What will you do when Earth’s largest active volcano erupts?

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In 2019, the Volcano Alert Level for Mauna Loa was elevated from “NORMAL” to “ADVISORY” due to increased seismicity and deformation at the volcano. This alert level does not mean an eruption is imminent, but it is a fact that Mauna Loa, which has erupted 33 times since 1843 (most recently in 1984), will erupt again. What will you do when it does? USGS Hawaiian Volcano Observatory geologist Frank Trusdell spoke about Earth’s largest active volcano in this Volcano Awareness Month program presented at Puʻuhonua O Hōnaunau National Historical Park on January 29, 2020. He talks about the current status of Mauna Loa, as well as potential volcanic hazards based on past eruptions, and describes how HVO is preparing for the next eruption of Earth’s largest active volcano.


Date Taken:

Length: 00:41:29

Location Taken: HI, US

Video Credits

Video Production: Katherine Mulliken, Geologist, USGS Hawaiian Volcano Observatory, kmulliken@usgs.gov

Transcription: Janet Babb, Geologist, USGS Hawaiian Volcano Observatory, jbabb@usgs.gov


Speaker:  Frank Trusdell, USGS Hawaiian Volcano Observatory


Thank you for coming. Afterwards if you have questions about those maps or any other questions, once the presentation is done, I’ll be here to entertain them.

Alright, so we're going to talk about Mauna Loa. I’m going to assume that people don't know anything about Mauna Loa. So, I'm going to start very basic. Then we’ll move through talking about its history, and then we'll talk about some of the potential hazards. Then we'll look at what's the current status of the volcano.


The Big Island of Hawaiʻi is comprised of five volcanoes. In the north is Kohala, and Kohala last erupted about 60,000 years ago. The next oldest volcano is Mauna Kea, and it last erupted about 4,000 years ago. Hualālai, last erupted in 1800–1801; and yet in 1929, it tried to erupt, and it failed.  Mauna Loa last erupted in 1984, and Kīlauea, as you know, last erupted in 2018. Lōʻihi is the submarine volcano, the next potential Hawaiian Island.


Mauna Loa comprises 51 percent of the surface area of the Island of Hawaiʻi. By itself, it's almost as large as all the rest of the Hawaiian Islands put together.


Where can lava effuse from on the mountain? Where can be the potential sources?  We're going to look at the structure of Mauna Loa.


At the summit of Mauna Loa, is the caldera. The caldera is roughly 6 km by 3 km (3.7 mi by 1.9 mi). These vents shown in blue are fissures from the summit. Mauna Loa has a northeast trending rift zone that heads off in this direction and lava flows from this volcano, and the distal part of the rift zone are underneath the younger flows from Kīlauea. Mauna Loa’s dominant rift zone is this south-southwest trending rift zone, and it extends for another 30 km (18.6 mi) offshore. But Mauna Loa’s anomalous vent system, or potential hazards from Mauna Loa that other Hawaiian volcanoes do not have, are this class of vents up here, called radial vents. We call them radial vents because, if you envision the summit caldera as the hub and the radial vents are the spokes, like on a bicycle wheel, they radiate away from the summit caldera in the north and northwest and west flanks.


In 2002, I was on an oceanographic cruise, and we mapped 11 additional radial vents just offshore here, of Kealakekua Bay, extending all the way down to Hoʻokena. These radial vents can occur from the submarine environment, all the way up to the caldera. If these vents pop out at low elevations, then your response time to an eruption is foreshortened.


In 1877, there was one such radial vent out here in Kealakekua Bay. These are some accounts of what happened. In the summit, there was a large cloud that rose up very rapidly from the summit and went to an elevation of 14,000–17,000 feet (4267–5182 m).

