Is it dust? Sun? Mississippi mud? Guess again.

This is the latest edition of Curious Mississippi, a service to the readers of the Clarion Ledger. Other questions answered by Curious Mississippi have surrounded potholes, cicadas, the international nature of Jackson’s airport and water availability charges. Last week, we looked at the expense of registering a car in Jackson.

Ever wondered why the streets in Jackson and other parts of Mississippi have a distinctive red-brown hue?

While some people may guess it’s dust or a street suntan or maybe just some classic Mississippi mud, the answer is far more simple. The asphalt is really, really old. 

“What you’re looking at out there is older asphalt, probably more than 15 years [old], maybe 20,” said Lance Yarbrough, associate professor of geology and geological engineering at University of Mississippi.

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Good asphalt can last up to 30 years, Yarbrough said, but it has to be maintained to beat out the Mississippi heat and rain. 

“After many years that asphalt ages out in a process called oxidation. All you’re doing is bleeding out bits of volatile materials and it doesn’t look black anymore. It goes to that stained brownish gray,” he said.

There are two main components of asphalt: the binder, which gives asphalt its distinct black coloring, and the aggregate, the sand and rocks mixed in. 

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Though brown roads in and of themselves are not dangerous, very old asphalt that goes unattended can lead to cracks and potholes. If you notice smooth blacktops in other cities outside of Jackson, it’s from updated maintenance and repaving.

“What you’re seeing now is that lack of maintenance, lack of money being able to do the maintenance,” Yarbrough said.

A close examination of one of Jackson’s tried-and-true roads reveals rocks in rainbow hues of white, red, orange, gray, black and white. 

Griffin Sullivan, materials engineer for Mississippi Department of Transportation, said that as the asphalt on the state’s interstates and highways age, the top layer of black binder wears off, revealing a new color below.  

“The color it changes to is actually just the color of the rocks that we use to make asphalt, so here in Mississippi, we use a lot of gravel to aggregate our asphalt, so it will take on the color of tan,” he said.

Exposure to sun and UV rays also contribute to the discoloration, Sullivan added. 

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Are brown roads unique to Mississippi? Yes and no. In states such as Alabama, where the main material in the aggregate is limestone, older roads will take on a gray tone. But Mississippi gravel, the local aggregate source chosen by asphalt companies for its economic viability and long-lasting road performance, fades to a reddish brown.

“It’s really dependent on the materials it’s made out of,” Sullivan said. “The only time a road is truly black is when it’s first constructed.”

Years later, all that’s left is the rock below.

Have a question? Email CuriousMississippi@ClarionLedger.com.