The largest volcano on Earth
October 2013

Volcanoes are not always the classic cone shape. In fact the biggest volcanoes are sometimes so large that they are not immediately recognizable. So the fact that largest volcano ever to exist on the Earth has only recently been discovered is less of a surprise than it might otherwise be.

Scientists from the University of Houston, led by Professor William Sager, published an article in Nature Geoscience last month describing this volcano. It is a massive 'shield' type volcano which has been named Tamu Massif (Tamu stands for 'Texas A&M University', e where Professor Sager started studying volcanoes some 20 years ago). The volcano covers an area of about 120 thousand square miles. If confirmed, this makes Tamu by far the biggest volcano every to be discovered on Earth. In fact, Tamu Massif is comparable to some of the giant volcanoes discovered on Mars, giving it a rightful place amongst the largest volcanoes in our Solar System.

Shield volcanoes are the largest volcanoes on Earth. They have very fluid lava which is composed almost entirely of basalt. This lava arrives at the surface at very high temperature, little changed from the time it was generated. The basaltic lava piles up into huge 'shields' with gentle slopes and convex outlines. Explosive eruptions are rare because about 90% of the volcano is lava rather than pyroclastic material (such as gas at explosive pressures). The Hawaiian shield volcanoes (Kilauea and Mauna Loa) are the most notable examples of shield volcanoes. But to put Mauna Loa in perspective, its size is approximately 2,000 square miles or about 1/50 the size of Tamu. For a more accurate comparison Tamu is about the size of New Mexico in the USA.

Another reason it took so long to discover is because Tamu Massif is a submarine volcano. It is located in the Shatsky Rise some 1600 km (990 miles) east of Japan. Shatsky Rise is an underwater mountain range formed 130 to 145 million years ago by the eruption of several underwater volcanoes. Until now, it was unclear whether Tamu Massif was a single volcano, or a composite of many eruption points.

The latest results come from the analyses of two distinct studies. The first set of data comes from analysis of core samples which were collected during the Integrated Ocean Drilling Program (IOPD) Expedition 324 which focused on Shatsky Rise Formation in 2009. The second set of data was from the seismic reflection studies carried out by two separate expeditions in 2010 and 2012. Using multichannel seismic profiles and rock samples to analyse the chemical composition of Tamu Massif, the researchers were able to make a model of the volcano. The results showed that the summit lies about 6500 ft (1980 m) below the sea surface and the base extends to a depth of 6.4 km (4 miles). The volcano is about 4,460 metres (14,620 ft) tall. Rather than being the product of several different volcanoes, the studies showed that the lava had come from a central point of origin, one - very large - volcano.

Here is how Sager describes this massive edifice: 'It's not high, but very wide, so the flank slopes are very gradual. In fact, if you were standing on its flank, you would have trouble telling which way is downhill. We know that it is a single immense volcano constructed from massive lava flows that emanated from the center of the volcano to form a broad, shield-like shape. It's shape is different from any other submarine volcano found on Earth, and it's very possible it can give us some clues about how massive volcanoes can form.'

Given the size of the volcano, it is reassuring to know that it is now extinct - and has been for over 100 million years.

Reference:

William W. Sager, Jinchang Zhang, Jun Korenaga, Takashi Sano, Anthony A. P. Koppers, Mike Widdowson, John J. Mahoney. An immense shield volcano within the Shatsky Rise oceanic plateau, northwest Pacific Ocean. Nature Geoscience, (2013). Published online Sept. 5 at http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1934.html.