by neufer » Sat Nov 28, 2020 6:42 pm
Ron-Astro Pharmacist wrote: ↑Mon Jan 26, 2015 11:05 pm
nigel wrote:John Severinghaus wrote:
Today's APOD about tall stone columns called Sever Strong Men.. suggests the tufa columns in Mono Lake California, formed under water when a spring of limestone loaded water comes from the ground into a very alkaline lake.
Was this area ever under an alkaline lake like the tufa?
No, apparently these are the product of differential erosion of schist (a metamorphic rock).
http://www.rgo.ru/en/photo/manpupuner-rock-formations
Then these may be AKA : The Seven Schisters
https://en.wikipedia.org/wiki/Schist wrote:
<<During metamorphism, rocks which were originally sedimentary, igneous or metamorphic are converted into schists and gneisses. If the composition of the rocks was originally similar, they may be very difficult to distinguish from one another if the metamorphism has been great. A quartz-porphyry, for example, and a fine grained feldspathic sandstone, may both be converted into a grey or pink mica-schist. Usually, however, it is possible to distinguish between sedimentary and igneous schists and gneisses. If, for example, the whole district occupied by these rocks has traces of bedding, clastic structure, or unconformability, then it may be a sign that the original rock was sedimentary. In other cases intrusive junctions, chilled edges, contact alteration or porphyritic structure may prove that in its original condition a metamorphic gneiss was an igneous rock. The last appeal is often to the chemistry, for there are certain rock types which occur only as sediments, while others are found only among igneous masses, and however advanced the metamorphism may be, it rarely modifies the chemical composition of the mass very greatly. Such rocks as limestones, dolomites, quartzites, and aluminous shales have very definite chemical characteristics that distinguish them even when completely recrystallized.>>
And schisters are not gneiss.
[quote="Ron-Astro Pharmacist" post_id=238827 time=1422313511 user_id=140444]
[quote="nigel"][quote="John Severinghaus"]
Today's APOD about tall stone columns called Sever Strong Men.. suggests the tufa columns in Mono Lake California, formed under water when a spring of limestone loaded water comes from the ground into a very alkaline lake.
Was this area ever under an alkaline lake like the tufa?[/quote]
No, apparently these are the product of differential erosion of schist (a metamorphic rock).
http://www.rgo.ru/en/photo/manpupuner-rock-formations[/quote]
Then these may be AKA : The Seven Schisters :lol2: [/quote]
[list]That was not [url=https://en.wikipedia.org/wiki/Gneiss]gneiss[/url] :evil: http://asterisk.apod.com/viewtopic.php?f=9&t=38158&p=281210#p281210 [/list]
[quote=https://en.wikipedia.org/wiki/Schist]
[float=right][img3=Schist showing "scaly" schistose texture, caused by platy micas]https://upload.wikimedia.org/wikipedia/commons/thumb/6/6f/Schist_detail.jpg/330px-Schist_detail.jpg[/img3][/float]
<<During metamorphism, rocks which were originally sedimentary, igneous or metamorphic are converted into schists and gneisses. If the composition of the rocks was originally similar, they may be very difficult to distinguish from one another if the metamorphism has been great. A quartz-porphyry, for example, and a fine grained feldspathic sandstone, may both be converted into a grey or pink mica-schist. Usually, however, it is possible to distinguish between sedimentary and igneous schists and gneisses. If, for example, the whole district occupied by these rocks has traces of bedding, clastic structure, or unconformability, then it may be a sign that the original rock was sedimentary. In other cases intrusive junctions, chilled edges, contact alteration or porphyritic structure may prove that in its original condition a metamorphic gneiss was an igneous rock. The last appeal is often to the chemistry, for there are certain rock types which occur only as sediments, while others are found only among igneous masses, and however advanced the metamorphism may be, it rarely modifies the chemical composition of the mass very greatly. Such rocks as limestones, dolomites, quartzites, and aluminous shales have very definite chemical characteristics that distinguish them even when completely recrystallized.>>[/quote]
And schisters are not gneiss.