Musing: The Red Roads of Wyomings

A couple of weeks ago, I drove up to Casper, Wyoming via I-25. Casper is one of the prime viewing spots for the Great American Eclipse which is going to take place starting about 11 am across much of the northern United States. (Several Wyoming cities are in the path of totality, from Casper, to Glenrock, Douglas, Glendo, Torrington and Lusk.)

I've been up to Casper before of course, but this is the first time I really paid attention to the road. I took I-25 all the way up, and if you do too you'll see the grey asphalt of paving that's many years old, black asphalt of brand new (within a couple of years) paving, and red asphalt.

Red tinted asphalt on I-25

The red asphalt is quite pretty. But I know nothing about the paving of roads (except what I don't like, which is that the city of Cheyenne seems to be re-paving roads that don't need to be paved, and neglecting to re-pave roads that do need to be paved! But that's a rant for a different time.) so I decided to do some research to learn about.it. I wanted to know how and why some of the road was tinted red.

I had assumed that the road pavers had actually used red rock for some reason, but not so.

Here's what I learned by researching on the web:

The stuff that is used to pave roads is called asphalt concrete, blacktop or pavement. (In England, just as a point of interest, it's called tarmac or macadam.)

It is a composite material, consisting of a mineral aggregate bound together with asphalt. This material is laid down in several layers, and then compacted.

The color of the road comes from the type of mineral aggregate used. And the different aggregates depend on the type of minerals used - sand, gravel, crushed stone, slag, etc.

The pink/red in the asphalt on Wyoming's roads (and on some other roads across the country, apparently) come from shale, which oxidizes - i.e. rusts, and turns bright red which fades to pink.

Well..that's what one source said. Another has said that the color comes from pink quartzite, which is stone, so there's no rusting involved!  (http://saltthesandbox.org/rocks/quartzitepink.htm) According to this source, many highways in western Minnesota and eastern South Dakota are pink.

So that actually sounds more logical...

Presumably this material is cheaper than the aggregate used to make the coal black asphalt, which is also used in several sections of I-25.