Tutulia is the largest and main island of American Samoa, 73 km ESE from Upolu. It is the third largest island in the chain. It 33 km long, up to 5 km wide, has an area of just over 140 km2, and a population of nearly 56,000. The highest point on the island is Matafao Peak, at 653 m. It is located some 150 ESE of the Manu’a Islands on a sea floor that is more than 4,000 m deep. It was built by 5 shield volcanoes along 2 -3 parallel rifts.
The Pago volcano is a shield on the center of Tutuila Island. The volcano was formed 1.54 – 1.28 Ma. It might have topped out as high as 1,200 m. A 10 x 5 km caldera formed some 1.27 Ma. Activity here stopped around 1.03 Ma with a series of trachyte eruptions. Construction of satellite Olomoana and Taputapu volcanoes on the E and W extensions of the main rift zone through Pago took place at around the same time as that of Pago.
Pago Pago Bay is a drowned river valley cut by a stream along the curved base of the caldera wall. A thick series of tuffs and lavas partially filled the caldera. There are several trachyte domes pushed thru the tuffs and lava flows to the surface. There are 600 dikes and 40 faults in the caldera. Several of the dikes were unusual, containing oriented xenoliths and one with bedded ash. The absence of a continuous reef around Tutuila is also due to rapid submergence.
There was a second round of vulcanism that erupted out of a 5 km N-S rift over a broad area of the southernmost part of the island. This formed a group of initially submarine tuff cones and subsequent aerial tuff cones. It also built the Tafuna-Leone plain. The Aunuu Island tuff cone on the SE coast was also built during this time. An ash layer in this part of the island covers a cultural deposit dated around 440 AD. Rejuvenated volcanism only covers 15% of Tutuila, which may indicate it has either started only recently or is in an earlier stage of tectonically rejuvenated volcanism.
A geothermal exploratory well was drilled on Tutuila Island in 2015. It dated shield lavas around 1.45 Ma. They were overlain by younger lavas 1.35 – 1.17 Ma. Rejuvenated volcanism began some 24 ka. The rejuvenated lavas may be related to tectonic uplift outboard of subduction zones and are similar to “petit spot” lavas erupted 150 km outboard of the Japan Trench and late stage lavas erupted at Christmas Island outboard of the Sunda Trench. The tectonic trigger for eruption of these lavas has also been suggested for the Hawaiian Islands, though the flexure there is due to the lithospheric loading of the large volcanic edifices themselves rather than proximity to a trench.
The Tulaga-Malumalu seamount volcanic complex is connected on the E edge of Tutuila by a 40 km rift. Malumalu looks young, with few slope failures and a near-circular footprint. Tulaga has been more eroded. It is highly eroded and appears to have been formed by large and continuous lava outpouring on its SE flank. There was a brief lapse in shield volcanism that created a shallow saddle between the two seamounts.
Three other smallish seamounts, Tama’I, Sodo and Lalulu are farther to the E. They have not yet been sampled and little is known of their origins. Tama’I is 30 km N of Tutuila. There are smaller seamounts in the space between the two.
Ofu and Olosega Islands are a complex of volcanic cones buried by lava flows from two coalescing shields. They are located some 96 km E of Tutuila. They have a combined length of 6 km and an area of 12 km2. Total population is around 500. Highest point is Mount Piumafua on Olosega at 629 km.
One of the volcanic shields is centered off the NW coast of Olosega near Sili Village. The other is centered at A’ofa on the northern coast of Ofu. There are older cones mostly aligned with the crest of the Samoan Ridge including a small cinder cone at Tauga Point on NW Ofu, a tuff cone at the western end of Samo’I Beach, a composite cone exposed in the cliff behind To’aga on southern Ofu, a breccia cone with associated intrusive plug at Fatuaga Point on eastern Ofu, and a final tuff cone at Maga Point on the southern tip of Olosega.
The underlying volcanic pile is constructed of lava flows from the two shields. The volcanic center on SW Ofu has flows dipping SW. It is located just N of the present northern shoreline. The summit of this shield collapsed to form a caldera. Fault boundaries of this collapse are exposed on the N coast of Ofu. The steep crescent shaped scarp and sloping platform is known as A’ofa. The caldera is 1.6 km in diameter. There are vertical dikes in the sea cliff formed by the collapse. The caldera has been partly filled with lava flows and pyroclastic material erupted after the collapse. Less than half of the caldera is still above sea level. There has not sufficient sonar exploration offshore to characterize the flank collapse of the caldera.
The second shield is on Olosega. It also has multiple dikes and a high cliff. The center of this shield is in the ocean beneath the ocean NW of Sili. The very existence of this second shield is still in dispute, as is a caldera from its collapse. High cliffs truncate lava flows on SE Ofu and SW Olosega, suggesting flank collapses into the surrounding ocean.
