The island is a stratovolcano with a granitic composite cone, dating from the late Pleistocene period, between 10,000 and 15,000 years ago. The island has a roughly circular coastline, 38.3 kilometers in length, with an average diameter of eight kilometers. The highest elevation Mount Oyama (雄山, Oyama-san) is an active volcano with a height of 775.1 meters. The mountain has been recorded to have erupted numerous times throughout history, and is mentioned as far back as Nara period written records. Over the past 500 years, it has erupted 13 times, including five times since the start of the Meiji period. A lava flow in 1940 killed 11 people, and other eruptions occurred in 1962 and 1983. On July 14, 2000, Mount Oyama began another series of eruptions, and by September, the island was completely evacuated. After a four-year period of volcanic emissions, residents were allowed to return permanently on February 1, 2005. After the eruption, there has been a constant flow of sulfur dioxide gas coming from Mount Oyama. The residents of the island are required to carry gas masks with them at all times, but they do not need to be worn constantly. Alarms go off if there is a dramatic increase in the levels of toxic gases in the air. Volcanic gas emissions from this volcano are measured by a Multi-Component Gas Analyzer System, which detects pre-eruptive degassing of rising magmas, improving prediction of volcanic activity.
Miyake-jima has a humid subtropical climate (Köppen climate classification Cfa) with very warm summers and mild winters. Precipitation is abundant throughout the year, but is somewhat lower in winter.
The island is administered by the Miyake subprefecture of the Tokyo Metropolitan government. As of June 1, 2016, the population of the island was 2451. Miyake Village (三宅村, Miyake-mura) serves as the local government of the island, and also encloses nearby tiny uninhabited Ōnoharajima west south-west of Miyake-jima.
Miyake village consists of the five traditional hamlets of Izu, Kamitsuki, Igaya, Ako and Tsubota, with Tsubota on the southeast coast as the administrative center.
Miyake-jima is home to the rare endemic Izu thrush (akakokko). The island is home to an unusually rich flora and fauna with several rare species of birds and animals, although its natural habitats are constantly under threat from human and volcanic activity. Underwater, the island is valued by divers for its coral reefs and marine fauna (including the dolphin population outside nearby Mikura-jima).
The island is accessible by over-night ferry, the Sarubia Maru or the Camellia, which is operated by Tōkai Kisen. The ferry departs from Takeshiba Pier, near Hamamatsuchō Station, Tokyo at 22:30 and arrives at Miyakejima at 5:00.
There are flights connecting Miyakejima Airport and Haneda Airport with an approximate flying time of 50 minutes. Flights are also operated from Chofu Airport. The area is prone to high volume of sulfuric gas and flights were suspended for almost eight years after the July 14, 2000 volcano eruption. Flights resumed during April 2008, after sulfuric gases in the air dropped to levels below 0.2ppm. There is a helicopter that arrives via Izu Ōshima. There are also two helicopter flights operated by Tokyo Island Shuttle which originate in Aogashima and Izu Ōshima and fly to Toshima, Miyake-jima, Mikura-jima, Hachijō-jima and Aogashima.