Call Us: US - +1 845 478 5244 | UK - +44 20 7193 7850 | AUS - +61 2 8005 4826

Exosystem: A relationship between two systems

High Islands

High islands, also called volcanic islands, are created as volcanic eruptions build up land over time. These eruptions begin under water, when hot magma is cooled and hardened by the ocean. Over time, this activity creates islands with a steep central peak—hence the name “high island.” Ridges and valleys radiate outward from the peak toward the coastline.

The island region of Melanesia contains many high islands because it is a major part of the “Ring of Fire,” a string of volcanoes around the boundary of the Pacific Ocean. This part of the Ring of Fire is on the boundary of the Pacific plate and the Australian plate. This is a convergent plate boundary, where the two plates move toward each other. Important volcanic mountains in Melanesia include Mount Tomanivi, Fiji; Mount Lamington, Papua New Guinea; and Mount Yasur, Vanuatu.

Low Islands

Low islands are also called coral islands. They are made of the skeletons and living bodies of small marine animals called corals. Sometimes, coral islands barely reach above sea level—hence the name “low island.” Low islands often take the shape of an irregular ring of very small islands, called an atoll, surrounding a lagoon. An atoll forms when a coral reef builds up around a volcanic island, then the volcanic island erodes away, leaving a lagoon. Atolls are defined as one island even though they are made up of multiple communities of coral.

The island regions of Micronesia and Polynesia are dominated by low islands. The Kwajalein Atoll in the Marshall Islands, for example, is composed of 97 islands and islets that surround one of the largest lagoons in the world, with an area of 2,173 square kilometers (839 square miles). The nation of Kiribati is composed of 32 atolls and one solitary island dispersed over 3.5 million square kilometers (1.35 million square miles) of the Pacific Ocean.