THIRTY kilometres southeast of the Big Island of Hawaii, more than a thousand metres below the surface of the Pacific, lies an island waiting to be born. For the past few hundred thousand years, Loihi volcano has been growing up from the seafloor. Now four kilometres high, in just 50 000 years it will emerge as the newest island in the Hawaiian chain. Because Loihi is relatively easy to reach, geologists at the University of Hawaii are in a privileged position. 鈥淲e鈥檙e hoping to see how a Hawaiian island develops from womb to birth,鈥� says Alex Malahoff, Director of the Hawaii Undersea Research Laboratory, based in Honolulu.
About 15 years ago geologists discovered that Loihi was an active volcano rather than a lifeless lump of rock. Since then, they have spent their time taking photographs, sending submersibles to retrieve rock samples, and leaving recording instruments on its summit for later collection and analysis. Now Malahoff and Fred Duennebier, Head of the Depaftment of Geology and Geophysics at the University of Hawaii, have more ambitious plans. Next year they intend to deposit a whole observatory bristling with instruments on Loihi鈥檚 summit.
HUGO or the Hawaii undersea geo-observatory will be powered remotely from the Big Island via a fibre-optic cable, which will also send back details of events as they happen. Malahoff and Duennebier are hoping to build up a picture of the very earliest stages in the life of a Hawaiian volcano. All the islands in the chain are volcanic and owe their existence to a hotspot or plume of hot rock welling up from deep inside the Earth. For the past 70 million years, as the Pacific plate has moved over this hidden furnace, new volcanoes have formed and older ones have died. As they studied the different volcanoes, geologists have formed a picture of how one matures. But even Kilauea on the Big Island, the youngest volcano on land, is already quite elderly. As a mere baby Loihi should help fill in the missing pieces.
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Loihi has already thrown up surprises. For instance, geologists knew that the younger volcanoes on the Big Island produce tholeiitic lava which contains plenty of silica and only relatively small amounts of alkali metals such as sodium and potassium. But the more venerable volcanoes such as Mauna Kea, which last erupted several thousand years ago produce alkalic lava, which has a much higher proportion of alkali elements. While alkalic lava reflects cooler temperatures, tholeiitic lava is born of mantle rock that has melted under intensely hot conditions. So geologists deduced that the switch in lava types must happen late in the life of the volcano, when it moves away from the hotspot and become less active.
Then in 1982 geologists had to think again when rocks dredged from Loihi revealed that, young as it was, it was producing both types of lava. They realised that the hotspot must be made up of concentric zones at different temperatures. Loihi must have formed while the crust was passing over the relatively cool outer edges of the hotspot, and must now be crossing over into a hotter inner region. Says David Clague, Chief 快猫短视频 of the US Geological Survey鈥檚 Hawaiian Volcano Observatory (HVO), 鈥渢he picture that we鈥檙e playing with at the moment is that, as a volcano approaches the hotspot you get a little bit of melting and make alkalic lava. As it progresses over the hot spot you really start melting a lot and make tholeiitic lava. Then you keep going and go back to alkalic lava again.鈥� A hotspot might have this structure if the outer edges of the plume contained rock drawn in from relatively cool, shallow parts of the mantle, while the rock in the centre came shooting up from deeper, hotter regions.
If anything, the geologists are even more disturbed by the way Loihi looks. Volcanoes on land slope down gently at the sides. But undersea photography has revealed that Loihi looks very rough indeed. 鈥淚t鈥檚 a real mess of a volcano,鈥� says Malahoff. On land, the lava is relatively thin, and runs out and down the slopes to create a characteristic smooth shape. But underwater, the lava breaks up as soon as it forms so that Loihi鈥檚 slopes are covered with rubble. 鈥淭hat tells us that the foundations of most of these Hawaiian islands are pretty unstable,鈥� says Malahoff. Many of the islands show signs of experiencing huge, catastrophic landslides in the past, and Malahoff suspects that the unstable foundations could be partly to blame.
Some of the most interesting findings have emerged in the past five years or so from instruments that were left on Loihi鈥檚 summit. For instance, temperature sensors left near a hydrothermal vent field on Loihi鈥檚 summit in 1991 show that the vents are much cooler than the 鈥渂lack smokers鈥� found on mid-ocean ridges. The water emerging from the vents 鈥� rather whimsically dubbed Pele鈥檚 vents after the Hawaiian fire goddess 鈥� is a mere 100 掳C, whereas black smokers can spew out water at up to 400 掳C.
