Chapter 14, Problem 14.1R

To determine

The formation of high islands, coral atolls, and barrier reefs.

Answer to Problem 14.1R

Larger and active volcanoes on the ocean floor produce high islands that rise several hundreds or even thousand feet high above the sea level. The barrier reefs are produced by reef-building corals in shallow water environment surrounding islands. An atoll is composed of narrow sandy islands, barrier coral reefs, and shallow central lagoons. It is formed by the erosion and sinking of an extinct volcano with the growth of coral reefs and formation of sand spits around a central lagoon.

Explanation of Solution

The two distinct processes form most of the islands either by volcanism or by Coral reefs. Coral reefs are aquatic ecosystem characterized by reef-building corals. The reef-building corals are colonies of corals that survive in warm, shallow, tropical seawater, and produce a hard skeleton of limestone.

High islands: Islands originated by the processes of volcanic eruption on the ocean floor are called high islands. The volcanoes that are larger, active, and recently active volcanoes create high islands. These are termed high islands because they rise hundreds or thousands of feet in elevation above the sea level. For example, The Hawaiian Islands with volcanic peaks ranging more than $4000\text{ meters (13,000 feet)}$ on Hawaii’s Big Island. Continental high islands are even larger and more geographically complex such as the Continental high islands of New Zealand, Papua New Guinea, and the Solomon Islands.

Low islands: The islands originated by the action of coral reefs on the ocean floor are called low islands. In contrast to the high islands, the low islands in lower elevation are usually smaller and are flatter.  The soil originated by coral action is less fertile than the volcanic soil of high islands and supports less varied plant life. The low islands originate as barrier reefs around or over sunken volcanic high islands and results in an atoll.

• Barrier reef - The barrier reef start as Fringing reefs. Fringing reef is formed by corals that surrounds the shore of an island or volcano in a shallow water environment. Here, the water qualities such as salinity, temperature, and turbidity are favorable for reef-building corals.  The extinct volcano gets eroded and sinks, the coral reef expands, forming a larger barrier reef. Barrier reefs are round reefs formed by corals separated from an island by a well-built lagoon. The term ‘barrier reef’ comes from the threat these structures pose to navigators approaching the island from the sea. As the island or volcano sinks, the reef grows upwards and maintains its position near to sea surface level. Example of barrier reef is Great Barrier Reef in Australia, the largest reef system in the world.
• Atoll - The barrier reef results in an atoll. An atoll is a circular reef formed by the complete submergence of a volcanic high island separated by a central lagoon. The coral reefs build up toward the surface and when the island becomes completely sunken, the coral reef continues to grow. The buildups of fragmented coral debris form narrow islands that enclose the central lagoon. These are large enough to facilitate human habitation. An example of an atoll is Kwajalein in Micronesia’s Marshall Islands, the largest atoll in the world.