There has been much talk about Mauna Loa lately. Reason for this is that the earthquake levels have become increasingly elevated in past years, and even more so within the past few months, with some alarming episodes of earthquake activity at the summit. We’ve had two strong swarms coupled with rapid inflation of the caldera, which happened in August and September. Each episode amounted to 5 mm of caldera extension as measured by HVO. This may not sound like much, but they did happen very fast which caused some substantial shaking at the summit of the volcano.
Mauna Loa has always created much interest, and even excitement. It is said to be the largest active volcano of the world after all, and that should count for something. Its eruption of 1950 had become almost a legend among fissure eruptions, and for a good reason. And yet Mauna Loa has been dormant for 38 years, which is a lot for a supposedly very productive shield volcano. Other shield volcanoes like Piton de la Fournaise, Kilauea, Etna, or Fernandina, have been far more active lately. Thing is that the activity of Mauna Loa is very variable over time. This is very obvious even for the past 200 years.
Mauna Loa is in its longest historical dormancy. At its lowest activity. But how did it look when it was at its peak? It has been over a century since then. We do have historical records of a time when Mauna Loa threw one big eruption after another. I will look into what to me is one of the most interesting times at Mauna Loa, the 1852-1868 interval.
At that time, there was a man called Titus Coan. He lived in Hilo, a town located on the slopes of Mauna Loa. Coan was a missionary but also had a burning passion for volcanoes. He never lost a chance to visit an active lava flow, and frequently sent observations on the activity of Mauna Loa and Kilauea to geologists of the time. Many descriptions in this article will come from him. These descriptions are mostly excerpts from the book “The Volcanoes of Kilauea and Mauna Loa on the Island of Hawaii: Their Variously Recorded History to the Present Time”, written by the geologist William Tufts Brigham, which has many other interesting passages as well. Some information also comes from the amazing “Catalogue of Hawaiian earthquakes, 1823-1959” by Fred W. Klein and Thomas L. Wright, particularly concerning the 1868 eruption. The account of Abraham Fornander in 1868 comes from the Hawaiian Gazette.
The eruption of 1852.
There had been a relatively small eruption in August 1851. This eruption came from the summit caldera of Mauna Loa, Mokuaweoweo and propagated into the uppermost Southwest Rift Zone. The dike that fed the eruption probably followed a similar path as more recent events in 1940 and 1949. This eruption was brief. It lasted 3-4 days.
It took only six months for Mauna Loa to rebuild its pressure, and erupt again in February 1852. It is normal that small summit eruptions are closely followed by another eruption, given that they don’t release as much pressure as larger rift eruptions. Still, the dormancy between the 1851 and 1852 eruptions is the shortest in historical time, and likely shows what was a very large influx of magma.
While the 1851 eruption had been on the opposite side of the mountain, the 1852 outbreak was in full view from Hilo. And thus Coan writes:
“At half-past three on the morning of the 17th ult. a small beacon light was discovered on the summit of Mauna Loa. At first it appeared like a solitary star resting on the apex of the mountain. In a few minutes its light increased and shone like a rising moon. Seamen keeping watch on deck in our port exclaimed, ” What is that ? The moon is rising in the West!” In fifteen minutes the problem was solved. A flood of fire burst out of the mountain, and soon began to flow in a brilliant current down its northern slope. It was from the same point, and it flowed in the same line, as the great eruption which I visited in March, 1843. In a short time immense columns of burning lava shot up heavenward to the height of three or four hundred feet, flooding the summit of the mountain with light, and gilding the firmament with its radiance. Streams of light came pouring down the mountain, flashing through our windows, and lighting up our apartments so that we could see to read large print. When we first awoke, so dazzling was the glare on our windows that we supposed that some building near us must be on fire ; but as the light shone directly upon our couch and into our faces we soon perceived the cause. In two hours, the molten stream had rolled, as we judged, about fifteen miles down the side of the mountain (actually it was probably half this distance or less). This eruption was one of terrible activity and surpassing splendor, but it was short. In about twenty-four hours, all traces of it seemed to be extinguished.”
Here Coan describes the typical Mauna Loa eruption onset. The dike intrusion will start at the summit where a complex of small magma chambers is located. The first fissure usually opens at high elevations on the mountain. Initial eruption is vigorous, long rows of lava fountains shoot from fissures, and floods of thin fluid lava encroach substantial areas, Lava makes glassy pahoehoe near the vents, but turns to thin sheets of scoriaceous aa lava downslope. The eruption in 1950 was an exaggerated version of this kind of activity.
