West coast of Ambrym at dawn. The dip in the middle is the collapsed caldera. Maroum is generating the most smoke while Benbow, slightly to the left, is putting out smaller smoke plume. Scale has been exaggerated vertically by 3xThis cross-section of Ambrym shows the basal shield, or the bottom part of volcano, in black. Benbow, Niri Mwembelesu (centre front) and Mwebelsu vent inside Maroum are all smoking

Ambrym is an amazing volcanic island in the centre of the of Vanuatu archipelago. Although it is constantly erupting, on the VEI scale (Volcanic Explosivity Index), it's generally around 0-1. That means its putting out a lot of smoke and some ash and the occasional rock or two, but is generally safe to be nearby. In recent years it has also oozed a slow lava flow from within the caldera. These daily incidents do not really define it as 'erupting', however Ambrym has certainly done that - 48 times since 1774. It also has a caldera that collapsed as a result of an eruption around 50AD* with a force of VEI 6 (10 times bigger than Mt. St Helens).

Ambrym is a shield volcano. That means it grew slowly, into a flattened dome shape, as successive lava flows built one on top of the other.

The light grey area on he diagram at left is called the tuff cone. A tuff cone is a type of volcanic cone formed when basaltic magma (that's lava while it's still underground) comes into contact with water. The black lines through it are faults. The darker grey is the ash, lava and ejecta that have filled the caldera since it collapsed. It's called the Ash Plain and includes three currently active cones. The larger active active cones are named Benbow and Maroum (the active vent in Maroum has moved slightly westward and is identified as Mbwelesu). The third, smaller cone at the southern flank of Maroum is called Niri Mbwelesu. This cone has only recently become 'active' again although to date it only sends up wafts of smoke. However three fingers of lava oozed from a fault in the ground just a few hundred metres to the south (identified as Niri Mbwelesu Taten) across the Ash Plain in 1988-89.(see diagram below) The black fault lines are cracks that allow the magma to rise up into lava lakes (see image below), or through side cones like Niri Mbwelesu and the 1998-99 lava flow that oozed from the ground. This ground flow is just one of many that have simply bubbled up from the major east-west fault line across the island and slowly spread across the Ash Plain in recent years. Such upwellings may have come directly from the magma chamber below or via underground faults from the main vents.

Ambrym is quite a large shield volcano. It may not seem so big when you read the figure, but think about it, the collapsed caldera, the Ash Plain, at the top is huge - 12 kilometres across! As for the Ash Plain, well, it's ash all right, but the term 'Plain' is a misnomer, for most of Ambrym's eruptions are Plinian.

Plinian eruptions are named after the Roman Naturalist, Pliny the Elder, who was killed by Mount Vesuvius in 79AD. Plinian eruptions are a description given to eruptions that occur when the magma is full of gas. This gas is most often superheated steam which has come from groundwater percolating into the magma chamber. You know what happens when water boils, and when it boils in something like a pressure cooker, if it overheats, there's a tremendous explosion and huge mess! In Plinian eruptions, the explosion is forced straight upwards, often at twice the speed of sound and as much as 20 miles high. The blast can be so powerful it literally shreds the magma to tiny fragments, or ash, that then falls in a suffocating blanket over everything. It's also characterised by masses of pumice, a light coloured frothy dacite rock so filled with gas bubbles that it floats on water.

The Ash Plain of Ambrym is the result of many such eruptions, plus the constant, smaller day to day belching of smoke and ash. Although much of the island is verdant tropical rainforest, places where the highly acidic ash rains down are generally barren.

In the diagram below, the Ash Plain is light brown and the darker brown are the flanks of Benbow and Maroum volcanoes. The red spots indicate active lava lakes.  The coloured areas are lava flows. Please note, this is valid only until 1998.

On the edge of Maroum, the beautiful spider web like filaments of volcanic glass called Peles Hair are in abundance. If you stand there long enough, or camp on the rim at night, warm spidery filaments of Peles hair land on you.

When Ambrym erupted in 1913, a fault in the volcano (black lines in the diagram below) opened up.

Instead of just Benbow and Maroum erupting, the entire island began spurting vents from Benbow westward.

When the eruption ended, these vents sealed up, but left very distinctive crater shapes called maars, in the landscape. These are the black half moon and circular shapes in the diagram. There are a lot more maars on Ambrym than indicated here, but these are the most obvious ones.

There are also underwater craters just off the coast.

Look closely at the lava lake photo; most of the steam and ash is coming from a ventabove the lava lake. There is also a large vent partly in shadow and full of steam, at the bottom middle/left of the photo.

Now, about that eruption in 1913 that tore the island in two, it was witnessed by a missionary and he said........NEXT PAGE

* Dating by Arctic core samples