I just concluded a study on the summit of Mauna Loa, looking at explosive deposits that are surrounding the caldera. When Wilkes went there in 1841 to look at the summit caldera and map, where he set up camp, he said there was just flow after flow. That's all he could see. If those of you who've been to the summit caldera and hiked to where the Wilkes camp is, it's right in the vicinity of the summit cabin. It's like you're walking down a beach. There are these sandy deposits that are there. So, we presume that during this eruption, that that material was deposited there. Subsequent to the summit phase of the eruption, there was a series of earthquakes. Then the people recorded a remarkable bubbling was seen in the sea 3 miles (4.8 km) south of Kealakekua and 1 mile (1.6 km) from shore. People went out there in canoes, and they found blocks of lava 2-feet (0.3 m) square that came up from below. Then they said it was red hot, and there was a sulfur smell in the air and fish had died. Some of these specimens were collected. If any of you guys know where those specimens are, we'd like to have a sample.


[audience laughs]


Anyway, in that same oceanographic cruise, we were offshore here, and we were able to go and map out what we think is the 1877 vents.


Here are pillow lavas from those vents. You can see they don’t have any heavy sediment on them. When we picked up the samples, they were very glassy and fresh-looking, they didn’t look altered.


Then we stumbled upon here, which looks like a vent with some drain-back.


Now we're going to look at the historical eruptions of Mauna Loa. The historical eruptions are the most well-documented. People were here that actually wrote these down and charting what happened during the course of these eruptions. The first eruption that we can find evidence of on the ground is 1843. The summit caldera is the source for about half of the eruptions.


That means, the summit, there's a breakout in the summit, lava flows will fill the caldera. Then some flows may actually leak out of the caldera, either to the north or the south, because that's where the topographic low is. But these lava flows don't generally threaten any communities.


The Northeast Rift Zone is the source for about 25 percent of Mauna Loa historical eruptions. You can see that the flows emanating to the north basically buttress up against Mauna Kea and there's a shallow valley-like structure between the two volcanoes. So, when the flows hit this natural drainage, they turn and head downslope in the direction of Hilo. The Southwest Rift Zone is the more dominant one, and it has flows that have basically gone to both sides.


If you look at the northeast rift, there seems to be a majority of the flows heading towards Mauna Kea. On the Southwest Rift Zone, we have flows on both sides and about 20 percent of the eruptions go down that rift zone.


There are three radial vent eruptions, probably the one that most people are going to be familiar with is the 1859. Started up here at 11,000 feet (3353 m), behind Hualālai in the saddle but high on the flanks of Mauna Loa and the flows go all the way down here to Kīholo Bay. The other flow I just talked about was the 1877 flow in the submarine environment. Then up here high on the flanks is a radial vent from 1852.


For Mauna Loa, we like to pride ourselves with this volcano. It’s probably going to be, when it's finally done with geologic mapping, the best geologic map of any volcano and the best dated. So far, we've been able to map over 500 different lava flows on it, and we're only mapping the surface, and we have over 300 radiocarbon ages. So, about 35‒40 percent of the flows have been dated.


Then we can take this information and compile a long-term history of the volcano. What we've done here is, we’ve broken out the lava flows into epochs of time, roughly 1000-year intervals, excluding the historical and near-historical lava flows.


The warmer the color, the more recent the lava flows. Here are the flows in the historical period of time from the Northeast Rift Zone. You can see that they had a tendency to go to the north. If you look at the prior epoch of time shown in orange, you can see that they go to the south. We don't completely understand why this is happening, but there seems to be a self-righting effect of erupting on one side and then the other side.


The other important thing about this is the distal part of the rift zone here has all these cooler colors, which means it hasn't erupted as frequently in time as the upper portions of that rift zone. So, the colors here… The hypothesis is that Mauna Kea and Kīlauea are squeezing the volcano, making a compressive force so that dikes won't intrude into the distal portion of the rift zone. The geologic record is suggestive of that because the colors are much cooler.


If you look at the Southwest Rift Zone, you can see the colors of lava flows, no matter what epoch of time, are more evenly distributed. There isn't a preference. Also, the entire length of the rift zone has been active in recent time. The one thing that stands out… well, there’s several things that stand out… but down here, you can see on the southeast coast these colors are old-age lava flows. The oldest rocks on Mauna Loa are these rocks here, known as the Nīnole Hills.


How many of you guys know where the Nīnole Hills are? Those of you familiar with the island, if you go to the black sand beach at Punaluʻu to see the turtles, when you turn around and you face mauka [toward the mountain], you see these large flat-topped mesas. Those are the Nīnole Hills. Those are the oldest rocks at the surface of Mauna Loa, and they've been dated at about 110,000 years ago.


