… but, it’s swollen.

This was originally going to be just a post, then I decided to make it into an article.

Not so OT… if you can extend your thinking

“Spall are flakes of a material that are broken off a larger solid body and can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure (as in a ball bearing). Spalling and spallation both describe the process of surface failure in which spall is shed.”

Essentially, those flakes that pop off of a surface due to stress is spalling. Here is spalling at a joint on a highway concrete joint.

I point this out because where I grew up there was an issue with slabs of concrete along the highway literally popping up/exploding off of the road bed. Highway 80 and Highway 49 were both made out of concrete slabs. At each joint between slabs, you would find tar impregnated felt that was used as an expansion joint. Over time, as the tar and felt wore away, it was replaced by road sediment/sand. Eventually, this locked the slabs in place and there was no mechanism to relieve the stress. Let the pavement get pretty hot… as the deep south tends to do, and you have a ready made, not so pleasant issue to deal with.

When a material heats or cools, it expands or contracts. How much it does depends on it’s linear coefficient of thermal expansion. (applies to solids) Once you get to liquids, it becomes a volume related coefficient, but the reason for it happening are the same. Thermally excited molecules.

In this plot, I calculated the volume that one cubic meter of material would have at a given temperature, reference to a 20°C starting point. The actual curves are most likely not straight linear functions, but this is a simplification in order to illustrate the point. As you can see, concrete is pretty high on the scale, though it’s composed of materials that individually, don’t expand as much (limestone is about the same as marble in its expansion coefficient)

Ruptured Highway in Wisconson.

Now… why the yammering about thermal expansion? Increased heat flow can (and does) expand the overlying material. This is best illustrated with the Hawaiian islands and the Hawaiian hot-spot swell structure. This (and possibly in conjunction with underplating) is a popular theory for the swells existence.

The swell extends further east than Hawaii, and starts to decline in intensity the further west that you go. (following the seamount chain). You can find other locations across the planet where this effects may be in force… the Canaries, Mid Ocean Ridges, etc.

Many of you have already seen my Hawaii Hotspot Animated Earthquake plot… here is is again in case you have missed it.

If you will note from the earlier thermal plot, not all material expands at the same rate. This will introduce additional stresses as one material expands faster than another, and natch, earthquakes can result from the added stress/strain.

Here is the swell around the Galapagos hotspot and the nearby associated spreading center. Image is from Canales et al overlaying the seafloor using Google Earth.

In order to get an idea of just how much uplift a region can have from a change in the geothermal gradient, I ran a really simplified model using the linear expansion coefficients for the materials that commonly make up a section of the crust. This was done in 500 meter layers using a combination of Gabbro, Sandstone, Shale, Slate, Mudrock. It is not specific to any particular location but should be in the ballpark for oceanic crust. It is a bit arbitrary in that is assumes a 20km thick crust, and that generally only occurs under volcanic islands. (If I remember correctly, the MOHO under El Hierro is about 17km down). Usually oceanic crust is in the 5km thick range. This was just a quick model to see what sort of effect the gradient could impart. Think of it as a happy medium between oceanic and continental crust.

It is important to realize that this is NOT expansion from magma intrusion, this is expansion from the change in the geothermal gradient. Heat flows pretty slowly in rock, much slower than in metal. Over periods of several thousands of years, the changes in the gradient can cause significant changes in the elevation of the surface.

So, the exploding chunks of highway on a hot summers day in central Mississippi could share a common physical cause to some earthquakes…. Thermal expansion.

Yeah, it’s a short article, but mainly I just wanted to point at this physical feature and provide food for thought.



Hawaiian hot-spot swell structure from seafloor MT sounding” Constable et al (2004)

Mantle P-wave velocity structure beneath the Hawaiian hotspot” Wolfe et al (2011)

Crustal thickness along the western Gala¤pagos Spreading Center and the compensation of the Gala¤pagos hotspot swell” Canales et al (2002)

Crustal structure, gravity anomalies and flexure of the lithosphere in the vicinity of the Canary Islands.” Watts (1994)

111 thoughts on “… but, it’s swollen.

  1. so a shallow sill could cause lateral expansion in the overlying rock creating stresses like your local road surface, interesting thought.

    • Rising heat from magma could cause these. Ground deformation may exacerbate the effect. A bit chicken & egg, I guess. 🙂

    • A shallow sill could of course give up more heat to overlying rocks and / or subglacial lakes and so cause smaller glacial floods. See Grímsvötn. The heated rock could also in expanding cause cracks in the ice, but I think for bigger glacier runs, an eruption would be necessary. And the ice by its weight and low temperature would act against the heat and expanding forces here, as I imagine. Glacier caps in Iceland are often 700 to 900 m thick, and Vatnajökull covers around 11% of the country, i.e. around 8.000 km2.

