Volcanic Hazards: Lahars

Lahars are called jökulhlaups in Iceland, or mudflows in other regions of the world. What are they? A lahar is a combination of water with volcanic ash and lots of other debris.

Image Wikimedia Commons. Mount St. Helens erupted often between 1980 and 1986. An explosive eruption on March 19, 1982, sent pumice and ash 9 miles (14 kilometers) into the air, and resulted in a lahar (the dark deposit on the snow) flowing from the crater into the North Fork Toutle River valley. Part of the lahar entered Spirit Lake (lower left corner) but most of the flow went west down the Toutle River, eventually reaching the Cowlitz River, 50 miles (80 kilometers) downstream.

Image by Wikimedia Commons. Mount St. Helens erupted often between 1980 and 1986. An explosive eruption on March 19, 1982, sent pumice and ash 9 miles (14 kilometers) into the air, and resulted in a lahar (the dark deposit on the snow) flowing from the crater into the North Fork Toutle River valley. Part of the lahar entered Spirit Lake (lower left corner) but most of the flow went west down the Toutle River, eventually reaching the Cowlitz River, 50 miles (80 kilometers) downstream.

The necesarry water can come out of many sources:

  • Glaciers are one source: IngeB wrote a really nice post on Icelandic glacier runs. When glaciers start running 1, Author IngeB, January 9th.
  • Snow can also be melted by volcanic action and start a lahar.
  • A lake can provide the necessary water too. This also happened with the latest eruption of Mount Saint Helens.
  • But lahars can also be created when torrential rains pour down on fresh or even old volcanic ash. This happens frequently in the tropics like in the Philippines, Indonesia or when a hurricane strikes Central America. This means a lahar can even happen quite some time after an eruption.

Wikipedia quote: “A lahar can cause fatalities years after its precipitating eruption. For example, the Cabalantian tragedy occurred four years subsequent to the 1991 eruption of Mount Pinatubo.”

“In particular, although lahars are typically associated with the effects of volcanic activity, lahars can occur even without any current volcanic activity, as long as the conditions are right to cause the collapse and movement of mud originating from existing volcanic ash deposits.”


This video was taken about a year after Merapi´s eruption.

The name originates in the Javanese language used in Java ( surprise ;) ), Indonesia, where it means lava or lava flow. In Sumeria people worshiped a god called Lahar, he was a god for cattle so I do not really see a connection with this volcanic hazard.


The video shows a dramatic lahar in Japan.

Lahars have the consistency, viscosity and approximate density of concrete. Impossible to escape if someone gets trapped by it. Lahars can be huge. The Osceola lahar produced by Mount Rainier (Washington) some 5,600 years ago resulted in a wall of mud 140 meters deep in the White River canyon, which covered an area of over 330 square kilometers  for a total volume of 2.3 cubic kilometres.

Another problem is that a lahar tends to solidify once it comes to a halt. With the potential to flow at speeds up to 100 kilometers per hour, and flow distances of more than 300 kilometers, a lahar can cause catastrophic destruction in its path.

A Jökulhlaup during the Eyjafjallajökull eruption 2010.

USGS has an article on Lahars, different reasons and their scenarios.
Wikipedia on Lahars.
Student´s paper

http://all-geo.org/volcan01010/2011/12/colima-lahar-videos/ a very informative link describing two different types of lahar, with videos, supplied by Sherine France. (schtevie edit)

Spica (a volcanoholic, but a layman)

About these ads

284 thoughts on “Volcanic Hazards: Lahars

    • A bit strange that is did a upwards jump around the june 2012 eruption. On would expect the opposite!

  1. On Hekla, reply to previous talk: When I hiked Hekla, there was mobile connection at all times. The hiking path starts from the north, so you tend to catch signal always. But if you go east, behind the mountain as seen from Reykjavik and the lowlands, the signal fades. But perhaps some mobiles might fail to have a signal sometimes. Bigger trouble is that it is often fog around the top of Hekla.

    You can actually run down the sandy/ashy slopes in about 1 hour (its like running down a beach slope), but walking at a normal pace takes 2 hours (more if you hike it when there is more snow and ice on the mountain). Still it is very risky because if an eruption starts, you cannot run away. Earthquakes might only be felt 30min before the eruption starts.

    On the most recent 3 eruptions, they were all near the top (not in the foot) – so I guess the farmer observation is not very correct. Although he could not see the eruptions (perhaps of cloudiness) they occur at nearly the top. Actually when I reached the top, the ground is still steaming from one of the most recent eruptions. And I saw some lava fields of 1990 and 2000 on the way up, quite near the top. However compared to 1947, they were smaller fissures that opened and they did not “blow” up the mountain like 1947 or 1104 did.

    Thus if an eruption starts, the explosive phase is the first thing to happen. Lava bombs will be flying everywhere. I guess there is nearly no change of surviving an eruption if you are on the mountain when an eruption starts. Because of a 99.9999% probability that you will be hit by flying rocks. Add risk of lightning bolts. Same thing if you hike near Katla (example, the Laugavegur hike near Ermstur).

