Mauna Loa keeps paradise interesting. It has erupted 33 times since 1843, with large eruptions happening on average once every 8 years. Over that time it has covered its slopes with 4 km3 of new lava. But those are just the most recent stirrings. Its older lava flows cover over half of the island of Hawaii. But after the large eruption in 1950, Mauna Loa became remarkably quiet. There have been only two eruptions since, one of which (1975) lasted less than one day and was the second smallest eruption of the 20th century. There was one significant eruption in 1984, but nothing since. But now the largest volcano on Earth is stirring. Inflation shows that magma is accumulating, just south west of the peak. Earthquakes are a daily occurrence and HVO has raised the alert level. There is trouble brewing in paradise.
Mauna Loa is the world’s second tallest volcano on Earth, after Mauna Kea. It started 5 km below the sea, taking perhaps a million years to reach its current height of 4.17 km above sea level. The total height, from base to summit, is therefore over 9 km, more than Mount Everest. To compare, the highest volcano on Earth is Ojos del Salado, on the Chile/Argentina border, which reaches 6,893 m above sea level. Mauna Loa is slightly cheating since the part that is under water benefits from the upward pressure from the water – it floats a bit. I can correct for this, and calculate how high a mountain would be that has the same pressure at its base, without the aid of water: this reduces Mauna Loa by 1800 m, and gives an equivalent height of 7200 m, almost the same as Ojos de Salado. This may not be entirely accidental. There is only so much weight a rock can bear, and both volcanoes will consist of very similar types of rock at their base. Rocks at the bottom, 9 km below the summit, are in danger of being crushed by the weight. On Mars, gravity is three times less and the same weight corresponds to a mountain three times taller – 22.5 km tall. In fact, Olympus Mons stands 22 km above the surrounding plain, almost exactly this number. So it seems this is indeed about the tallest a volcano can get. Mauna Loa and Mauna Kea are almost the same height, differing by only 24 meter. They are running into their limits.
Although it is slightly lower, Mauna Loa is a lot bigger than Mauna Kea. Mauna Loa is a shield volcano: its fast flowing lava has spread far, building a large mountain with a shallow slope. It is much wider at its base than Mauna Kea, has much more volume and therefore much more mass. Mauna Loa, the largest volcano on Earth, is so large, it is very hard to see. You can’t see it above the horizon: it IS the horizon. I remember reading the story of someone in Africa trying to chase an elephant out of their garden at night. In the flash light she failed to see any elephant, just the greyish sky. Only then did she realize the sky was the elephant, so large the eye couldn’t see it. Mauna Loa is this proverbial elephant. The only place from where you can get a good feeling for its tremendous size is from the summit of Mauna Kea (not an easy place to get to either.) From here, at night sometimes you can see the distant, angry eye of Pu’u’O’o, but in the day Mauna Loa dominates the sky line. (Unexpectedly for an astronomical observatory, you can’t see the stars that well from Mauna Kea. It is too high and the eye and the brain are badly affected by the lack of oxygen. You become half blind and too dim to realize.)
The top of Mauna Loa has its crater, called Moku`aweoweo Caldera. It consists of three partly overlapping craters, of which the central one is the largest. Together they are 6 by 2.5 km in size. The caldera isn’t that old. It formed because of a large flank eruption which emptied the shallow magma reservoir. This was the eruption which formed the Pana`ewa flow field, which Hilo is build on.
Mauna Loa is a very elongated mountain, much longer in the south west-north east direction. In fact, the name Mauna Loa means ‘long mountain’. The long ridge follows a double rift zone. There is a caldera at the top (showing the mountain used to be a little higher); the rifts extend from the caldera towards the south west and towards the east, running from the south eastern point of Hawaii to (almost) Hilo, a distance of close to 100 km. The south west rift bends by 40 degrees where it reaches an altitude of 2400 m. At this point a number of eruptions have build a satellite shield.
Mauna Loa eruptions tend to begin near the summit, but quickly migrate to the rift zones, down slope. Both rifts can erupt anywhere along their length. Individual rift eruptions can occur along a section as short as tens of meter, to a staggering 20 km (as happened during the large 1950 eruption). Individual eruptions typically last a week, erupt 0.2 km3 of lava and cover 20 km2. The lava moves fast, covering up to 5 km per hour on the steepest slopes, and can reach the sea (on the west and south side, at least), in less than a day.
The 1984 eruption is a good example. It began three years earlier, with slowly increasing earthquake activity, culminating in an M6.6 event. About 5 months after this, 25 March 1984, the eruption began, after 2 hours of tremors, initially at the summit, but within hours the eruption migrated first to the south west rift zone, changed its mind and moved to the north east rift zone. A few hours later, lava curtains erupted 7 km from the summit, and later that day, the eruption moved to a 2 km section 10 km from the summit. From here lava flows quickly advanced to Hilo. Eventually the flow reached a length of 30 km, coming to within 7 km of Hilo. As the eruption diminished, the active lava stayed closer to the point of eruption. After some tense days, the eruption ended on April 15.
