Sometimes the earth splits open for no apparent reason. It’s pretty frightening to witness giant, miles-long ground cracks that makes the surface feel unstable and we humans feel rather small and vulnerable to be swallowed up by the ground. Many “mysterious earth” websites and those who believe that the End Times are upon us will focus on these earth fissures as if they are unprecedented, unexplainable, and worthy of panic and prayers.

Not quite. A recent example of a ground crack scaring the bejeezus out of people was a giant fissure reported in Arizona by LiveScience and Science Alert a week ago. The opening in the soil was up to 3m across, 25-30 ft deep and 2 miles long, not too shabby. A drone dramatically traced it from the air.

While this particular ground fissure was formed in 2013-2014, it got attention from sensationalist outlets like Daily Mail, (Signs of the Times) and Coast to Coast AM because it’s scary and feeds into the narrative they promote, earth fissures unrelated to seismicity are rather common. In fact, the Arizona Geological Survey monitor 26 areas mostly in Central Arizona where 170 miles of fissures are mapped. Why here? Well, the cause is pretty clear: people have overpumped the aquifers and now the ground is compacting and sinking. Fissures have been noted here since the late 70s related to extensive groundwater withdrawals in the Sonoran desert. The only water available in the desert for irrigation is deep wells. Fissures were first noted near Eloy in 1929. Officials here call it “groundwater mining” because of the removal of so much of the commodity.

Arizona has a “fissure hotline” to report new ground cracks because they pose serious hazards to livestock and off-road travelers in particular. They are particularly dangerous during and after heavy rain when the flow washes in the sides and creates a trap of mud. In 2007, a horse died in Arizona after being trapped in a muddy gouge. Rain will also cause differential settling in clayey material.

A map showing known earth fissures in Central Arizona.

A great piece here describes how these fissures were not a problem 200 years ago but are common today as we use groundwater faster than it can be replenished. The cracks often appear parallel to edges of bedrock that are more stable.

Credit: C. Wanamaker
Credit: C. Wanamaker

In other Western states like Utah, and in Mexico, the ground cracks are sometimes thought to be fault-related. Instead, the fissures are a visible manifestation of the ground subsidence as a result of a water table depleted by 100 feet or more causing damage to buildings, roads, bridges and railroads. According to the Land Subsidence and Earth Fissures in Cedar Valley (Utah Geological Survey),  the ground subsides several feet but moves only inches a year. Cedar Valley land dropped 2.4 inches per year. Continued groundwater pumping will only exacerbate the hazard.

In 2014, several outlets repeated a story of an “apocalyptic” crack in Hermosillo, Mexico, that many feared was related to movement on the San Andreas fault. Careful examination showed that this desert area had nearby irrigated fields suggesting that, once again, groundwater over withdrawal from arid areas was the likely culprit.

Not all earth fissures are related to subsidence. Some rare ones are due to upheaval, like this limestone bulge that very loudly occurred in 2010 in Michigan, uprooting trees. The 360 ft long, 5 ft deep and up to 30ft wide crack resulted from a violent break in the surface resulting from “popping up” of the near surface rock layers due to unloading of pressure. Again, this was not fault-related.

So, the earth will sometimes split open unexpectedly and understandably freaking out the nearby farmers and townspeople. But the geologists know there is probably a good reason for it. Instead of assuming it’s the beginning of the end of the world, they seek out the natural reasons, reasons that sometimes we humans had a hand in bringing about.

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