Ambrym Volcano

Description
Ambrym, a large basaltic shield volcano with a 12-km-wide caldera, is one of the most active volcanoes of the New Hebrides arc.

Scoria cones are dotted across the island. Marum and Benbow cones within the caldera are the main active vents, with glowing lava lakes inside.

Eruptions have occurred almost yearly during historical time from cones within the caldera or from flank vents.

  •  Elevation: 1334 m / 4376 ft
  •  Latitude: 16° 15' 54.26'' S
  •  Longitude: 168° 7' 17.50'' E


Type
Ambrym includes four main volcanic land forms

1. Shield volcano -  A wide gently-sloping stack of lavas and deposits from explosive eruptions

2. Caldera - An area of the top of the shieldvolcano that has collapsed as magma below ground is erupted to the surface  

3. Scoria cones produced by two eruption styles:

  • Strombolian - bubbles of glowing magma burst spraying bombs of lava into the air
  • Vulcanian - explosions within the magma fragment it to fine ash which blows downwind

4. Maars - Wide low craters produced by local explosive eruptions as magma has reached surface or ground water – mostly found near the coastline.

The central east-west axis of the island hosts a concentration of eruption vents down the middle of the island.
Magma in the lava lakes is up to 1000°C near the surface, and bombs remain very hot for up to an hour from eruption. Ash is not hot.

Monitoring
There are two seismographs, and one camera at Ambrym, monitored remotely by VMGD in Port Vila. The data and photos can be seen at:
http://www.geohazards.gov.vu/

Volcanic history
Ambrym is a long lived volcano, with the northern portion containing many overlaping volcanic cones.
The caldera cuts through the old volcano strcuture, possibly producing a large explosive eruption about 1900 years ago.  
Marum and Benbow cones are the most active vents inside the caldera and produce lava flows that pond on the caldera floor or spill through gaps in the caldera rim.  

Other eruptions (e.g., 1896, 1917, 1950) formed a scoria cones, maars and lava flows outside the caldera, along the E-W ridge.
Many villages have been abandoned after being overun by lavas.
Large ash-eruptions occasionally occur from Marum and Benbow (e.g., 1977, 2005) covering crops and villages in the west and north with ash.  

Cause
The volcanoes of Vanuatu are created by subduction of the Indo-Australian plate below the Pacific plate under Vanuatu.
A large magma chamber is located a few km below the caldera, feeding Marum and Benbow.

Safety

Bombs can land in any zone at any time.
The only way to avoid all risk of bombs is to stay out of Zone B.

Stay out of the permanent exclusion zone – the danger is extreme
The exclusion zone may be extended  during periods of larger or more frequent explosions to include Zone A, or to include Zone A & B.

Check which zone(s) are closed in the latest bulletin - click on ‘Ambrym’ at:
http://www.geohazards.gov.vu/

Flying bombs are always a danger - Wear a hard hat, it will help protect you, but it will not stop larger bombs.
Watch for bombs in the air especially after explosions. Stand still unless you see bombs that are not moving left/right or up/down – these are coming towards you, and you should avoid them.  

Bombs fly very fast – even though they look to move slowly at first. The time from an explosion to bombs landing at the rim is often only a few seconds.

Falling into the crater – beware that the edge of the crater is slippery and unstable.
 

Yasur Volcano

Yasur Volcano – Yenkahe Caldera

Description
Yasur, located at the SE tip of Tanna Island, is a mostly unvegetated 361-m-high scoria cone with a nearly circular, 400-m-wide summit crater.  
Yasur is the most frequently visited of the Vanuatu volcanoes.
It has been in more-or-less continuous activity since Captain Cook observed ash eruptions in 1774.  
This style of activity may have continued for the past 800 years.

Type
Yasur is a basaltic scoria cone that typically has up to five active vents.

These produce a combination of two eruption types

  • Strombolian - bubbles of glowing magma burst, spraying bombs of lava into the air
  • Vulcanian – explosions that burst through a blocked vent, generating showers of bombs and also clouds of fine ash, which blow downwind

Erupted Magma is up to 1000°C and bombs remain very hot for up to an hour from eruption. Ash is not hot.

Yasur’s summit is 361 m above sea level and the crater is 600 m across at its widest point.

Volcanic history
Yasur is the youngest of a group of Holocene volcanic centres constructed inside a larger volcanic caldera.

Yenkahe Caldera
The Yenkahe caldera is a 4-km-wide, horseshoe-shaped depression that collapsed during two large eruptions that blanketed almost all of southern Tanna in thick ash and scoria. This deposit forms the cliffs along the SE and S coasts.

Yenkahe hill
This huge block of ground beside Yasur has been pushed up, probably by magma underground.  

Hot springs
Groundwater heated by the magma comes up fractures around the Yenkahe hill.

Earthquakes and ground movement
Earthquakes in this area have raised the Yenkahe hill and Port Resolution Harbour more than 20 m during the past 300 years.

Cause
Vanuatu volcanism is caused by subduction of the Indo-Australian plate below the Pacific plate under Vanuatu.
A large magma chamber is located a few km below Yenkahe caldera, feeding Yasur and pushing up Yenkahe hill and ajacent areas.

Monitoring
There are a seismograph, two cameras and a microbarometer stationed at Yasur, monitored remotely by VMGD in Port Vila.
The data and photos can be seen at: http://www.geohazards.gov.vu/

Safety
Bombs can land in any zone at any time.
The only way to avoid all risk of bombs is to stay off of Yasur.

Stay out of the permanent exclusion zone – the danger is extreme

The exclusion zone may be extended - during periods of larger or more frequent explosions to include Zone A, or to include Zone A & B.

Check which zone(s) are closed in the latest bulletin - click on ‘Tanna’ at:
http://www.geohazards.gov.vu/


Flying bombs are always a danger - Wear a hard hat, it will help protect you, but it will not stop larger bombs.
Watch for bombs in the air especially after explosions. Stand still unless you see bombs that are not moving left/right or up/down – these are coming towards you, and you should avoid them.  

Bombs fly very fast – even though they look to move slowly at first. The time from an explosion to bombs landing at the rim is often only a few seconds.

Falling into the crater – beware that the edge of the crater is slippery and unstable.