The geography of Indonesia is dominated by volcanoes that are formed due to subduction zones between the Eurasian plate and the Indo-Australian plate. Some of the volcanoes are notable for their eruptions, for instance, Krakatau for its global effects in 1883, Lake Toba for its supervolcanic eruption estimated to have occurred 74,000 Before Present which was responsible for six years of volcanic winter, and Mount Tambora for the most violent eruption in recorded history in 1815.
Weh Island is located in the Andaman Sea, where two groups of islands, the Nicobar Islands and Andaman Islands, are scattered in one line from Sumatra to the north up to the Burma plate. The Andaman Sea lies on an active moving small tectonic plate (microplate). A complex geological fault system and volcanic arc islands have been created along the length of the sea by the movement of the microplate.
The island is just 350 meters (1,150 ft) off the northernmost tip of Sumatra. The island is small at only 156.3 km˛, but mountainous. The highest peak is a fumarolic volcano and is 617 meters (2,024 ft). The last known eruption is estimated to have occurred in the Pleistocene age. As a result of this eruption, the mountain partially collapsed, was filled by the sea and thus a separate island was formed.
At nine metres depth (29.5 ft) close to Sabang city, underwater fumaroles emerge from the seabed. A volcanic cone is found in the jungle. There are three solfatara (mudpot) fields on the island: one is 750 m (0.5 mi) southeast of the summit and the others are 5 km (3 mi) and 11.5 km (7 mi) northwest of the summit on the western shore of Lhok Perialakot bay.
There are four islets surrounding Weh Island: Klah, Rubiah, Seulako, and Rondo. Among those, Rubiah is well known for diving tourism, because of its coral reefs. When traveling to Saudi Arabia was only possible by sea, Rubiah was used as a place of quarantine for Indonesian Muslim during the Hajj pilgrimage season.
