Volcanoes of the Solomon Islands – Part 2 of 2

Kana Keoki, Coleman Seamount and Rendova Volcano from Google Maps. Kana Keoki is left of center toward the edge of the image with a dual peak. It also looks a bit eroded. Neighboring Coleman Seamount has a clearly defined crater and looks a bit younger. Screen capture from Google Maps

Kana Keoki Volcano – New Georgia Islands

Kana Keoki is an active dacitic submarine volcano 47 km SW of Rendova Island.  It was first reported in 1987.  The volcano is located S of the trench (on the Indo-Australian side) that marks the convergent margin of the Indo-Australian and Pacific Plates.  It rises 3,000 m from the seafloor within 700 m of the surface.  It is surrounded by an apron of volcaniclastic debris.

Coleman Seamount Volcano – New Georgia Islands

The Coleman Seamount volcano is a young submarine volcano 20 km SE of Kana Keoki, 47 km SSW of Rendova volcano.  It was discovered during a research cruise in late 1985.  It is also on the Indo-European portion of the subducting plate, south of the trench.  The volcano is building on the inactive Woodlark spreading center.  It is relatively pristine and appears to be more recently active than neighboring Kana Keoki.

Heavily eroded Mbareke, Vangunu and Nggatokae volcanoes. Mbareke and Vangunu share the Vangunu Island, with the breached Vangunu crater more visible on the south side of the island, just below the white lettering label. Mbareke is the heavily eroded lobe on the upper center of the image, upper right lobe on the island. Nggatokae is at the lower right constructing a single volcanic island ringed with coral reefs. Image screen capture from Google Maps

Mbareke Volcano – New Georgia Islands

Mbareke volcanic is a heavily eroded volcanic edifice making up the northern half of Vangunu Island just off the SE end of New Georgia Island.  It is 81 km ESE from Rendova volcano.  It has a core of basaltic andesites, andesitic lavas and breccias overline by a thick pile of basalt lavas.  There is an andesitic plug on the southern rim of the summit crater.  There are no known historic eruptions or any in the last 10 ka from this volcano.  Like Rendova, it dates no older than 2.6 Ma.

Black sand beach on Vangunu islands used for leatherback sea turtle nesting. Eggs are relocated to increase survivability of the young. Species is endangered. Image courtesy Jino, et al, Oct 2018

Vangunu Volcano – New Georgia Islands

Vangunu volcano makes up the southern half of Vangunu Island, some 19 km SW of Mbareke Volcano.  It has a deeply eroded central crater breached to the south.  Like Mbareke, there are no known historic eruptions or any in the last 10 ka from this volcano.  Like Rendova, it dates no older than 2.6 Ma.

Nggatokae Island from the air. Image courtesy Airbnb

Nggatokae Volcano New Georgia Islands

Nggatokae volcano is an island volcano neighbor to Vangunu Island to the ESE.  It is some 23 km ESE from Vangunu.  It is a well-formed eroded volcanic cone with a breached central crater and scoria cone on its northern flank.  Like its neighboring Mbareke and Vangunu volcanoes, there are no known historic eruptions or any in the last 10 ka from this volcano.  Like Rendova, it dates no older than 2.6 Ma.  It is considered to be an extinct volcano by the Solomon Encyclopedia. 

Seamounts south of Vanugu Island. Kavachi is at the red marker. Unnamed Volcano is clearly visible NE of Kavachi. Line of other seamounts stretches out to the NW from Kavachi. Note that this are has not been mapped in detail. Screen capture from Google Maps

Unnamed Volcano – New Georgia Islands

Unnamed volcano is a seamount some 24 km SSE from Vangunu volcano.  It was first mapped during a 1979 cruise some 9 km NE Kavachi.  It appears to be recently active with youthful andesitic rocks dredged from the location.  Unnamed is part of a E – W chain of seamounts immediately N of Kavachi.  It is the largest, has a flat-topped summit within 70 m of the surface.  The summit is ringed a coral reef.  Thermal plumes were detected in 1992 from one of the pair of summit craters.

Underwater eruption at Kavachi, May 2000. Image courtesy CSIRO News Corp Australia

Kavachi Volcano – New Georgia Islands

Kavachi is one of the most active submarine volcanoes in the SW Pacific.  It is some 32 km S of Vangunu, 9 km SSW of Unnamed.  Kavachi is a shallow basaltic to andesitic volcano that built multiple ephemeral islands up to 1 km long.  Its first recorded eruption was 1939.  Residents of neighboring Nggatokae and Vanguna islands reported fire on the water in the direction of Kavachi before 1939 which may be possible eruptions.  The island rises from the seafloor 1.2 km below the surface on the N and deeper to the SE.  There are frequent phreatic explosions that eject steam, ash, and incandescent bombs.  Lava flows have been observed on the temporary islands. 

