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I saw this rock formation near Hveravellir, Iceland. It is probably of volcanic origin and looks like a dome. It is nearly symmetric and appears to consist of hardened lava maybe, with several very big cracks that divide it into sections. It is quite far away from the lava flows of the old, very flat Strytur volcano that is about 2 km away.

I have several hypotheses of my own, but I am not a geologist. Could it be a small "failed volcano"?

enter image description here

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  • $\begingroup$ Can you see what's inside? $\endgroup$
    – Gimelist
    Sep 2, 2014 at 9:37
  • $\begingroup$ Could this be an extinct fumarole? $\endgroup$
    – user889
    Mar 28, 2015 at 9:23
  • $\begingroup$ It is in the area of thermal springs and gas vents, so may potentially be. $\endgroup$
    – h22
    Mar 28, 2015 at 16:04

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This is a lava tumulus. Tumuli are a very common feature in pahoehoe lava flow fields. There is a classic paper by Walker (1991) describing their morphological characteristics and formation process. Here is an image from the article, you can see that it is almost identical to your photo:

enter image description here

Tumulus B20, 2.8 m high, near Holei sea arch on south side of Kilauea.

Tumuli are formed by inflation of the cold, stationary crust which is lifted upwards by the influx of new lava underneath. From what I see on satellite images, they are common around Hveravellir.

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  • $\begingroup$ Exactly:'nother foto for comparison en.wikipedia.org/wiki/Pressure_ridge_(lava) $\endgroup$
    – user18607
    Jan 9, 2020 at 10:07
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    $\begingroup$ @ebv Tumuli and pressure ridges are two different things, but it's true that the second term has been used to refer to tumuli in the past. Walker discusses this confusion in his paper. Tumuli form "without any lateral shortening or evidence for lateral compression", while pressure ridges form, as their name suggest, by compression. Also tumuli are punctual features, while pressure ridges are linear features, going all the way across the flow, generally perpendicular to the flow direction, like at Big Obsidian Flow for example: goo.gl/maps/KKZigxV4AU2CepDBA $\endgroup$
    – Jean-Marie Prival
    Jan 9, 2020 at 12:54
  • $\begingroup$ Aww. Thanks for pointing that out. I blame wikipedia because the image looked so similar (just a bit fresher than the weathered image in the OP) and they named it "tumulus". Walker is even in the references. But of course, the word of an expert weighs more than any wikipedia article, no question ! $\endgroup$
    – user18607
    Jan 9, 2020 at 13:04
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    $\begingroup$ @ebv Of course I did not mean you were wrong: Wikipedia is. Their picture is a tumulus, no question. The text also seems to correspond to tumulus formation. Only the title of the article is misleading... I don't blame them, terminology evolves all the time and what I consider the correct name today will probably be obsolete when I'm old... $\endgroup$
    – Jean-Marie Prival
    Jan 9, 2020 at 13:16
  • $\begingroup$ @Jean-MariePrival The filename says it: en.wikipedia.org/wiki/Pressure_ridge_(lava)#/media/… $\endgroup$
    – Keith McClary
    Jan 11, 2020 at 23:13
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The vast majority of magma never even makes it out to the surface - most is simply crystallised at depth in magma chambers which dead-end several kilometres below the surface, or are injected as dykes or sills within the host strata.

By OrbitalPete, posted on reddit.com

If magma rose up below that mound it probably got closer than several kilometres to the surface; and the theory could be tested with one drill hole. I'm assuming the mound itself is not lava as such but bedrock that's been forced up.

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  • $\begingroup$ How "forced up"? Why the land around has not been forced up? It is not very big, somewhat 20 meters in diameter at most. $\endgroup$
    – h22
    Sep 3, 2014 at 8:06
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    $\begingroup$ Without drilling this is all guesswork but there could be a crystallized plume which is only that size. $\endgroup$
    – mistermarko
    Sep 3, 2014 at 9:17
  • $\begingroup$ Read up on it - the best way to investigate this may be using reflection seismology. It could produce a 3d map of differences in the rock below the surface. $\endgroup$
    – mistermarko
    Sep 4, 2014 at 0:35

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