What do you think this is ?

[mistermack]

Browsing Google Earth, I came across this area in Siberia.

It has a clear circular shape, showing concentric rings, and it's about 10 km across.

It's miles away from the nearest road, in a barren very rocky part of the world, so it's not human activity.

 

I've narrowed it down in my own mind, to either an ancient volcano, or impact crater.

Impact craters that show are very rare, so the odds are against it, but what do you think?

It's worth a look :

 

https://www.google.co.uk/maps/@70.9198194,103.0433649,15024m/data=!3m1!1e3?hl=en

 

Or on Google Earth :

70 deg 55 min 30 sec N

103 deg 05 min 30 sec E

Edited January 26, 2017 by mistermack

[imatfaal]

It is in a volcanic region - but I must admit I am having trouble getting concavity/convexity sorted out in my head

It is just a cicruclar mountain I think. There is no concavity I think. If you look at the picture of the river nearby there is very obvious stratification - that is what we are seeing and that makes it look concave .

The tributary streams of the Kotuy run straight through the white band - this makes me think the white band is just that rather than being the rim of a crater. Streams do not run up one side of a caldera/crater and down the other

Now I have downloaded google earth I am sure. You can plot a path from the centre to the river, then right click on the path to view elevation profile. It is a round hill on a raised platform - and there is no concavity

[mistermack]

Yes, I agree. But it's in an area that's been swept many times by glacial ice-sheets.

So any hollow would have been filled long ago with debris dragged along at the base of the ice sheets.

The centre does seem to be loose material, as the streams have eroded it quite easily.

[studiot]

A screenshot would be useful to those of us without your special equipment.

 

[Sensei]

A screenshot would be useful to those of us without your special equipment.

Here you go.

But I have no idea about what "special equipment" you're talking about.

Google Earth/Maps used to work even on WinXP and single core 15 years old machine..

Edited January 26, 2017 by Sensei

[arc]

I believe what you are seeing in that image is this area looks to be limestone or similar soluble rock and this feature is the result of the underlying limestone slowly subsiding and creating a depression that collected water.

 

https://en.wikipedia.org/wiki/Karst

Karst topography is a landscape formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves.

 

 

What you see as rings surrounding this feature are where waves have left evidence of the changing shoreline of the original lake that had formed in the slowly developing shallow depression as the subterranean limestone subsided.

 

As the ground slowly sank the water's depth and diameter slowly increased until this basin, which is called a Polje, reached an area where the seasonal high water could over top the basin's edge, eroding the terrain and draining to a still lower area. You can see these in the image.

 

Area A is likely the first to over-top the outer edge due to its advanced erosion and substantial size compared to the other smaller erosional features D that all appear to drain assorted regions of the basin to the main branch of the Kotuy river system B1 to the S.W of the basin. Area A's drainage channel C runs to the N.E. into a separate branch B of the Kotuy river and likely emptied the vast majority of the basin's water.

 

https://en.wikipedia.org/wiki/Polje

A polje, in geological terminology, is a large, flat-floored depression within karst limestone, whose long axis develops in parallel with major structural trends and can become several miles (tens of kilometers) long. Superficial deposits tend to accumulate along the floor. Drainage may be either by surface watercourses (as an open polje) or by swallow holes (as a closed polje) or ponors. Usually, the ponors cannot transmit entire flood flows, so many poljes become wet-season lakes. The structure of some poljes is related to the geological structure, but others are purely the result of lateral dissolution and planation. The development of poljes is fostered by any blockage in the karst drainage.

A polje or karst polje covers the flatbottomed lands of closed basins which may extend over large areas, up to 1,000 km². The flat floor of a polje may consist of bare limestone, of a nonsoluble formation (as with rolling topography), or of soil.

Edited January 27, 2017 by arc

[Acme]

I believe what you are seeing in that image is this area looks to be limestone or similar soluble rock and this feature is the result of the underlying limestone slowly subsiding and creating a depression that collected water.

 

...

Except that it is not a depression. Go to Imatfaal's post #2, follow his directions, and see this is a hill. Result attached. (Nice tip Imatfaal!)

[arc]

Oops, somehow I completely missed imatfaal's entire post. Those rings reminded me of when we were in Montana and saw the ancient shoreline "Bath tub rings" down the sides of the valley that were left by Glacial Lake Missoula.

 

 

Nice job imatfaal, +1. I withdraw my supposition.

[mistermack]

Some great comments on this. Arc, I agree, it does look like the multiple shorelines that somethimes happen when a glacial lake gets dammed to different levels. There's one in Scotland that I thought of straight away. It's quite famouse, and looks like ancient roads cut into the hillside.

In this case though, it would have to be a huge lake, with a very circular island in it, to produce this effect. It seems like too many coincidences are needed for that to be the answer.

I'm beginning to think it is an ancient volcano, that maybe pushed up the sedimentary layers into a circular bulge, and the weathering then exposed the layers in that shape.

It's a fascinating area, incredibly rocky, if you look at some of the photos of the area.

This is one taken down by the river. There are loads of weird towers left standing, like thousands of Stonehenge stones. Anyone know what they are called, and how they are formed?

Edited January 27, 2017 by mistermack

[studiot]

According to my admittedly coarse scale geology map, this area of the Central Siberian Plateau is a tongue of Qaternery material extending from a much larger area comprising the Yenisei-Irtysh basins., to the west.