The eruption was not over, the dike was making its way underground. It took about three days before the dike broke out from an elevation of 2560 meters in the Northeast Rift Zone. No earthquakes are reported to have been felt in Hilo or anywhere else on the island during the eruption or dike propagation. So any seismic activity must have been relatively weak. Once again, Coan describes:
“At daybreak on the 20th of February, we were again startled by a rapid eruption bursting out laterally on the side of the mountain facing Hilo, and about midway from the base to the summit of the mountain. This lateral crater was equally active with the one on the summit, and in a short time we perceived the molten river flowing from its orifice direct towards Hilo. The action became more and more fierce from hour to hour. Floods of lava poured out of the mountain’s side, and the glowing river soon reached the woods at the base of the mountain, a distance of twenty miles (more like 4-5 miles in reality, 20 miles would put it in the very outskirts of Hilo!). Clouds of smoke ascended and hung like a vast canopy over the mountain, or rolled off on the wings of the wind. These clouds assumed various hues, —murky, blue, white, purple or scarlet—as they were more or less illuminated from the fiery abyss below. Sometimes they resembled an inverted burning mountain with its apex pointing to the awful orifice over which it hung. Sometimes the glowing pillar would shoot up vertically for several degrees, and then describing a graceful curve, sweep off horizontally, like the tail of a comet, further than the eye could reach. The sable atmosphere of Hilo assumed a lurid appearance, and the sun’s rays fell upon us with a yellow, sickly light. Clouds of smoke careered over the ocean, carrying with them ashes, cinders, charred leaves, etc., which fell in showers upon the decks of ships approaching our coast. The light was seen more than a hundred miles at sea, and at times the purple tinge was so widely diffused as to appear like the whole firmament on fire. Ashes and capillary vitrifactions called Pele’s hair fell thick in our streets and upon the roofs of our houses. And this state of things still continues, for even now while I write, the atmosphere is in the same yellow and dingy condition; every object looks pale, and sickly showers of vitreous filaments are falling around us, and our children are gathering them.”
Coan made an ascent towards the eruption site. For four days his party cut their way through thick forests with a long knife, and walked through jagged fields of scoriaceous aa lava. Eventually they reached the eruption site on the 27th.
“After half-past three p. m. I reached the awful crater and stood alone in the light of its fires. It was a moment of unutterable interest. . . I was ten thousand feet above the sea, in a vast solitude untrodden by the foot of man or beast; amidst a silence unbroken by any living voice, and surrounded by scenes of terrific desolation. Here I stood almost blinded by the insufferable brightness; almost deafened by the startling clangor; almost petrified with the awful scene. The heat was so intense that the crater could not be approached within forty or fifty yards on the windward side, and probably, not within two miles on the leeward. The eruption as before stated, commenced on the very summit of the mountain, but it would seem that the lateral pressure of the embowelled lava was so great as to force itself out at a weaker point in the side of the mountain ; at the same time cracking and rending the mountain all the way down from the summit to the place of ejection. The eruption first issued from a depression in the mountain, but a rim of scoriae two hundred feet in elevation had already been formed around the orifice in the form of a hollow, truncated cone. This cone was about half a mile in circumference at its base, and the orifice at the top may be three hundred feet in diameter. The eruptions were not intermittent but continuous. Volumes of the fusion were constantly ascending and descending like a jet d’eau. The force which expelled these igneous columns from the orifice, shivered them into millions of fragments of unequal size, some of which would be rising, some falling, some shooting off laterally, others describing graceful curves; some moving in tangents, and some falling back in vertical lines into the mouth of the crater. During the night the scene surpassed all powers of description. Vast volumes of lava, at a white heat shot up continuously. . . A large fissure opening through the lower rim of the crater gave vent to the molten flood which constantly poured out of the orifice and rolled down the mountain in a deep, broad river at the rate, probably, of ten miles an hour.”
The eruption of 1852 was somewhat unusual. The lava carried many olivine crystals, which is called a picrite. Of the historical eruptions of Mauna Loa only the 1868 eruption was more picritic than 1852. These olivines had perhaps settled at the bottom of a magma chamber that at the time existed at the summit of the volcano.
Another oddity was the strong fountaining. The circumference of a volcanic crater roughly approximates the height of the lava fountains that have formed it, which is something I found out by comparing historical eruptions with known fountain heights to their craters. The vent of the 1852 eruption must have formed by fountains reaching 200-250 meters in height. One contemporary description estimated the fountains to range from 70 to 210 meters in height. In a later ascent, Coan also describes the large amount of pumice that the eruption produced:
“We found it ten miles from the crater, and it grew more and more abundant till we reached the cone, where it covered the entire region to a depth of five or ten feet.”’