Hawaiian Archipelago: trail of a hot spot
Hawaii is at the end of a long chain of islands and sea mounts. Close to Hawaii are the Windward islands, or Hawaiian archipelago. Further away are the Leeward islands, terminating near Midway. Beyond that, the chain becomes a series of sea mounts, which makes an angle of 120 degrees with the other chain: these are called the Emperor sea mounts (named after Japanese emperors). The mounts get progressively older further from Hawaii, with the oldest one about 80 million years. The chain terminates at the Obruchev Rise, very close to the Aleutian subduction trench, where it meets Kamchatka. There may have been more, older sea mounts, now subducted into oblivion.
The Hawaiian chain is the type specimen of a hot spot chain. The hot spot is supposed to have been stationary or near stationary, while the Pacific plate drifted over it. The sharp bend in the chain shows a sudden change of direction of the Pacific plate, about 45 million year ago. Previously, the plate was going north, but now it is moving to the north west. What caused this change? That is not really known. It happened at about the time India collided with Asia, and perhaps this is related although that was quite a distance away. It may also have been caused by the onset of subduction along the Asian plate boundary. After the change of direction, for a few million years no islands were formed.
Hot spots may come from plumes deep into the mantle, or they can be shallower, affecting mainly the upper mantle. Opinions differ. In the case of Hawaii, seismographs, used to map out the mantle, have detected the hot region to 1500 km depth, perhaps more. That is half the depth to the core-mantle boundary. It seems likely it extends to the bottom of the mantle. It came from the deep. But other data do not agree, and a more recent study has only found a pancake of heat, underneath the crust, not extending into the deep.
In any case, not all the lava it erupts comes from the deep mantle. It seems that over time, Mauna Loa has erupted a decreasing fraction of 3He, and that is a sign that a fraction of recycled crust is being included in the magma. Perhaps as the hot spot is moving away (and Mauna Loa is no longer directly over it, as it was when it began), the magma includes an increasing amount of shallower melt.
How and where did the hot spot get started? Its origins have sadly been lost in the subduction trench. Or perhaps not. Could the hot spot have started at the Obruchev Rise? The Rise is near an old spreading centre, abandoned in the Cretaceous when the centre jumped north. The ridge was at the western edge of the Fallaron plate, which has since largely been subducted underneath America. This puts the hot spot, at its earliest known location, directly underneath this spreading ridge. Much like Iceland! Did the spreading centre start the plume? Or did the plume split the plate? This is solidly in the realm of speculation. But it is interesting to think of tropical Hawaii starting off like Iceland. Perhaps there is hope for Reykjavik.
During its 80 million year history, the eruption rate has been remarkably constant, at about 0.015km3 per year. For comparison, over the past 200 years the eruption rate has been about 0.023km3 per year from Mauna Loa. Kilauea’s eruption rate should be added, which doubles the amount. So the current rate of activity is a bit above average for the hot spot.
Volcanoes provide a very temporary surface and are not the best place to build expensive structures. A good example was the Etna volcano observatory, build too close and unfortunately destroyed by the lava they were trying to observe. They wisely relocated to Catania, for safer viewing. It is surprising to find a scientific institute located near the top of Mauna Loa, and even more surprising that it is not there to study the volcano. The Mauna Loa Observatory studies our air and our Sun – not the shaky ground underneath their feet.
The Observatory is located 5 km north of the summit, 700 meter below it. It is so high up in order to stay away from any pollution (natural or otherwise) coming up from below. The inversion layer in the atmosphere normally keeps that locked up. The air comes in from the sea, again as clean as anywhere on Earth. It is an ideal pace to measure how our air is changing. Since 1956 the amount of CO2 in our atmosphere has been measured. There are several places around the world where this is done, but Mauna Loa has the cleanest record. The buildings, offices and domes are build on stark, black lava fields and even after 50 years, gives an impression of a temporary incursion in an unforgiving land. The total lack of any vegetation is an advantage, as plants affect the local CO2. The famous Keeling curve, showing how rapidly we are changing our atmosphere, is measured here, a monument to our fragility and willingness to take inordinate risks. It fits the environment, but if Mauna Loa were to erupt this direction, the scientists may only have an hour to get away. There is also a solar observatory here. In general, only astronomers put their most expensive instruments on top of active volcanoes. A certain disregard of the world they live in may play a role. There is a massive astronomical observatory on Mauna Kea. Again, their only access road goes over the recent flows from Mauna Loa. One could question the wisdom of building the one escape route where it is most likely to be cut off.
