MANKATO — 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.

These Swiss-cheese-like stretches of Madison Avenue were chosen because they had generated the most complaints, Street Supt. Jim Braunshausen said. The city has been using so-called “cold mix” asphalt to patch holes, but that variety isn’t suited for larger patching jobs.

First, crews spread a special kind of oil on the exposed road to act as glue for the asphalt mixture. Using shovels, rakes and a contraption that’s sort of like a giant wheelbarrow, a smooth layer of oil-covered rocks (the asphalt) is spread across the road.

At this point, it’s still hot — up to 280 degrees — but is still more like little rocks than road.

This next part is key.

As the oil cools, it hardens. Steel drum rollers pack the asphalt down.

Wednesday’s weather, warm enough to keep the asphalt from hardening before it was laid but with a cool breeze that hardened the asphalt in only a few hours, was ideal.

As the asphalt cools to below 200 degrees, it becomes clumpy. By 100 degrees, it’s drive-able.

By the end of the day, the city was estimating that more than 200 tons of asphalt would be laid on Madison Avenue.

The long-term goal is to keep Madison Avenue going for two more years, when it’s slated for a total replacement, a much more expensive and time-consuming process than this sort of patching.

It looks like hard work, hot and noisy, but he says crews are happy to mix up what has for months been a winter-dominated routine.

“This is what we’ll be doing all summer,” Braunshausen said.