Agrigan is an active stratovolcano. It is the tallest volcano above sea level of all the volcanoes in the volcanic arc at 965 m. The submarine portion is 4,000 m tall. It is topped with a 1 x 2 km, 500 m deep, flat floored caldera. Last confirmed eruption was 1917. The island is 8 km long. The caldera floor is surfaced by fresh-looking lava flows and has two cones likely built during the 1917 eruption cycle. The caldera is breached to the NW, with a lava flow that extended to the coast and created a lava delta. The 1917 eruption deposited large blocks and up to 3 m of ash and lapilli on a village on the SE coast, prompting evacuation.
An overflight Aug 1990 observed increased fumarole activity on the volcano. Nine residents were evacuated. There were no earthquakes felt. A field team visited around Oct 1 for a week. They found weak fumarole activity at the summit, sulfur deposits and boiling hot springs, overall a low level of activity with no sign of a recent increase. The hot spring terraces located below the boiling hot spring were mostly dry, indicating activity was more vigorous in the past. Fluctuations in flow volume from the spring may be tied to seasonal rainfall. They installed a regional network of short period seismometers on Pagan, Alamagan, Anatahan and Agrigan with a receiving station on Saipan. A team visited the volcano again in May 1992 and found little variation. A 2007 M 3.3 earthquake was reported July 16. No sulfur smell or any other signs of volcanic activity was noted. There were observations of possible volcanic plumes in Feb 2012 and Jan 2013. Neither the USGS nor NOAA confirmed a volcanic source for these satellite observations. No hot spots or thermal alerts were observed. The clouds were then interpreted as weather rather than volcanic-related.
Alamagan is the emergent summit of a stratovolcano in the central Mariana Islands. It has a 350 m deep summit crater. There is a graben cut on the SW flank. There are two lava flows, one to the N coast and a lava platform on the S flank. Pyroclastic flow deposits have been dated around 870 AD, a VEI 4. An earlier eruption took place 540 AD. There are reports of a pair of eruptions in the late 19th Century that are not confirmed.
The volcano has an active hydrothermal system with active fumaroles and hot springs. The seismic station was installed in Sept 1990. There was a small group of residents evacuated in Dec 1998 after the volcano showed signs of activity. There were no tremors, rumblings, no eruption noises before the evacuation. The speculation is that rain penetrating fissures in the volcanic edifice after an extended dry season triggered a greater than normal amount of steaming. The seismic station installed in 1992 was not operational due to battery failure and funding issues (no money to replace the batteries).
Guguan is the tip of a massive stratovolcano topping out at 287 m above sea level. The island is just under 3 km in diameter with an eroded volcano to the S, a caldera with a post-caldera cone in the middle, and a volcano to the N. The northern volcano has three coalescing cones and a breached summit crater with lava flows to the W and NW. The high point of the island is the southern rim of the caldera. The only known eruptions from Guguan took place 1882 – 1884 and built the northern volcano and the lava flows from it that reached the coast. The volcano has active hydrothermal system with solfataric activity.
Zealandia Bank is a pair of pinnacles a kilometer apart rising from a submerged bank to near sea level. It is located between Gugan and Sarigan Island. Andesitic rocks were dredged from the southern peak with some evidence of coral growth. Freshly broken pahoehoe was dredged from the western flank. Most recent eruptive activity is unknown, though a 2004 survey detected fumaroles.
Sarigan is a stratovolcano with an aerial portion that formed a 3 km island. Its cone tops out at 538 m with a 750 m summit crater. The summit crater contains a small ash cone. The most recent eruptions built lava domes near the south crater rim. Lava flows from the domes reached the coast and extended into the sea. The northern flow overtopped the crater rim on the N and NW sides. They are sparsely vegetated. There was a seismic swarm on Sarigan Aug 9 – 18, 2005.
