Question

how were the andaman and nicobar islands formed?

Answer 1

Andaman and Nicobar Islands were formed due to collision between Indian Plate and Burma Minor Plate [part of Eurasian Plate][Similar to formation of Himalayas].
Andaman and Nicobar Islands are southward extension of Arakan Yoma range [Myanmar][Arakan Yoma in itself is an extension of Purvanchal Hills].
Lakshadweep Islands are coral islands. These islands are a part Reunion Hotspot volcanism.

Answer 2

Your answer is

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Andaman  and Nicobar Island is a island arc complex. Island arcs are formed by collision of 2 oceanic lithosphere. In the case of Andaman and Nicobar Islands, the Indian plate collided with the Burmese plate. About 100 million years ago the Indian plate broke off from Gondwana and started moving Northward and it encountered the Burmese plate and as the subduction occured between the two plates and the islands started rising.
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