A Tear in the Earth's Crust
Beneath the waters of the Andaman Sea, geological activity unfolds. This region is a back-arc basin, a depression that formed behind a volcanic island arc due to the immense forces of plate tectonics. Specifically, it is the boundary between the small Burma Plate and the Sunda Plate. These two microplates began to break away from the larger Eurasian Plate millions of years ago, a process driven by the relentless northward push of the Indian Plate colliding with Asia. This collision created the Himalayas and set in motion a chain of tectonic events, including the formation of the Andaman Sea starting around 3 to 4 million years ago.
The primary feature of this boundary is the Andaman Sea Spreading Center, an underwater rift where new oceanic crust is born. Here, the seafloor is pulling apart at a full spreading rate that has varied over time, with current estimates around 38 millimeters (3.8 cm) per year. This process began in earnest about 4 million years ago, initially at a slower rate of 16 mm per year before accelerating around 2 million years ago. The spreading center itself is a series of rift valleys and volcanic seamounts, morphologically marked by a 400–600 meter deep rift within a basin that is about 3,000 meters deep. This entire system is part of a larger tectonic puzzle that connects the Sagaing Fault in Myanmar to the Great Sumatra Fault in the south via a series of spreading segments and transform faults.
A Seafloor in the Making
The creation of new crust at the Andaman Sea Spreading Center is a dynamic process. As the plates separate, magma from the mantle rises to fill the gap. High-resolution seismic surveys have imaged this process in detail, revealing magma lenses at different depths within the crust. These magma chambers feed intrusions, like sills, that are injected into the thick layers of sediment blanketing the seafloor. Much of this sediment comes from the massive Irrawaddy River in Myanmar, which deposits so much material that it buries the developing rift valley.
This heavy sediment load creates unusual geological features. The faults that form the axial graben, or the central rift valley, are very steep, with dips between 65 and 75 degrees. The sediment also appears to influence how the faults develop, allowing them to remain active for longer and become more widely spaced than at unsedimented mid-ocean ridges with similar spreading rates. While much of the magmatic activity happens below the seafloor, it has also given rise to volcanic islands. Barren Island, the only active volcano associated with the Indian subcontinent, results from the subduction of the Indian Plate beneath the Burma Plate, which forces magma to the surface. Its first recorded eruption was in 1787, and after a quiet period of nearly 150 years, it became active again in 1991 and erupts periodically.