      On dynamics of Grímsvötn eruptions, see: Dynamics, stratigraphy and proximal dispersal
      of supraglacial tephra during the ice-confined 2004 eruption at Grímsvötn Volcano, Iceland
      by T. Thordarson & M. T. Gudmundsson, e.a. (2010), p.3 http://link.springer.com/article/10.1007%2Fs00445-012-0583-3?LI=true#page-1 🙂

      • Ooh Yes! Amazing what can live where! Strange life in Lava tubes. Life forms in volcanic habitats where theoretically they shouldn’t be able to survive. Thank you IngeB :). Some bed-time reading for me.

  2. Interesting article. Thank you, Geolurking. Thermal expansion must contribute to the ground deformation caused by rising magma 😕

  3. Thinking of getting more 3D plotting software. I like the one I use very much as it runs with Excel so I can do calculations in Excel. But I can’t add the topography to plots. It is also cube based which is OK for small areas but not for larger ones (e.g. Northern Europe).

    Octave looks promising but do you need any programming skills? & what support is available – eg for a time poor novice getting started?

    Igor looks good but not researched the cost.

  4. Hi

    Here is the second teaser (yeah I know, this is taking too long). End of video (with the age of the quakes tomorrow)

    up to the eruption day. The magma reservoir density is awesome. We see this is no hotspot but a nice subduction area.
    I think this will help define the future zooming.

    There are aspect changes in the end of the video, as I went back to surfacing instead of meshing. Also changed the color and orientation of some of the text. Tell me if you like it or if i have to go back to former settings.

  5. Some zooming will ne needed I think around Tolbachik as there is (I think) a sill at 5 km depth (and you’ll see later on the next video that there is a string of EQ at ground level)

  6. Igor is also cube based (haven´t found a way to change that yet). It has extensive documentation and there is a good user exchange website (igorexchange), but it requires a little programming for special purposes. If you are academic staff Igor comes a bit…uuhm…”cheaper” https://www.wavemetrics.com/order/order.htm
    Main reason for chosing it was I use it at work so already knew the basics.

    • that’s a really good one. Love watching the development of the dike at 30 km depth. It starts off as one straight column and then spreads out laterally, yet when the view moves around to the south you can still see how thin it is from that angle.

      • Thanks.

        Yes it is very thin, even the reservoir is quite flat. When I get some time I’ll do some zooming on different parts (reservoir, dikes). also since august I think you can see 2 sills under Tobalchik at 5 and mater 0 km

        • Possibly the flatness of the subsurface reservoirs is a little exagerated, because some earthquake accumulate at a technical default depth (-0.9, 0.1 and 3.1 km)? But maybe that are real values, don´t know.

  7. Not only heat, but also freezing can cause deformations. In dutch, we even have a proverb: “het vriest stenen uit de grond”, litt: it freezes stones out the ground. This can happen when stones are surrounded with sediment with a different thermal gradient. In Belgium it happens specially with old cobbled streets. In cities the old cobblestones are often still underneath the tarmac and I’ve seen them coming ‘out’ the tarmac when it freezes. Probably in Iceland it happens to (in floodplains etc).

  8. Thank you for this educating post!
    Trying to follow your train of thought: for 700°C temperature increase one gets 1 to 3% volume increase, depending on the material. That would be around 0.3% extension in each direction, right (third root)? That would give 3 m per km elevation if the layer was heated from 20 to 720 °C and if it could freely expand horizontally.
    If there is an average geothermal gradient of 35°C per km (700°C/ 20 km, going along well with what Wikipedia says, 25°C/km) each heating by 35the top most layer would expand 1 to 3% /20, i.e. 0.05 to 0.15%, giving 0.01 to 0.04% elevation (10 to 40 cm per km)

      • What I meant to say was:
        If there is an average geothermal gradient of 35°C per km (700°C/20 km, going along well with what Wikipedia says, i.e. 25°C per km), each successive km would have an additional elevation of 0.01 to 0.04% (third root of 0.05 to 0.15% volume increase per 35°C). That would amount to 10 to 40 cm per km per gradient “step”, totalling to about (20+19+18+…)x 10 to 40 cm = 19 to 76 m if I calculate correctly.

        Pretty close to your 50 m :-).

        What I think the asphalt also does, that it cannot accomodate for the horizontal expansion and therefore goes floppy.

        • Asphalt, yes. It does become less stiff with the heat. That’s why you generally don’t see blocks of it popping up.. unless they just paved over an older concrete road. Before doing that, it’s usually best to widen/rework the gaps… though the best method is to remove it since the slabs will always cause a lot of deflection in the joints as the road ages… and make that section of asphalt more prone to failure and pot-holes.

