The Indo-Australian Plate is a major tectonic plate that includes the continent of Australia and surrounding ocean, and extends northwest to include the Indian subcontinent and adjacent waters. It was formed by the fusion of Indian and Australian plates approximately 43 million years ago. The fusion happened when the mid-ocean ridge in the Indian Ocean, which separated the two plates, ceased spreading.
The eastern part (Australia) is moving northward at the rate of 5.6 cm (2.2 in) per year while the western part (India) is moving only at the rate of 3.7 cm (1.5 in) per year due to the impediment of the Himalayas. This differential movement has resulted in the compression of the former plate near its centre at Sumatra and the division into the Indian and Australian Plates.
A third plate, known as the Capricorn Plate, may also be separating off the western side of the Indian plate as part of the continued breakup of the Indo-Australian Plate.
Recent studies, and evidence from seismic events such as the 2012 Indian Ocean earthquakes, suggest that the Indo-Australian Plate may have broken up into two or three separate plates due primarily to stresses induced by the collision of the Indo-Australian Plate with Eurasia along what later became the Himalayas, and that the Indian Plate and Australian Plate have been separate since at least .
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