The other portion down here is down by South Point. This area hasn't been inundated by lava in a while. But if we look over to the other side of the rift zone, you can see warm colors. The distance between the rift zone and this coastline is shorter compared to this distance from here to the coast on the southeast side.


Another thing from one of our studies that we just concluded… well, several years ago… was that the rift zone of Mauna Loa used to extend from the summit caldera past the Nīnole Hills, out past the green sand beach. So, the original orientation of the rift zone used to be over here. Then when we had a catastrophic landslide on the east side of the island, we actually had the rift zones migrate over to where there was a free surface.


Then, if we look over on this side here, all these dark flecks that I'm showing here are the radial vents, and the radial vents are spread over times from present all the way up until about 2,000 years ago. If there are any older radial vents, they're probably buried, because lava flows on this side are fairly young. As I said, the radial vents can occur at almost any elevation.

To put this in context, the missionaries were here when you see the red colors. The Hawaiians got here under the yellow color regime.


We're looking at eruption rates now.


Here's a map showing the slope of the volcano, the steepness. The warmer the colors that we have here, the steeper the slopes. These big numbers here are average effusion rates in millions of cubic meters per day. On the southwest side of the island, we have high effusion rates, we have very steep slopes. The combination of those two results in abbreviated time from when the vent opens to the lava actually hitting the ocean.


See that… these are hours, 3 hours … 18, 24, 29, 3½. The one anomaly is the one in 1926. In 1926, the eruption broke out right at the rift crest, so lava went both ways for a while. Then eventually the eastern lobe dried up, and lava then—the bulk of it—went into the southwestern lobe, and that’s where it threatened the town of Miloliʻi. So, high effusion rates and steep slopes result in rapid advance of lava flows.


On the Northeast Rift Zone, we got basically about half the effusion rate. The town of Hilo is located there. The one thing the Hilo side has that the Kona side doesn’t is that up here in the Saddle Road area, you have this hint of green. The slopes are gentle. The lava flows come down from the rift zone very rapidly. Then they hit gentle topography, and they start to pancake. The flow front velocity actually slows down. Yet, the source doesn't know what the flow front is doing. It just keeps shunting lava.


So, if the flow front doesn't move rapidly enough, you create a log jam situation at the inflection point. Lava ponds, and the lava will jump out and make a subsidiary flow. It did this three times in 1984.


So, the topography up here is facilitating a strategy of divide and conquer. The one lava flow that actually made it within the city… actually this is it… the one that actually made it right into almost downtown Hilo is the 1880–1881 lava flow. That was a slow, oozing pāhoehoe lava flow and it took over 200 days to get down there. We really have only one benchmark for the radial vent eruption. That one was in 1859. The ʻaʻā phase of this lava flow hit the ocean in 8 days. The pāhoehoe hit it in 120 days. So, ʻaʻā, ʻaʻā, ʻaʻā, ʻaʻā, ʻaʻā, for 8 days; 120 days; 200 days.


We're going to take a tour of some of the (couple) flows from the Southwest Rift Zone of Mauna Loa, because that's where we are, the province. We're going to take a look at the 1926 lava flow and then the 1950 lava flows. Here you can get your bearings on the years of the different eruptions along the Southwest Rift Zone.


In 1926, here's a view of the Hoʻōpūloa lava flow and this is the regional village of Hoʻōpūloa. Miloliʻi is a little bit farther to the south. You can see the wall of lava was about 30 feet (9 m) high, and 1,000 feet (305 m) wide, and you can see here the orientation map.


Here's a view from the actual harbor there. So, the lava flow, you can see how tall it is and it just grinds and pulverizes and burns up everything in its way. The lava then came right down and it's right in the middle of the village.

You can see this… it's taken out a few of the homes there.

Now it's 40 feet (12 m) high and 2,000 feet (610 m) wide.

Here is the lava flow superimposed on what we have as the current [village]... Here's Miloliʻi village. This is the first extension of that and then this is the current (Miloliʻi village). Most of the people live over here in the subdivision. This is mostly the Hawaiian community from the regional fishing village.