This is shown in yellow on the map.

This tongue is sandwiched between much older formations to the north and south, first Palaeozoic rocks shown blue and purple followed by some Pre-Cambrian, shown brown.

 

This is suggestive of a former syncline. (Youngest rocks in the middle) on a much larger scale than the 10km feature in the OP.

 

http://web.arc.losrios.edu/~borougt/GeologicStructuresDiagrams.htm

 

The rivers run through the Quaternary material, which must be softer than the older rocks and Imatfaal has show to be lower.

Not only lower but the Google pictures show that the rivers have cut quite deeply into the plains with the older rocks forming background peaks.

 

The Google maps also show much meandering, typical of low gradients.

 

https://www.google.co.uk/search?q=river+kotuy&biw=1366&bih=679&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiA89Sfn-LRAhUkLMAKHf09DigQ_AUICCgD

 

 

The point about the ages is that known igneous activity preceded the Quaternary deposition, so the feature in question may be a remnant intrusive dome the radial lines could be old dykes, although the river seems to have cut straight through one (perhaps following an earlier glaciation).

I bet there are some lovely rapids there.

Edited January 27, 2017 by studiot

[mistermack]

I'm picturing something similar, a granitic intrusion, pushing up the sedimentary rocks, and then getting exposed when they are weathered. The wikipedia page for Laccolith gives a good suspect :

 

 

[LizW12]

So much good info here

[studiot]

 

I have found some useful further information but need help posting to comply with the wikimedia licence attributing the sources.

 

Please help.

 

http://www.mantleplumes.org/WebpagePDFs/Siberia.pdf

 

https://commons.wikimedia.org/wiki/File:Extent_of_Siberian_traps_german.png

 

https://commons.wikimedia.org/wiki/File:Extent_of_Siberian_traps_german.png#/media/File:Extent_of_Siberian_traps-ru.svg

Edited January 27, 2017 by studiot

[Acme]

...

I have found some useful further information but need help posting to comply with the wikimedia licence attributing the sources.

 

Please help.

 

http://www.mantleplumes.org/WebpagePDFs/Siberia.pdf

 

https://commons.wikimedia.org/wiki/File:Extent_of_Siberian_traps_german.png

 

https://commons.wikimedia.org/wiki/File:Extent_of_Siberian_traps_german.png#/media/File:Extent_of_Siberian_traps-ru.svg

Like this?

 

Extent of Siberian Traps by Kaidor

 

Russland Relief by Kaidor

[arc]

 

I'm beginning to think it is an ancient volcano, that maybe pushed up the sedimentary layers into a circular bulge, and the weathering then exposed the layers in that shape.

It's a fascinating area, incredibly rocky, if you look at some of the photos of the area.

This is one taken down by the river. There are loads of weird towers left standing, like thousands of Stonehenge stones. Anyone know what they are called, and how they are formed?

 

 

I would really like to understand how this terrain was shaped from the last glacial period. Was this area just caped with ice and all that we see here is the debris of this process? How did those "towers" of rock survive this period?They look rather delicate to have resisted a mile of ice bearing down on them. My guess would be the huge amounts of ground down rock material filled this whole area and these towers were buried in it to quite a depth.Think sand and powdered rock. As the glaciers melted most of this fine material was transported out through normal erosion processes revealing the still fixed and hardest to move materials that we see now.

Edited January 28, 2017 by arc

[mistermack]

I'm thinking that they are small scale granitic intrusions, where the magma forced it's way into gaps and solidified, leaving these shapes of harder rock standing, as the softer rock breaks up around them.

I think you're right, they must have been buried at the last ice age, and only exposed in the last ten thousand years or so, as the softer rock breaks up and rolls away.

Since the area gets very cold in winter, the action of ice breaking up the rocks is probably pretty rapid. But granite would be more resistant.

[studiot]

I'm thinking that they are small scale granitic intrusions, where the magma forced it's way into gaps and solidified, leaving these shapes of harder rock standing, as the softer rock breaks up around them.

I think you're right, they must have been buried at the last ice age, and only exposed in the last ten thousand years or so, as the softer rock breaks up and rolls away.

Since the area gets very cold in winter, the action of ice breaking up the rocks is probably pretty rapid. But granite would be more resistant.

 

 

If you read the translated russian pdf I linked to it tells us that the lava flows were basaltic, not granitic

[mistermack]

 

 

If you read the translated russian pdf I linked to it tells us that the lava flows were basaltic, not granitic

Yes, but that doesn't exclude the possibility of granitic intrusion from below, after the basaltic flow occurred.

I read some of what was linked, and granitic intrusions were also mentioned for the area.

It looks likely that the "towers" are of a different composition to the rock that surrounds them.

It's only speculation, could well be wrong. But there does look to be more than one type of rock there.

[mistermack]

Looking again at some of the pictures, it seems likely that the rocks that are visible are volcanic tuff, rather than basaltic flows.

That would explain the obvious layering that you can see in google earth. The Siberian Traps event went on for about a million years, and there are large areas of tuff in the area, according to wikipedia. Tuff is an ash deposit, which becomes solidified as the rain turns it to mud, and more layers are deposited on top.

Tuff is a soft rock, so weathering is fairly rapid, So the "towers" of harder rock were probably buried during the last ice-age and exposed more recently by weathering.