    Yes Hekla does have a deep chamber. (or several). Anywhere between 11-17km, but Carl once said there could be one just beneath the mountain, very shallow. I also think the dike used for recent eruptions might be blocked now. I often said that this might have to do with the fact that there were major earthquakes “near” Hekla in 2000 and 2008 which could have changed its eruption pattern. Volcanoes do not run the same eruption pattern for ever!

    As far as I know, (I live 45km west of Hekla) no one here has any warning sign about Hekla. But we live in a safe location. The only type of eruption that can affects us more seriously is very strong ones like 1104 – because of ocasional flying rocks hitting ground more than 40km away. In 1510 (I guess year is correct) a rock killed one person in Skáholt 40km away. But usually with most eruptions, rocks never fly more than 20km away. And even at that distance, like in 1947 big one, only hot but small pieces of ash and gravel fall.

    • ” I also think the dike used for recent eruptions might be blocked now.”

      If that were the case, would we not be seeing much more seismicity? It’s well-accepted that Hekla has been pressurising and is ‘ready'; if the main feeder dyke were blocked you would expect a lot of seismicity as magma pressurised old, cold rock?

      • Hekla already has changed its ultra-short-time behavour, by increase of 30% over the previous short-time average between eruptions. These quakes we see now are small ones, Micro-quakes, same or similar phenomenon was observed at HAU (or in HAU area) “in the last days” before the 2000 eruption. This and faint gossip, already passed on here at VC, indicate Hekla is blowing out like a ballon, at increasing rate, soon to burst (there already was a warning in July 2011). Does short time average for Iceland indicate no eruption again this year? None known in year 2012, but will 2013 be the same? Last centuries average was one about every 3 – 4 years.
        “”*period not specified in old document I once read

  2. Alpine version of a mudflow. Look at the blocks in the second half of the video. That stuff has quite a transport capacity, and imagine the energies you have to deal with when it hits a house…
    If I have time I look for better vids. That’s the first that appeared on youtube… But I think worth a look. Can you imagine that it’s frustrating when you know that that can run into a habited area and the people just don’t care for your concern for their safety? And if something happens they’ll all say the government and administration didn’t do enough to protect them… No kidding with that kind of phenomenon. But there are so many villages situated on the feet of slopes where such beasts can come down…

    • Here an example from 1993. Nearly looks like an simple “flood”-phenomenon, but that stuff was heavily charged with “coarse” material as can be seen in the end of the vid. Again, heavy stuff transported, impact energy on houses or other infrastructure quite high…

      • And here I just discovered that the colleagues from Kanton Bern have tried to make a kind of audiovisual abstract of our job. For those who understand a bit of german or just like pictures of landscapes and hazardous stuff…

        • Thanks for posting this Geo Loco. It’s just what I imagined you job entailed. Diverting possible disasters. However it must be difficult sometimes to convince the pen pushers of neccesarry action, I could imagine. Switzerland does well in it’s planning and approach to the forces of nature. No huge dramatic shouting, like iceland just getting on with what needs to be done.
          I am also surprised that I could actually understand a little of the Narrative. Pctures help but I must have soaked up more German than I realised!!

          • I like these colleagues a lot, and although my first thought was “what the hell did they put into that video”, I have to say they once more proved to be great pros. Quite a nice explanation, even if logically containing the specificities of their cantonal legal system. Someone who has no clue but is confronted with them can get an idea of what they do prevention wise in 10 minutes. Doesn’t mean that I’ll take money from my projects that actually intend to protect people to make that kind of commercial, but I admit that it’s well done. But I didn’t expect less from them. Geeez they are one good team, where intelligence meets “good-guyness”.
            Well German is good, but when I see the last propaganda videos from North Korea, I think I should start studying korean… If we believe them they’re going to reach world domination quite soon… Sad story.
            Ehm, what is this here about? Ah yes, volcanoes, and beauties coming out of their morning shower to drink steaming hot chocolate or coffee… Back to that… :-)

          • The big difference between Iceland and France, Germany, Austria, Italy and Switzerland is the density of the urbanized area. In some valleys we have only very small room to let nature act.
            But then, Iceland has volcanism and seismic activity that can quickly touch wide areas at once.
            Just a fabulous subject, how mankind tries to deal with nature when it get’s busy…

    • Wow, that is frightening, it is not only transporting those MASSIVE rocks but even breaking them up as they tumble!

    • A sober reminder of what can happen. We’ve just had 24 hours of rain again, but thankfully I don’t live in a flood area or in a steep, narrow valley. The more I see of what Nature can do the more I change my idea of an ideal house site. Fairly high, flat plains seem to be the best option – in a non-siesmic area! (Greetings! GeoLoco!)

