“We haven’t been on a holiday for a long time, dear hubby, I hear there is a new island coming up soon, somewhere off Hawaii…?” True, but…hmm… If the hot-spot theory is correct, the next island in the Hawaiian chain should form E or SE of the Island of Hawai’i. There is abundant evidence that a new volcano exists at Lö’ihi. We have not known long enough about it to have measured its island-building progress, but sure, one day people will wander the beaches of Lōʻihi Island (if we are still there).
Until 1978, Lōʻihi was thought to be an inactive volcano that had been transported to its current location by sea-floor spreading. The seafloor under Hawaiʻi is 80–100 million years old and was created at the East Pacific Rise, an oceanic spreading center. From there it moved to its present location 6,000 km (4,000 mi) west, carrying ancient seamounts with it. When scientists investigated a series of earthquakes off Hawaiʻi in the 1970s, they discovered that Lōʻihi was an active member of the Hawaiian-Emperor seamount chain.
Lōʻihi is a seamount, or underwater volcano, on the flank of Mauna Loa, 35 km (22 mi) off the Hawaiian SE coast. Sitting on a steep slope, its northern base is 1,900 m (6,200 ft) below sea level, but its southern base is a more substantial 4,755 m (15,600 ft) b.s.l.; its summit is about 975 m (3,000 ft) below sea level. Lōʻihi is predicted to rise above the surface very soon, i.e. in 10,000 to 100,000 years!
It is the youngest volcano in the Hawaiian-Emperor seamount chain, a long trail of volcanoes created by the Hawaiʻi hotspot. The distance between the summit of the older Mauna Loa and the summit of Lōʻihi is about 80 km (50 mi), which is, coincidentally, also the approximate diameter of the Hawaiʻi hotspot. Lōʻihi consists of a summit area with three pit craters, an 11 km (7 mi) long rift zone extending N from the summit, and a 19 km (12 mi) long rift zone extending SSE from the summit.
The summit’s pit craters are named West Pit, East Pit, and Pele’s Pit. Pele’s Pit is the youngest of this group and is located at the southern part of the summit. The walls of Pele’s Pit stand 200 m (700 ft) high and were formed in July 1996 when its predecessor, Pele’s Vent, a hydrothermal field near Lōʻihi’s summit, collapsed into a large depression. The thick crater walls of Pele’s Pit – averaging 20 m (70 ft) in width, unusually thick for Hawaiian volcanic craters – suggest that its craters have filled with lava multiple times in the past. Seismic data indicate that the deepest earthquakes beneath Lö’ihi merge with the deep earthquakes beneath neighboring Kïlauea. This downward convergence implies that Lö’ihi, Kïlauea, and Mauna Loa all tap the same deep magma supply. The triangular zone defined by the summits of these three active volcanoes perhaps can be taken to lie over the postulated Hawaiian hotspot.
SUBMARINE PRE-SHIELD STAGE
When a volcano is created near the Hawaiian hotspot, it begins its growth in the submarine preshield stage, characterized by infrequent, typically low volume eruptions. The type of lava erupted in this stage of activity is alkali basalt. Due to stretching forces, the development of two or more rift zones is common. The lava accumulates in a shallow magma storage reservoir. Lōʻihi Seamount is the only example of a Hawaiian volcano in this stage. It is thought to be some 400,000 years old and presently transitioning from the submarine preshield stage (thought to last about 200,000 years) into the submarine phase of the shield stage. All older volcanoes have had their preshield stage lavas buried by younger lavas, so everything that is known about this stage comes from research done on Lōʻihi Seamount.
Lōʻihi had its last eruption in 1996, before the earthquake swarm of that summer. Nobody has watched it erupting, but photographs taken by deep-sea cameras show that Lö’ihi’s summit area has fresh-appearing, coherent pillow-lava flows and talus blocks. Pillow-lava fragments dredged from Lö’ihi have fresh glassy crusts, indicative of their recent formation. In fact, since 1959 the HVO seismic network has recorded large earthquake swarms at Lö’ihi during 1971-1972, 1975, 1984-1985, 1990-1991, and 1996, suggesting major submarine eruptions or magma intrusions into the upper part of Lö’ihi. The July-August 1996 swarm was by far the most energetic seismic activity in Hawaiian recorded history to date, involving more than 4,200 earthquakes, 95 of these with magnitudes of 4.0 or larger.
Seismic swarms during 2001-2006: The Lö’ihi web site reported that a seismic swarm was detected at Lö’ihi’s summit with earthquakes up to M 5.2 on 13 September 2001. Activity continued for a couple of weeks, with 4 events >M 4 at depths of 12-13 km. No >M 4 earthquakes were detected at Lö’ihi during 2002-2004. An M 4.3 earthquake occurred on 23 April 2005 at ~33 km depth beneath Lö’ihi, and earthquakes of M 5.1 and 5.4 occurred on 13 May and 17 July 2005, respectively, both at a depth of 44 km. The U.S. Geological Survey Advanced National Seismic System measured a small swarm of about 100 earthquakes (the largest 3 events were ~M 4, and between 12 and 28 km deep) that occurred beneath Lö’ihi on 7 December 2005. An earthquake (estimated M 4.7) occurred on 18 January 2006, roughly midway between Loihi and Pahala (on the S coast of the island of Hawaii).
