<p>This is a lava <a href="https://volcanoes.usgs.gov/vsc/glossary/tumulus.html" rel="nofollow noreferrer">tumulus</a>. Tumuli are a very common feature in pahoehoe lava flow fields. There is a classic paper by <a href="https://doi.org/10.1007/BF00298155" rel="nofollow noreferrer">Walker (1991)</a> 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:</p>
<p><a href="https://i.sstatic.net/AwZd7.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/AwZd7.png" alt="enter image description here" /></a></p>
<blockquote>
<p>Tumulus B20, 2.8 m high, near Holei sea arch on south side of Kilauea.</p>
</blockquote>
<p>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.</p>

CC BY-SA 4.0
2020-01-09T12:54:53.437Z
@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: https://goo.gl/maps/KKZigxV4AU2CepDBA
Jean-Marie Prival
CC BY-SA 4.0
2020-01-09T13:16:30.427Z
@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...
Jean-Marie Prival
CC BY-SA 4.0
2020-01-11T23:13:51.260Z
@Jean-MariePrival The filename says it: https://en.wikipedia.org/wiki/Pressure_ridge_(lava)#/media/File:Tumulus-inflationary_cave.jpg
Keith McClary
CC BY-SA 4.0
2020-01-09T09:55:26.607Z
2021-11-08T15:08:12.460Z
2021-11-08T15:08:12.460Z
Jean-Marie Prival
<p>I saw this rock formation near <a href="https://www.google.ch/maps/place/Hveravellir/@64.5253208,-18.9692902,7z/data=!4m2!3m1!1s0x48d3ebf93b256813:0x341fbb98dbd52317" rel="noreferrer">Hveravellir</a>, 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.</p>

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

<p><img src="https://i.sstatic.net/NY2zb.jpg" alt="enter image description here"></p>

CC BY-SA 3.0
2014-09-02T09:37:01.940Z
Can you see what's inside?
Gimelist
CC BY-SA 3.0
2015-03-28T16:04:05.227Z
It is in the area of thermal springs and gas vents, so may potentially be.
h22
<volcanology><geomorphology>
How did this rock dome (pictured) form?
h22
# How did this rock dome (pictured) form?

(Asked by: username_0 on 2014-08-31)

I saw this rock formation near [Hveravellir](https://www.google.ch/maps/place/Hveravellir/@64.5253208,-18.9692902,7z/data=!4m2!3m1!1s0x48d3ebf93b256813:0x341fbb98dbd52317), 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](https://i.sstatic.net/NY2zb.jpg)

**Tags:** volcanology, geomorphology

> **Comments:**
> **username_1**: Can you see what's inside?  
> **username_0**: It is in the area of thermal springs and gas vents, so may potentially be.  

---

## 1 answers

### Answer by username_2 on 2020-01-09. Score: 5

This is a lava [tumulus](https://volcanoes.usgs.gov/vsc/glossary/tumulus.html). Tumuli are a very common feature in pahoehoe lava flow fields. There is a classic paper by [Walker (1991)](https://doi.org/10.1007/BF00298155) 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](https://i.sstatic.net/AwZd7.png)](https://i.sstatic.net/AwZd7.png)

> 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.

> **Comments:**
> **username_2**: @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: https://goo.gl/maps/KKZigxV4AU2CepDBA  
> **username_2**: @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...  
> **username_3**: @username_2 The filename says it: https://en.wikipedia.org/wiki/Pressure_ridge_(lava)#/media/File:Tumulus-inflationary_cave.jpg  

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