After summit collapse of the shields, volcanism decreased to allow a 100 m sea cliff to be cut around the islands. At about the same time, a gravity collapse destroyed the A’ofa caldera on the southern side of the island pair. There was a similar collapse N of the islands in the vicinity of the Sili caldera. Recent volcanism on SW Ofu built the tuff cone on Nu’utele Islet. There were a few thick a’a lava flows were erupted from post-caldera cinder cones. Recent erosion built a 3 m bench around the islands.
At least five original cones on Ofu and Olosega Islands were buried by lavas from the A’ofa and Sili shields. The multiple tuff cones covered by lava flows, either predate construction of the two shields or they may be parasitic cones on the flanks of the shields.
The Nu’utele and Nu’usilaelae islets off the W cost of Ofu are the erosional remains of a tuff cone, originally about a kilometer in diameter and 100 M above sea level. It was centered off the SW shore of Nu’utele. The eruption took place near sea level after an extensive period of erosion that cut a sea cliff around the two islets. There are no coral fragments or evidence of a submarine vent found, but the eruption that formed the islets may have been at least in part underwater.
The most recent eruption was a submarine eruption in 1866, 3 km SE of Olosega, along the ridge connecting Olosega with Ta’u Island.
Ta’u is the next island to the E in American Samoa. It is located some 11 km E of Ofu-Olosega. The island is 44 km2, with a population of 790. The highest point on the island is Lata Mountain at 931 m. Ta’u Island is the remnant of a shield with two smaller, later shields on NW – NE trending rift zones. The NW rift zone extends seaward to Ofu and Olosega Islands as the regional Samoan Ridge. The NE rift zone continues some 6 km offshore. Estimated age less than 0.5 Ma. There are no Samoan legends that mention volcanic eruptions on Ta’u island. The base of the island is about 2,700 m below the surface of the ocean, giving the entire pile a thickness of nearly 3,700 m.
The lavas on Ta’u are relatively fresh. Lavas from vents along the Manu’a Ridge gradually built the island above sea level. This is thought to be a long period of relatively quiet, frequent thin lava flows from rift zones which built a base shield volcano. The lavas that built this shield are exposed from 400 – 900 m above sea level and are referred to as the Lata Shield.
The summit of this shield collapsed to form a caldera, after which somewhat more explosive eruptions began out of cinder cones within the caldera an on the shield flanks. These eruptions were not as frequent, and erosion became more effective. The island has a long history of intermittent lava flows even before the formation of the caldera and should be considered to be in the shield-building portion of its life cycle.
The Lata caldera is only partially filled with lava flows. There are extensive deposits of ash and lapilli tuff. There are two main benches within the main summit caldera. There are suspected large-scale flank collapses on the southern slopes of the island. The caldera has no southern rim. There is a depression that extends to more than 2,400 m below sea level down the flank of the island. Faulting associated with the collapse of the main shield allowed paths for later magmas to reach the surface.
Two smaller shields built out the NE and NW portions of the island. The Tunoa shield on the NW is located along a regional rift zone. The Luatele shield is located on a minor rift zone on the NW slope of the main Lata shield. The summits of both new shields collapsed to form depressions which were subsequently filled. Volcanism subsided considerably following construction of these two new shields and erosion built a 60 m sea cliff around the island. Later lava flows spilled over the cliff in places. One location, these flows built a large area of new land in front of a sea cliff. There is a tuff complex with 2 – 3 cones on the NW corner of the island. The base of this complex is some 200 m below sea level.
Vailulu’u Seamount is an active submerged volcano. It is located some 45 km E of Ta’u in American Samoa. It was discovered in 1975. There is an active volcanic cone inside the summit crater named Nafanua. Its summit is a 400 m deep, 2 km diameter crater. It rises from the 5,000 m sea floor to within 590 m of the surface. There has been no observed volcanic growth or collapse since its discovery.
The overall shape of Vailulu’u is dominated by two rift zones extending E and W from the summit, defining a lineament parallel with the track of the Samoan hot spot. There is a third, slightly well-defined rift extending SE from the summit along with multiple minor ridges extending from the lower slopes. Rift zones and ridges in the southern sector are better developed than those on the N. One paper notes “… Vailulu’u a stunning similarity to a “Young Ta’u” island.” The edifice shows signs of slope collapse and mass wasting. These are located near the emergence point of the western rift where it narrows with amphitheater-shaped scars on both the N and S sides of its upper slopes.
The crater and rim are oval-shaped with two well-developed pit craters in the northern two-thirds of the crater. The crater wall has a scalloped appearance suggesting multiple mass wasting crater collapse events.
There are several historic events that suggest activity at Vailulu’u. Sound-fixing equipment recorded explosions in July 1973. There was a strong earthquake swarm in the vicinity of the volcano Jan 1995 NW of the volcano. Dredging pulled up fresh volcanic, pristine volcanic glasses from the slopes of the volcano. Some of these were dated around 5 – 10 years.
Oceanographic sampling in Mar 2000 measured temperature and turbidity in and around the crater detecting what they interpreted as thermal plumes. Particulate matter does not appear to be ejected from the crater above the rim, but there is sufficient activity within the crater to keep it from settling down to the floor. They are carried away by local currents, creating a smog cloud several kilometers around the vent.