Deep heat
Malahoff believes that this is because of the unique geology of Loihi. At mid-ocean ridges, where lava wells up to form new seafloor, seawater heats up when it seeps down into the chamber holding the lava. Then, when the water tried to escape it is trapped by large rocks on the ridge surface, and reaches high temperatures before it finally cracks the rocks and emerges. But the surface of Loihi is so broken up that the seawater is not trapped by the rocks. This also influences the type of marine life found near the vents. At mid-ocean ridges, clams and strange creatures like giant tube worms bask in the warmth of the emerging water, but at Loihi only bacteria are seen clustering around the vents.
Instruments placed on Loihi鈥檚 summit have also given tantalising hints of activity inside. Seismometers based on the south coast of the Big Island and operated by the HVO have been picking up earthquakes on Loihi for years 鈥� this was one of the first hints that Loihi wasn鈥檛 dead in the water. But in 1991, the HVO detected an earthquake swarm at Loihi at the same time as a sensor on Loihi鈥檚 summit registered a sudden increase in pressure. Malahoff believes that these two events signalled an underwater eruption. The earthquakes revealed that hot lava was on the move -fracturing colder rock and sending out seismic waves. Meanwhile, the pressure sensor recorded an increase in the amount of water above the volcano. This meant that the summit of Loihi had dropped by about 30 centimetres 鈥� just what you would expect if the volcano had pumped itself up with lava, and then deflated on eruption.
Sadly, Malahoff didn鈥檛 catch the volcano in action because he did not recover the pressure sensor until six months later. Moreover, although the HVO seismometers picked up the large-scale activity, there was no functioning seismometer on Loihi鈥檚 summit to pick up the subtle details, or to pin down the movements of the lava. 鈥淲ith no seismometers working out there and with only one pressure sensor, it鈥檚 very hard to know exactly what was going on,鈥� says Duennebier. 鈥淲e know it happened, but we can鈥檛 say where it happened, or how big it was, or whether some other place moved five times as much.鈥�
In 1992, Malahoff took a Russian submarine to the base of the volcano, and photographed recent lava flows on the ocean floor. But he has still not been able to pinpoint the exact source of the lava, or watch an eruption in action.
HUGO could solve all of these problems. First, says Malahoff, it will widen the horizons of the existing land-based instruments. For instance, combining the HUGO seismometers with the data from the HVO array will give a 3D picture of the moving lava and should help researchers decide whether the lava is stored in a magma chamber, as it is in the older subaerial volcanoes. The HVO data already hint that some of the 1991 earthquakes were concentrated at a depth of about 18 kilometres and others at somewhere between 5 and 12 kilometres below Loihi鈥檚 summit, way below the base of the volcano, suggesting that the eruptions could have come from two different magma chambers.
Also, seismometers placed temporarily on Loihi have picked up peculiar small-scale features too weak to be seen by the HVO array. Malahoff believes some of these features could reflect rock cracking from pressure changes as the lava moves around, and others could be picking up mini explosions from hydrothermal activity. He says HUGO鈥檚 array should pick up many more of these features to give an intricate picture of what is going on inside Loihi. Having instruments on Loihi could also give new clues to the size and location of the hotspot.
HUGO will also be recording Loihi鈥檚 activities as they happen. That way, if something interesting starts up, the submarine can zip down to the summit with more instruments in tow. It may even be possible to watch an eruption from the sub, although that could be rather hairy. 鈥淚 don鈥檛 know if you鈥檇 want to hang around for too long if something like that started up,鈥� says Brian O鈥機onnor, a diver who is training to pilot the submersible. They might also investigate the aftermath of a landslide. 鈥淲e鈥檙e pretty sure that landslides occur here regularly,鈥� says Duennebier, 鈥渁nd it would be a lot of fun to go out there and see what had happened. Sometimes a landslide will actually uncover a magma chamber, in which case you鈥檒l get a very nasty eruption. That would be really exciting.鈥�
It鈥檚 in the box
The central feature of HUGO is a 2-metre-long titanium junction box containing a series of plugs for different instruments. Initially, HUGO should have an array of seismometers, as well as temperature sensors, pressure sensors and hydrophones (for detecting distant earthquakes, as well as listening to whale songs). There are also plans for a 鈥渃hemical sniffer鈥� to sense changes in water chemistry around the hydrothermal vents. The box will be connected to the shore by the 47-kilometre cable donated earlier this year by AT&T which is the kind used regularly for transocean communication. But Duennebier admits that using it this way is an experiment. 鈥淣o-one has ever had one of these cables end in the middle of the ocean before,鈥� he says.