South Sarigan Seamount is located 12 km S of Sarigan Island. It tops out 184 m below sea level. There was a short, explosive submarine eruption in May 2010 that put a plume some 12 km into the atmosphere. The summit is irregular with multiple peaks, with possibly a young cone 350 m below the surface. The summit and flanks suggest a frequently active volcano. West Sarigan Submarine Volcano is located about the same distance to the West of Sarigan Island as South Sarigan.
The May 2010 explosive eruption took place 28 – 29 May. The most energetic phase lasted about 10 minutes and ejected the plume over 12 km above sea level. There were a pair of moderate earthquakes before the eruption and multiple smaller shocks. There was a M 4.8 quake at the time of the eruption. Initial activity was observed the previous day with a patch of discolored water and possible light colored debris on the surface. Alaska Volcano Observatory noted tremor on Alaskan seismic stations at the same time. The are of disturbance had grown significantly before the May 29 explosion. Seismic activity decreased immediately after the eruption. Only a few scattered events took place after May 31. There were multiple felt earthquakes on neighboring Sarigan Island during this and explosions were heard. Small tsunamis were reported as increasing ocean wave activity during the night.
The seamount was studied in 2013. It dramatically changed the shape of the northern crater, forming a breached crater 350 m in diameter and a substantial deposit on the W flank. The crater floor dropped some 200 m beneath the pre-eruption summit. The South Sarigan event is one of the first instances of am explosive, relatively deep, submarine eruption that breached the ocean surface that qualitative data exists before, after and during the eruption. The eruption was dominantly andesitic. We know very little about the eruptive history of most submarine arc volcanoes.
Anatahan is a 9 x 4 km island in the northern Marianas located 120 km N of Saipan. It is located near the southern end of the volcanic arc. The submarine portion of the volcano is huge, rising some 3,700 m from the sea floor, measuring 35 km at the base. The island is topped with a 5 x 2.3 km caldera. This caldera has a recent 1.4 x 1.2 km, 200 m deep crater in the eastern part of the caldera. First known activity on the island took place in 2003. It had frequent eruptions since then. The 2003 eruption was on the flank some 70 m above sea level. There is a neighboring submarine volcano within 460 m of the ocean surface on the NE flank of the volcano. There are numerous other submarine vents on the NE – SE flanks. Recent lava flows are sparsely vegetated indicating recent, though undated eruptions. The May 2003 eruption formed a new crater inside the eastern caldera.
East Diamante is a dacitic submarine volcano. It is topped with an elongated caldera with a rim on the NE side. There is a large post-caldera cone on the SW caldera rim and a complex of lava domes in the center of the caldera. Several areas of hydrothermal activity with black smokers were observed in the central dome complex at depths of only a few hundred meters. These are the shallowest black smokers yet discovered. The summit of the volcano is 127 m below the surface.
Ruby is an active basaltic submarine volcano located near the southern end of the volcanic arc NW of Saipan. It was first detected in 1966 via sonar during an eruption. A 1995 eruption produced subsurface explosions, fish kill, sulfurous odors, bubbling water and volcanic tremor. The summit depth was originally estimated at 549 m below the surface. This was refined in 1985 to 230 m. By Oct 1995, it was measured at 185 m, though that depth is unconfirmed. On Oct 1995, it was measured at around 60 m, though this could have been a reflection off the eruptive plume. Two seismic stations nearest to the eruption in Saipan 50 km SE and Pagan 130 km N were too distant to measure smaller movements. Tremor did appear at the time of the eruption and on the next day. A fish recovered at the eruption site had ash in its gills. News of the eruption raised concern about a possible local tsunami Oct 25, 1995.
Esmeralda Bank is a submarine volcano some 40 km W of Tinian Island. It tops out within 43 m of sea level. The volcano has three summit cones along a N-S line. The highest, middle peak is topped by a 3 km caldera that is open to the W. There are several parasitic cones on the volcano. Frequent sulfur boils and water discoloration have been observed attributed to either eruptive activity or solfataric activity. There are no known recent eruptions. A pilot report May 1987 reported a heavy boil on the ocean surface over Esmeralda feeding a zone of discolored water, the first report in years.
