The polar vortex currently sweeping across the Midwest toward the eastern United States has caused dangerously cold conditions this week, causing the deaths of 11 people and even more injuries due to frostbite. Adding to the terrifying conditions in some states, the sounds of whirling winds and rattling trees were punctuated by thunderingly loud booms. Residents can rest assured: Those loud noises were caused by “frost quakes,” which are horrifyingly named but are fortunately a safe and natural byproduct of the strange physics of water.
What is a “Frost Quake”?
The breaking noise associated with a “frost quake” is the sound of soil that’s been compressed at such high pressure that it suddenly breaks, causing a loud cracking sound. That’s the frost quake, or cryoseism, the scientific name for a crack in the soil that was previously saturated with water or ice. All that pressure is caused by water that has permeated far below the surface of the ground that suddenly freezes because of a massive blast of cold temperatures, like a polar vortex.
As anyone who’s ever made ice cubes has witnessed, water expands when it freezes. That’s why it floats: its molecular structure expands once the temperature dips below 4 degrees Celsius, causing it to become less dense and rise above liquid water. Most liquids contract as the temperature drops, making water especially unusual — and particularly frightening to have in the ground during a cold snap.
Here’s how Andrew Leung, a Ph.D. candidate at the University of Toronto Scarborough (a place very familiar with cold snaps), explained frost quakes in 2014, not long after the region wintered through an intense ice storm.
When water enters soil leading to saturation and suddenly freezes, the water becomes ice and expands inside the soil. Slowly, all the spaces in the soil are used up which cause the soil pressure to build. Eventually the pressure reaches a critical point and the soil gives way to release the pressure. As the pressure is being released, it causes cracking or thundering noises. Snow is an effective insulator, so shallow or no snow cover is also a necessary requirement for this deep freezing.
Fortunately, according to Leung, they’re only “mild nuisances” and don’t pose much of a risk, other than disturbing people trying to sleep through the cold.
People weathering the cold, however, might want to be prepared for the opposite of thundering winter booms: the creepy whispers of frozen Lake Michigan, another victim of the polar vortex, which you can hear below.