Table 1 Chronology of Ambae’s volcanic activity from ref. ²¹ with details on the 2017–2018 eruption (this work)

Explosion at the summit and formation of the Voui cone and the Manaro Ngoru crater

Fissural eruption on the western flank, along N50 fractures, damaging the Ndui Ndui region. Formation of a palagonitic tuf cone in the lake Voui.

Lahar event causing victims.

Eruption? (and/or earthquake) triggering a landslide killing about 100 people.

Vapour emission, strong fumarole activity.

Strong vapour emission

Strong activity on Solfatara

On the 13 of July—3 boiled zones identified in lake Voui and acid impact on vegetation on Voui crater rim.

Increase of vegetation damage around Voui.

On the 4th and 5th December large quakes were felt on the island.

On the 3rd of March ash plume was seen. High tremor recorded, high degassing from lake Voui.

Surtseyan eruption and spectacular colour change in lake Voui

 Eruption phase 1

September 5–10, 2017

A large explosion at the summit was heard up to 10 km away from the Ambae summit (September 5). It marked the onset of the eruptive event. A large quantity of SO₂ (38 kt) was detected by the OMI above the Ambae volcano during the following days. The activity alert level raised from 2 (major unrest) to 3 (minor eruption), following the Vanuatu Volcanic Alert Level System (VVALS).

 Relative calm phase

September 10–21, 2017

A short period of relative calm followed the onset event. During this period the gas emission returned to its background level.

 Eruption phase 2

September 22–November 23, 2017

On September 22, 2017, MODIS detected the first thermal anomaly with a very high thermal energy release (318 MW) within Lake Voui (September 22). Inhabitants have reported the appearance of a sustained glow at the summit in relation to the new lava flow on Ambae after that of Ndundui, some ~350 years ago. The activity alert level was raised from 3 to 4 (moderate eruption—according to VVALS) on September 23.

On September 25, 2017, the national Government imposed the first state of emergency, prompting the evacuation of the entire population of the island. Lava flows continued in the Voui crater.

On October 3, 2017, observation flight confirmed the sustained eruptive activity, which was restricted to the crater lake Voui. A lava flow continued to spread from a well formed cone and a strombolian manifestation was observed on 3 active vents.

On October 6, 2017, another observation flight confirmed a continuous eruption but with no further increase, in comparison to the October 3 observation. The eruptive activity alert level downgraded from 4 to 3.

From October 10 to November 23, 2017, lava continued to flow from the new cone; another period of large SO₂ release was recorded by satellites, mainly from October14 to November 23. A field survey at the summit on the October 17 confirmed this persistent strong degassing sustained by a strombolian activity. MODIS thermal alert, which was issued continuously since September 22 in relation to lava flow and strombolian activity, has ceased on November 9, although an alert with less thermal energy was issued on November 17. The 1^st state of Emergency ended on October 27, allowing the population to return. However on their return, they found themselves exposed to stronger eruptive manifestations in comparison to that of September. Ash deposits reached the coast whilst acid rain devastated the vegetation to the north-west and western part of the island. Crops were destroyed and drinking water reservoirs were contaminated.

 Relative calm phase

November 24, 2017–early January 3, 2018

A relative calm was observed from end November 2017 to early January 2018. MODIS thermal alert ceased and no ash fall was observed. Gas emission reduced to near background level. Volcanic activity level downgraded to 2 (December 7). A field survey at the summit (January 5), confirmed that lava flow had stopped.

 Eruption phase 3

January 4–April 11, 2018

A new strong thermal energy release (368 MW) occurred on January 4, which marked the beginning of the eruption phase 3. It persisted until April 11. The SO₂ emission exhibited another extreme degassing phase from March 19 to April 14. Heavy ash falls were witnessed, especially towards the northern and southern parts of the island. On 6 April, residents living in the north-western area witnessed a short nightfall at about midday. Due to cyclone Hola (March 6–7) many lahars have occurred. A village (Waluebue) in the northern part of Ambae was destroyed by a lahar during the night of 30 March 2018. The alert level was again raised to 3 on April 4.

 Relative calm phase

April 15–June 20, 2018

No ash fall and acidic rainfall reported. A field survey at the summit on May 23, confirmed the lull of activity. The summit was covered with ash deposits, that were up to almost a meter thick in some places. The lake was divided into two parts by the newly-formed cone and in both parts the water was red. It was possible to reach the rim of this new cone for gas measurements. On June 7, the alert level was downgraded to 2.

 Eruption phase 4

June 21–August 4, 2018

A new and more powerful eruptive activity was witnessed. It commenced on June 21 with a thermal alert and followed by an extreme degassing phase that started on July 17. Heavy ash falls caused a nightfall at mid-day on July 25⁸². Houses collapsed under overload of ash. The alert level was raised to 4 (July 21), and a 2nd state of emergency was issued (July 26). The whole population was again evacuated from the island. The strong degassing ceased on August 3 while the last thermal alert was released earlier, on July 26. This final violent event lasted about 2 weeks and was the final phase of this year-long eruptive activity.