Here's some other pictures of that. This is taken from almost the same place, you can see the lava flow there. There's the new houses on top of the flow. There's the thickness of the lava flow.


Now we're going to take a little tour of the 1950 eruption. Some of the people say that the 1950 eruption was Mauna Loa’s most spectacular eruption, because the effusion rates were just basically, as a volcanologist would say, “out of this world” because it was many orders of magnitude higher than what we've been able to see in the recent past.


Here we have the fissure vent. The fissure was 21 km (13 mi) long and effusing lava across the fissure system. At different points in time, it activated… if you take the original trace… all the way back 21 km (13 mi). The eruption had effusion rates on the order of 1,200 to 1,800 cubic meters per second (1570‒2354 cu yds/sec). During the Pu‘u ‘Ō‘ō days we had about 2 cubic meters per second (2.6 cu yds/sec). And in the 2018 eruption, we had about 350 cubic meters per second (458 cu yds/sec). Mauna Loa still was three times higher in its effusion.


This is the view from Hoʻokena. We're looking at those lava flows, and you can see, they came down across the highway and entered the ocean in three different locations. The question is, if you live in South Kona, which way would you drive?

[audience laughs]


Alright. We can talk about that later.


Here's a clip of the lava coming across the upper highway there. Look at the size of these blocks.


[audience] Wow.

[Frank] Look at that one.


[audience chatters]


Yeah, look at that part.

The average rate of advance of this flow was on the order of 10 km (6 mi) an hour.


[audience chatters]


You can look at it one more time.


[audience chatters]

The other question is how many of you would like to be standing right… oops. How many of you would like to be standing right there?


[audience laughs]


Budding volcanologists.


[Man … Is that the front of the flow?]

[Frank] No, that's just looking at the channel from the mauka [inland] highway. It's going from mauka to makai [seaward] and you’re watching it in transit.


[Woman … How far away are those people from that?]

[Frank] I'm guessing that they’re standing on a ridge here, and then it goes down and goes back up. So, this part here has overflowed the ridge that was holding the lava in that channel. But there's still another ridge right here. You can’t really see it in this video.


Let's talk about lava inundation maps. Some of the things from learning from what we gain from the long-term history. We decided that we needed to make some products that people can use, be helpful for planning purposes for emergency managers. Instead of them being top down, where the lava flow is going to come, and then shift downward, we identified communities on the flank of the volcano, and we went from bottom up. We tried to delimit segments of the rift zone that can shed lava into the different communities. Then we lump them into those lava inundation zones.


As I said, we have these maps, and they're online. You can download them, if you want to.


To illustrate how these maps are intended to be used, here's an example from Hilo. This is the lava inundation map that we created for Hilo. This yellow zone right here represents the rift zone. The rift zone has both positive and negative topography, so it can act as lava diversion structures.


The path of a flow is not very predictable when you have complicated topography. So, if you read the maps, one of the caveats is, the lava has to actually exit the rift zone in order for the inundation zone to be valid.


We had an eruption in 1984. If you were the emergency manager, the idea is we're going to use this map for planning. I have limited resources on the Big Island of Hawaiʻi. Where am I going to put my resources for a 1984-like eruption? Using this map, you can delimit an area up here that would be impacted, or one there, or a different one here—instead of, in 1984, they closed the roads all the way down here, all the way over there, all the way across the Saddle Road.


This is what people saw in 1984 from Hilo. All the people of Hilo saw this. They look out their windows and they're like, “ahhh.” They see this glow, and everybody is in kind of a state of uneasiness.


[audience laughs]


Yet, if the maps existed, some people could go to sleep at night and not worry that the lava was coming to their house.


This is what happened in 1984. The fissure propagated down here. The initial breakout sent lava flows in this direction. Here’s the Kūlani Correctional Facility. They thought, “oh, the lava flow is gonna get to the prisoners and how we gonna get the prisoners out of there?” One idea floated was they're gonna take the Robert’s buses, drive them up there, load the prisoners on and drive them away.  