Kavachi built ephemeral islands at least seven times 1950 – 1982.  All were short-lived due to wave action, subsequent eruptive activity, and slumping.  Maximum size was around 150 m.  Explosive eruptions were fountains, jets, mounts of seawater with differing proportions of volcanic debris, shallow-water phreatomagmatic activity, subaerial lava flows, scoria eruptions and probably Strombolian activity.  Some aerial lava flows did form wave-resistant caps on the island from time to time, but the island returns below sea level within days to weeks.  There are no records of tsunamis caused by slumping on neighboring islands.  The island is considered a hazard to shipping.  

Plume of discolored water drifting from Kavachi. Image courtesy NASA EOS-1 satellite, Jan 2014, via The Watchers blog

There was a Jan 2015 biological expedition to Kavachi by National Geographic that discovered sharks living inside the subsurface crater of Kivachi.  Results of this expedition were picked up by international media, breathlessly describing the find as a “Sharkcano”, a term borrowed by the infamous Sharknado series of schlock SciFi movies here in the US that aired 2013  – 2018. 

Timing for the expedition was excellent, as the team arrived during a period of quiet of the volcano.  They confirmed a second peak on the massif, diffuse-flow venting from that second peak, a megafauna community living inside the active crater, and gathered samples and data on magma sources and characteristics of Kavachi’s active volcanism. 

Image of sharks and rays living inside crater of Kavachi volcano taken during 2015 expedition. Image courtesy InHabitat, July 2015

The summit is an oblong, pockmarked crater 120 x 75 m with a 24 m rim.  Deepest portion of the crater floor is around 70 m.  The second summit is about 1.3 km SW and rises to 260 m below the surface.  The volcano has active hydrothermal plumes that caused physical discomfort for divers in its caustic waters.  There was measurable increases in CO2, temperatures, and variations in acidity (pH) of waters around the plumes. 

Biology in and around the crater included extensive microbial mats, tube worms, multiple species of fish and zooplankton, apparently associated with the active venting.  There were two species of shark viewed by suspended cameras, suspected to be swimming up from deeper in the crater floor. 

Shuttle image of NW Guadalcanal showing Gallego volcanic massif at the Northern tip of the island. Photo is oriented generally NW to the top. Image courtesy NASA, 2001 via Smithsonian GVP

Gallego Volcano – Guadalcanal Island

Gallego is a group of heavily eroded cones covering a large area of NW Guadacanal Island, some 200 km SE of Vangunu Island and volcano.    Mount Roundhead is a small apparently recent volcano.  Local traditions mention a historic volcanic eruption, but this activity may have been an eruption at neighboring Savo around 1991.  Andesitic Mount Esperance is thought to be active within the last 2,000 years. 

Savo Island and volcano. Image courtesy Solomon Times, July 2014

Savo Volcano – Guadalcanal Island

Savo Island is a forested andesitic to dacitic stratovolcano some 30 km NNE of Gallego.  The 485 m tall island volcano has a shallow summit crater measuring 1 x 1.5 km.  There are recent lava domes on the crater floor and its NE rim.  Older domes are located on the flanks.  Pyroclastic flows and lahars have traveled down valleys cut into the flanks from the summit to form debris fans along the coast.  There are thermal areas in the summit crater, S to SE flanks and offshore.  These include steaming ground, fumaroles, small geysers, and hot springs.  Boulder and sand beaches and even seawater higher than atmospheric heat.  A population of megapodes uses the heated sand to incubate their eggs.  Like their population on Simbo, residents have decimated the population by over harvesting the eggs.  Thermally heated streams descend from the central lava dome.  Steaming hot streams are close to boiling at their source high in the crater. 

Fumarole and hot spring area on Savo volcano. Image courtesy PBase.com

The volcano was in eruption when Spanish explorers arrived in 1568.  Pyroclastic flows during this activity killed most people on the island.  There are oral traditions that the eruptions expanded the island on the northern side.  There were two other eruptive periods during the mid-17^th century and mid-19^th century.  Volcano Discovery calls it one of the most dangerous volcanoes in the Solomon Islands.  Severe earthquakes shook the island in the late 1910s.  The locals were on the verge of evacuating, but ultimately did not.  It is currently quiescent but viewed as particularly dangerous to nearby Honiara on Guadalcanal.

Savo Island is the 6 km diameter emergent summit of a stratovolcano.  It has erupted at least three times within recorded history.  The 3,000 inhabitants have an extensive oral tradition of past events.  Block and ash flow deposits are dominant on the island.  Most of these appear to have been a result of dome collapse during the 1560 – 1570 eruption.  There are eyewitness descriptions and crater morphology that suggests similar deposits formed by dome collapses or eruptive column collapses in later eruptions 1830 – 1840.  The vent repeatedly sealed likely to inward collapse of hydrothermally altered rocks.  The block and ash flows were hot enough to char vegetation up to 3 km from the vent, many of them with ash cloud surges.  The greatest volcanic risk from Savo is future block and ash flows, ash cloud surges, and lahars. 

Image courtesy Science Source

Tinakula Volcano – Santa Cruz Islands

As we previously mentioned, Granyia covered Tinaula Volcano nicely in her Nov 2017 Volcano Hotspot post.  There is also extensive coverage by the Smithsonian GVP and Volcano Discovery.  It is one of the most frequently active volcanoes in the Solomon Islands.  It is located another 680 km mostly to the ESE from Gallego in the Santa Cruz Island portion of the Solomons. 

