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Geology/Rock question


I have been trying to find someone to help me identify this rock. I found it on a beach in in the Upper Peninnsula of Michigan. It looks like the whole inside is all shiny green. I really hope you can help me out. I appreciate any information you can provide me.



Hi Jason,
You have a piece of amygdaloidal vesicular basalt.  Got that?  

Well, let's break it down.  Basalt is a black or dark grey extrusive igneous rock (think lava).  The name "Basalt" is from "basanites" which is Latin for "very hard rock".   Your rock is cool because it is a part of the Portage Lake Volcanics formation laid down in Michigan's U.P. about 1 billion years ago.  At that time that part of the U.P. split apart and huge volumes of lava spilled out.  Incidentally, this is happening right now in the mid Atlantic.  Crustal plates are being pushed apart and basalt is being laid down at ocean ridges.  In fact, all the oceanic basins are floored by basalt.   

If there is a lot of gas dissolved in the "magma" (the name for underground lava) it will leave holes in the rock as the basalt hardens; like your rock.  If that rock, with the holes in it, stays underground for a few millennia mineral-laden groundwater may flow through it and deposit the minerals in the holes.  Those mineral blebs in your rock are called "amygdules", Greek for almonds.  The minerals in your rock are probably epidote or chlorite.

Here's an image similar to yours but with different minerals;

If you want to know more about the geology of the Keweenaw Peninsula of Michigan;

Hope this helps.


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C. Robert Reszka, Jr.


I can answer any general geology question (rocks, minerals, stratigraphy, geomorphology etc.). My expertise is in the geology of the Michigan Basin, PreCambrian, Paleozoic and Recent. I can answer questions concerning mining and petroleum exploration and production and the laws concerning those activities. I can also answer questions concerning stratigraphy of the Michigan Basin. I will also answer questions about mineral and rock collecting in the Basin. I won`t be able to answer many specific questions on hydrology, geophysics or geochemistry. I may be able to answer very general questions in those venues.


I have been working for the State of Michigan for 36 years as a Geologist and a Resource Analyst. I have experience with Subsurface Geology and Petroleum Geology, mining in Michigan, and Sand Dune Mining and Protection issues.

Michigan Basin Geological Society

Decade of North American Geology.
Bedrock Geology of Michigan

BS Wayne State University

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