Question

Reef-building corals are responsible for creating atolls-ring-shaped structures that extend from the surface of the ocean to depths of thousands of meters. These corals, however, can only live in warm, sunlit water no more than about 45 meters deep. This presents a paradox: How can corals, which require warm, sunlit water, create structures that extend to great depths? Explain the apparent contradiction.

Short answer

Corals grow in shallow water and geologic subsidence creates deep structures.

Step-by-step solution

Understand Coral Growth Requirements

Reef-building corals require warm and sunlit water, which limits their growth to shallow waters no more than 45 meters deep. This sets the initial conditions of where corals can thrive and start building structures.

Recognize Coral Growth Mechanism

Corals grow upwards and expand laterally over time. As they build, they form a calcium carbonate skeleton that becomes the base for new coral life. This growth pattern contributes to the overall height and structure of coral reefs.

Identify Geologic Process Involved

The ocean floor can gradually subside or sink due to geologic processes such as tectonic activity. While the corals maintain their growth near the surface, the base they build upon can sink deeper over thousands of years.

Explain Atoll Formation Process

As the volcanic island or landmass upon which the corals originally formed sinks, corals continue to grow upwards, staying within the required 45 meter sunlit zone. Over time, the original island may completely submerge, leaving only a coral ring – an atoll – around the lagoon.

Resolve the Paradox

The paradox is resolved by understanding that it is not the corals themselves that are built at great depths, but rather, the previously elevated land sinks leaving behind extensive coral structures that began in much shallower water.

Key concepts

Atoll Formation

Atolls are fascinating structures formed by coral reefs that appear as ring-like islands enclosing a lagoon. The process behind the creation of an atoll begins when a volcanic island emerges from the ocean. Reef-building corals start to colonize the shallow waters around the island, no deeper than 45 meters, since they need sunlight and warmth to survive.
As the volcanic island slowly subsides or sinks due to geological forces over thousands of years, the corals, which thrive in the warm sunlit zone, continue to grow upward and outward. This vertical growth allows them to keep pace with the subsiding land, maintaining their presence within the sunlit zone. Eventually, the original volcanic island may completely submerge beneath the ocean surface, leaving just the ring of growing corals. This results in the formation of an atoll, a coral-formed, ring-like structure encircling a central lagoon.

Reef-building Corals

Reef-building corals, also known as hermatypic corals, are primarily responsible for the construction of coral reefs, including atolls. These corals are made up of tiny, individual coral polyps that work together to form large colonies.
Each polyp secretes calcium carbonate, which creates a hard skeleton that provides a foundation for future coral generations. This skeleton forms the intricate architecture of coral reefs. Here are some unique characteristics of these corals:

• They thrive in warm, shallow waters, close to sunlight, which fuels the photosynthesis of their symbiotic algae, zooxanthellae.
• Reef-building corals have a symbiotic relationship with zooxanthellae, which live within their tissues and supply nutrients via photosynthesis, enhancing the coral's energy supply.
• The corals receive their vibrant colors from these algae, adding beauty to the structures they form.

The intricate skeletons left behind by these corals build up over numerous generations, forming massive reef structures like those found in atolls.

Geologic Processes

The formation of coral reefs and atolls is significantly influenced by geologic processes that occur over vast periods. Key among these processes is the subsidence, or gradual sinking, of the geological base supporting the coral reef.
This subsiding base is often the result of tectonic activity, such as plate movements beneath the Earth's surface. As these plates shift, they can cause parts of the ocean floor to sink, creating a downward motion for land masses like volcanic islands.

• As the island sinks, corals grow vertically, maintaining their habitat within the necessary sunlight and warmth.
• The balance between the sinking land and coral growth is crucial; the corals must grow fast enough to remain in the uplifting zone, allowing them to survive and continue constructing reefs.

Over thousands or even millions of years, these geologic processes transition a once-visible island into a submerged structure, framed by the coral reef that thrives above it.

Submarine Volcanic Activity

Submarine volcanic activity plays a crucial role in the initial formation of atolls. When a submarine volcano erupts, it builds up layers of lava and ash on the ocean floor that can eventually form islands.
These volcanic islands provide the perfect platform for the initial colonization by reef-building corals. Here’s how volcanic activity contributes to coral reef formation:

• The newly formed volcanic island offers a hard, stable surface where coral larvae can settle and begin to grow.
• As the volcano becomes inactive, geothermal heat may still influence the water around it, offering conditions favorable for coral growth.
• Even as the volcanic structure erodes and subsides with geological processes, coral reefs persist by growing upwards to stay within sunlight exposure.

Without this volcanic activity, many coral reefs, especially atolls, would not have the solid base necessary to begin their incredible life cycles. It is the interconnection of volcanic events and coral adaptation that leads to the mesmerizing ring-like atoll formations we see in warm oceanic waters.