NW Rota 1 is a submarine volcano located some 64 km NW of Rota, 100 km N of Guam. It tops out 517 m below sea level. It was discovered in 2003 and found to be hydrothermally active. The second visit in 2004 discovered a smallish ongoing eruption from a small pit on the upper portions of the S flank. This ejected a plume several hundred meters high containing ash, rock particles and molten sulfur droplets that stuck to the surface of the remote submersible. The active vent was 20 m in diameter, 12 m deep, and funnel shaped. The submarine volcano has structural lineaments cutting across the summit of the edifice and down the NW and SW flanks.
West Rota lies 56 km to the W of Rota. It is topped with a 10 km diameter caldera, similar in size to Crater Lake on top of Mount Mazama in Oregon. The caldera floor is 1,600 m below sea level. It is inactive and lacks a strong hydrothermal plume in the waters above it. A 2003 survey identified features of a violent explosive eruption some unknown time in the recent past. The estimated date is some thousands to tens of thousands of years ago.
Tracey Seamount is located some 40 km W of northern Guam. It tops out at 900 m below sea level. While listed as probably extinct, fresh pumice was dredged from its summit in 1971. It is a simple cone that rises from a base around 3 km deep and tops out around 600 m below the surface. It is by far the largest submarine volcano in this region, with a 250 km3 volume some 10% the size of large Mariana Arc volcanoes like Pagan and Agrigan. It summit has a small crater with a felsic plug or resurgent cone.
Forecast Seamount lies some 70 km W of the southern tip of Guam and 19 km NW of Seamount X. It has an active hydrothermal system with temperatures up to 200° F, one of the highest temperature vent systems known in the Marinara arc. The vent community species differ from those found further N in the volcanic arc. It was active around 500 ka.
Alphabet Seamounts
The Alphabet Seamounts are a group of at least 6 seamounts S of Tracey Seamount where the Mariana Trough spreading ridge thins the crust to 6 -7 km allowing magma to reach the surface building seamounts in a different way than those along the volcanic front. Western seamounts A and B erupt magmas more similar to back-arc basin basalts than eastern seamounts X and C which erupt more arc-like magmas. X and C are located around where the next arc volcano should be located. Hydrothermal activity is widespread.
Seamount A is the smallest, located a few kilometers E of the spreading ridge. It is 1 km wide and 500 m tall. Seamount B is centered on what may be a fault scarp / spreading ridge that connects Forecast and Seamount Z.
Seamount C is a shield with a well-developed crater some 50 km SW from Tracey Seamount. It rises to 2,700 m and is 6 km in diameter from N – S. There are 6 km long ridges that extend from the shield W toward the spreading ridge and E into a zone of ESE-trending ridges generated in response to N – S directed extension. Seamount C is located at the junction separating the E – W Mariana Philippine spreading center to the N and the tectonically more complex area to the S, making its location near a triple junction-like setting.
Seamount X is the largest, located some 70 km WSW from Guam. It tops out 1,230 m below sea level. Hydrothermal activity was detected during a 2003 survey. The hydrothermal system supports a diverse array of sea life. A 2006 survey found thick deposits of sulfur covered with thousands of lobsters. The summit is cut by an elongated caldera. It is a simple cone with a 10 km diameter base.
Seamount Y is a complex volcano. Its older part is around 7 km in diameter, with steep sides that rise 500 m from the sea floor. This circle is cut by a N – S ridge that rises to 2,300 m and associated faults. The volcano is clearly affected by strong E – W extension.
Forecast is conical, about 7 km across, and rises to 1,600 m below the surface.
Seamount Z is the second smallest seamount, a conical volcano with a faulted western slope. It is 8 km across and rises to 2,400 m.
