No diver worth his or her salt ever turns down the opportunity to dive a fantastic reef. These incredible structures are amazing through and through – not just for their often abundant and colorful marine life, but for their geologic structures. What caused those tunnels and swimthroughs, caves, canyons, and walls we so love to explore? Enjoy this quick peek into reef geology, and you’ll find you have a new appreciation for your favorite dive sites.
What Happened Before the Coral Grew
Depending on where you dive, you’ll notice that the reefs you enjoy have different profiles. Some are gently rounded, others have fingers of coral, some are almost maze-like, and others are pockmarked with tunnels. While each of these reefs underwent a specific process during its early formative period, most have some things in common.
Most were formed during the final years of the Pleistocene era, when melting ice caused global sea levels to rise, in turn flooding continental shelves around the world. As water levels increased and erosion occurred worldwide, soil was washed away, revealing rocky structures that varied from walls to peaks and valleys that are similar to mountain peaks and various rock formations found worldwide. Where limestone substrate was abundant, coral took hold.
Offshore, where volcanic islands eroded, the same process occurred, and where oceanic islands dissolved over time, reefs grew upward, keeping pace with rising sea levels. Some reefs never became well established and are known today as drowned reefs; others, found deep beneath the sea, are still places of mystery which we know very little about. In some exceptional cases, reefs are formed due to tectonic plate movement, where plates have collided and caused undersea mounts to rise.
Tunnels, Walls, Arches, and More
Most reef walls you’ll see while diving are composed of skeletal coral remains with living coral atop them; they are still growing in many cases, and despite their sheer faces, they are the perfect place for the life that has adapted to growing there to cling on. Caves and tunnels are sometimes volcanic in origin, having been formed when massive gas bubbles remained submerged as the rock around them cooled quickly. Sometimes, sea caves and tunnels are an extension of cave systems that can be accessed from shore; at other times, these structures have been formed by the relentless friction water causes over time. Arches form the same way, and pinnacles normally began their lives as rock formations or hills with rocky cores. In areas where coral, algae, and anemone growth is minimal and you can see the rocks’ structure, you can often see striations and bands much like those we see in topside rock formations.
While most reefs have their origins in geology, they are now biological wonders. The largest of the world’s reefs, the Great Barrier Reef, has in fact been named the world’s largest living structure, and others, including the New Caledonia Barrier Reef and the Mesoamerican Barrier Reef System are not far behind. Big or small, shallow or deep, these incredible structures are vital to life on earth. No matter which reefs you explore, you are experiencing a priceless piece of planetary history.