THE 1996 ERUPTION AND SEISMIC EVENT
(Compiled from the research of many individuals)
– There was at least one, possibly two eruptions at Lö’ihi in 1996, based on radiometric dating of young lavas collected by manned-submersible from Lö’ihi
– The summit of Lö’ihi has undergone some drastic changes in 1996, including the formation of a third new pit crater and deposition of large quantities of glassy sands and gravels on the southern part of the summit platform
– New vents issuing the hottest waters ever observed at Lö’ihi (up to 200 ° C) are present in the new pit crater
– The water column above Lö’ihi has signatures of enhanced hydrothermal activity at or near the summit
Unfortunately, the Lö’ihi pages on the website SOEST (Volcanological Center Hawaii) are not updated after 2005. I read elsewhere, that the current seismological equipment for Lö’ihi, stationed on land, can only detect earthquakes over M 4.5 or so. Perhaps there haven’t been any events of that magnitude so there was nothing to report. Which is not to say that the volcano hasn’t erupted since and nobody noticed. In fact it was mentioned that there may have been an eruption in 1998/99 but no hard evidence has been forthcoming.
Lōʻihi Seamount’s first depiction on a map was on an US Survey Chart in 1940. At the time, the seamount was non-notable, being one of many in the region. A large earthquake swarm first brought attention to it in 1952. That same year, geologist Gordon A. MacDonald hypothesized that the seamount was actually an active submarine shield volcano, but, because the earthquakes were oriented east–west (the direction of the volcanic fault) and there was no volcanic tremor in seismometers distant from the seamount, Macdonald attributed the earthquake to faulting rather than a volcanic eruption. Without evidence the idea remained speculative. In 1978, an expedition studied intense, repeated seismic activity in and around the Lōʻihi area. Rather than finding an old, extinct seamount, the data collected revealed Lōʻihi to be a young, possibly active volcano. Observations showed the volcano to be encrusted with young and old lava flows as well as actively venting hydrothermal fluids.
From that time on, numerous expeditions have brought an enormous amount of oceanographic, volcanologic, geologic and biologic data to light. A number of underwater observatories had been installed (and lost again), and in more recent times many dives with submersibles have been undertaken.
Hawaii Undersea Geo-Observatory (HUGO) was a a permanent University of Hawaii un-manned research “station” on Lōʻihi Seamount. The HUGO project, directed by UH Prof. Fred Duennebier, was the first undersea volcano observatory. It communicated with and received power from a shore station of the Big Island of Hawaii via a permanent glass fiber cable more than 40 km long (!). It gave scientists real-time seismic, chemical and visual data about the state of Lōʻihi, which had by then become an international laboratory for the study of undersea volcanism. The cable broke in October 1998, effectively shutting HUGO down. On January 19 of the following year, HUGO was visited by the submersible Pisces V. Thereafter, the observatory functioned for four years before it went dead again in 2002.
Monterey Bay Aquarium Research Institute (MBARI)
In the image on the right, a bathymetric model of the New Pit Crater is pictured which formed during the 1996 seismic event. The crater is roughly 300m deep and formed in a collapse event. It is located in a site that was previously the high-point on the summit, the Pele’s vents area of hydrothermal activity. The pinnacles on the western side of the pit are near vertical spires of material left behind during the pit collapse and have been observed from the Pisces V submersible to be quite precarious.
Since the pit collapse, hydrothermal activity has resumed in the the pit, forming new chimneys of minerals built up at the places where these fluids issue onto the seafloor. Activity is occurring in at least 2 sites, Lohiau vents (the left “x” in the image above) and Forbidden vents (the right “x” in that image). Exit emperatures of over 200 °C have been recorded within the Forbidden vent’s chimneys. This vent had, within the course of a year, spawned new hydrothermal chimneys comprised mostly of Pyrite and Barite.
Prior to the seismic events in 1996, Lōʻihi hydrothermal vents emitted only low temperature fluids (<30° C) which precipitated iron oxide and iron-rich clay minerals. After the seismic swarm, high – temperature sulfide minerals were found in the glass sands near the margins of the newly formed pit crater and vent fluids with extreme temperatures up to 200° C were measured. Some glass grains have rounded edges but show hardly any alteration as might be expected from reaction with hot, acidic fluids. This suggests that reactions occurred rapidly, probably while glass shards were ejected into a turbulent current. The small, euhedral crystals do not appear to be pieces of chimney-like structures or stockwork from deeper in the hydrothermal system. Most likely these crystals precipitated from a hot hydrothermal plume, violently ejected through deep fractures and fissures along the walls of the collapsing pit crater.
In “Lōʻihi Seamount – Part 2: Gourmets of Heavy Metal” we will go a bit OT and visit the dwellers – and in particular the iron-eaters – on the slopes of Lōʻihi.
Enjoy! – GRANYIA
SOURCES & FURTHER READING
– Wikipedia, Lōʻihi Seamount
– Wikipedia, Evolution of Hawaiian volcanoes
– Hawaii Center for Volcanology – SOEST – Lö’ihi Volcano pages
– USGS, Hawaiian Volcanoes Revealed
– Loihi Seamount: The Next Volcanic Island in the Hawaiian Chain
– A.S. Davis, D.A. Clague, R.A. Zierenberg, C.G. Wheat, B.L. Cousens (2003) Sulfide formation related to changes in the hydrothermal system on Lōʻihi Seamount, Hawai’i, following the seismic event in 1996, The Canadian Mineralogist, 41: 457-472.