          You’re also dead on the money about the cumulative volume change per unit of depth.

          As for the effect in other settings… these “swells” have formed over millennia of being exposed to higher thermal gradients. Heat doesn’t move through rock that quickly… that’s why only 100 to 200 meters down it can be quite hot several hundred years after volcanic activity.

          • Asphalt will run with the heat, my father was watching a bonfire after VE day, the bonfire was on the road at the top of the hill, as the heat built up the asphalt began to melt and the entire bonfire began to slide taking the asphalt into the fire and stripping the road of it’s cover as the bonfire made its way down the hill.

          • Back in 1996 I was working a fire out of Phoenix Az.(We HATED that tanker base at
            Sky Harbor.). Brand new intense black asphalt ramp. Ok about 110f out Ramp temp 122f
            interior of aircraft 130f. Blew tire on landing.The procedure -park aircraft out of the road,
            take jack stand (Large flat steel plate to spread the weight of oh 45,000lbs of the left
            side.)I started jacking the left main gear. The plate kept sinking deeper and deeper into
            the ramp. Called the tanker base manager. “OH we hadn’t thought about that.”
            You need to park it on the other side of the Tanker base.” Now, we start to move it
            and the DC-7 is sinking into the Ramp!! the left side in particular due to the lack of
            a tire on that dual .Dinosaur in the tarpits! We got out to the old concrete ramp
            and there were serious gouges in the Asphalt. Later we got airborne and managed
            to escape due to a bit of a problem with #2 engine seems it started to run rough
            our mechanics were in Winslow… 94f and 20kt breeze- Heaven on earth…

  9. Ehm, Chippewa Falls is largely a flood plain from the Chippewa river. State Highway 29 largely follows the river. This means plenty of clay. Swell & shrink appears naturally in that type of soil, not onlydue to heat, but mainly water contents. And then when the ballastbed is not right, the effects can be huge. Actually the extreme heat may have shrunken the roadbed more than the concrete top layer simply by drying out the clay. http://www.citg.tudelft.nl/fileadmin/Faculteit/CiTG/Over_de_faculteit/Afdelingen/Afdeling_Bouw/-_Secties/Sectie_Weg_en_Railbouwkunde/-_Leerstoelen/Leerstoel_Wegbouwkunde/-_Onderwijs/-_College_Dictaten/doc/CT3041_UK_Hoofdstuk_1.pdf

    • Try a clay that can expand up to 200%, and as a rule usually expands by at least 130% when it gets wet.

      Welcome to Yazoo Clay. Part of the Tertiary Jackson Group and technically, “calcareous fossiliferous mudrock”


      This stuff is rock hard and tough when it’s dry. You can beat on it all day long with a pick and make little progress. Get it wet… and it’s slime city.

  10. Hello everybody,
    Yesterday I received a message from Spica asking me to look after the blog next week. As I do not know any details and haven’t seen her around afterwards, I consider the possibility that she got the flu and wisely went to bed to get well soon. She did mention the word “fever”. If she is OK, she will let us know.
    In the meantime I will “look after” the blog.
    To be continued.

    • Oh, that’s sad news to hear Spica is poorly. Hope it is nothing more than the flu (bad enough I know). Gute Besserung Spica!!

      And thanks Sissel! Crikey I hope this process of attrition stops sometime soon. We’re losing so many good people!! Carl, MIA, Geoloco, missing by inaction, Spica, sick, etc.. 😮

    • @Spica, If you are poorly then please get well soon Spica and thanks so much for the work that all you dragons do for this blog and to you Sissel fot taking over at short notice.
      As for myself, I am around but brain not functioning well because of pain so nothing sensible to post. Condition has finally been diagnosed as Trigeminal neuralgia and at the moment I am sitting here like Pudsey Bear with a cloth wrapped round my eye and ear to stop the cold triggering the severe pain. Look an idiot but part of a set perhaps. 😉
      Love to all of you but specially anyone unwell at the moment.

      • Oops almost forgot, a fascinating article GeoLurking, explains all the lumps out of the edges of the concrete slabs I stare at as I wait for a bus. I suppose starts as chips in the summer expansion, then freeze/thaw weathering in the winter, I do love the diverse things I learn on here.

      • oh by gimminy, just researched TN in wikipedia.. you poor thing! I once had a bout of Bell’s Palsey, which, by way of total contrast, is utterly painless, but I remember the feeling of helplessness about it and for some reason, problems with the face somehow mean more than other body parts, don’t ask me why. My best wishes to you Newby. Hope treatment works for you. bruce.

        • Many thanks Bruce. Keeping the area warm is helping more than the pills at the moment although I thankfully have a cautious doc, so he is starting me on a very low dose. First time in my life I don’t like the prospect of a prolonged cold spell. 🙄

    • @Spica, get well soon. If it is ‘flu, try that elderberry syrup that Sissel recommended.

