The Three Largest Active Volcano on Earth

This is the Big Island of Hawaii, which is about 75 miles (120 km) in diameter. The largest volcano toward the center is Mauna Loa, which comprises most of the island. Up and to the right Mauna Kea can be seen. 

The three largest volcanoes in physical size
The world’s largest volcanoes in physical size are not the ones that could produce the very largest eruptions. The largest eruptions are produced by supervolcanoes.
The three largest volcanoes in physical size are Mauna Loa and Mauna Kea, which comprise most of the Big Island of Hawaii, and Mount Teide, which comprises part of the island of Tenerife in the Canary Islands. Each are shield volcanoes, although Mount Teide is actually a stratovolcano sitting on top of a large shield volcano.
The three supervolcanoes, Taupo, Toba, and Yellowstone, are all smaller in physical size but are capable of far larger and more dangerous eruptions. However, Mauna Loa and Mount Teide are still considered two of the world’s most dangerous volcanoes because they have a history of destroying towns on their respective islands and will likely do it again sometime

Mount Teide, Canary Islands, Spain: Third-largest active volcano on Earth
Located on the island of Tenerife, which has over 900,000 residents, in the Canary Islands, this potentially dangerous volcano last erupted in 1909. In 1706 the primary town of the island and several other villages were destroyed by an eruption. Christopher Columbus witnessed an eruption when sailing past Tenerife in 1492.

The peak of the mountain stands at an elevation of 12,198 feet (3,178 meters), and is the tallest peak in the Canary Islands, which are part of Spain and located in the northern Atlantic Ocean (see a map showing the location below).

The Canary Islands consist of multiple shield volcanoes, and Mount Teide is considered a stratovolcano which is part of, and sits on the top of, the shield volcanoes that formed Tenerife anciently. As a stratovolcano, it has potential for explosive eruptions similar to Italy’s dangerous Mount Vesuvius, which it closely resembles in structure.[

The island formed millions of years ago, and begins from the ocean floor at a depth of about 12,400 feet (3,240 meters), and thus overall is about 24,600 feet in height. However, not all of the island is the Teide volcanic complex. The portion that is the active volcano Mount Teide has an estimated volume that ranks third after the two Hawaiian volcanoes discussed below.

Mauna Kea, Hawaii, USA: Second-largest active volcano on Earth
I’ve seen many websites incorrectly state that Mauna Kea is the largest active volcano in the world, although the largest is actually its neighbor on the same island, Mauna Loa. The confusion comes from the fact that the peak of Mauna Kea is taller. It is the tallest volcano in the world, counting from the sea floor below, although not the largest by volume.

The most recent eruption of Mauna Kea was about 4,500 years ago. By geological standards, this is recent and the shield volcano is still considered active and is expected to eventually erupt again.[8][11]

The estimate for the volume of Mauna Kea is about 3,200 cubic km (770 cubic miles). Although this is considerably larger than Mount Teide in the Canary Islands, it is far less than Mauna Loa, discussed next.

Mauna Loa, Hawaii, USA: Largest active volcano on Earth
No active volcano on Earth matches the size of Mauna Loa, which comprises most of the Big Island of Hawaii.  This large, broad shield volcano originates about 16,400 feet (5,000 meters) below the surface of the ocean, and rises to a height of 13,679 feet (4,169 meters) above sea level.  Thus the total height from the ocean floor is over 30,000 feet (9,000 meters).[3]

The volcano is so massive that it pushes the Earth’s crust down where it sits, and the true bottom of the volcano goes down even further, making the overall height of the volcano actually over 56,000 feet (over 17,000 meters). Even without counting the underground portion, it is easily the tallest mountain on Earth. The total volume of Mauna Loa is estimated at 75,000 cubic km (18,000 cubic miles).[3]

Mauna Loa is considered highly dangerous due to its recent history of eruptions, and the potential damage that could be caused, in future eruptions, to towns located on the island.  The most recent eruption occurred in 1984, and in hundreds of years it has not been known to go so long without an eruption.[3]

Mauna Loa destroyed two villages during the 20th century.  The city of Hilo is partially built on lava flows from an eruption which occurred during the 19th century.[3]

There is one extinct volcano larger than all of them
In the northern Pacific Ocean there is an extinct shield volcano that is the same approximate size as the US state of Arizona or the island of Great Britain, dwarfing even Mauna Loa. However, it has been extinct since the end of the Jurassic Period, having last erupted about 145 million years ago. It is entirely submerged beneath the ocean, with its peak 6,500 feet (1,980 meters) below sea level.[5]

The slopes are very gradual. With the immense area that it covers, it rises just 14,620 feet (4,460 meters) from the sea floor. Due to this shield volcano’s very gradual sloping and its location beneath the surface of the ocean, it was not recognized as a giant extinct volcano until 2013. The estimated volume is an astonishing 2.5 million cubic km (600,000 cubic miles)