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.

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