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Geology/Rock ID?


"the rock"
"the rock"  
I was wondering if you could help me ID this rock?   It's been "in my family" for almost 30 years in Iowa (midwest US) and we take it with us whenever we move.  The vein appears to be granite but unsure about the rest.  The black part of it is eroding faster than vein.

Thanks : )

Hi Cammy,
Rocks like this always look interesting, don't they?  This is an igneous rock, as you may already know.  The black part looks to be "Gabbro".  That's an igneous rock that's made up of all the dark, heavy minerals.  It looks like that other black rock, "Basalt".  Both are made of the same minerals, but the crystals/grains that make up the rock are larger in a gabbro.  And that's because the gabbro remained underground and "cooked" for a longer time than the basalt.  Some other things can cause generate larger crystals but "time" is usually the most important.  Gabbro, along with basalt, makes up most of the oceans bedrock.

Here's pretty good definition of gabbro;

Your gabbro has an "intrusion" in it.  That's where another rock has filled up cracks in the base rock.  You are correct, the light colored rock is granite.  You seem to know about that so I won't go into it, except to say geologists call that intrusion a "granitic dike".  

You noticed that the gabbro is eroding faster than the granite. That's because the gabbro is made up of less competent #softer# minerals, in the greater part, than the granite.  

Here's a good picture of differential weathering:

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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