The summit outbreaks of 1940 and 1949 may have had slightly higher fountains, however the high fountains of the 1852 eruption were probably sustained for much longer, probably for ~20 days that the eruption lasted. And the amount of pyroclastic material erupted in 1852 I think might well be the greatest among historical fissure eruptions of Mauna Loa.
Estimates from a publication called “Holocene eruptive history of Mauna Loa Volcano, Hawaii”, put the volume around 0.18 km3 or 182 million cubic meters. That makes the 1852 a more or less “normal sized” rift eruption.
There was a large earthquake, estimated at M 6.2, felt in Hilo, 20 days after the eruption ended. It is hard to know if it was or was not related to the volcanic activity. It is possible that it was a flank slip in response to the dike that had been intruded.
The eruptions of 1855 and 1859
After a dormancy of 3.4 years, Mauna Loa erupted again. 1852 had been a very deep draining event of the volcano, so it is remarkable that it erupted in such short notice. In comparison, 38 years have passed since the low elevation Northeast Rift Zone eruption in 1984, and while Mauna Loa is probably near erupting, we still don’t know how many more years it may take. Given that 1852 reduced at least as much pressure as 1984 did, then we can see that the supply in the 1850s was probably over ten times higher than it has been since the 1984 eruption.
The next eruption was that of 1855. This eruption was also in the Northeast Rift Zone but was of a very different character to its predecessor. 1855 is the longest flank eruption of Mauna Loa, it lasted 15 months. This difference was at least partly a matter of elevation. While the 1852 eruption happened at 2560 meters, the 1855 eruption happened at 3200 meters. It could also be a matter of the 1852 eruption having filled up the rift, allowing for the 1855 eruption overflowing more and becoming sustained. Like the 1852 eruption, no earthquakes are reported at the time of the 1855 eruption.
After the initial more violent fissure stage, the 1855 eruption stabilized and sent lava into tubes that fed a slowly growing field of pahoehoe lava. The lava came quite close to Hilo, rising concerns, but stopped before doing so.
Because of the topography, most Northeast Rift Zone lava flows will head towards Hilo, but the vast majority stop before doing so. Historically, only the 1880 lava flow would have hit part of Hilo at its present size, and before 1880 the earlier lava flows to have reached the present-day location of Hilo were two closely spaced events around 500 AD.
Coan visited the 1855 lava flow many times. Here he describes the lava tubes feeding the flow. The orifices he speaks of are what we now know as lava tube skylights:
“Early on Saturday, the 6th, we were ascending our rugged pathway, amidst steam and smoke and heat which almost blinded and scathed us. At ten we came to open orifices down which we looked into the fiery river which rushed furiously beneath our feet. Up to this we had come to no open lake or stream of active fusion. We had seen in the night many lights, like street lamps, glowing along the slope of the mountain at considerable distances from each other, while the stream made its way in a subterranean channel, traced only by these vents. From 10 a.m. and onward, these fiery vents were frequent, some of them measuring ten, twenty, fifty, or one hundred feet in diameter. In one place only we saw the river uncovered for thirty rods and rushing down a declivity of from ten to twenty-five degrees. The scene was awful, the momentum incredible, the fusion perfect (a white heat), and the velocity forty miles an hour. The banks on each side of this stream were red-hot, jagged and overhanging, adorned with burning stalactites and festooned with immense quantities of filamentose or capillary glass, called Pele’s hair. From this point to the summit crater all was inexpressibly interesting. Valve after valve opened as we went up, out of which issued fire and smoke and brimstone, and down which we looked as into the caverns of Pluto.”
Using lava thicknesses which I measured in Google Earth with the help of elevation contours I got a rough estimate of the volume erupted in 1855. The approximate volume is 0.22 km3 or about 217 million cubic meters. Parts of the flow are buried under 1880 lava though and those might add a bit of volume. As such it is among the larger eruptions of Mauna Loa.
Following the 1855-56 event, 2.1 years elapsed before the next outbreak in 1859. The 1859 eruption was unusual because it did not happen along the rifts but actually on the north flank of the mountain and is thus referred to as a radial eruption. Once again the eruption had no felt seismicity on the island. The eruption started at the north side of the summit on January 23, presumably in the uppermost Northeast Rift Zone (NERZ). Soon, more fissures opened down the mountain to the west.
Initial eruption was vigorous. A large open channel of lava travelled 49 kilometres in ~7 days, plunging into the ocean. It reached the northwest coast of the island in Kiholo Bay. It is quite rare for lava flows to reach the northwest coast, and judging from radiocarbon data, this was the first time this happened in ~3000 years. A radial eruption in the north flank is in itself unusual. Only two radial fissures have opened in the north flank of Mauna Loa since the extensive summit overflows of 200-800 AD, 1859, and an event that probably happened in the 17th century.
