Sunda Shelf

Map of Sunda and Sahul.

Geologically, the Sunda Shelf is a southeast extension of the continental shelf of Southeast Asia. Major landmasses on the shelf include the Malay Peninsula, Sumatra, Borneo, Java, Madura, Bali and their surrounding smaller islands. It covers an area of approximately 1.85 million km². Sea depths over the shelf rarely exceed 50 metres and extensive areas are less than 20 metres resulting in strong bottom friction and strong tidal friction. Steep undersea gradients separate the Sunda Shelf from the Philippines, Sulawesi, and the Lesser Sunda Islands.

Definition

Biogeographically, Sundaland is a term for the region of Southeastern Asia which encompasses these areas of the Asian continental shelf that were exposed during the last ice age. Sundaland included the Malay Peninsula on the Asian mainland, as well as the large islands of Borneo, Java, and Sumatra and their surrounding islands. The same steep undersea gradients that mark the eastern boundary of Sundaland are identified biogeographically by the Wallace Line, identified by Alfred Russel Wallace, which marks the eastern boundary of the Asia's land mammal fauna, and is the boundary of the Indomalaya and Australasia ecozones.

The shelf has resulted from millennia of volcanic activity and erosion of the Asian continental mass, and the build up and consolidation of debris along the margins as sea levels rose and fell.

The seas between the islands cover relatively stable ancient pene-plains that are characterised by low seismicity, low isostatic gravity anomalies and no active volcanoes with the exception of Sumatra, Java, and Bali, which while connected to the Sunda Shelf, belong geologically to the young Sunda Arc orogenic system (i.e., the Sunda Mountain System). During glacial periods, the sea level falls, and great expanses of the Sunda Shelf are exposed as a marshy plain. The rise of sea level during a meltwater pulse 14.6 to 14.3 kbp was as much as 16 meters within 300 years.

Present sea levels submerge the Pleistocene Molengraaf River System - three vast submerged river systems that drained much of Sundaland during the last glacial maximum 18,000 to 20,000 years ago. Generally, the paleo-rivers are extensions of present-day river systems and may be interpreted to follow topographic lows in a down-slope direction. During the driest of the Pleistocene era the catchments forming west Borneo and the majority of Sumatra emptied through the Great Sunda River which arose between Belitung Island and Borneo flowing north east between the North and South Natuna Islands. North Java and south Borneo emptied in an easterly direction between southern Borneo and Java.

To the east of the Sunda Shelf is the Sahul Shelf. Separating these two regions of shallow seas is Wallacea, which encompasses Sulawesi and the thousands of smaller islands making up Nusa Tenggara and Maluku. Within Wallacea lie some of the deepest seas in the world with depths of up to 7,000 metres. Passing between Bali and Lombok, and Borneo and Sulawesi, the Wallacea is marked by a transition zone of flora and fauna first described by Alfred Russel Wallace. The complicated history of island formation on the Sunda Shelf and changing landbridge connections with mainland Southeast Asia have resulted in a high degree of endemism and local distribution discontinuities, discussed at Sundaland, the biogeographical province that has resulted from these changes.

The exposure of the Sunda Shelf during eustatic sea level changes has effects on the El Niño oscillation.

W. Earle in 1845 was the first to describe the general features of the Sunda and Sahul Shelves, which he termed the "Great Asiatic Bank" and the "Great Australian Bank" respectively.

See also

• Sundaland
• Sunda Shelf mangroves - (ecoregion)

External links

• Voris, H., and C. Simpson, 2000 and 2006, [1] The Field Museum, Chicago, Illinois.