      @Sissel, I am sure that you will do a good job.

    • Get well soon – Spica and Newby! Thank you Sissel for taking over – I’m amazed at the work the dragons do for us – thank you all.

  11. 15 Days…

    I figured out a way to commit the perfect murder. Again, you know, you gotta think of something. You pick one guy up by his ankles, and you kill another guy with him. they both die and there’s no murder weapon, man.

    “What happened here, Sarge?”

    “I don’t know, it looks like a pedestrian accident to me.”

    “They must’ve been moving at quite a clip!”

    George Carlin

  12. And thanks for the piece Geolurking, I’m appreciating your recent broadening of the theme, I particularly liked the article about the tectonics of the Azores. It’s helpful to have an insight into the “additional” mechanisms that may feed into (or out of) volcanic activity… I’m schtill working on that Mental Model 😀

  13. Hi

    here is the (too long) awaited for end to the “Tolbachik saga”.
    The new part is after about 2 mn. I have changed the long.lat setting to get a more precise view and then there is the second part with the age of the earthquakes and the rotation.

    I will supply a zoom on Tolbachik only this evening.

  14. A get well soon card for all poorly Volcanoholics Newbie, Spica especially http://i186.photobucket.com/albums/x96/wildjinny/volcanic-strength_medium1.jpg
    Thank you Sissel for taking over at short notice. Greatly appreciated.
    @dfm….Thank you too for the terrific plots. I still battle with sea- sickness whilst watching but they are soooooo fascinating.
    Now since we had the first Snowfall of the Christmas season here today I am sending this to you all as it is such a lovely winter song……. The subtitles are useful and I am slowly learning more Icelandic so I can sing along.

  15. On the original post, and reply to Diana:

    it is quite well known that the Icelandic hotspot bulges laterally in a few directions as it approaches the crust. It extends as a plume towards the southwest and northwards at more shallow depths (by shallow I mean 50-100km deep or more). It spreads nearly 600km southwest. So, the hotspot goes from beneath Bardarbunga towards Hamarinn, Katla, Hekla, hengill, Reykjanes and well into the sea over the southwest. And at same time, also towards Askja, Krafla and Tjornes.

    This lateral swelling of hotspots explains a lot of stuff.

    • That is totally unlike what I expected to see.

      Quakes “began” in the upper sill and then as they progressed the lower sill responded… both accumulating quakes at about the same time and rate.

      (Warning layman’s opinion) So, the pressure was there, and as the upper vent began to release the pressure, the upflowing magma changed the stress patterns and you get the quakes. Interesting.


      “Dmitry Melnikov” over at Eruptions linked to EO-1 ALI satellite image of lava flow. Taking that image and aligning it with the terrain in Google Earth, you get a nice over all view.

      • Naturally I’m busier than a one armed paperhanger, and duck in and out of my
        two favorite volcano sites. Get well those who are sick and great work, people..

      • Yes I noticed also this point when making the video (I had not seen it on the more general ones, but I slowed the image rate to be able to see the “red dot” movements.
        Strange isn’t it ?
        Also the number of “local” earthquakes is very limited (250 or so) compared to the total…..(2800 or so).

        Nice picture

        found this. they will not need a barbecue….

  16. Come one everyone! Do not disappear, please!
    Spica, get better! Carl come back, now!
    Lurk, keep on doing this brilliant job!
    (Where is Geoloco when you need him?)
    Very little time to read and comment, but I’m lurking from behind!

      • Well that was a hilarious accompaniment to my #Coffee. Lurking, Geoloco and of course Allan………Dashig in and out on unsuspecting Dragon, Old Lady and the world in general. I empathise with The Old lady :D:D…..” But I don’t understand!” is one of my silent cries as I try to unravel the mysteries of mathematical formulae and deep discussions on the relativity of Lava to the cubed power squared minus the depth over strain and velocity…………. ROFL

    • Oh! Renato You lurk so well…..You give this Blog a touch of that hot, sensuous South American passion 😀 …….. Don’t work too hard it’s nearly weekend .

    • Oh… it could be a number of reasons. But you’re welcome to pull up a chair and join in. 😀

      BTW, it tends to get nutty on Fridays with the riddles and the Evil Alan puzzles.

    • Because your SATNAV has malfunctioned and you ended up here rather that there?
      Welcome to VC where we love people with questions 😀 😀

    • Maybe you were too busy brushing sage? 😉 OK poor joke but here you can forget all about sage and enjoy reading all the clever peoples comments (I am not one of them 😦 ) and with a bit of luck you will be a clever one and if not, like me you can still really enjoy things here. A big welcome.

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