Fortunately, 24 hours later these flows stopped already, and the main lava flow headed off in this general direction. So, now, if you're the Civil Defense director, you basically have a very limited geographic region if you need to evacuate or how many beds you’re going to supply, and so on and so forth.


Those other people in Hilo that lived in Waiākea would look out their window and go, “wow, the lava flow was coming, but it was not coming to my house,” so then they go to sleep, instead of everybody having sleepless nights.


So that's the intent of how these are supposed to be used.


We have other tools that we’re using now to actually further delimit. We use these steepest lines of descent, and they actually… several lines can be within one inundation zone. So, we can use those lines to further guide where the flows might impact. We can also do some flow modeling things, so we are building up our tool set.


Now I'm going to move over to monitoring, what we do and what's the status of Mauna Loa. This is a map showing all of our sensors across the Island of Hawaiʻi. The various sensors are represented by the different dots here on the map. You can see that Mauna Loa and Kīlauea are the two most active volcanoes and, therefore, they have the densest array of monitoring equipment.


Even though my boss is in the back, I'll say this. It should be no surprise that Mauna Loa is going to erupt. All right. That's my personal opinion.


[audience laughs]


Anyways … these are the sensors that we have on Mauna Loa specifically. As I said, we are monitoring these volcanoes. But if, let’s say, if Hualālai was to show some signs of reawakening, then we would put additional instruments to better monitor that volcano. Right now, Hualālai is not showing any signs of life. We have a seismic station there that’s a 24/7 sentry that sends back data. All Quiet.


Now I'm going to do a series of cartoons, sort of a general way of how we monitor volcanoes. Let’s call it the layperson’s guide to volcanic monitoring. Before there's any ingress of magma to the reservoir we have our sensors out here, collecting data. If there's no ingress of lava, we basically show no change, really sort of the status quo.


We can look at elevation changes. We can look at X-Y changes on these GPS stations. We have tiltmeters that tilt upward when there’s influx of magma to the reservoir.


Now the magma starts to come in, ever so slowly, into the reservoir. We actually notice deformation changes or these changes in these instruments first before we have increased seismicity. So, we have influx of material into the volcano, and as more material comes in, then we notice elevation changes, we notice increased tilt, and there might be an occasional earthquake.


But for eruption forecasting, it's when this magma comes into the reservoir and starts to stress the edifice and generate many more earthquakes, is when we start to look at forecasting the eruption. No earthquakes … there’s not going to be an eruption; you gotta have earthquakes, to have an eruption.


Then we stress the edifice, we noticed these changes all appear.


If there's a failure or there's a propagation of molten material, the way we can track these earthquakes, the direction and the depth of these earthquakes will track where the molten material is. If you had instruments on the distal flank of the volcano, these would be all going up and the tilt will tilt away from this area, and in response the summit is providing magma on the distal area so the summit will drop correspondingly.


What did it look like precursory seismicity before the 1975 and 1984 eruptions? Why do I just choose all of these based on all the 33 historical eruptions? Because these are monitored in the modern era. The other ones had older instruments, a lot of them were people-felt earthquakes and would write them down and their observations. But these ones we actually collected seismic data, etc.  


As the volcano starts to have ingress of molten material, we start to see these deeper earthquakes on the northwest side, and the color is reflective of the depth, if you look at the charts here. Then, as this... the volcano becomes more and more distended, then the migration of earthquakes comes in a shallower and shallower environment. Then eventually we have an eruption. These are earthquakes of magnitudes greater than magnitude-1.6. The reason why we use the cutoff of 1.6 is because the older instruments aren't as good as the new ones. So, the new instruments can get negative magnitude, whereas the old instruments would never even shake under a negative magnitude. In order to compare apples to apples, we take the new data, we filter it at 1.6, and … the punch line isn't there yet.


[audience laughs]


Right now, we're just looking at seismicity, a plot of seismicity on Mauna Loa for the last week. We see there's about 14 events. This is looking at the earthquakes per week over a 6-month period, and this is looking at earthquakes per week, over a 1-year period. If we look at this, the 1-year period, you can see we have higher numbers of earthquakes. Right about here is about 100 earthquakes. We go up to a 100, approach 100, and then we drop back down, go up to 100, drop back. So, we've got this sort of sawtooth pattern of seismicity on this volcano.


