At its southeastern reaches, the Andaman Sea narrows to form the Straits of Malacca, which separate the Malay Peninsula from the island of Sumatra.

Ocean floor tectonics

The Andaman Sea, showing tectonic plate boundariesRunning in a rough north-south line on the seabed of the Andaman Sea is the boundary between two tectonic plates, the Burma plate and the Sunda Plate. These plates (or microplates) are believed to have formerly been part of the larger Eurasian Plate, but were formed when transform fault activity intensified as the Indian Plate began its substantive collision with the Eurasian continent.

As a result, a seafloor spreading centre was created, which began to form the marginal basin which would become the Andaman Sea, the current stages of which commenced approximately 3-4 million years ago

Volcanic activity
Within the sea to the east of the main Great Andaman island group is Barren Island, an active volcano (the only presently active volcano associated with the Indian subcontinent). Its volcanic activity is due to the ongoing subduction of the India Plate beneath the Andaman island arc, which forces magma to rise in this location of the Burma Plate. The volcanic island of Narcondam which lies further to the north was also formed by this process; however it has not recently been active.