Duennebier is sanguine about siting the junction box in an area that is susceptible to eruptions and landslides. 鈥淲e鈥檒l put it in an area we know to be relatively flat, and where nothing seems to be happening right now,鈥� he says. 鈥淥f course there鈥檚 no guarantee that something won鈥檛 be happening in the future, but if you want to go to the interesting areas you have to take your chances.鈥� One way to explore the more interesting areas with less risk to the cable 鈥� the most expensive part of the HUGO equipment 鈥� is to run secondary junction boxes off the primary one, and put these boxes in the more dangerous areas. There are even plans for an acoustic system, so that certain experiments could bypass direct connections altogether. Ships could drop simple apparatus to the seafloor, and the data could be broadcast to the junction box as sound waves, and transported along the cable to shore.
Malahoff鈥檚 submersible, Pisces V, will carry the junction box and cable down to the summit, and then use its external manipulator arms to switch and service instruments. Pisces can reach depths of up to 2000 metres. Gleaming from its recent refurbishment, the sub can now be recovered and serviced on board a support ship after each dive, rather than being towed to shore on a raft as in previous dives. This means that geologists can sit over the site for several days, making repeated dives, which will be very useful if something鈥檚 going on that they really don鈥檛 want to miss. Meanwhile, the ship, a reincarnated seismic survey boat, is being fitted with laboratories where geologists can examine samples recovered by Pisces while engineers are preparing it for its next dive.
The sub鈥檚 pressure vessel, which houses the crew of one pilot and two scientists, is a stainless steel sphere just 7 feet in diameter. Sitting inside, with sun streaming through the three small windows, it seems cosy, and not uncomfortable. But it鈥檚 easy to see how, in the chill half-light 1000 metres down, it could be a very different story. Duennebier is excited but nervous about the prospect of his first trip in the sub. 鈥淚 do tend to get claustrophobic,鈥� he admits. According to O鈥機onnor, the pilot usually tries to give the scientists jobs to do as the sub descends, to take their minds off all that water above them. Safety is paramount. At these depths, water would burst in at the speed of a bullet with just a pin prick in the outer shell. The crew take a packed lunch with them 鈥� dives last up to 8 hours 鈥� but lockers in the pressure vessel hold emergency supplies of water and food. A gas cylinder bleeds fresh oxygen into the vessel and extra cylinders contain oxygen for up to three days in case the sub becomes stranded.
One of the most difficult parts of the project was designing connectors that would hold the instruments firmly in place, yet could be manipulated easily using Pisces鈥� remote arms. 鈥淭he first one had to be pushed in and rotated to lock it in place,鈥� says Duennebier. 鈥淯nfortunately, we didn鈥檛 realise that the submersible arm is very strong. So they plugged the instrument in, turned it and destroyed the whole thing 鈥� they literally ripped [the box] apart.鈥� In the new version, the suhmersible arm drops the instrument onto a tray, and pushes it gently into the connector.
The refurbished sub will be tested in the water this month for the first time. If all goes well, the junction box and cable should be in place by next summer. Malahoff says that this could be the perfect time for a blitz on Loihi. Over the past few years seismic activity seems to have been hotting up. The HVO seismometers measured a burst of earthquake activity on Loihi in 1970, another in 1974, and then nothing until the 1991 swarm. But there was another swarm of smallish earthquakes in 1993 and another this year. 鈥淚 think it鈥檚 pumping itself up for a major volcanic eruption,鈥� says Malahoff. If so, he hopes that HUGO will be ready in time. 鈥淵ou never know what a volcano will do,鈥� he says 鈥淭hey certainly keep you on your toes.鈥� (see Map)
