When I was creating and compiling my list of the largest eruptions of the holocene period, I noticed some interesting trends. Unsurprisingly, Kamchatka and the Aleutian islands were hotbeds for large eruptions. But there was one reason I was not expecting that nearly matched Kamchatka and the Aleutians despite being monumentally smaller in size. That region would be New Britain.
New Britain: The Most Active Holocene Region For Large Eruptions By Volume
Going off the Global Volcanism Program’s list, of the 14 active volcanoes on the mainland area of New Britain, there were 6 volcanoes which formed large calderas in the last 10,000 years- those volcanoes being:
- Witori (multiple caldera events)
While there may be some regions in the world that may have had more caldera forming events by volume, if you go strictly by caldera formations per volcano, New Britain is far and away in the lead. With 6 caldera forming events in this period, over 42% of the volcanoes in New Britain formed calderas during the past 10,000 years – some which had multiple events.
If you extend the volcanism to other volcanoes in the arc that aren’t on New Britain island proper (such as Long Island volcano), you get an even more ridiculous record of activity.
The extreme level of activity seen at New Britain in the past 12,000 years (the holocene period) blows away the activity level of any other similar sized region, with the exception of the Taupo volcanic zone. The fact that we can make a comparison to the Taupo volcanic zone here alone speaks to the how active New Britain has been.
Why Has New Britain Been So Volcanically Active?
It seems that as you would expect, there are a variety of factors that may be influencing the high level of activity in New Britain.
Before getting into the details, I will say that New Britain is an extremely complex region from a tectonic perspective.
Rapid Subduction Playing a Role?
Subduction is occurring on the New Britain Trench at an approximate rate of 80 – 150 mm per year depending on which side of the island you are on (1). Not surprisingly, the region of most intense subduction is towards the eastern end of the island, where Rabaul finds its home
This is an extremely high subduction rate, which likely supplies a higher volume of melt to the mantle and crust.
The increased magma volume allows for bigger volcanoes to form at a quicker rate, and also allows for explosive gases to accumulate in the volcanoes. In short, it accelerates the normal life-cycle you would get at a subduction arc-based volcanic system.
New Britain: An Extremely Complex Tectonic Setting
The subduction and tectonic setting around New Britain is extremely complex, and it likely plays a role in the volcanism of New Britain. I am going to try to simplify this at least somewhat so that it can be understood.
From this image, in just the small region around New Britain and Papua New Guine, we have multiple subduction zones, a spreading center, and three microplates (small tectonic plates). Trying to make sense of this all is difficult – the below is a quote from a phys.org article on the region around New Britain.
Subduction occurs when one tectonic plate moves beneath another plate. In this area, there are actually three plates involved, two of them subducting beneath the third while sliding past each other. The Australia Plate and the Solomon Sea/Woodlark Basin Plate are both moving beneath the Pacific Plate. At the same time, the Australia and Solomon Sea/Woodlark Basin Plates are sliding past each other. The Australia Plate moves beneath the Pacific Plate at about 4 inches a year and the Solomon Sea Plate moves beneath the Pacific Plate at about 5.5 inches per year. As if this were not complicated enough, the Australia and Solomon Sea plates are also moving in slightly different directions.
The spreading of the very young Woodlark basin likely contributes to the push from the Australian plate, which perhaps is part of why subduction is so rapid beneath New Britain. Additionally, the very old Solomon sea plate (adjacent to the Woodlark basin) subducts at a steep angle that is ideal for magma formation beneath New Britain. This steep subduction angle is likely a result of the crust being colder and more dense (due to age), which also influences why it is subducting in the first place.
Additionally, there may be some minor spreading / crustal thinning in New Britain, as certain studies suggest an upwarping and thinning of the crust in central New Britain by as much as 8km. Thinning of the crust would likely help to increase magma production rates here. (3)
High levels of hydration in the subducting Phillipine slab?
One interesting tidbit I found that may be playing a role in the high level of volcanic output is that the subducting slab may have an extra high level of water in it due to serpentinization. Serpentinization is a processes whereby rock and crust material is changed, with the addition of water into the crystal structure of the minerals found within the rock.
It is fairly well known that the subducting Solomon sea plate is highly hydrated and serpentinized as evidenced by a double seismic zone in the subducting slab along with other factors.
This could mean that there is even more magma production as well as additional volatiles in the magma than you would normally get.
New Britain is one of the most volcanically active regions in the world relative to its size. The intense eruptive activity can be attributed to a variety of factors.
The primary factors include:
- Extremely intense plate convergence and subsequent subduction leading to higher volumes of magma production.
- Steep subduction angle which is ideal for the formation of magma.
- Potential thinning which may lead to increased magma production and larger volcanoes.
- Significantly higher amount of water in the melt, which leads to increased magma production and increased volatiles in the magma.
(1) Estimation of current plate motions in Papua New Guinea
from Global Positioning System observations – http://onlinelibrary.wiley.com/doi/10.1029/97JB03676/pdf
(2)Magma Genesis in the New Britain Island Arc: Further Insights into Melting and Mass Transfer Processes – https://academic.oup.com/petrology/article-lookup/doi/10.1093/petroj/39.9.1641
(3) Structural Profiles in the New Britain / New Ireland Region – https://d28rz98at9flks.cloudfront.net/12761/Rec1972_080.pdf
(4) A remarkable pulse of large-scale volcanism on New Britain Island, Papua New Guinea – https://www.researchgate.net/publication/226912549_A_remarkable_pulse_of_large-scale_volcanism_on_New_Britain_Island_Papua_New_Guinea