The unofficial start to southern Minnesota’s road construction season can perhaps be traced to a few minutes before 8 a.m. Wednesday.

That start may not have the astronomical certainty of the seasons, but it’s when the first hot asphalt of the year fell into the first waiting dump truck en route to patch a road.

In this case, that was a particularly bumpy section of Madison Avenue.

It took just a few hours for the finely crushed rock in W.W. Blacktopping’s storage bins to be transformed into smooth driving surface.

Here’s how it happened, and who helped it along:

Let’s start at W.W.’s lot, skipping the story of how the raw materials got there. Suffice it to say the rock was mined from a Le Sueur quarry, Johnson Aggregates, and the oil was refined at the Pine Bend Refinery in Rosemount.

It ends up in the capable hands of Rob Hermel, who manages the company’s outdoor plant, the first to open in southern Minnesota this year. The company uses most of the asphalt it makes but sells about 15 percent.

The rock is sorted by size, from fine sand to three-quarter-inch-across gravel, and put in four large bins.

Like a chef assembling ingredients, Hermel tells a computer just how much of each sort of rock will go into a particular blend of asphalt. The smaller rocks are more expensive because there’s more work and waste involved in crushing them to that size, Hermel said.

The different types of rock, called “aggregate” by the industry, are mixed in a cylindrical bin that’s parallel with the ground but sloped down. Some recycled asphalt, typically scraped from the top of roads and parking lots, is added in to substitute for part of the oil, which comes in next. It’s not exactly motor vehicle oil but more a thick sludge byproduct of the refining process.

Though oil only makes up about 6 percent of the total weight of the asphalt — yep, asphalt is basically crushed rock covered with oil — it’s the most expensive part.

A large flame heats the rock-and-oil mixture, and it comes out in a thick slurry that vaguely resembles a steaming pile of caviar heated to about 320 degrees Fahrenheit.

It’s dumped into a waiting dump truck and hauled directly to the site.

Wednesday that site was Madison Avenue, where a 12-person crew headed by Foreman Joe Grabianowski turned the rock into road.

Earlier in the week, city crews had scraped the top 2 inches or so from the road, which is only about a fifth of the asphalt’s depth.