The world of volcanism is not about being equal. There are small volcanoes, there are big volcanoes and then there is Mauna Loa. And until someone actually proves that the Tamu Massif is one single volcano and not a Large Igneous Province or a multiple volcano area I will continue to refer to Mauna Loa as the world’s largest single volcano.
The numbers are just staggering. It is built out of 75 000 to 80 000 cubic kilometers of lava, it is 17 kilometers high (roughly) and it is so heavy that it has pushed down the underlying crust a whopping 8 kilometers. And obviously that translates into both large and numerous eruptions.
Mauna Loa is between 600 000 to 1 000 000 years old and that gives us between 0.12 and 0.075 cubic kilometers of erupted lava each year. Obviously it does not erupt every year, so some eruptions are pretty darn impressive. And before anyone starts to say that all eruptions are large, some of them last for just a day and are rather piddly.
Mauna Loa is so large that it creates its own gravitational field. Due to the volcano being so massive that it is bending the crust it is also forming its own tectonic scheme with a major NE/SW trending rift zone that transects the shallow top caldera. The SW part is 100 kilometers long and that can erupt at all parts of its length (but no eruption has been witnessed that opened the entire rift). The NE part is shorter and has historically only erupted from its top 20 kilometers.
Many people say that Mauna Loa is cyclical and that it shifts between mainly having rift eruptions and mainly having top vent eruptions and that we are currently in a rift eruption part of the cycle. I find this a fair bit of a conjecture that remains to be proven. Of the historically known eruptions 38 percent have been top vent eruptions, 31 percent from the northeast rift, 25 percent from the southwestern rift and 6 percent from vents on the northwest side. There seem to be something to the theory right?
Well, not really. The rift zone is really deep, pretty much as deep as the volcano is high and the eruptions there are feed from dyke injections from a deep seated source. The top vent eruptions are fed from a magma reservoir at roughly 4.6km depth. So, in effect we are looking at 3 different eruption feeding systems that probably are joined at the bottom, and as such we should see those percentages at any given time.
Another thing we need to think about before we ponder the current activity is that Mauna Loa is big. Hm, yes I just said so, but we have to ponder another effect it has. The entire rift is spreading at an average of 10 centimeters annually and basically all of that motion is in one direction. And that direction is against Kilauea. Now the size of Mauna Loa should start to sink in, after all it is shoving poor Kilauea a full decimeter to the side every single year. And this has an effect and creates a cycle of its own. Normally high activity in Mauna Loa means low activity in Kilauea and vice versa, but then comes cycles where they act like they are two mating tubes of toothpaste squeezing toothpaste out of each other.
The 1984 eruption was a co-eruption with Kilauea and the signs that we see now are reminiscent of those seen prior to that eruption.
I have also read lately that Mauna Loa leaves few signs before erupting, that is not true. Both the 1975 and the 1984 eruption was accurately predicted years in advance of the Hawaiian Volcanic Observatory. And there is no reason to not believe that they will be able to predict the next one as accurately, but that the warnings will be issued closer to the eruption this time around.
Classic volcano prediction model
Few volcanoes lends itself so well to be predicted using a classic approach as Mauna Loa, probably due to many of the techniques being developed there due to the hard work of the scientists at HVO.
At Mauna Loa you follow the seismic record to see what the earthquakes can tell us and you watch how the mountain deforms as magma moves in the system. You also need to know how the mountain is deforming due to its weight and size and the spread of the rift zone. If you do not know the last part and take that into account, you will see things that are not volcanic and interpret it as volcanic. You also need to remember that all that “natural” deforming that is going on produces its own set of tectonic and intra-mountain earthquake schemes. As always, most things at a volcano are not volcanic in nature per see and it is important to know what the effects will be due to those things.
Now, imagine that you are a big nice warm blob of magma. You have spent millions of years travelling up the mantle upwelling that forms the stationary hotspot that created the Hawai’ian-Emperor Chain. Your speed of travel is about the speed of a growing nail.
As you softly and slowly hit the underside of Mauna Loa you exert an ever increasing amount of pressure, just as a pillow hitting the underside of a mattress in slow motion. As the pressure becomes too large you start to be squeezed into the magma conduit leading up from the Mohorovic discontinuity (mantle/crust boundary). As you slowly travel upwards you will start to pressure the sides of the conduit apart causing short duration tectonic earthquakes that are easy to recognize on a seismometer since the initial break is so sharp. Here at Mauna Loa there are two ways you can go, either you are crushing the side of the conduit at depth and go upwards as a dyke intrusion into the rift zone or you continue upwards into a magma reservoir.
