When Ambrym blew up a thousand years ago, it ejected 25 cubic kilometres of earth into the atmosphere. Read an amazing story written by a missionary about Ambrym's last really big eruption in 1913 by clicking here.

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).

West coast of Ambrym at dawn. The dip in the middle is the collapsed caldera. Maroum is causing the most smoke while Benbow, slightly to the left, is putting out smaller smoke plume.
Photo: Sonny

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.

This cross-section of Ambrym shows the basal shield, or the bottom part of volcano, in black.
Photo : Sonny

The light grey area 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 page 3) The black fault lines are cracks that allow the magma to rise up into lava lakes (see page 3), or through side cones like Niri Mbwelesu and the 1998-99 lava flow that oozed from the ground.

Benbow, Niri Mbweselu and Maroum in cross section

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.

Benbow, Niri Mwembelesu (centre front) and Mwebelsu vent inside Maroum are all smoking.
Photo: Sonny

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.

Ambrym's Ash 'Plain'
Photo: Sonny

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 falls in a suffocating blanket over everything. It's also characterised by masses pumice, a light coloured frothy 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.

North flank of Benbow
Photo: Sonny

Without vegetation to hold the ash in place, the frequent tropical rainstorms carve great gouges in the land, forming surrealistic moonscapes. Walking through the maze like river flows cut in the Ash Plain is an awesome experience.

It can also be frightening when it begins to rain and there is nowhere to escape from the water gushing through the gullies.

It is not possible to climb the sides as the loosely compacted ash and scoria simply crumbles in your fingers when you try to climb it.

Of the 48 eruptions in the last two hundred years, most have been of the explosive Plinian type and most from a central vent, although some have been from lateral vents. Ten eruptions involved lava lakes and fifteen produced significant lava flows.

In 1894, six people were killed by volcanic bombs and four people were overtaken by lava flows. In 1913, at least 21 people, but likely it was far more, were killed during an explosive eruption. The eyewitness description of it on page 4 has all the elements of a Plinian eruption - but with a few added twists, for this eruption tore the island from one side to the other.

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.

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

Benbow lava lake (facing south east)
Look closely at this photo; most of the steam and ash is coming from a vent on above the lava lake. There is also a large vent partly in shadow and full of steam, at the bottom middle/left of the photo.

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.

* Dating by Arctic core samples

Next: Lopevi Island Volcanoes