When Captain Cook first sighted Yasur in 1774, he likened it to Stromboli in the Mediterranean because of it's constant glow at night, like a beacon.

There are different types of volcanoes, and sometimes, the same volcano will erupt in different ways. To distinguish between types of volcanoes and eruptions, they are given names.

Coincidental to Cook's descriptions, Yasur often erupts in what Volcanologists call a Strobolian type eruption, throwing out clots, or fountains of magma from a central crater.

Yasur Volcano at dawn. Note the fog over Lake Siwi at the base of volcano
Photo : Sonny

For the most part, however, Yasur is more a Vulcanian type. That's where large blocks and glowing fragments of new, viscous lava are thrown out. And this is what most people see when they stand on the edge of the Yasur and look in. Generally, the blocks are not thrown too high so it is not dangerous.

Because it has erupted in different ways over the years, Yasur is made up of different types of rocks and is called a strato (meaning layers or strata) volcano.

Yasur and Lake Siwi
Photo : Sonny

In other words it has built from both lava flows and pyroclastic material. Pyroclastic eruptions occur when the magma rises up into a dome that eventually gives way like an avalanche.

In fact in French it's called nuee ardente - glowing avalanche. The pyroclastic flow is a combination of crystals and fragments of lava, glass shards, pumice, ash and steam in a superheated rolling, broiling cloud that can move over 100 miles per hour down the side of the volcano.

Such pyrocalstic flows do not do that much damage around Yasur simply because the volcano is not that big, or tall. However Yasur has been known to cause a tsunami and sometimes the ash clouds have reached 6,500 feet (2,000 m). With all that ash falling on parts of the island, combined with the acidic nature of the fallout, a great deal of damage can be done to gardens that people need to live. Because the people living there mostly have no cash income, but live from their gardens, many times in the last few years Tanna has had severe food shortages.

The other aspect of ash falls is that it builds up into a thick mud, almost as hard as concrete. But when it rains a lot, the ash mud can turn into a landslide and bury whole villages.

This cross-section of upper mantle and crust beneath Yasur shows the eastward-moving Australian Plate, here made of oceanic crust and upper mantle (black), is subducted beneath the westward-moving Pacific Plate (brown). Yasur is on the Pacific Plate.

When earthquakes occur their epicentres are in the the subducted plate. By looking at these, scientists can tell that the subducted plate goes down at least 150 miles (250 km) into the asthenosphere (light brown/orange).

Magma is made where the mantle in the asthenosphere is changed because of fluids leaving the subducted plate. It then forces its way up through fractures or faults in the overriding plate. Some of these faults come all the way to the surface to become volcanoes.

For those living on the island, it's common knowledge that Yasur is most active around late February to April. This time of year is the wet season. A lot of rain means a higher groundwater table, possibly allowing water to come into contact with hot rocks around the magma chamber.

Explosive phreatic (water and hot rock - not magma) eruptions are known to occur, but whether this has anything to do with the wet season is only speculation. However, it is a fact that this time of year is the most spectacular and the volcano is generally very quiet at the end of the dry season.

This night photo shows three active vents , one behind the other from this angle (facing east). The vents are bright yellow, the middle one being hardest to see because the front one is most active. The vents do not erupt all at the one time, however this photo was taken by time lapse, over about 2 minutes. You can tell the third vent was the last to erupt because some of the lava projectiles were still in the air when the camera shutter closed, leaving red trace lines that have not touched the ground.
Photo : Sonny

One of the most frequently asked questions is , has anyone ever fallen in? The answer is no, possibly because hot air is constantly upwelling, almost blowing people back away from the edge. Sometimes it can be so forceful, camera tripods are knocked backwards.

A few years ago, during March and April, three people were killed over the course of two weeks when they stood on the downside of spurting lava. It was during the Wet season and Yasur was more active than usual. Sadly those who died had gone to areas at the edge of the vent that tourist guides normally kept people away from, knowing it was potentially dangerous. In the first incident, a Tannese ni-Vanuatu from another part of the island had gone to the volcano alone, was hit in the leg by a piece of lava and instead of going to a doctor, went to seek 'bush medicine' treatment; he eventually bled to death.

In the second incident, both a guide and tourist were killed because the tourist had insisted on going to a dangerous area and the guide accompanied her. Both were hit by erupting pieces of lava and died instantly.

Thereafter the upper rim of the volcano was closed until activity quieted down some weeks later. However the lesson here is, stay on the path in dangerous places. Volcanoes are not theme parks, they are nature at its most awesome - and unpredictable.

Next: Ambrym Island Volcanoes