Tectonics
Tectonics of the Marianas Islands is driven by subduction of the Pacific Plate under the Philippine Sea Plate. The deepest point in any ocean on earth is the Challenger Deep portion of the Marianas Trench to the east of the volcanic arc. The Marianas are the southern portion of the mature Izu – Bonin – Mariana volcanic arc system. Throughout most of its length, there is a regular arrangement of trench, 200 km wide forearc, active volcanic front, and back-arc region. The back arc region is extensional, forming rifts, a back arc basin. Volcanic cross chains are found in the northern Izu portion of the system. The Mariana portion has an actively spreading back arc basin, the Mariana Trough.
This is the location where the very oldest part of the Pacific Plate is subducted. Some have described this portion of the Philippine Sea Plate as the Mariana micro plate. It is separated from the rest of the Philippine Sea Plate in the west by a divergent boundary with numerous transform fault offsets. The NE portion of this microplate is defined by the Izu – Ogasawara Trench, north of the Marianas. The eastern boundary is defined by the Mariana Trench along the Marianas. The western portion of this microplate is defined by the Mariana Trough to the west. The Mariana microplate is separating from the rest of the Philippine Sea microplate around 3 cm/year. The Pacific Plate is subducting around 6 – 10 cm/yr.
For the first 100 km, the subducted plate is at a gentle angle. From 100 – 680 km, the angle is almost vertical. There is an ongoing dispute about what is taking place here. On the one hand, is the sheer weight of the vertical segment of the place pulling the driving force for plate movement? Or are the plates pushed away from the mid-ocean ridge by forces generated at the ridge. Final possibility is that of slab rollback by the Pacific Plate.
Active volcanoes are 80 – 230 km above the subducting Pacific Plate. About a quarter of these lie behind the magmatic front.
The southern portion of the Mariana arc is an unusual nest of small volcanoes to the west of Guam and to the south, different from the typical convergent margin volcanic systems of the southern Marianas. This is a region of complex deformation and enhanced magmatic flux. This deformation is also reflected in the unusual shape of Alphabet Seamount volcanoes, elongated E – W in the south (Seamount C) the NW (Forecast and Seamount Y). In this part of the arc, the Pacific Plate is subducting at right angles to the Philippine Sea Plate at around 3 cm/yr. It is also undergoing diffuse extension as the Mariana Trough spreading center changes from an axial rift to the north to a rapidly, complexly opening magma-rich system to the south.
This system starts where the next major Mariana volcanic structure should be, some 75 km S of the Tracey Seamount. Rather than the 75 km or so spacing along the rest of the arc, these volcanoes are smallish, 15 – 20 km apart, some 130 – 168 km above the subducted Pacific Plate. They appear to be located on weak lithosphere disrupted by distributed extension of the southern portion of the Mariana Trough.
Conclusions
The Mariana Islands are part of an active, tectonically driven volcanic arc that has been in motion for over 35 Ma. The larger and older islands to the S no longer show active volcanism, while the newer islands to the N and seamounts to the S and W do. The region is complex with a healthy magma supply. While most of the systems are andesitic and basaltic, the occasional dacite does show up. Eruptions are both effusive and explosive above and below the surface of the ocean. Subsidence does not appear to be an issue along the volcanic arc. Volcanic islands and seamounts are more likely than not to have active hydrothermal systems.
Additional information
Oregon State – Basic Geology of the Mariana Islands
Wiki – Izu – Bonin – Mariana Arc
Schmidt Ocean Institute – Hydrothermal Hunt at Mariana – The Tectonic Neighborhood
How the Mariana Volcanic Arc ends in the south, Stern, et al, Island Arc (2013) 22,133-148
Physical volcanology of the submarine Mariana and Volcano Arcs, Bloomer, et al, May 1989
Mariana: The Tectonic Neighborhood, AGU Blogosphere, ihwang, Nov 2015
New submarine eruption in Northern Mariana Islands, USGS, May 2014
New submarine eruption in Northern Mariana Islands, USGS, May 2014
Nomination Title: Mariana Trench National Marine Sanctuary, 2016
Mariana serpentine mud volcanism exhumes subducted seamount materials: implications for the origin of life, Fryer, et al, Jan 2020
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