      • Hi Talla.
        Fairly high, but gentle slope, bedrock directly under the ground. If it’s too flat, then there always can be water accumulation.
        Problem with fairly high is water sources often are not handy… I’m quite happy with where my house stands, but if I wanted my own water, I’d need to drill dozens of meters…

      • Hi Talla, even up there you can get hurricane force winds. ;)
        Dry here this morning and even the sun trying to shine! Definitely the sunniest place in the UK.

    • criseh! ( and all) Only 4 links per comment are allowed by wordpress. if you add more, you´ ll end up in the spam box. So to prevent that, if you got more than 4 links… write 2 comments. OK?

    • Ctab and Ccum are interesting, especially in spectral mode (2). there is a increase on the power spectral density and a spike for lower frequencies (1Hz). However I take this with a pinch of salt for themomenr as one does not see the same trend in CHIE And there is a warning for strong seas….(5 m high waves today)

  3. Sigh! This is deja Vu! Avcan report 3.4 tremor El Hierro. They are asking the citizens as usual to fill in the Online form provided by IGN. From the comments it would seem either the locals are not filling them in or as one comment says they ARE filling them in but getting no results published, no information from IGN or other authorities. Avcan followers are pretty sure there is magma on the way up again if not already submarine eruption.
    At present I can get no further information off the ING site. no updates but there sure have been plenty of quakes already this morning. I can get no map no signals.

  4. Manfred Betzwieser says there were 115 earthquakes yesterday and 40 today when he wrote his blog entry. Most quakes happened at a depth of 15 or 17 km some smaller quakes in 2 km were only “pressure release” quakes he thinks. Deformation happened at the surface which is visible on 2 GPS graphs, but he also mentiones that the deformation was already up to 12 cm in some places which is not visible on the graphs. He also says, that there might be some small tremmor visible in the charts. Magma is on the move.
    http://elhierro1.blogspot.com.es/2013/03/vulkan-beben-der-starke-ml38.html

  5. Something is happeing on El Hierro again, big increase in signal in the last 20 minutes.

  6. HI

    Here is the update (I have not done all the rotations so it will be for later).
    Addition are the earthquake count on the left and the hour of the event on the right.
    Data from Ign up to 10h58 today. The 3.8 is quite shallow

  7. They seem to have scaled down the 24-hour tremor graph by about 50%. EQ’s of M2.5 would previously swing up and down 2 hour-lines (if you get what I mean), now only swing up and down 1. The hourly graph shows them roughly 3x stronger then the 24 hours graph now.

    Whether the noise in between the distinct EQ’s can be called (harmonic) tremor already, I don’t know, but it surely was not there the last days when the reactivation started. They could also be thousands and thousands of M0-M0.5 quakes for example.

    • Just forget about the <0.5 EQ's. If something "flows" down there it's just normal to have a bit of movement. 0.5 is really nothing energy wise.

      • True, but I imagine that a flow, without actually fracturing anything, would probably have a harmonic component to it due to the geometry of the flow path and density/viscosity of flowing material. I could not really make out any harmonic components, so that’s why I can’t say if it’s just a flow going somewhere without fracturing anything (conduit?), or a flow going somewhere while fracturing something (sill/dyke?). By <M0.5 I meant lots of small quakes, but maybe they can actually be M1.0 or so or even a bit more to generate this more or less constant signal.

    • HI

      they have probalby rescaled because of the 3.8 of yesterday. For tremor… Mc Nutt says 2-3 Hz so we could have some, but there is still a lot of Volcano tectonic events if you look at the shape of the events on the hourly detail.

    • Well I would not call it a feeling, but I saw some quakes going up and that is worrying fomr my point of vue

  8. The scaling down also becomes clear when you check out the other stations on nearby islands. The signal is increasing in all cases… could be weather related :)

    • not only that, they are not listing but 1/3rd of them visible. From 00:00 to 01:00 last night (today) I counted about 33+ quakes on the hourly graph, yet they only show (list) 11 quaks. *this got stuck in my PC, forgot to push enter!*

    • I saw it, like a darker ressish brown cloud hovering; now it[‘s gone. It may have been the light playing tricks with the clouds?

  9. ‘reddish’, that is. (I bought a silicon keyboard protector with the keys printed onto it, but anyone with nails, no matter how long or short, will type the paint off within a few weeks…)

    • There would be a swarm of small quakes and an onset of tremor as well as marked strain movements before there should something to be seen like an eruption column or so. See: http://hraun.vedur.is/ja/hekla/
      BTW there is also a new webcam an Mjóaskard (same link).

      • ah, Mjóskarð camera is old one, has been out of action all winter
        else Inge probably has onset og Hekla eruption bingo on target
        (does NOT start with red glow)

  10. The quakes are getting shallower, even one at just 1km depth now!
    1199270 22/03/2013 14:15:34 27.7290 -18.0817 1 1.9 mbLg SW FRONTERA.IHI

  11. After a long research i’ve discovered the real matter under El Hierro. Xray scan revealed a secret room under Hierro structure, here’s a video:

Comments are closed.