Coral reefs appear as solid, rock-like structures, yet they are built by tiny, soft-bodied animals called coral polyps. This unique biological construction creates a durable habitat that withstands the constant forces of the ocean. Despite their apparent solidity, corals can be surprisingly fragile when handled, making their physical durability a complex question. This exploration will delve into the fundamental materials and structural engineering that determine how hard coral truly is.
The Fundamental Composition of Coral
The hard structure of a coral reef is an exoskeleton built primarily from a mineral compound known as calcium carbonate. This is the same chemical compound found in limestone and seashells, but the coral skeleton forms a specific crystalline structure. The mineral takes the form of aragonite, which is a denser and slightly harder polymorph of calcium carbonate than the common form called calcite.
Coral polyps create this aragonite framework through a process called calcification. They draw calcium and carbonate ions from the surrounding seawater into a tiny enclosed space beneath their body. The organism then regulates the chemistry within this space to encourage the precipitation and growth of the aragonite crystals. This intricate biological control allows the coral to build its solid, yet porous, skeleton.
Quantifying Coral’s Physical Strength
To measure the intrinsic hardness of the material itself, scientists use standard mineralogical scales, placing coral’s aragonite on the Mohs scale of hardness. Aragonite typically registers a hardness of 3.5 to 4.0, meaning a common copper penny (Mohs 3.0) cannot scratch it, but a steel knife (Mohs 5.5) can easily mark its surface. This value reflects the hardness of the crystal, but it does not account for the coral’s overall structural strength.
A more relevant measure of durability is compressive strength, which describes how much pressure the material can withstand before crushing. Coral skeletal material exhibits compressive strengths ranging from approximately 12 to 81 MN/m\(^2\) (meganewtons per square meter). For context, this places coral material above the strength of common engineered materials like construction concrete, which typically has a compressive strength around 32 MN/m\(^2\).
However, the difference between coral and concrete is porosity, the measure of empty space within the structure. Coral skeletons are highly porous, meaning the raw material must be stronger to compensate for the voids that reduce the material per unit volume. Skeletal density, which is inversely related to porosity, becomes a direct indicator of strength, as denser coral structures are consistently more difficult to break.
Why Coral Hardness Varies Greatly
The term “coral” encompasses thousands of species, and their growth form fundamentally dictates their mechanical properties. Massive corals, like brain corals or boulder corals, grow slowly and form dense, dome-shaped colonies. These forms prioritize structural integrity, building skeletons with high bulk density that act like solid rock, making them resistant to crushing forces and wave action.
In contrast, branching corals, such as staghorn corals, grow rapidly by extending thin, tree-like structures. These forms have lower overall skeletal density and a higher surface area, making them structurally strong against the constant drag of water flow. They are much more brittle and susceptible to fracture from a sudden, sharp impact, like a heavy storm or physical contact.
The mechanical requirements of the environment often shape the coral’s internal architecture. Material from branching colonies may sometimes show a higher intrinsic strength than that from massive colonies, but the overall structural strength of the massive form is greater due to its sheer volume and density. This variation means a massive coral is less likely to be damaged by a passing ship, while a branching coral is better designed to withstand the hydraulic pressure of a strong current.
Processes That Weaken Coral Structure
Despite the inherent strength of its aragonite composition, a coral structure is constantly under threat from processes that compromise its physical integrity. One significant factor is bioerosion, the destruction of the skeleton by living organisms. Organisms like boring sponges, clams, and worms physically tunnel into the calcium carbonate structure, dramatically increasing its porosity and reducing its strength.
Chemical dissolution is another major weakening factor, primarily driven by ocean acidification. When the ocean absorbs excess carbon dioxide, the water chemistry changes, making it more difficult for corals to build and maintain their skeletons. This environmental stress impedes the process of thickening and reinforcing the skeletal walls, resulting in a less dense and structurally weaker skeleton.
The resulting less-dense structure becomes far more vulnerable to mechanical stress from the environment. Storms and strong waves, which the reef would normally withstand, can easily break apart these weakened skeletons. This combination of biological and chemical attacks means that the strength of a coral reef is not a static property but a dynamic balance between the coral’s ability to build and the forces attempting to break it down.