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May 22, 2016 Weekly Geology Guess

May 22, 2016

Greetings from the Bluff Park   Porch, way up yonder on Shades Mountain (1,109′) in Alabama:

With my discussion on Alabama Geology, I have gotten the geologic cart before the horse.  I have not included a basic discussion regarding rocks and all the various kinds of sedimentary, metamorphic, and igneous rocks.

We will continue with Igneous Rocks.  Sorry, this will not be easy nor pretty.  R


Minerals and Rocks – The Good Life from the Ground
“If it ain’t grown, it has to be mined”
Igneous Rocks


>    This is one of the best and simple explanations of Igneous Rocks.

–    “Intrusive igneous rocks crystallize below Earth’s surface, and the slow cooling that occurs there allows large crystals to form. Examples of intrusive igneous rocks are diorite, gabbro, granite, pegmatite, and peridotite.”
–    “Extrusive igneous rocks erupt onto the surface, where they cool quickly to form small crystals. Some cool so quickly that they form an amorphous glass. These rocks include andesite, basalt, obsidian, pumice, rhyolite, scoria, and tuff.”
>    “As it turns out, most of the ocean floor is basalt, and most of the continents are granite. Basaltic crust is dark and thin and heavy, while granite is light and accumulates into continent-sized rafts which bob about like corks in this “sea of basalt.” When a continent runs into a piece of seafloor, it’s much like a Mac truck running into a Volkswagon. Not very pretty, but at least there’s a clear winner. And the seafloor basalt ends up in pretty much the same position as does the VW – under the truck (or continent, as the case may be). This may seem like a drag for the basalt, but remember that it isn’t all that happy on the surface anyway, and this gives it the heat it needs to re-melt and try to complete the differentiation process which was so rudely interrupted at the spreading ridge. If successful and allowed to continue, what’s left behind is a “purified” magma with most of the iron, magnesium, and other heavy elements removed. When it cools, guess what forms? And the continental land mass just got a wee bit larger.”  by Geoman.
*    And based on the info above, guess where did the train wreck occur in Alabama?  AL Piedmont.
HA!  I thought that I would just simply tip toe through the patch of Alabama igneous rocks.  It was not to be.  They are a complex lot of intrusive and extrusive igneous rocks that have been tortured [metamorphosed and faulted].
I will try and divide the igneous rocks by the major grouping in the State, see first attachment of the “Alabama Piedmont Geology”.  For location, see second attachment regarding roads.
Travel Note:  Along US Highway 280 In Alexander City, the road cut with the American Flag Pole on top is a nice exposure of Alabama Granite.
Dadeville Complex consists of intrusive granitic bodies and extrusive volcanic sequences, to name a few.
Age:  Taconic U-Pb age of 460 ± 4 Ma.  by FARRIS, David W

And to further muddy up the complex granitic waters of Alabama >>>
Hatchet Creek Group; Pinchoulee Gneiss (Precambrian to Paleozoic)
Pinchoulee Gneiss – medium to fine-grained migmatitic, locally garnetiferous biotite-feldspar gneiss, commonly saturated with granitic pods.
Hissop Granite (Precambrian to Paleozoic)
Hissop Granite – mesocratic to leucocratic, strongly lineated granite to granodiorite.
Opelika Complex; Bottle Granite (Precambrian to Paleozoic)
Bottle Granite – leucocratic, fine to medium-grained, well-foliated quartz monzonite to granite; locally porphyritic.
Rockford Granite (Precambrian to Paleozoic)
Rockford Granite – leucocratic granite, granodiorite, and trondhjemite; locally well foliated, numerous pegmatites. Includes unnamed granitoids in Chilton County.
Uchee Complex; Phenix City Gneiss (Precambrian to Paleozoic)
Phenix City Gneiss – biotite-epidote quartz diorite gneiss and biotite-hornblende gneiss; locally includes migmatitic amphibolite; cut by numerous granitic veins.
Zana Granite (Precambrian to Paleozoic)
Zana Granite – quartz monzonite to granite with strong gneissic texture, cut by small pegmatites and aplite dikes, generally elongate; semiconcordant to foliation of country rock.
All of the other groups in the Alabama Piedmont are metamorphic in nature.
Academic Note:  for those that are professional involved in geology, especially the Alabama Piedmont; this is no slight for the significant work that you have done.  I am attempting to keep a very complex and sometimes contentious issue, as simple as possible for the non-geologist.  Thanks, R
For a more in depth review of igneous rocks, go to


References include:-

–    Wikipedia


Attached is a handy guide to rocks by the USGS (US Geological Survey).  Simple in terms of Petrology, but still detailed.  R



Editors Note:  It is the intent of this site to keep this discussion as simple as possible, so as to educate the interested general public and not to discuss with the geology crowd the latest geologic theories and nuances.  Thanks, R


“No copyright infringement intended.
The rights belong to their respective owners”
Copyright Disclaimer: Under Section 107 of the Copyright Act 1976, allowance is made for “fair use” for purposes such as criticism, comment, news reporting, teaching, scholarship, and research. Fair use is a use permitted by copyright statute that might otherwise be infringing. Non-profit, educational or personal use tips the balance in favor of fair use.


Author’s Request:  If you see any pictures that you know the source and photographer, let me know immediately.  Thanks!  R

Images have been searched by TinEye Reverse Image Search.


Next week we will slither out from underneath the Piedmont into the Alabama Coastal Plain.


AL Geo. Map, Piedmont

Alabama Geologic Map of the Piedmont

AL-P.  Loc.-Roads

Road Map of the Alabama Piedmont

Alabama Granite, Russel Farms, 11-23-13,  (22). by R. McDaniel

Alabama Granite, Russel Farms. 11-12-2013. by R. McDaniel

Alabama Granite, Russel Farms, 11-23-13,  (23). by R. McDaniel

Alabama Granite, Russel Farms. 11-12-2013. by R. McDaniel

Alabama Granite, Russel Farms, 11-23-13,  (24). by R. McDaniel

Alabama Granite, Russel Farms. 11-12-2013. by R. McDaniel

Basalt. Cape Cod, MA. 05-27-2010. USGS.

Basalt. Cape Cod, MA. 05-27-2010. USGS.

Basalt. Eagle Creek, Colombia River Gorge, OR.

Basalt. Eagle Creek, Colombia River Gorge, OR. by Unknown

Volcanic, Extrusive-Basalt Lava Flow.  Newberry Crater, OR. 05-30-2003

Volcanic, Extrusive-Basalt Lava Flow. Newberry Crater, OR. 05-30-2003. by R. McDaniel

Volcanic, Extrusive-Basalt.  Newberry Crater, OR. 05-30-2003

Volcanic, Extrusive-Basalt. Newberry Crater, OR. 05-30-2003. by R. McDaniel

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