The 1859 developed into a sustained effusion similar to 1855. Lava built a field of tube-fed pahoehoe that slowly crept towards the coast of the island and travelled those 49 kilometres once again, but this time more slowly. It formed a small lava delta by the sea. Eruption lasted 10 months. It is the second-longest flank eruption in historical times after 1855.
I have estimated the volume of the 1859 eruption at 0.36 km3 or 363 million cubic meters. I did so by measuring the lava delta thickness, and also the average thickness across different areas of the flow. I think this estimate might be more spot on than other estimates of subaerial volume. I did however miss the upper NERZ lava outbreak, which might raise the volume by a tiny amount, only later realized where this initial flow was located. The 1859 eruption is probably the largest historical flank eruption of Mauna Loa.
Two different fissure systems fed the 1859 eruption, which are offset from each other. This is strange. Normally fissures from a dike line up quite well. Eruptions in Piton de la Fournaise and the Galapagos often have offset fissure systems because they are not fed by normal dikes, instead intrusions there start as horizontal or inclined sheet of magma that rotate into vertical dike segments, sometimes more than one. So it is an interesting possibility that the 1859 eruption may have been fed by a more complex intrusion similar to those of the Galapagos. Hawaiian rifts are under strong tensional stresses due to flank slip. But 1859 was beyond the rifts. Perhaps it propagated through an area of different stress leading to a different intrusion geometry.
One might ask why exactly did the 1859 eruption go radial. There must be a reason for everything, after all. I think I may know the reason. 1852, 1855, and 1859 can all be considered Northeast Rift Zone events. It is the only time in which multiple NERZ events have taken place in a row. 1852 found a rift under strong tensional stresses and travelled far downrift, more so than other historical events. 1855 instead found a more compressed rift after the 1852 intrusion and erupted higher up. 1859 may have found the rift so compressed as to be un-intrudible and was simply deflected out of the rift onto the north flank.
Questions breed more questions. Why didn’t the volcano send the intrusions into the Southwest Rift Zone, or simply kept them within the summit? The volcano has different magma chambers under the summit, which are evidenced by the morphology of the summit caldera, Mokuaweoweo, and the late historical eruptions. The largest pit in the center of Mokuaweowo is the Inner Pit, now buried by lava flows, it usually produces eruptions in the summit caldera but it may be able to send them into the Northeast Rift, and rarely in radial directions down the west flank. Three pits lie to the south, the South Pit, Lua Hohonu, and Lua Hou, they are underlain by magma storage, which has produced long dikes into the Southwest Rift Zone (1950 and 1926 for example). And at the northern end of Mokuaweoweo lies the North Pit, which last seems to have been the source of a 1942 intrusion that went into the Northeast Rift, although I wouldn’t be surprised if it participated in the 1984 event together with the Inner Pit. If the 1852, 1855 and 1859 were sourced from the North Pit, it would explain why they all went into the Northeast Rift, and why 1859 was then deflected onto the north flank.
Questions breed even more questions. Why would the North Pit have sourced three intrusions in a row? As I will show in a future article at some point, Mokuaweoweo collapsed around 1710, destroying all five main magma chambers Mauna Loa had at the time, including the North Pit storage. All upper NERZ (Northeast Rift) prehistoric eruptions are cut by the caldera faults of Mokuaweoweo. The Inner Pit was probably the first to reactivate and erupt onto the deepest part of Mokuaweweo after 1710. The SWRZ had also reactivated around 1800 AD, producing two eruptions far down the SWRZ. But the NERZ may not have seen much, or any activity, until 1843. It is possible that the North Pit plumbing was recovered just before 1852, and that it then went to produce some sort of initial rampage of activity as this portion of the plumbing was “uncorked”. Or at least so goes my far-fetched speculation.
Considering the reported eruption volumes of 1852, 1855, and 1859, together with an estimated 1851 erupted volume of 35 million cubic meters, then 0.8 km3 were erupted from Mauna Loa in the 1850s. Doing a rough approximation of the dike intrusion volumes as 2 km deep sheets that are 2 meters thick and as long as needed to reach the lowest eruptive fissure, then the volumes of the 1851, 1852, 1855 and 1859 dike intrusions are respectively 24, 79, 52 and 48 million cubic meters. That would put the total volume of magma supplied to Mauna Loa in the 1850s at almost exactly 1 km3, similar or maybe just slightly lower than the magma influx Kilauea has shown during its long 1983-2018 Pu’u’o’o eruption. A Hawaiian volcano at its peak activity.
To be continued: The 1868 eruption