Tag Archives: igneous

Geo-pic of the week: Igneous Dike

igneous dike

 

100 million years ago, during the Late Cretaceous Period, a preponderance of igneous activity occurred in the continental region now known as Arkansas.  In fact, all of the igneous rocks discovered in the state were emplaced around that time.  Some of them are well known, such as Magnet Cove, located east of Hot Springs, or the diamond-bearing intrusion near Murfreesboro.    There are also lots of smaller igneous intrusions like the one shown in the picture above. 

Small igneous intrusions are found throughout the Ouachita Mountains.  There are so many small intrusions that new ones are regularly discovered.  Weathering at the earth’s surface has typically destroyed the original rock’s characteristics and what remains is mostly soft clay because the minerals that make up the intrusion are unstable under surface conditions. 

If you happen to notice an unusual-looking body of rock that cuts across the strata of a road cut or other rock outcrop when you’re exploring the Ouachita Mountains, it’s likely that you have seen a Cretaceous igneous dike.

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Geopic of the week: Arkansas Bauxite

 

bauxite enhanced

Pictured is a piece of bauxite, a sedimentary rock for which the central Arkansas town of Bauxite is named.  It’s also the principal ore of aluminum.  At times in the past century, Arkansas bauxite was the source of as much as 90% of the aluminum produced in the US.

Bauxite is a chemical sedimentary rock that formed in Arkansas from weathering of igneous rock.  During the Eocene (55 – 34 million years ago), North America was nearer the equator, and the tropical climate concentrated aluminum in rocks exposed south of Little Rock.

Following WW2, production of aluminum from Arkansas bauxite declined due to availability of rich international ores.  Mining of bauxite for aluminum ceased altogether in the state in 1981, though there is still a significant quantity of reserves.

 

To see more views of Arkansas Bauxite, click here

Statemap Field Blog—Nov 12-14, 2013

Hello all!

This week we took a break from our regular field area, and were treated to a three days of field trips relating to the sparse though important igneous rocks of Arkansas led by our own Mike Howard.  Mike, who is regarded as the preeminent expert on the mineralogy of Arkansas, will be retiring in a few weeks after a 39 year career at our agency,

We stopped at numerous locations in Pope, Conway, Pulaski, Saline, Hot Spring, and Pike counties.  The first day, we headed up to the northernmost igneous intrusion in the state which is north of Dover just off Highway 7 on Dare Knob.  From there we stopped at several carbonatite sills (igneous intrusions that follow sedimentary bedding planes) in the Arkansas River Valley near Oppelo.

2013-11-13 0052013-11-13 0012013-11-13 0132013-11-13 014The next day we started out with a visit to the 3M quarry on Granite Mountain.  Granite Mountain is actually a relatively small exposure of an igneous batholith which extends to an area of over 250 square miles in the subsurface.  Also, it is not actually granite at all, but nepheline syenite.    It is being processed mainly into roofing granules.  The other large active quarry in the area, Granite Mountain Quarry, produces mostly aggregate materials.

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From there we went to Bauxite where we saw a large olistolith in the syenite.  A xenolith is a piece of the country rock that is incorporated into the melt of an igneous intrusion.  In this case the country rock was probably Jackfork Sandstone as evidenced by tight folding in a quartzitic rock slightly metamorphosed by contact with the igneous body.   From there we made several stops in the Magnet Cove area, where we saw broken phenocrysts (large crystals in a finer matrix) of pseudoleucite, carbonatite, garnet, rutile and pyrite.2013-11-13 0372013-11-13 040

We also saw the largest barite pit in the U.S. (now abandoned), and a nearby vein of smoky quartz that was easily accessible.

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The next day we drove to the diamond mine at Murfreesboro, with which Mike has had an association since even before it was made a state park.  I found a good barite crystal, but unfortunately, no diamonds.

These trips were attended by almost the entire staff, and we much appreciated getting to see these rare rocks and minerals with knowledgeable commentary by the state’s most expert resident.  We will miss having him at the survey to answer our many questions, but hope that he enjoys many years of well-deserved leisure activities.  Thanks, Mike!!

Until next week, I’ll see you on the outcrop!