Sunda Shelf (geological formation)

The Sunda Shelf is an established platform of underwater rock in Southeast Asia. It extends from the continental mainland of Asia and is partly covered by relatively shallow waters from the South China Sea, the Gulf of Thailand, and the Java Sea. Several islands are part of the Sunda Shelf, including Borneo, Java, and Sumatra. Scientists say that the shelf was once above water when it likely served as a land bridge for human and animal travel. The shelf became economically significant in the 1950s, when large pockets of natural gas and petroleum were found in the area. The presence of the shelf affects the patterns of weather and climate in the area.

Background

The Sunda Shelf is a geographical formation known as a continental shelf. Continental shelves are protrusions from larger land masses that extend under the water of a continent’s shoreline. They are stable parts of the continent and included as part of the continent’s geographic boundary. A continental shelf extends out into relatively shallow water—usually five hundred feet deep or less—until it drops off sharply. This is known as a shelf break. The area beyond the break is known as a continental slope, which descends to the main sea floor.

Continental shelves often bear signs of rivers as well as plant and animal life that indicate they were originally dry land. Over time, a combination of rising sea levels and a buildup of organic material (such as plant and animal remains) and inorganic matter (such as rock and sediment) push the surrounding seawater over coastal areas. This results in the formation of continental shelves.

Continental shelves can benefit humans in many ways. The organic matter that helps form them often creates rich deposits of natural resources. Petroleum and natural gas are created by the decay of plant and animal materials under pressure from sediment that is deposited over them. This makes them valuable sources of energy.

In addition, shallow waters over continental shelves provide ideal living conditions for many types of sea life. Sunlight, calmer waters, and the sediment that comes from the fresh water rivers on the nearby land provide ideal conditions for the micro-lifeforms like shrimp and algae that other fish eat. As a result, many types of aquatic plants and animals live on continental shelves. Some studies have indicated that as much as ninety percent of the world’s fish depend on these shelves.

The Sunda Shelf was first identified by George Windsor Earl, a nineteenth-century British navigator and author. Earl, born about 1805, spent much time in the seas between Asia and Australia and coined the term indu-nesian for the area between Asia and India; this later became Indonesia, the term used to the present day. Earl reported the location of the shallow water shelf off the coast of Asia in a paper presented to the Royal Geographical Society in 1845.

Overview

The Sunda Shelf covers about 690,000 square miles (1.8 million square kilometers) off of the coastline of Southeast Asia. It is the second-largest continental shelf in the world, after the Siberian shelf, that extends into the Arctic Ocean. The Sunda Shelf extends from Vietnam to the island countries of Borneo and Java. These islands, along with Sumatra and Malaysia and a few others, are the visible portion of the Sunda Shelf that lies above water. The remainder of the shelf is below relatively shallow water measuring about 330 feet (100 meters) in depth.

While the shelf is tectonically stable—meaning it is not prone to earthquakes or other shifts—it is surrounded by less stable areas. Its west and south boundaries are defined by the Java Trench, which is sometimes known as the Sunda Trench. This deep undersea depression is part of the Ring of Fire, an area that sees frequent seismic activity such as earthquakes and volcanic eruptions. A 2004 earthquake in this area unleashed a devastating tsunami. Just outside the shelf’s edge are the lesser Sunda Islands, including Bali, Lombok, Sumbawa, Flores, Sumba, Timor, the Alor archipelago, the Barat Daya Islands, and the Tanimbar Islands. These are volcanically active islands.

Scientific examination has revealed evidence that the entire shelf was above the water line in the past. The remains of streams and rivers cross the underwater portions of the shelf and indicate the land around them was once dry. The remains of plant and animal life that formed the petroleum and natural gas reserves also point to a time when the shelf was above water.

The portions now underwater are thought to have provided a land bridge that enabled people to travel between areas that are now islands and the mainland. Some experts believe the submerged areas—sometimes referred to as Sundaland—may have been the source of the Asian population. This is known as the Sundaland theory.

The underwater shelf also has an impact on the climate in the region. Day-to-day weather and longer-term climate patterns are affected in part by the amount of heat and moisture in the air. The relatively shallow waters over the shelf allow sunlight to keep the water warmer there than it is over deeper ocean waters. This affects the amount of rainfall and the wind patterns in the area, impacting the weather and climate. In addition, both the underwater and above-water portions of the Sunda Shelf help mitigate the effect of tsunamis on the nearby mainland.

The plant life on the Sunda Shelf is part of this mitigation effect. For example, the shelf is home to more than two dozen different species of mangrove. Mangroves are trees and shrubs that are able to live in areas with salt water and brackish water, or water that has more salt than fresh water but not as much as seawater. The mangroves help limit the erosion of soil during the cyclones that often occur in the area. They also help to dissipate the power of tsunamis and tidal surges that might otherwise cause more damage to the mainland.

Bibliography

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