If we were going to have an erupting volcano, we would expect this number to just keep going upward. The blue would go up—ignore the red line because that's a cumulative—but we'd expect these to go up and up and up. In 1974, we had on the order of a few hundred earthquakes per day. This is per week. In 1984, we had in the mid- to… we had like 50, 60, 80 earthquakes per day. And this is per week, right now.


Now we're looking at seismicity from now—and then you say, well that sort of looks like the other two.


And it does, it does look sort of like the other two—if we're looking at two years of seismicity.

Let's see what the other indicators tell us.

Now we're looking at deformation. The earliest deformation that we had was using a laser ranging device and measuring the length across here. Imagine two dots on a balloon. You inflate the balloon and then you measure the dots, before and after, and you see the line, actually the dots, will be farther apart. It's reflective of inflation. Nowadays we have GPS, so we don't need to use the laser ranging device. From the GPS, we get a very large synoptic view where we can see down the flanks. You don't even have to see across the summit of the volcano, the world's largest volcano we can measure across the flanks of the volcano. That really enhances our ability to monitor this volcano.


Here we have three years of GPS data. You can see, here's the summit caldera. You see the GPS vectors on this side are smaller than the ones on that side. At the base of each arrow is a GPS station. Down here is the scale of motion of these and arrows.

You can see that the volcano is moving towards Kīlauea faster than it is moving in, like a balloon, a radial pattern.

If we look at one year, we start to say, oh, look... these arrows are starting to show a radial pattern, so Mauna Loa is slowly inflating.


If we look at six months, we can see a radial pattern here. These vectors are still a little bit smaller than those, but the pattern is more radial, so we have slight inflation of the mountain.

We also have other tools in our tool kit. This is satellite data. We have a satellite that passes over, and then it passes over another time, and we're looking at elevation changes between time A and time B.

So, we’re looking at this bull’s-eye pattern. Each one of these fringes, let's say from magenta to magenta to magenta, is about 1.5 cm (less than 1 inch) of change. So, we have 1-2-3-4-5… we have about 7.5 cm (3 in) of uplift.


That was from January to June 2019—last year.


We just recently got a new one, which is a 1-year view of Mauna Loa. What you see here is, you see these same fringes out here except, you look on Mauna Kea, an inactive volcano, you can see it has fringes as well. Those fringes over Mauna Kea are due to atmospheric conditions. The clouds and the moisture in the air can give the impression that there's deformation.


In reality, if we're looking at the changes for Mauna Loa, we’re really just looking at this little area right in here, because these are also atmospheric. We have to ignore that, and you can see there aren’t as many fringes as the prior record. So, it's still inflating, but the rate of inflation is less.


So, what's happening? We have small inflation at the summit. We have above background seismicity intermittently. We have slightly above background seismicity in the summit and in the upper Southwest Rift Zone. But it's sort of episodic, I showed you that sawtooth pattern.


What are we missing in order to have an eruption of Mauna Loa? We really need to have more consistent and persistent seismicity. The rate should just be going gradually up and up, and instead of us counting earthquakes per week, we should be counting earthquakes per day. Then we expect and hope to see increasing rates of deformation and seismicity as we get closer to the actual eruption.


What is HVO doing in response to these above-background levels at Mauna Loa?


We are upgrading and increasing, we're maintaining always, and upgrading our monitoring equipment or adding new instrumentation.

We're starting to do internal planning. People on our staff, most of them were born after the [1984] Mauna Loa eruption. So, they have no idea what that is.

So we got to build up and plan for what may happen. We're starting to coordinate with our partners in CD. We're looking into different kinds of assets, to include drones.

We are going out and briefing essential personnel, people that should be briefed. And then we have community outreach, like this, Volcano Watch, and other things that we have—web presentations and community meetings. So, people should be aware of the hazard. You should stay informed. The take-home message is an eruption is not imminent.


In 2002, we had a meeting like this where we said there's a slight swelling and this is what the press did.


[audience laughter]

The take-home message is, an eruption is not imminent.

Any questions?


[audience applauds]