Loihi Volcano (Seamount). Hawaii.

Introduction.

Loihi Seamount is an active submarine volcano built on the seafloor south of Kilauea about 35 km from the Hawaii Island. The seamount rises to 969 m below sea level. Loihi is the newest volcano in the Hawaiian-Emperor seamount chain, a string of volcanoes that stretches over 5,800 km northwest of this volcano.
Location: 18.92 N 155.27 W.
Volume: 660 km3.
Like the volcanoes on the Island of Hawaii, Loihi has grown from eruptions along its 31-km-long rift zone that extends northwest and southeast of the caldera.
At its summit, Loihi Seamount stands more than 3,000 m above the seafloor, making it taller than Mount St. Helens was before its catastrophic 1980 eruption.
Loihi last erupted in 1996, before the earthquake swarm of that summer.
During 1996, a new pit crater was formed at the summit of the volcano, and lava flows were erupted. Continued volcanism is expected to eventually build a new island at Loihi; time estimates for the summit to reach the sea surface range from roughly 10,000 to 100,000 years.
Hawaiian Volcano Stage: In transition between pre-shield and shield stage.
A diverse microbial community resides around Loihi's many hydrothermal vents.

The summit of Loihi volcano.

The summit of Loihi volcano is marked by a caldera-like depression 2.8 km wide and 3.7 km long. The summit platform includes:
- three collapse pits or craters occupy the southern part of the caldera;
- sediment-free glassy lava;
- low-temperature hydrothermal venting.
The most recent pit (Pele's Pit) formed during an intense earthquake swarm in July-August 1996. This crater is about 600 m in diameter and its bottom is 300 m below the previous surface. Pit Crater is located in a site that was previously the high-point on the summit, the Pele's vents area of hydrothermal activity. Since the pit collapse, hydrothermal activity has resumed in the pit, forming new chimneys of minerals built up at the places where these fluids issue onto the seafloor. Exit temperatures of over 200 °C have been recorded within the Forbidden vents chimney's chimneys.
An arcuate chain of small cones on the western edge of the summit extends north and south of the pit craters and merges into the crests of Loihi's prominent rift zones. Deep and shallow seismicity indicate a magmatic plumbing system distinct from that of Kilauea volcano.

Piton de la Fournaise Eruptions.

Unlike most active volcanoes in the Pacific Ocean that make up the active plate margins on the Pacific Ring of Fire, Loihi and the other volcanoes of the Hawaiian-Emperor seamount chain are hotspot volcanoes and formed well away from the nearest plate boundary. Volcanoes in the Hawaiian Islands arise from the Hawaii hotspot. Loihi is the youngest volcano in the chain and is the only Hawaiian volcano in the deep submarine preshield stage of development.
Loihi began forming around 400,000 years ago and is expected to begin emerging above sea level about 10,000–100,000 years from now.

Loihi volcano is in transition between the preshield and shield volcano stage.

Loihi volcano Bathymetric map. The 3 depressions in the summit area are "pit craters"; the lower-left most crater (the arrow points to Pele's Pit) was formed in July 1996.	Ocean bottom observatory (OBO) at Pele's Vents

