Explanation:

THE BARREN AND NARCONDAM ISLANDS, north of Port Blair, are volcanic islands(Note: Barren Island is the only active volcano of India.).The Andaman and Nicobar islands archipelago consists of approximately 265 small and large islands.
Andaman Islands:The Andaman islands are divided into three main islands i.e. North, Middle and South.Duncan passage separates Little Andaman from South Andaman.The Andaman group of islands in the north is separated by the Ten Degree Channel from the Nicobar group in the south.Port Blair, the capital of Andaman Nicobar Islands lies in the South Andaman.
Nicobar Islands:Among the Nicobar islands, the Great Nicobar is the largest. It is the southernmost island and is very close to Sumatra island of Indonesia. The Car Nicobar is the northernmost.Most of these islands are made of tertiary sandstone, limestone and shale resting on basic and ultrabasic volcanoes [Similar to Himalayas].Some of the islands are fringed with coral reefs. Many of them are covered with thick forests. Most of the islands are mountainous.Saddle peak (737 m) in North Andaman is the highest peak.
Note: In Andaman and Nicobar island group, the three type of Islands are found – the extension of continent, the Volcanic Islands and Coral Islands.