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Rock or fossil
Rock or fossil  

Rock or Fossil
Rock or Fossil  
I found this rock near the Colorado River in Rifle Colorado this past summer. I am just starting to collect rocks and have several unique ones. Can you tell me anything from these pictures?
Thank You!

Hi Tanna,

Nice specimen you have there.  It's a bit blurry but I am sure it's what we call a "porphyry".  That's an igneous rock, one that was completely melted and then lithified.  

When rocks are heated enough to melt, all the various elements are able to move around.  When the melted rock (magma) begins to cool enough to turn into a solid (lithify), mineral crystals form.  If it cools very quickly, the crystals are very small, sometimes too small to see with the unaided eye.  If it takes a long time to harden,the crystals can become very large.

Sometimes the rock crystals start forming and begin to grow very large, but then the magma is pushed up to the surface quickly and the rest of the melt solidifies at a much faster rate.  Then you are left with a rock that has big crystals embedded in a groundmass of much smaller ones.  We call this rock a porphyry.

That's what you have.  The groundmass is probably a rock called Andesite while the large crystals are probably Plagioclase feldspars.

Here's a site that has igneous rocks.  One looks much like yours.

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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