The mid-life crisis of Mauna Loa
I mentioned that Mauna Loa seems to be losing its hot spot. How do we know? Part of the evidence comes from the changing isotopes, the reducing fraction of 3He compared to 4He. But there is a more direct indication. Mauna Loa has stopped growing.
Mauna Loa’s lava covers half of Hawaii, to the coast and into the sea. The eruptions still often reach the sea, at least on the west side of the island, but they only rarely reach the Hilo area, even though this area is entirely build on Mauna Loa’s lavas. Over the past 100,000 years, the lava flows have not been as vigorous and have not reached as far as previously. Still, Hilo should not be complacent. In 1880 lava reached with 2 km of Hilo Bay.
How far lava can flow depends on the cooling rate. The flow stops where the lava solidifies. The higher the flow rate, the slower the cooling (large bodies stay warm for longer), and the further the lava flows reach. This process is very clear in Kilauea. When the eruption rates go up, the lava flows extend, as they did last year. When they decline, lava stays closer to the point of eruption, mostly less than 5 km this year. Holuhraun did the same thing: when the flow began to diminish, the flow field stopped expanding. Lava from Mauna Loa now rarely reaches Hilo Bay, perhaps only once per 4000 year. It mostly stops 5-10 km from Hilo. One flow did cover the entire urban area and the bay, perhaps 1000 year ago, but this one erupted from a vent very close to Hilo to begin with. Most of the lava now flows closer to the rift, and less lava reaches the sea. This makes the mountain grow steeper: it is beginning to enter its post-shield phase. This may be related to the growth of Kilauea, competing with it for lava resources.
The weight of the mountain pushes the crust below down. The whole island subsides, at around 2 mm per year. So far lava deposition has just about kept up with this, but it may not for much longer. The baton is being passed and Mauna Loa will sink.
The 30-year silence of Mauna Loa has coincided with the continuing eruption of Kileauea at Pu’u’O’o. This eruption started in 1984 and is still going. But eruption rates have slowly declined. The central crater of Kilauea shows inflation and this may be due to an ebbing flow through to Pu’u o’o. It is argued that Mauna Loa and Kilauea are separate volcanoes, from two different hotspot tracks, Kilauea connecting to Mauna Kea. This seems less likely, and although the feed systems are not identical, they are not independent either – they are too close together. The question will be settled soon: if they are independent, a Mauna Loa eruption would not affect Kilauea. If there is connection between them, you would see Kilauea changing when Mauna Loa erupts. Will in the future Kilauea take over and rival Mauna Loa in size? Perhaps they really are too close. There is another candidate waiting out at sea, which is more likely to become the next giant.
After 20 years, activity underneath Mauna Loa resumed in 2004, with a series of deep earthquakes. The mountain began to inflate at the same time. Things calmed down again, but in 2014 earthquakes moved to shallower levels, and inflation increased. Magma is now accumulating at 3-5 km depth, below the south west rift. Activity is continuing at an elevated but non-critical level: it has all the appearance of building up to an eruption, but it does not appear to be imminent. Will it be a year, five years, longer? After the previous burst, in 2004, nothing happened for a decade. The current episode could equally go off the boiler. But the magma is now shallower, and the inflation more focussed. This is beginning to look as if it is closer to deciding to erupt.
An eruption is likely to start within hours of strong tremors. Initially it will be close to or at the summit, before rapidly migrating. As the current inflation is a little south west, the most likely migration is in this direction. The lava could flow either side of the ridge. For a normal-size eruption, Highway 11 will be cut within one or two days, possibly at several locations if the eruption is large, and lava will flow into the ocean shortly after. Judging from the time line of the various flows since 1843, Kealakekua Bay would seem next but this is guess work – Mauna Loa does not work like that. As the magma chamber empties, the risk of larger, possibly damaging earthquakes increases. The south side of the mountain is prone to slipping and the risk of this increases when the pressure changes. Conversely, earthquakes can also affect internal magma flows and an earthquake can hasten or stop an imminent eruption. The double 1868 earthquake (M7.1, M7.9) disrupted the magma supply for decades.
Purely based on the past record, there is a 50% chance of an eruption within 8 years. I would expect something within a decade, most likely from the south west rift zone. When it happens, it would be advisable to run away, and not, as Hawaiians normally do, run towards the eruption to get a better view. Mauna Loa is not like Kilauea. It is fast and furious, best viewed from a large distance. Paradise can wait.
Hawaiian Volcano Observatory; Much of the material and several figures in this post came from them
Hawaii’s island chain, with exam questions at the end! Source of some of the figures and material used in this post