Loihi volcano consists of the three different parts.
1. A summit area with three pit craters: 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 high and were formed in July 1996 when its predecessor, Pele's Vent, a hydrothermal field near Loihi's summit, collapsed into a large depression of Pele's Pit - averaging 20 m in width, unusually thick for Hawaiian volcanic craters—suggest its craters have filled with lava multiple times in the past.
2. An 11 km long rift zone extending north from the summit.
Map of a north–south ridge, trending slightly east of south. Draws lines through areas of a given water depth with an arrow pointing to Pele's Pit. At its peak, Pele's pit is about 1,000 meters below sea level; further south the ridge gradually descends about 3,500 metres to the sea floor.
Loihi's north–south trending rift zones create a distinctive elongated shape, from which the volcano's Hawaiian name, meaning "long", derives. The north rift zone consists of a longer western portion and a shorter eastern rift zone.
3. A 19 km long rift zone extending south-southeast from the summit.
Observations show that both the north and south rift zones lack sediment cover, indicating recent activity. A bulge in the western part of the north rift zone contains three 60–80 m (200–260 ft) cone-shaped prominences.
The seafloor under Hawaii is 80–100 million years old and was created at the East Pacific Rise, an oceanic spreading center where new sea floor forms from magma that erupts from the mantle. New oceanic crust moves away from the spreading center. Over a period of 80–100 million years, the sea floor under Hawaii moved from the East Pacific Rise to its present location 6,000 km west, carrying ancient seamounts with it.
When scientists investigated a series of earthquakes off Hawaii in 1970, they discovered that Loihi was an active member of the Hawaiian-Emperor seamount chain.
Loihi is following the pattern of development that is characteristic of all Hawaiian volcanoes. Geochemical evidence from Loihi lavas indicates that Loihi is in transition between the preshield and shield volcano stage, providing valuable clues to the early development of Hawaiian volcanoes. In the preshield stage, Hawaiian volcanoes have steeper sides and a lower level of activity, producing an alkali basalt lava.
Radiometric dating was used to determine the age of rock samples from Loihi. The oldest dated rock is around 300,000 years old. Based on the samples, scientists estimate Loihi is about 400,000 years old. The rock accumulates at an average rate of 3.5 mm per year near the base, and 7.8 mm near the summit. If the data model from other volcanoes such as Kilauea holds true for Loihi, 40% of the volcano's mass formed within the last 100,000 years. Assuming a linear growth rate, Loihi is 250,000 years old. However, as with all hotspot volcanoes, Loihi's level of activity has increased with time; therefore, it would take at least 400,000 years for such a volcano to reach Loihi's mass. As Hawaiian volcanoes drift northwest at a rate of about 10 cm a year, Loihi was 40 km southeast of its current position at the time of its initial eruption.

Seismic Activity.

Loihi volcano generates earthquake swarms, the most intense of which occurred in 1996.
Loihi is a young and fairly active volcano, although less active than nearby Kilauea. Most earthquake swarms at Loihi have lasted less than two days; the two exceptions are the 1991–92 earthquake, lasting several months, and the 1996 event, which was shorter but much more pronounced. Both of the earthquakes followed a pattern of activity that began on the flank and migrated to the summit. The depth of the 1996 earthquakes as 6 km to 8 km below the summit, approximating to the position of Loihi's extremely shallow magma chamber. This is evidence that Loihi's seismicity is volcanic in origin.
Low level activity is periodically punctuated by large swarms of earthquakes, each swarm comprising up to hundreds of earthquakes.
The majority of the earthquakes are not distributed close to the summit, though they follow a north–south trend. Rather, most of the earthquakes occur in the southwest portion of Loihi.
The largest recorded swarms took place on Loihi in 1971, 1972, 1975, 1991–92 and 1996.

A swarm of 4,070 earthquakes between July 16 and August 9, 1996.

The largest amount of activity recorded for the Loihi seamount was a swarm of 4,070 earthquakes between July 16 and August 9, 1996. This series of earthquakes was the largest recorded for any Hawaiian volcano to date in both amount and intensity. Most of the earthquakes had moment magnitudes of less than 3.0. More than 40 earthquakes had moment magnitudes of greater than 4.0 and a 5.0. The swarm altered 10 to 13 km2 of the Loihi's summit. The southern portion of Loihi's summit had collapsed, a result of a swarm of earthquakes and the rapid withdrawal of magma from the volcano. A crater 1 km across and 300 m deep formed out of the rubble. The event involved the movement of 100 million cubic meters of volcanic material. A region of 10 to 13 km2 of the summit was altered and populated by bus-sized pillow lava blocks, precariously perched along the outer rim of the newly formed crater. "Pele's Vents," an area on the southern side, previously considered stable, collapsed completely into a giant pit, renamed "Pele's Pit".
The studies demonstrated that the most volcanically and hydrothermally active area was along the southern rift. Dives on the less active northern rim indicated that the terrain was more stable there, and high lava columns were still standing upright. A new hydrothermal vent field (Naha Vents) was located in the upper-south rift zone, at a depth of 1,325 m.

Seismic swarms during 2001-2006.

A seismic swarm was detected at Loihi'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 Loihi during 2002-2004.
An M 4.3 earthquake occurred on 23 April 2005 at ~33 km depth beneath Loihi, 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 Loihi on 7 December 2005. A more recent 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).

Loihi volcano (Seamount). Video.