The Mariana Islands are the southern segment of the Izu – Bonin – Mariana (IBM) arc system that stretches some 2,800 km from Tokyo in the north to south of Guam to the south. We discussed volcanic activity in the Izu portion of this system with our Myojin – Sho, Kita – Bayonnaise Rocks post March 2021.
The Marinara arc stretches 1,370 km. It includes at least 76 volcanic edifices grouped into 60 volcanic centers. At least 26 of them (20 submarine and 6 aerial) are hydrothermally active. The 15 islands are volcanic, with the older islands to the south. Submarine volcanoes top out as close as 50 m below sea level to over 1,800 m below.
Politically they are grouped into the Commonwealth of the Northern Mariana Islands and the territory of Guam to the south. Both are politically associated with the United States. The northern group of 10 volcanic islands are uninhabited. The southern group of 5 larger, coraline limestone islands are inhabited (other than Aguijan).
Total area of the islands is 1,036 km2. Total population is around 220,000. The islands are generally wooded (at least originally). Vegetation is dense, resembling the Carolines and Philippines. Fauna is similar to that of the Carolines. The climate is damp and generally tropical, with temperatures controlled by persistent trade winds. It is milder than the Philippines with average highs and lows ranging 29° – 23°. Average rainfall monthly ranges between 31 – 61 cm/month. Typhoons are not uncommon.
Humans arrived around 1500 – 1400 BC by migrants from the Philippines. There was a second migration from the Carolines that took place sometime 0 – 1000 AD. A third migration from Southeast Asia took place by 900 AD. The first people on these islands may have made what was at that time the longest uninterrupted ocean-crossing voyage in human history. Tinian may have been the first of the islands settled.
Magellan discovered the islands for the western world in 1521, landing on Guam. Traffic grew over the following centuries. Most of the original native population died from European diseases introduced by the Spanish. The islands were repopulated during the 19th Century. Spain ceded Guam to the US in 1898 and sold the remainder of the islands and Caroline Islands to Germany in 1899.
In the 20th Century, world wars visited the Marianas, with a Japanese invasion early in WWI. The League of Nations awarded all Germany’s islands in the Pacific north of the equator to Japan. Japan administered the islands until WWII when they were retaken from Japan by the US in a series of incredibly hard fought and bloody campaigns. The US has administered the Marianas ever since. The people of the Northern Marianas negotiated Commonwealth status with the US in 1972.
The main islands are Guam, Saipan, Tinian, and Rota. Saipan is the northern most large, inhabited island. The remaining islands to the north of Saipan are volcanic edifices, 10 of them listed as active. None of these are inhabited.
Volcanic activity in the Marianas is monitored by the US Geologic Survey (USGS). Along with weekly updates available via subscription, there are two webcams currently available. One for Pagan and one for Anatahan. Pagan is the only one that displays an image as of this writing.
- Farallon de Pajaros
- Maug Islands
- Pagan (to be covered in a future post)
- Farallon de Mediniola
- Esmeralda Bank
With the exception of Esmeralda Bank, all of these are north of Tinian and Saipan. Of these, Pagan is the most recently active. Volcano Discovery lists 24 volcanoes. The additional 9 islands are all submarine volcanoes and seamounts. The review of Mariana Islands is generally listed from N to S.
The larger islands, Guam, Rota, Saipan, Tinian are located just to the east of the current volcanic arc. All started out as volcanic islands, some as old as 35 Ma. That volcanic activity moved to the west, leaving stable, limestone encrusted (and for the most part), coral encircled islands.
The string of volcanic islands and seamounts along the volcanic arc continues to the north of the Marianas. Volcano Discovery classifies the neighboring string as the Japanese Volcanic Islands and draws the dividing line between the two regions just N of Farallon de Pajaros.
As the volcanic systems N of Farallon de Pajaros appear to be located within the US Mariana Trench Marine National Monument, I will include a few of them in this review.
