科学家宁愿其归类为单一类别。这两种类型都发生在陆地附近，其中大堡礁与陆地的距离比边缘礁更远，深度更大。珊瑚礁和堡礁在所有海洋的珊瑚礁区都是常见。 不同海洋的不同种类的珊瑚礁可能有着不同的起源和历史。 人们对珊瑚礁起源的最大兴趣都集中在环礁。多年来，人们推测这种珊瑚礁如何能够在离最近的陆地几英里深的深水中产生。当发现礁珊瑚不能在50-70米以上的海水深度生活时，那么人们对这种兴趣就会增强。这导致了关于环礁起源的某些理论的发展。这里只需要讨论一个问题 - 达尔文的沉降理论 - 边缘珊瑚生长在新形成的火山岛的海岸上，这些岛屿从深海被推到水面。这些岛屿通常开始下沉，如果下沉速度不太快，珊瑚礁的生长就会跟上下沉的速度。随着岛屿消失在海底，珊瑚礁的生长将形成一道屏障礁，最终形成一个环礁，岛屿就会消失在海洋下面。岛屿消失后，珊瑚在外面继续生长，并将珊瑚礁保持在水面。岛内曾经是平静的水环境和高沉降。这些条件都阻止了珊瑚持续剧烈的生长已经泻湖的发展。这个理论将所有三种礁类与演化序列联系起来，但并不是对所有堡礁和边缘礁类型的解释。 由于目前环礁的表面特征没有提供任何火山基的证据，在达尔文理论发展的几年后，人们提出了其他解释，并且环礁起源的整个概念也被卷入了关于珊瑚礁起源的争论中。如果达尔文的理论是正确的，那么就必须假定，通过向下钻探环礁会产生一层一层的礁石灰岩，直到最后才会遇到火山岩。能够钻探环礁的底部并解决问题的能力必须等到20世纪中叶1953年。拉德和其他地质学家报告在马绍尔群岛的艾瑞沃托克环礁钻孔，钻入1283米的礁石灰岩，然后撞击火山岩。这证明了达尔文的理论是正确的。这种理论的正确性已经通过发现平顶山或平顶山而得到增强，目前，它们的顶部有数百或数千米深的海底，但是它们的顶部已经在海平面以下数百或数千米的顶部，但在它们的表面上仍有浅水珊瑚。显然，这些山脉下沉得太快了，使得珊瑚礁无法在海面上生长。 虽然沉降理论将所有三种类型的礁岩类型联系在了一个连续的序列中，但并非所有的礁堡和边缘礁都可以用这种机制来解释。 事实上，礁堡和边缘礁出现在大陆边缘和非火山岛周围的原因很简单，就是这些地区为珊瑚礁的生长提供了适宜的环境条件，并为开始生长的珊瑚礁提供了合适的基质（表面）。 印度尼西亚群岛，菲律宾，新几内亚，斐济和大部分加勒比群岛周围都存在许多的珊瑚礁，因为在浅水区有一个适宜的基质，它们可以在那里生长。在这些地区没有大面积塌陷，不能将这些珊瑚礁最终成为环礁。