Cryoseisms and depth hoar in the January cold

surface hoar crystal snow ice
Hidden beauty in fallen snow – magnified surface hoar crystals. / Image Matt Sturm

Laura Nielsen for Frontier Scientists

Cold hit hard this month. January 6 and 7, 2014, brought startlingly frigid temperatures to southern Canada and the United States, weather that swept through the Midwest and then eastward. The U.S. National Weather Service recorded widespread subzero temperatures; on January 7th over fifty U.S. sites measured record low temperatures compared to past measurements on the same calendar date.

These are weather effects: short-term changes, as compared to the longer-term trend of Earth’s warming climate. Just a month prior in December 2013, dozens of U.S. sites tied or beat past record high maximum temperature records and experienced record warm daily minimum temperatures. In December the weather was unusually warm, while in January it has been unusually cold.

That unusual cold can bring unexpected surprises for residents accustomed to warmer winter temperatures. Explosive booming or cracking noises heard in the night, for instance, are a part of the frozen world that can seem quite mysterious.


A cryoseism, sometimes called a frost quake, happens when water freezes underground and causes the ground to crack. Water can seep through soil and infiltrate cracks and crevices in stone and bedrock. An abrupt temperature drop– especially one from above-freezing to below-freezing temperatures– causes the water to turn to ice. As water freezes it expands. The rapidly-forming ice takes up more space and applies increasing pressure to surrounding stone, which can cause a localized underground event. Rocks crack suddenly due to the heightened pressure. It’s a process much like water seeping into cracks in pavement, freezing, expanding, and causing potholes, except it happens below the surface of the ground and on a larger scale. When cryoseisms cause bedrock to split it can sound like a small explosion and result in small tremors. They’re not independently dangerous to people, but the noises that result can be intense.

Sometimes the startling noises (booms usually heard in the dark because it gets colder at night) are accompanied by flashing lights from flickering light bulbs. Eerie. John Ebel, professor of geophysics at Boston College, explains that electrical changes can happen in rocks when they are put under pressure. Those electrical charges can play with local electric systems.

The science of cold can explain the not-so-mysterious effects of cryoseism. It can also uncover a world of beauty.

depth hoar crystal
Example of a depth hoar crystal collected from the base of a snowpit. The crystal is imaged on the left with light, and on the right with Low Temperature Scanning Electron Microscopy. / Courtesy Agricultural Research Service, U.S. Department of Agriculture


Whether glimpsed in person or through photography, you’ve probably seen a snowflake up close. The unique configuration of their ice crystals are indeed lovely, but let’s go deeper to a part of snow that few people ever take a good look at.

Matthew Sturm, professor of geophysics, University of Alaska Fairbanks Geophysical Institute, is a scientists who’s passionate about snow. “Everyone knows what a snowflake looks like,” he says, “but that is not what’s on the ground.” Snow that accumulates on the ground undergoes a transformation. Extremely cold winter air temperatures contrast with the very slightly warm ground, which can retain heat from the sun that is later insulated by a blanket of snow. The snow closest to the lingering ground heat melts slightly and refreezes in big striated “Crystals that are called depthhoar; the Native term or Inupiat word would be pukak.” Sturm says: “These crystals are glittery, the edges are razor sharp, they are the most beautiful,” and “Under a microscope they are utterly fantastic.” Pukak crystals are so large that Sturm’s graduate students used to simply press a digital camera to the eyepiece of a standard microscope to capture stunning pictures.

Snow has a profound effect on the ecosystems it touches. “Snow affects everything and everybody, particularly in the North,” Matt Sturm points out. Setting aside scheduling growing times for plants, impacting nesting migratory birds, and placing energy demands on animals that must work hard to live through the cold seasons, snow also governs planetary energy balance. A snow-and-ice covered land is a cooler one: pristine snow can reflect about 90% of sunlight back into space and sea ice about 70%. Warming climate means less overall ice and more time when non-snow-covered dark land and dark ocean surface absorb heat from our star. It’s a positive feedback loop. And despite our recent cold weather, the overall trend of a warming climate is still soundly impacting our planet.

Frontier Scientists: presenting scientific discovery in the Arctic and beyond


  • ‘Cryoseisms Explained’ The Weather Notebook (accessed January13 2014)
  • ‘Deep Freeze Recap: Coldest Temperatures of the Century for Some’ The Weather Channel (January10 2014)
  • ‘‘Frost quakes’ blamed for waking GTA residents with a boom’ Jodee Brown, staff reporter, The Star (January03 2014)