How Do They Dye the Chicago River Green for St. Patrick's Day?


It wouldn’t be a St. Patrick’s Day celebration in the Windy City without 400,000 spectators crowding the banks of the Chicago River to “ooh” and “aah” at its (temporarily) emerald green tinge. But how do officials turn the water green?

First, a bit of history: The dyeing tradition became an annual thing nearly 60 years ago, in 1962, but its real origins go back even further. In the early days of his administration as Mayor of Chicago, Richard J. Daley was a man on a mission to develop the city’s riverfront area. There was just one problem: The river itself was a sewage-filled eyesore. In order to get to the bottom of the city’s pollution problem and pinpoint the exact places where waste was being discarded into the waterway (and by whom), Daley authorized the pouring of a special green dye into the river that would allow them to see exactly where dumping was occurring.

Fast-forward to late 1961 when Stephen Bailey—part of the Chicago Journeymen Plumbers Local, the city’s St. Patrick’s Day Parade chairman, and a childhood friend of Daley’s—witnessed a colleague’s green-soaked coveralls following a day of pouring Daley’s dye into the Chicago River. That gave Bailey an idea: If they could streak the Chicago River green, why not turn it all green?

Three months later, revelers got their first look at an Ecto Cooler-colored river when the city poured 100 pounds of the chemical into the water. They got a really good look, too, as the river remained green for an entire week.

Over the next several years, the same practice was repeated, and again it was carried out by the Plumbers Local. The only difference was that the amount of dye used was cut in half over the next two years until they finally arrived at the magic number: 25 pounds of dye = one day of green water.

Unfortunately, the dye that was intended to help spot pollution was an oil-based fluorescein that many environmentalists warned was actually damaging the river even more. After fierce lobbying, eco-minded heads prevailed, and in 1966 the parade organizers began using a powdered, vegetable-based dye.

While the exact formula for the orange powder (yes, it's orange until it's mixed with water) is kept top-secret—in 2003 one of the parade organizers told a reporter that revealing the formula would be akin to “telling where the leprechaun hides its gold”—there are plenty of details that the committee lets even non-leprechauns in on.

The dyeing process will begin at 9 a.m. on the morning of the parade—which this year is Saturday, March 16 (it's always held on a Saturday) when six members of the local Plumbers Union hop aboard two boats, four of them on the larger vessel, the remaining two on a smaller boat.

The larger boat heads out onto the water first, with three members of the crew using flour sifters to spread the dye into the river. The smaller boat follows closely behind in order to help disperse the substance. (The best place to catch a glimpse is from the east side of Michigan Avenue, or on Upper and Lower Wacker Drive between Michigan Avenue and Columbus Drive.)

Approximately 45 minutes later, voila, the Chicago River is green—but don’t expect it to stay that way. These days, the color only sticks around for about five hours. Which is roughly the same amount of time it takes to get a perfectly poured pint of Guinness if you venture out to an Irish pub on St. Patrick’s Day.

A version of this article first ran in 2017.

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What Caused Pangea to Break Apart?

Emily Devenport:

There's another way to look at this question. People tend to think in terms of supercontinents forming and then breaking up again due to convection currents in the mantle, hot material rising and causing rifts in weaker spots, possibly in old sutures where the continents were shoved together—but what is really happening is that ocean basins are opening and closing, and the ocean has an active role in subduction.

The opening and closing of an ocean basin is called a Wilson Cycle. It begins when hot material rising from the mantle stretches the overlying crust. As molten material rises, a rift is formed. The rift is widened as material continues to squeeze into it. If that rifting goes on long enough, through a broad enough swath of a continent, ocean water will eventually flow into it, and an ocean basin begins to form. The upwelling of hot material will continue to rise through that thinner area of crust, pushing the plates apart. The Atlantic Ocean is an example of a basin that is well along in the Wilson Cycle; eventually subduction is going to begin at its margins, and the whole shebang will pivot.

This will happen because at the edge of continents, sediments accumulate. The weight of those sediments, combined with the weight of the water, drives the heavier, denser edge of the oceanic plate under the continental crust, which is fatter and lighter. Eventually subduction begins, and the basin begins to close again. The Pacific Ocean is an example of a basin that's closing.

If you look at a map of the oceanic rift zones, you'll notice that the one in the Atlantic is pretty much in the middle of that ocean, but the Pacific rift zone has been pulled all the way over to North America above Central America. Subduction is actively occurring on all margins of that plate.

The simple picture is that the continents are moving toward each other across the Pacific Ocean while the Atlantic Basin continues to widen. The truth is more complicated. When plates subduct, the water in the crust lowers the melting point of those rocks, so partial melting occurs. The partially melted material begins to rise through the overlying rocks, because it's less dense, and decompression melting occurs. Eventually, the upwelling of hot material forms plutons and volcanoes above the subduction zones. Fore-arc and Back-arc [PDF] basins can form. As the oceanic crust is pulled under the continental plate, island chains and other chunky bits get sutured to the edge of the continent along with sediments, making it larger. Our world is ~4.6 billion years old, so our continents are really large, now. They're unlikely to rift through the ancient cratons that formed their hearts.

