By height above sea level:
* Mauna Kea in Hawaii, USA, is the tallest volcano measured from its base on the ocean floor to its summit, reaching around 10,200 meters (33,474 feet). However, its summit is only 4,205 meters (13,796 feet) above sea level.
By height above surrounding terrain:
* Mount Kilimanjaro in Tanzania, Africa, is the tallest free-standing mountain in the world, rising 4,885 meters (16,024 feet) from its base to its summit. It's also the highest mountain in Africa.
By overall volume:
* Tamu Massif in the northwest Pacific Ocean is believed to be the largest volcano by volume, with an estimated volume of 1.8 million cubic kilometers (430,000 cubic miles).
So, depending on how you define "tallest," the answer could be Mauna Kea, Mount Kilimanjaro, or Tamu Massif.
