Our earth is an enormous thermal machine, the disintegration of its radioactive elements supplying the energy that moves its continents at its surface. Volcanic activity results from the continual movement of matter within our planet, generating volcano ranges in the expansion zone (ridge) generally under the ocean surface, or those of convergence, or subsidence. “Hot spot’ volcanoes represent a third category.
Vanuatu’s volcanoes are located in one of these areas of convergence, where the phenomenon marks the earth’s surface with a trench about 7000 meters deep. The Australian plate plunges under that of the Pacific and fuses with it, generating magma that reaches the surface and creates volcanoes.
Lava in the subsidence areas is thick and causes about 25% of the world’s explosive volcanic activity. The Hebridean archipelago’s volcanoes, created by subsidence, form part of the Pacific’s Ring of Fire’. Volcanic activity can cause catastrophes on a local level, or even go so far as to influence climatic change on a worldwide scale. The New Hebrides Archipelago (the geographic name of Vanuatu) is in the shape of the Greek letter
The Western part is about 25 to 14 million years old, while the Eastern part is between 14 and 4 million years old. The central part of the range began volcanic activity at the start of the Quaternary period, a little more than a million years ago.
Tanna’s oldest part (appearing over a million years ago) is in the north of the island. Today’s active area is located in the south of the island at the edge of the pseudo-caldera, (large volcanic crater), and began some 100,000 years ago. Yasur Volcano is an active element in the Yenkahe complex of volcanic activity, which emerged from the sea dozens of millions of years ago. A vast reservoir of molten lava between the villages of John Frum (Sulphur Bay) and Port Resolution (where Captain James Cook first set foot on Vanuatu), feeds into Yasur’s eruption vents via a network of faults and large gas pockets measuring over a metre in diameter form a few hundred metres under the volcano’s crater, which is about 400 by 700 metres.
Coral reefs form on what the volcanic activity produces, at depths ranging from just at the surface to a few meters deep. In this way, over the near thousand kilometers of the archipelago stretching from north to south, the ‘islands of ashes and coral’ are formed.