Kasuga is a line of three seamounts trending SSE from the volcanic front of the Izu – Bonin – Marianas Arc. Kasuga 1 is a conical stratovolcano that rises within 598 m of the surface. It is located SE of Fukujin submarine volcano. Japan Meteorological Agency lists it as an active volcano due to floating pumice south of the volcano in 1959. Water discoloration was reported near it Nov 1975. Sea floor here is 3,000 m below the surface. There are a series of flank vents on the southern side of the volcano. The summit does not have a caldera or hydrothermal activity. The volcano is mantled by volcanic rocks and what appear to be well preserved lava flows from a flank eruption. It may be the oldest of the three seamounts. Kasuga 2 (Minami Kasuga) is the largest of the three volcanoes in the chain. It tops out within 170 m of the surface. There are two subsidiary cones low on the eastern flank. It is a more complex volcano with prominent ridges and lava flow fields. Dating the material on the slopes indicate less than 8,000 and likely less than 1,000 to a few centuries old. There are active hydrothermal fields at the summit, the base of summit ridges, and on the lower flanks. Kasuga 3 is the southern-most of three submarine volcanoes forming the Kasuga seamount chain oriented generally SSW from the volcanic front. It tops out some 1,325 m below sea level. There are no known eruptions from this chain and no observed hydrothermal activity as of 2004. Kasuga is located some 183 km NNW from Farallon de Pajaros.
Eifuku is one of the northernmost submarine volcanoes in the volcanic arc. It tops out at 551 m below sea level. The volcano has likely not erupted in the last 10 ka and may be extinct. There are no recent earthquakes and it is dated somewhere in the last few million years old. It is part of a chain of submarine volcanoes with Daikoko and NE Eifuku. NE Eifuku is a small submarine volcano at the NW end of the chain and tops out at 1,535 m below the surface. It has a vigorous hydrothermal system with CO2 venting from white smokers. This is one of the two only places that natural liquid CO2 emissions have been observed. The Champaign field was discovered in 2003 at the headwall of a steep slope failure scarp on the SW side of the volcano. Eifuku is located some 122 km NNW from Farallon de Pajaros. NW Eifuku is another 15 km NW from Eifuku.
Daikoku is a conical seamount along a ridge SE of Eifuku. It rises within 323 m of the surface. The summit is topped with two large craters, warm water, and particulate matter suspended over plumes from the summit craters. It has a steep-walled 50 m wide crater on the N flank, some 75 m below the summit. The crater is at least 135 m deep and emits cloudy hydrothermal fluid. A 2006 NOAA expedition observed a convecting black pool of partly solidified black sulfur some 420 m below the summit. Gasses, particulates and liquid sulfur were bubbling up from the back edge of the pool and topped with a hardened sulfur crust.
The seamount was visited multiple times 2003 – 2014, finding an explosion took place in that interval creating at least two new craters. Hydrothermal activity continued, building sulfur chimneys by 2016. S Daikoku is located some 64 km NNW from Farallon de Pajaros. Daikoku itself is located 113 km NNW from Farallon.
Farallon de Pajaros (Uracas) is the northernmost aerial volcano in the Northern Marianas. It tops out at 360 m. The island is 2 km wide and is topped with a small caldera. There is an older edifice, visible on the SE and southern flanks near the coast. Flank fissures fed lava flows during historic times forming lava platforms along the coast. Both summit and flank vents were active during historic time. Eruptions have also been observed from nearby submarine vents and neighboring Makhahnas seamount, which rises to 640 m of the surface some 10 km to the SW. The summit is sparsely vegetated due to recent eruptions. They were so frequent in the 19th Century that the island was referred to as the “Lighthouse of the western Pacific.” The most recent eruptive activity was vigorous fuming observed May 1992. No lava seen. There was another instance Aug 1990. Normal fuming and discolored water were observed in Sept 1981. The Smithsonian GVP lists 17 eruptions 1967 – 1864. Most of these were confirmed VEI2 eruptions. Two instances of yellow streaks were observed on the ocean 175 and 200 km from Farallon de Pajaros (Uracas) Island in Sept 1975. No active volcano or the location has been identified as yet. Chamorro is a seamount located along the volcanic front some 34 km NNW from Farallon.