What will happen if subduction begins on the eastern side of North America before the Pacific Basin closes? The margin next to California is a transform fault; it's not subducting. Will it eventually push itself under that part of North America again, or will the transform zone get bigger? The hot spot that was driving the ancient Farallon Plate under North America was eventually overridden by the southwestern states (Arizona, New Mexico, etc.) forming a rift zone. Will it continue to rift or poop out?

There are computer models predicting what supercontinent may form next. They will continue to change as our understanding of tectonic processes gets more accurate.

This post originally appeared on Quora. Click here to view.

What Do Those Recycling Symbols and Codes Mean?

Earth Day is here again, serving as an annual reminder of the need to reduce, reuse, and recycle our way to a better planet.

When it comes to the last part of that familiar three-”R” mantra, most people know enough to separate certain items from the rest of their garbage, but much of our modern recycling routine remains a mystery. From the recycling symbol itself to what those numbers on plastic containers actually mean, there's a lot you can learn from your trash before it becomes someone else's treasure.

An International Symbol With An Earthy Origin

The universal recycling symbol—three folded arrows that form a triangle, with the head of one arrow pointing to the tail of the next—was created in 1970 by University of Southern California student Gary Anderson as part of a contest tied to the very first Earth Day. Each arrow of the design represents one of the steps in the recycling process: collecting the recyclable goods after use, breaking them down and reforming them, and then packaging new products in the containers.

Originally designed as an inverted triangle, the symbol was later rotated to the pyramid-like orientation commonly used now.

The Number Game

The American Society of Plastics Industry first began using numbers inside the recycling symbols on plastic containers in 1988 as a way to assist with sorting them. The "Resin Identification Code" uses seven numbers to identify the type of synthetic material used to manufacture the container, with the higher numbers representing less commonly used plastics.

Here's a primer on each of the codes:

1. Polyethylene Terephthalate (PETE/PET)
Usually accompanied by the letters "PETE" or "PET," this resin is generally used for soda bottles and other containers for edible and non-edible goods. When it's not being used to manufacture containers, you might recognize it by another name: polyester. (Yes, it's the stuff that insulates your jackets.) It's also one of the most widely accepted forms of plastic in curbside recycling programs, though the amount of useable material available for new products after breaking down this plastic is relatively small.

2. High Density Polyethylene (HDPE)
The second most widely used resin for plastic bottles, HDPE is a stiff, strong material with a high resistance to chemicals, which has made it the go-to plastic for food items like milk and juice, as well as household cleaners and trash bags. It's also easy to break down in the recycling process and easy to reform, making it one of the most efficient consumer plastics. Most curbside recycling programs have no problem with accepting products made from this plastic.

3. Polyvinyl Chloride (PVC)
First discovered in the 19th century, PVC is commonly used in building materials today—especially pipes and plumbing material—due to its strength and chemical resistance (although it's occasionally used for some household products). It has a nasty habit of releasing highly carcinogenic toxins into the atmosphere when it's burned, so recycling is a significantly less appealing option for PVC disposal, and it's usually not accepted by curbside recycling programs.

4. Low Density Polyethylene (LDPE)
This plastic is becoming more common today, especially for manufacturing squeeze bottles and grocery bags. Plastics made from LDPE are usually very strong, and they're regularly used as sealants because of this quality. While they weren't included in curbside recycling programs at first, plastics made from LDPE are now becoming more commonly accepted.

5. Polypropylene (PP)
Regarded as one of the “safest” plastics produced today, PP is generally used for squeezable bottles, bottle caps, and straws. Along with LDPE, it's also used for food-storage containers that can be reused over time. It has an extremely high melting point, so it's one of the best consumer plastics for items that will be exposed to heat. Like LDPE, it's becoming more common for curbside recycling programs to accept items made from this plastic.

6. Polystyrene (PS)
More commonly known as styrofoam, this type of plastic is not only notoriously difficult to recycle, but it's also been shown to leach dangerous toxins over time into anything packaged in it—and even greater amounts of toxins when it's burned. This is the resin usually found in disposable serving trays, egg cartons, and cups, and it's rarely accept by curbside recycling programs due to the danger it poses and the difficulty of recycling it. Basically, this is the worst of the bunch.

7. Everything Else
There are countless other plastics, but very few of them are easily recycled in curbside programs, making this category the catch-all for everything that could conceivably be broken down and reformed, but might be better off reused or reformed in some way that doesn't require a chemical process. This category encompasses everything from bulletproof material to those large water jugs on office coolers, and is rarely included in curbside recycling programs.

Safety In Numbers

For anyone wondering which plastics are safe to reuse in their current form, it's widely accepted that HDPE (2), LDPE (4), and PE (5) can be reused multiple times for edible items, as they're generally resistant to chemicals, haven't been shown to degrade, and don't leach dangerous substances into their contents.

This story first ran in 2013.