Tectonic setting of New Guinea and the Solomon Islands. Image courtesy Holm, et al Mar 2016

Tectonics

The Solomon Islands have been constructed via the collision between the Pacific Plate, moving generally west at around 10 cm/yr in the north, 7 cm/yr to the south and east, and the Indo-Australian Plate, moving generally NE at 6 cm/yr.  The northern section of this collision has created multiple microplates from Indonesia all the way east to Vanuatu, Fiji and Tonga.  Add to the mix the presence of the huge Ontong – Java Plateau (OJP) Large Igneous Province (LIP), and you end up with a complex tectonic region.

There are three tectonic segments in the Solomons.  To the E, it represents a steeply subducting In-Australian Plate beneath the Pacific Plate.  The subduction trench is the New Hebrides Trench and more pronounced as you travel S of the Santa Cruz Island portion of the Solomons toward Vanuatu.

The second tectonic segment is a short strike slip segment starting in the vicinity of the Santa Cruz Islands where the Indo-Australian Plate moves generally eastward against the Pacific Plate which is moving generally westward.  The transition between the two regimes takes place just S of the Santa Cruz Islands.  The New Hebrides Trench transitions into the Solomons Trench around this location. 

Schematic of Ontong Java Plateau interaction with Solomon Islands. Image courtesy Mann and Taira, Oct 2004

The third tectonic segment transitions back to subduction of the Indo-Australian Plate beneath the Pacific Plate.  Subduction in this portion is relatively shallow and not particularly steep.  The New Georgia Island group is one of four places where an active or recently active spreading ridge has been subducted beneath an island arc.  Subduction of the Woodlark spreading center is most recently driving uplift of the forearc islands above sea level with rates 5 – 7.5 mm/yr.  The collision of the Coleman Seamount may have terminated the spreading of the spreading center entering the trench, converting it into an active strike-slip fault zone. 

The main portion of the Solomon Islands are located on what is known as the Solomon block.  It is roughly 1,200 x 250 km, oriented roughly NW – SE, and surrounded by relatively deep ocean floor.  It is bounded by the North Solomon Trench to the NE and the San Cristobal Trench system to the SW.  The present-day Solomon Trench system is quite close to the New Georgia group of Islands, with at least two newly active seamounts on the SW side of the trench. 

Schematic of major tectonic features with Gallego, Kavachi and Simbo volcanoes labeled. Image courtesy DJ Smith, July 2014

The Ontong Java Plateau is located generally to the N of the Solomon block on the Pacific Plate.  Interaction between the two blocks has also been complex, with initial subduction of the Pacific Plate beneath the Indo-European Plate taking place some 30 – 40 Ma.  This subduction locked, as the OJP was too thick to subduct around 25 – 30 Ma.  The SW dipping slab detached off in places 20 – 15 Ma and the Indo-Australian Plate began subducting beneath the OJP some 12 Ma.  Arc volcanism in the Solomons began about this same time, creating the older line of islands to the NE in the central Solomons.  A second line of volcanic islands emerged some 4 – 2 Ma as activity shifted SW which is where we are today.

Kavachi in eruption Oct 2014. Image courtesy Gun Menezes, Pintrest

Conclusions

The Solomons will continue to be volcanically active, with current activity slowly migrating SW into the ocean.  Most of the new activity is building seamount volcanoes SW of the New Georgia island group in the central Solomons.  As long as the subduction continues, so will earthquakes and volcanic activity.   

Simbo Volcano looming over huts on the island. Image courtesy Mapio.net

Additional information

WorldData.info – Volcanoes on the Solomon Islands

Volcano Discovery – Volcanoes of the Solomon Islands (16 volcanoes)

WOVO.org – Honiara Solomon Islands

The eruptive history and volcanic hazards of Savo, Solomon Islands, Petterson, et al, 2003

Smithsonian GVP – Kavachi

Fire Mountains of the Islands – 11.  Cooke-Ravian and a Volcanic Resurgence: 1971 – 1979

Exploring the “Sharkcano”: Biogeochemical Observations of the Kavachi Submarine Volcano (Solomon Islands), B Phillips, Oct 2016

Oceanography, Vol 26, Number 4 – Exploring the “Sharkcano”, Phillips, et al, 2016

The elusive Cook volcano and other submarine forearc volcanoes in the Solomon Islands, Exon, et al, 1986

Kavachi, and active forearc volcano in the western Solomon Islands reported eruptions between 1950 and 1982, Johnson and Tuni, 1987

Global tectonic significance of the Solomon Islands and the Ontong Java Plateau convergent zone, Mann and Taira, Oct 2003

Post 8 Ma reconstruction of Papua New Guinea and Solomon Islands:  Microplate tectonics in a convergent plate boundary setting, Holm, et al, Mar 2016

The role of slab melting in the petrogenesis of high-Mg andesites: Evidence from Simbo volcano, Solomon Islands, Konig, et al, Oct 2007  

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