As with any active region, there are multiple seamounts unrelated to the Samoan chain. This determination is made by dredging rocks from the flanks and comparing them chemically with lavas from the Samoan hot spot. These include Rose Atoll, Malulu, Papatua, and Waterwitch Seamounts, four seamounts spread over 950 km near the track of the Samoan hot spot that are erupting magmas unrelated to those from Samoa. Worse, a 2010 paper by Jackson, et al suggests that the Samoan region might have volcanic remains from three currently active hot spots overlain by the currently active Samoan hot spot.
Rose Island Atoll is located 100 km E of the Vailulu’u Seamount. It has a well-developed reef system and does not fit the description of a young, active volcano. Its volcanic rocks are distinct from those erupted from Samoa. It is an uninhabited atoll, also part of a US Marine National Monument.
Malulu Seamount is 56 km W of Rose Atoll, 50 km E of Vailulu’u. Like Rose Island, it consists of chemically distinct rocks from the Samoan hot spot. It does not anchor the active end fo the Samoan hot spot.
Papatua Seamount is located some 50 km S of Tutuila. This seamount is not geochemically Samoan and may be related to a second hot spot acting on the Paific Plate before the arrival of the Samoan hot spot.
The final seamount is in the WESAM volcanic province, some 800 km W of Vailulu’u. The Waterwitch seamount has rocks chemically similar to Rose and Papatua.
Samoa is surrounded by water at a depth of up to 5,000 m. Tectonics of the region are driven by the collision of the Indo-Australian Plate into the Pacific Plate. The Samoan chain of islands lies on the Pacific Plate side of the collision zone, just N of a corner in the Pacific Plate. S of that corner lies the Kermadec – Tonga Trench running S to New Zealand. Pacific Plate movement here is generally E to W, as it subducts under the Indo-Australian Plate. The trench curves toward the west south of Samoa, extending and connecting to the Vitaz Trench. Subduction in this region changes into a strike-slip regime to the W of Samoa.
The Samoan chain of islands is thought to be related to a hot spot / mantle plume much like the Hawaiian Islands. Unlike the Hawaiian Islands, it has some unusual features. For example, the islands on either end of the chain are active and the western islands are larger. The easternmost island is an active seamount. The chain consists of alkali rather than tholeiitic lavas. Continuing volcanic activity on Savai’i is thought to be the result of deformation of the plate margin due to bending of the Pacific plate as it approaches the Tonga Trench subduction zone. The alkalic volcanism is associated with geochemistry of underlying mantle.
Recent activity on the island and neighboring Upolu present a problem to the hot spot model of volcanic activity. The Samoan Islands are thought to be the result of an active hot spot producing magma while the Pacific Plate slides E to W across it. As such, the oldest volcanoes ought to be the westernmost ones and the newest ones on the newest, eastern end of the chain. The discovery of an active submarine volcano, Vailulu’u on the eastern end of the island chain tends to support the theory.
If the theory is correct, Savai’i and neighboring Upolu ought to have the oldest volcanic rocks in Samoa. It also has some of the youngest with the 1905 – 1911 eruption sequence. A research team gathered volcanic rocks from the deep offshore flanks and rifts of Savai’i in 2005. They dated these rocks at about 5 Ma. So, the question becomes what reactivated volcanism on the two islands? Current speculation is that proximity to the Tonga Trench has reactivated volcanic activity by reopening ancient fracture zones.
The location of Samoa close to the sharp bend in the Pacific Plate is thought to trigger two types of volcanic activity – plume (hotspot) and rejuvenated (post-erosional). Combined with the motion of the Pacific Plate westward, the hotspot creates islands much like the Hawaiian chain, progressively newer islands to the E as magma from the hotspot drives volcanic activity. Dominant hotspot volcanism ended for around 500 ka and rejuvenated activity, the current activity on Savai’i, Upolu and Tutuila began. Stress and tension from the interaction between the two plates at the corner generated the rejuvenated volcanism. In Hawaii, rejuvenated volcanism begins after a 0.5 – 2 Ma hiatus. It is geochemically distinct from the earlier, shield building volcanism.
Konter and Jackson in 2012 make a case that a tear in the pacific Plate at the corner and a subsequent opening fracture N of the newly formed Vitaz Trench is what is allowing mantle fluids to rejuvenate volcanic activity in Savai’i and Upolu, increasing the volume of rejuvenated volcanism in Savai’i. The Samoan islands are roughly 100 km N of the intersection of the Kermadec – Tonga Trench and the Vitaz. Resulting upward mantle flow under the elastic portion of the plate may cause some small amount of decompression melting.
Samoa is an example of an active volcanic system with a surprising tectonic twist. Future activity on Savai’i should be monogenetic eruptions of rejuvenated magmas. And the hot spot continues to build an active seamount at the eastern end of the volcanic chain.