Ahyi is a submarine volcano that tops out at 75 below sea level. It is located 18 km SE of neighboring Farallon de Pajaros (Uracas) and currently classified as dormant. Last known eruption was 2001. Water discoloration has been observed over the summit. A fishing boat in 1979 felt shocks over the summit and observed an upwelling of sulfur-bearing water. The 2001 explosive eruption was detected seismically at a location near the volcano.
The most recent eruption from Ahyi took place April – May 2014. It was recorded by seismometers in the Marianas Island. Scuba divers doing coral reef surveys on Farallon de Pajaros 20 km NW of Ahyi felt and heard submarine explosions. At the same time, NOAA noted mats of orange – yellow bubbles up to a kilometer from the shoreline (what shoreline?). Seismic signals were detected 10 per hour until May 8 and sporadically until May 17.
An investigation of the summit in May found the minimum depth was now 90 m below the surface, a significant change from the 60 m measured in 2003. A new crater 100 m deep formed at the summit, replacing the previous summit cone. There was a distinct landslide chute down the SE slope, moving material from the top and depositing it on the lower flanks. There were significant particle plumes indicating vigorous hydrothermal activity. Plumes were found S and W of Ahyi at 100 – 175 m deep. A Dec 2014 visit discovered CO2 gas bubbles from the summit, imaged the new summit crater and measured a thick plume of particles near 150 m deep close to the new crater.
Maug Islands are three small islands up to 2.3 km long that mark the N, W and E rims of a submerged 2.5 km caldera 68 km SSE from Farallon de Pajaros. The highest point reaches 227 m above sea level. The caldera floor is generally 200 m below sea level and the non-aerial segments of the rim are 140 m below sea level. There is a twin peaked lava dome in the caldera that rises within 20 m of the surface. This is a twin volcano massif with Supply Reef 11 km N. The inner walls of the caldera on all three islands expose lava flows and pyroclastic deposits cut by radial dikes. There are bedded ash deposits on the outer flanks of all three islands. There are no known eruptions since their discovery in 1522. Coral reefs on the islands and the central domes are poorly developed, but do exist, suggesting a rather long period of quiet. A recent expedition detected possible hydrothermal activity below the ocean surface.
Supply Reef is a conical submarine volcano located 10 km NE from the Maug Islands. It is connected to that edifice by a low saddle that tops out at 1,800 m. The volcano tops out within 8 m of the surface and has corals on the crater rim indicating it has been some time since its last eruption. Several submarine eruptions were detected by sonar originating within 15 – 25 km of Supply Reef.
Hydrophone stations at Eniwetok, Wake and Midway recorded acoustic activity 11 – 12 Mar 1969. Locating placed the source some 30 km S of Farallon de Pajaros, in the vicinity of Supply Reef. A passing fishing boat reported a submarine eruption with 3 audible explosions and discolored sea surface. There was a 3 km zone of discolored water observed Sept 1985 near the 1969 eruption site. There was another report of discolored water and seismic events in Nov 1985. The most recent activity took place Sept and Dec 1989. There were more seismic, audible, and discolored water in the vicinity of Supply Reef. The Sept event went on for 21 hours with 109 seismic events recorded. The Dec event went on 22 – 24 and 26 – 27 Dec. Getting a precise location by hydrophone and seismic triangulation is difficult, and required visual identification of discolored water to determine where the eruptions took place.
Asuncion is a single asymmetrical stratovolcano forming a 3 km wide island. The aerial portion tops out at 857 m with high sea cliffs on the NE flank. Cliffs on the SE flank are only a few meters high. Southern flank has a large landslide scar. There are ash deposits on the southern and western flanks that may be recent. There was an explosive eruption in 1906 that produced a lava flow that traveled halfway down the western and SE flanks. Reports of earlier historic eruptions are unconfirmed. There are a pair of neighboring seamounts, Cheref and Poyo 30 and 50 km SE that have not yet been investigated.
We will cover everything south of Asuncion in Part 2 starting with Agrigan.