Either way, as you arrive at the depth where there are aquifers you will start to pressure water out of the way causing a fluid signature to appear on the seismometers as you cause earthquakes. It varies from volcano to volcano when and if this will happen, but if memory serves this start to happen at around 4 to 5 kilometers depth at Mauna Loa.
At around 2 kilometer the water will instead start to flash into superheated dry steam rapidly breaking the rock in small explosions. This shows up as pearl chains of small earthquakes that some people call “popcorn” earthquakes. This is when you might be starting to cause phreatic detonations above you. Imagine, you are causing destruction two kilometers above you? Isn’t that pretty neat! Mauna Loa most often does not suffer from this stage.
If this is a water rich volcano or you are a volatile rich blob of magma you may cause phreatomagmatic explosions as you mix with water and other volatiles and you may have a pre-eruptive stage that contains mixed fresh lava and water. This is not really a phreatic detonation, nor a true eruption, this is called a phreatomagmatic explosion and can be seen as pre-eruptive fluids. This is the main type of eruption if the volcano is covered by a glacier, and Mauna Loa has not had one of these types of events since the last ice age.
Now you are close to the surface at a depth where water will separate as dry steam and other gasses will nucleate and the magma starts to swell rapidly. Now nothing can stop you and you pick up speed rapidly as you joyously travel towards the opening of your choice. This is the stage when we see more and more harmonic tremor indicating rapid movement of magma.
On your way up you have moved the bedrock and that will show itself as deformation and inflation of the mountain. When you are still at the lower parts of the volcano the deformation will be diffuse and widespread, but as you travel upwards it will be more indicative of where you are traveling and different parts of the volcano will start to inflate at different rates. And at the end the inflation will be ever more rapid and place specific, so much so that during the last few seconds prior to an eruption the inflation will be counted in meters to tens of meters per second. Think of a bubble getting ready to burst.
At Mauna Loa the magma is hot and relatively low in volatiles, so it will normally not be explosive. But there are enough of volatiles to cause the lava to form curtain shaped fire fountains that can be pretty impressive.
One last thing to remember about your travels as a magma blob, as you move you will leave a trail of earthquakes that can be plotted. And if someone plots your road of travel it will form a distinct earthquake track. Following tracks like this is like getting a trail of postcards from various stops on the way and it is the best way to follow an eruption before it happens.
The combination of over the time more distinct deformation and inflation put together with the earthquake track gives volcanologists a way to both predict when and where an eruption is most likely to occur. And the closer to the surface the happy magma blob traveler get, the easier your arrival at the terminus will be to predict.
Now that you have a feeling for how you would behave as a happy warm magma blob we are ready to start looking at the activity that has happened in the last few years.
Few people have missed that Kilauea is currently erupting. This has seemingly increased the pace of the previous deformation (and inflation) happening at Mauna Loa. In the last few months magma has slowly moved upwards until the earthquakes has started to transform into bursts of harmonic tremor.
The deformation has also started to be more distinct. This deformation may even have affected Kilauea in return pushing the level of the lava lake up. If so we really have two tubes of toothpaste being very intimate indeed.
Now a bit of a warning here, it is not uncommon that a proto-eruption putters out at this stage, or that it will take quite some time before the magma reaches the point of no return (think several years).
If on the other hand the pressure from below and the joint squeezefest between Kilauea and Mauna Loa continues there will be lava gushing forth in a rare tandem eruption in the not so distant future.
Now over to a composite analysis of the earthquakes that have taken place, as we can see there are two spots on the image below that looks promising. One is the likely spot for an eruption judging from both deformation and earthquake data, and the other is a false positive caused by regular deformation and strain caused by the magma deforming the magma reservoir. Can you spot which one is the probable location?
The answer Is that it is the smudge of earthquakes on the southwestern part that is the current cause for worry in my opinion, the other swarm is most likely compression earthquakes caused by the expanding magma reservoir under the caldera.
So, if we sum it up. We have deformation that over time has become more distinct, a formative stack of earthquakes that is likely to be caused by moving magma, we have a gradual shift in earthquake types, and now we have intermittent harmonic tremor (that has been going for a while).
If we take all of these things into account we see that it is likely that an eruption will occur and that it is most likely to occur at southwestern rift zone, or it could occur at the caldera (due to evidence of an expanding magma reservoir).
Activity could though stop or delay, and the question is also at exactly what depth has the magma risen to, is it close enough soon to the surface to be buoyant enough to be unstoppable? Because, when that happens things will happen quickly.
My personal opinion is that it could happen quite soon, especially if we take into account that Kilauea is being squeezed so hard now that the famous lava lake is rising. Perhaps time soon to plan for a dual lava watching holiday.
CARL REHNBERG (links and aid by JESPER SANDBERG)