Need help identifying this rock, Found in northern MN

[ReevesN76R]

 

[jim mcnamara]

The image is not visible for me. If you need help getting your image to post, let us know. We can help.

 

[berkeman]

@jim mcnamara it looks like he was able to upload it now.

@ReevesN76R -- Welcome to the PF. Can you take a closeup picture of the marks on the rock? Are those marks (like drawn), or are they something embedded?

 

[BrushyCanyon]

Looks like a porphyritic basalt.

http://www.geo.mtu.edu/KeweenawGeoheritage/BlackLavas/Basalt_Textures.html

Large phenocrysts are plagioclase feldspar, groundmass is typically very fine grained olivine and pyroxenes.

BC

 

[ReevesN76R]

@jim mcnamara it looks like he was able to upload it now.

@ReevesN76R -- Welcome to the PF. Can you take a closeup picture of the marks on the rock? Are those marks (like drawn), or are they something embedded?

They are embedded.

 

[davenn]

Looks like a porphyritic basalt.

http://www.geo.mtu.edu/KeweenawGeoheritage/BlackLavas/Basalt_Textures.html

Large phenocrysts are plagioclase feldspar, groundmass is typically very fine grained olivine and pyroxenes.

BC

yes, that was my diagnosis as well

 

[davenn]

So @ReevesN76R

are you happy with the answer ?

 

[Edelmere]

The porphyritic basalt seems to be correct. I have been collecting rocks and minerals for 55 years.

 

[CWatters]

Cool rock.

 

[GeologyRocks1909]

a rhombic porphyry most likely - how come you found it in Minesota, tough? They are more common in the scandinavian region around Oslo, Norway.

 

[Baluncore]

how come you found it in Minesota

The rock is rounded and river worn, so it was not taken directly from an outcrop. It has probably been carried by a glacier or an ice sheet during the last glacial period. It may have traveled a long way south from some volcanics in Canada.
I have found identifiable rocks in unexpected places, but only where glaciation was available to transport them.

 

[256bits]

The Great Lakes region is rich is rocks and ancient volcanism.
Just over a billion years ago the continent was on the verge of breaking up,
https://en.wikipedia.org/wiki/Midcontinent_Rift_System
http://academic.emporia.edu/aberjame/student/woodbury2/rift2.html
https://en.wikipedia.org/wiki/Geology_of_Minnesota

 

[gleem]

It looks like what I have known as a "pudding stone"

 

[Baluncore]

It looks like what I have known as a "pudding stone"

Pudding stone is a sedimentary "conglomerate" of rounded pebbles in a fine matrix. It looks like a cut slice through a plum pudding.

The rock pictured was volcanic, having a fine green basalt matrix with large phenocrysts of feldspar. A rock with phenocrysts is described as a porphyry, or as being porphyritic. The phenocrysts are crystallised out of the melt over many years as the magma gradually rises up towards a volcanic vent. The fine grained matrix indicates that it cooled quickly once it reached the surface which is why it is known to be a volcanic rock.
https://en.wikipedia.org/wiki/Phenocrysts

 

[ReevesN76R]

Does the rock have any value?

 

[jim mcnamara]

As is, not really worth much.

If you like rock cutting and polishing, it would sell for a fair amount after it was polished on a cut through the middle.
Go to a local natural history museum store or one of the places that sell display quality minerals. You can get a good idea. Some mineral collectors use a rock tumbler only to polish. No cut.

My wife did this for fun and the results were amazing.