This post grew out of the GVP summary for mid-February as Ambrym’s current eruption was winding down. One thing kind of led to another …..
Ambrym Island hosts an active volcano with a pair of active vents, Mount Marum and Benbow. They occupy a 12 km diameter caldera created during a VEI 6 eruption around 50 AD. Both active vents host lava lakes.
Activity around Feb 21 was associated with what was described as a minor eruption from a new vent inside the caldera. Over the following couple weeks, activity decreased and the Alert Level is now listed as a 2 out of 5. Hazardous areas are defined as areas near the new vent and downwind areas prone to ashfall. http://www.geohazards.gov.vu/
The last eruption started in 2009 and is listed as ongoing.
Like Hawaii, Ambrym gets a significant amount of rain, which is heavier on the windward side and lighter on the leeward side. Average difference between the two sides is about half. Average rainfall is around 210 cm yearly. Given the soft nature of what has built the island, this leads to significant erosion. It also has the effect of making escape from rising water down the steep slopes of the cut ravines problematic as the soil is soft and difficult to grab hold or climb up.
Ambrym Island is part of the nation of Vanuatu, a collection of volcanic islands situated equidistant north of New Zealand and east of Australia. The island is just under 680 km2 and the volcano is some 1,300m high. Total population of the island is around 7,300 as of 2009.
Volcanic activity is driven by the subduction of the Australian Plate under the Pacific Plate. Basic movement by the Australian Plate is easterly. Basic movement of the relevant portion of the Pacific Plate is WNW. The region is tectonically complex with a spreading center in the Pacific Plate to the east of Vanuatu. The subduction trench to the west of Vanuatu is called the Vanuatu or New Hebrides Trench. The Tonga Trench sits east of both Vanuatu and the spreading center in the Pacific Plate.
One of the things that makes New Zealand so geologically volatile is that the Australian Plate is subducting under the Pacific Plate to the south of it and the Pacific Plate is subducting under the Australian Plate to the north of it. Crossover of the subduction zones is roughly in the center of the country. The Taupo Volcanic Zone sits north of this crossover area.
Vanuatu is similarly complex, with the triangular area bounded by the subduction trench to the west and the inactive trench segment southeast of it sometimes being referred to as the Fiji Plate. There is a discontinuity bounding the north of the area that may or may not be defined as a plate boundary. Because of the subduction to the west of Vanuatu, it is a very geologically active nation, with a large number of large caliber earthquakes.
Ambrym Volcano is primarily a basaltic volcano with andesitic output. The edifice constructed is a combination of lava flows and pyroclastic deposits, with the pyroclastic debris predominating. It is one of the most active volcanoes in the world. There is enough effusive output to support the lava lakes in the two craters. However the majority of eruptions is explosive, usually from a central vent, and also produces lava flows. Unlike Hawaii where lava flows predominate, the majority of the stuff that has built Ambrym (and its neighbors) is ash and pyroclastic materials, making the island itself very prone to invasive water from the surrounding ocean, which in turn lead to more explosive style of eruptions. http://www.vanuatu.travel/index.php/en/things-to-do/volcanoes/112-ambrym-volcano
The caldera is referred to as the ash plain, as it is mostly covered with pyroclastic debris.
There are rifts on the island which feed magma to the surface. This has led to phreatomagmatic eruptions and craters on either end of the island where the magma meets with seawater near the surface. Craters are typically 1000 m in diameter with rims some 100 m high. The most recent eruptions of this nature on Ambrym were 1894, 1913, 1937 and 1954. Similar features show up on other Vanuatu islands.
At least one paper describes the central caldera as a subsidence structure much like the calderas of the Hawaiian and Galapagos Islands. There is some dispute over that description given the VEI 6 eruption some 1900 years ago.
Geologic evolution of the island is poorly understood.
Ambrym is and will continue to be an active and dangerous volcano, capable of very large Plinian eruptions. For a basaltic system, it produces a significant amount of pyroclastic output. Local tectonics are complex and active. It is a most interesting part of the world.
Note: I am no expert; merely an amateur who is interested in how things work and why. Corrections and suggestions are always welcome and encouraged.
A Nobel Prize for Volcanology – Dr. Stephen Sparks
In January, Dr. Stephen Sparks was awarded the 2015 Vetlesen Prize. This may not be “news” anymore, but as we read so much of Dr. Spark’s work we thought it is something important to mention on a volcano blog:
(OSU Volcano World) A Nobel Prize for Volcanology
Volcanology has a new Nobel Prize winner! Dr. Stephen Sparks has received the 2015 Vetlesen Prize, awarded by the G. Unger Vetlese Foundation and Lamont-Doherty Earth Observatory. The Vetlesen Prize is considered to be the Nobel Prize in earth sciences, awarded to individuals who have made significant scientific achievements in establishing a clearer understanding of the Earth and its workings. Currently located in the University of Bristol, UK, Dr. Sparks received this great honor because of his work in improving our understanding of global volcanic hazards. His work on Montserrat’s Soufrière Hills volcano has helped goverments asses danger from volcanic eruptions and improve their evacuation and hazard response. Internationally, he has helped with the improvement of volcanic risk assesment, and has been awarded several honors because of his work.
For more about Dr. Sparks and his work, read this article:
Geologist Who Modernized Volcanology Wins the 2015 Vetlesen Prize
This video is well worth watching!