Andaman Sea

The Andaman Sea, historically referred to as the Burma Sea, is a marginal basin of the northeastern Indian Ocean. It is bounded to the north and east by the coastlines of Myanmar and Thailand, and to the west by the Andaman and Nicobar Islands, which separate it from the Bay of Bengal. Southwards, its waters approach the northern end of Sumatra, with the Strait of Malacca forming its principal southern outlet. For centuries, the sea has played a vital role in regional fisheries and maritime trade, while its coral reefs and islands remain major tourist destinations. The December 2004 Indian Ocean earthquake and tsunami caused extensive devastation to the sea’s coastal settlements, fishing infrastructure and tourist facilities, reshaping the region’s socioeconomic landscape.

Geography and location

The Andaman Sea occupies the region between approximately 4°N–20°N and 92°E–100°E. Its central position in the eastern Indian Ocean makes it oceanographically and geologically significant, although it remained relatively unexplored until recent decades. To the south of Myanmar and west of Thailand, the sea is enclosed by island arcs and submarine ridges associated with the Indo-Burmese plate boundary. The Strait of Malacca, between the Malay Peninsula and Sumatra, forms a narrow and shallow southern passage, influencing water exchange and sediment transport.
The International Hydrographic Organization defines the sea’s boundaries using capes and island chains in Sumatra, the Nicobar Islands and Thailand. These limits include narrow channels and straits that ensure separation from the Bay of Bengal. Exclusive economic zones of Myanmar, Thailand, India and Indonesia overlap within the basin, underlining its political and economic relevance.

Geology and topography

The Andaman Sea displays contrasting bathymetric features. The northern and eastern margins are dominated by the wide continental shelf of Myanmar and Thailand, extending to the 300-metre isobath and causing about 45 per cent of the basin to be shallower than 1,000 metres. Silt transported by the Irrawaddy River contributes heavily to the sedimentation in these regions, creating extensive shallow areas. In contrast, the western and central portions reach depths of around 3,000 metres, with less than 5 per cent of the area extending beyond this. Submarine valleys east of the Andaman–Nicobar Ridge contain isolated pockets deeper than 4,000 metres.
The continental slope off the eastern shelf is steep, particularly between 9°N and 14°N, where depth drops by roughly a kilometre over a short horizontal distance. The sea floor comprises pebbles, gravel and sand, and a series of sea mounts create complex relief patterns. Water exchange with the Bay of Bengal occurs through several major channels: the wide but shallow Preparis Channel, the deeper Ten Degree Channel between Little Andaman and Car Nicobar, and the Great Channel between Great Nicobar and Banda Aceh.

Plate tectonics and seismicity

The Andaman Sea sits atop an active tectonic boundary where the Burma Plate meets the Sunda Plate. These microplates were once part of the Eurasian Plate but became distinct as transform faulting accelerated during the collision of the Indian Plate with Asia. This tectonic activity generated a back-arc basin, giving rise to the marginal sea. Basin formation is considered to have begun around 34 million years ago.
The region experiences frequent seismic events, including major megathrust earthquakes. At least six earthquakes since the late eighteenth century—occurring in 1797, 1833, 1861, 2004, 2005 and 2007—have reached magnitudes of 8.4 or higher. The devastating 26 December 2004 event resulted from a massive rupture along the boundary between the Burma Plate and the Indo-Australian Plate. Approximately 1,300 kilometres of the fault zone slipped, raising parts of the sea floor by several metres and generating a tsunami with waves tens of metres high. Around 280,000 lives were lost across Indian Ocean coastlines, making it one of the deadliest natural disasters in recorded history.

Volcanic activity

Volcanism in the Andaman Sea is linked to the subduction of the Indian Plate beneath the Andaman island arc. The most prominent volcanic feature is Barren Island, located east of the main Great Andaman group. With a diameter of about three kilometres and a summit rising over 350 metres above sea level, it represents the only active volcano in South Asia. After nearly two centuries of dormancy, it resumed eruptive activity in 1991, with further episodes occurring from 2008 onwards.
Further north lies Narcondam Island, also volcanic in origin. Although no historical eruptions have been recorded, geological evidence indicates that it formed through similar subduction-related processes.

Sediment transport and deposition

The Andaman Sea receives substantial sediment input from the Ayeyarwady (Irrawaddy) and Thanlwin (Salween) rivers. These rivers collectively deliver several hundred million tonnes of sediment annually. Modern studies identify key depositional patterns:

• A major mud wedge up to tens of metres thick has formed in the Gulf of Martaban, extending into the Martaban Depression.
• Little sediment accumulates on the shelf directly off the Ayeyarwady delta.
• A mud drape, thinning seaward, covers the western Myanmar Shelf and likely channels fine sediments into the deep Andaman Trench.
• Total Holocene sediments offshore amount to several tens of billions of tonnes, with estimated historical deposition representing roughly one-third of present-day riverine sediment discharge.
• Sediment movement demonstrates bidirectional transport, governed by tides and seasonally reversing monsoon winds, unlike the predominantly unidirectional systems of the Mekong or Yangtze.

These processes shape the basin’s turbid waters and nutrient distribution, influencing marine ecosystems and coastal geomorphology.

Climate and seasonal variation

The climate of the Andaman Sea is strongly influenced by the South Asian monsoon, which causes a reversal in prevailing wind patterns. From November to February, northeasterly winds dominate, with stronger intensities in the western basin. Wind speeds decrease during March and April, then shift to powerful southwesterlies from May to September, reaching sustained speeds of around 15–20 knots during June to August. October marks the transition back to the northeasterly regime.
Air temperatures remain relatively stable, averaging between 26°C in February and 29°C in August. Precipitation is substantial, driven by monsoon dynamics, and contributes to seasonal freshwater input, variations in salinity and changes in surface circulation. These climatic patterns help sustain diverse marine habitats, including coral reefs and mangrove ecosystems.