Where does the sand come from?
The satiny soft coral sand found on the majority of St. John’s beaches comes, almost entirely, from the coral reef community. This is the main reason why our sand is so much finer and softer then the sand found on most continental beaches, which comes from terrestrial sources, such as the weathering of rocks.
Most of our sand is produced by the force of waves and currents acting on the coral reef as coral, calcareous algae, (algae with a hard exoskeleton) the shells of various sea creatures and sea urchin spines (which make up those little black grains of sand) are gradually broken down into sand sized grains.
In addition, reef-grazing fish, such as parrotfish, produce a significant amount of the sand found on our beaches. Parrotfish exist on a diet of algae, which they scrape off the surface of coral rock with their fused teeth that look like a parrot’s beak. They then grind this coral and algae mixture to a fine powder. The algae covering the coral are absorbed as food. The remainder of their meal passes through their digestive tracts and is excreted in the form of sand.
Parrotfish are not shy and by donning a mask fin and snorkel, you can easily observe them at work and even hear the sound of their beaks scraping against the coral, then every so often you may witness them relieving themselves of the indigestible portions of their meal in the form of a fine sand that will settle slowly to the bottom of the reef.
Other grazing fish, such as the blue tang, perform the same function. The amount of sand produced in this manner is considerable – about one ton of sand per acre of reef per year.
How does sand get to the beach?
Sand is basically a waste product of the coral reef. This waste, which would otherwise suffocate the coral, is removed by the action of waves and currents over the reef. This sand collects in a kind of storage area around the perimeter of the reef.
During the winter, storms and cold fronts coming from North America and from over the central Atlantic generate large ocean swells. When these reach the north shore of St. John, they become steeper and break on the shore. This winter phenomenon is called ground sea and it serves to move the sand from the storage areas around the reef deposited it on the beach.
In the summer the same process can occur on the southern coasts, caused by the action of the trade winds or by tropical storms or hurricanes coming from the southeast.
How is sand lost from beaches?
Although sand is regularly brought to the beach from the sea, it is also consistently being lost from the beach. Because most St John beaches are found within bays protected by headlands or points on both sides of the beach, sand is not washed laterally along the coast and lost in this manner, as is the case on the beaches of the continental United States.
However, sand from the drier upper portion of the beach is often blown by winds past the line of vegetation where it will stay forever in the form of soil.
On the wetter lower beach, sand is constantly washed back and forth by waves. This makes the grains get smaller and smaller. When they get so fine that they go into suspension, they are washed back out to sea and lost.
Hurricanes or strong tropical storms are other natural phenomena that could result in sand loss. Large storms may either take away or add sand to existing beaches. They may even create new beaches. In general, extremely high ground seas and hurricanes accompanied by high tides will send large amounts of sand past the vegetation line or wash it back out to sea so far that the depth of the water will be too deep for the sand to be recycled by ordinary ground seas. Moreover, these storms often destroy large sections of reef, reducing the sand supply for years to come.
The lost sand will be replaced reef community and the beaches will remain in their sandy state. That is, as long as the dynamics of sand production and sand loss are in balance. This balance can be disturbed by natural causes such as hurricanes or coral diseases or as a result of interference by human beings in the natural order of nature. This interference can create a more insidious and continual imbalance, then imbalances caused by natural factors.
Removing sand from the beach or the sea floor can have extremely long lasting effects. For example, dredging operations take sand from sand storage areas, preventing it from reaching the beaches in times of ground seas or tropical storms.
Taking sand from the beach can also be irreversible. When St. John first began to experience the boom of tourism with the resultant construction of roads and buildings, a great deal of sand was taken from the beaches to make concrete. The loss of sand in this manner was so dramatic that the beaches never recovered and some of north shore beaches are now considerably narrower than they used to be. (For instance the now narrow Big Maho Bay used to be on of the widest beaches on St. John.) The process of recovery from this interference is extremely slow, and if the dredging or the mining of sand is continual, the sand beach will be replaced by rocky shoreline.
The worst threat to beaches comes from damage to the coral reef.
It is important to remember that a healthy coral reef is responsible for the continued existence of our beaches, and those factors that negatively impact the reef, such as pollution or runoff caused by irresponsible development will eventually lead to the disappearance of our beaches, which are, perhaps, St. John’s the most valuable resource.
St. John Weather
Scattered showers. Mostly sunny, with a high near 81. East northeast wind 20 to 22 mph. Chance of precipitation is 30%. New precipitation amounts of less than a tenth of an inch possible.