Science Explains Why Boston’s Molasses Tsunami Was So Bad

In 1919 wave of molasses in the North End killed 21 people. A Scientific American investigation explains why.


The Great Molasses Disaster of 1919, in which a burst tank sent a giant wave of molasses rushing through the North End, often conjures images of a horror movie parody in which people run in slow motion from a wave that’s moving … well, as slow as molasses. When you read that 21 people died, you wonder whether any of them had more sense than the henchman who got crushed by Austin Powers because he refused to move out of his path for like 20 seconds.

That, of course, is not the case. The wave was perhaps 7.5 meters high and 50 meters wide at its peak and it moved at a frightening 35 miles per hour. (Jamaican sprinter Usain Bolt broke the world record at a top speed of 27.79 mph.)  Scientific American writer Ferris Jabr was already at work on a project about how microbes move through viscous liquids when, while walking through the North End, he spotted a plaque commemorating the disaster. Fluid dynamics, he thought, might help explain an event that’s often tough for those of us who’ve never seen fast-moving molasses to comprehend.

He presented his findings in an article for Scientific American, writing that molasses was, for several reasons, a particularly unfortunate substance to have coursing through the streets of Boston on that unusually warm January day.

“It totally sort of blows away the old thing of ‘slow as molasses’ because molasses can in fact move very quickly if you get it at the right pressure,” Jabr says in an interview. “That’s because not only did the whole tank rupture, but it’s a five story tank, so molasses falling this distance had gravity work on it as well.”

He describe more technically the difference between water and molasses in his story:

Molasses is a non-Newtonian fluid, which means that its viscosity depends on the forces applied to it, as measured by shear rate. Consider non-Newtonian fluids such as toothpaste, ketchup and whipped cream. In a stationary bottle, these fluids are thick and goopy and do not shift much if you tilt the container this way and that. When you squeeze or smack the bottle, however, applying stress and increasing the shear rate, the fluids suddenly flow. Because of this physical property, a wave of molasses is even more devastating than a typical tsunami.

It’s hard to imagine how the consistency of toothpaste would change if a giant squeezed 7.5 million liters of it from a tube while you were standing in front of the opening, but if you do, you might not wonder so much at the results when 7.5 million liters of molasses caught up with the people of Boston that day.

But it’s not just that molasses moves faster under pressure than its standing consistency would suggest that makes this disaster so bad. It’s what happens when molasses returns to a standstill, at which point it becomes exactly as unpleasant a substance in which to be swallowed as you might expect. “It’s not going to do what water does,” Jabr says. Again, he explains in his article:

[A] man immersed in molasses will not get anywhere with the kinds of symmetric swimming strokes that would propel him in water. Each repetitive stroke would only undo what was done before. Pulling his arm towards himself would move molasses away from his head, but reaching up to repeat the stroke would push the molasses back where it was before. He would stay in place, like a gnat trapped in tree sap.

Water, meanwhile, generally stays where you’ve displaced it with your arm when you go to move that arm forward, making it a rather pleasant liquid through which to do the front crawl. Even strong swimmers caught up in the flood that day had trouble removing themselves using strokes that might have worked at the beach.

The molasses flood is always an historical event that seems whimsical until you pause to consider it for more than five minutes, at which point it does become rather horrifying. But read Jagr’s work in full and you realize why it’s so horrible. Molasses is thin and fast exactly when you don’t want it to be—when it has been violently released from a faulty storage tank while you’re walking through a city street, for instance. And it’s thick and nearly impossible to move through also exactly when you don’t want it to be—when you’re drowning in it.