If you've ever seen a pile of rocks that left you saying, "Wait...what?", chances are good that you were looking at what geologists call precariously balanced rocks, or PBRs. PBRs seem to defy physics - they look like they were stacked by someone with a huge crane and a lot of patience. Surprisingly, these impressive formations are totally natural!
You might be lucky enough to have a PBR in your area. Maine's Acadia National Park, Big Bend in Texas, D.L. Bliss State Park in California, and Westchester County in New York all boast impressive balanced rocks. There are some jaw-dropping examples in Zimbabwe and Australia, too.
There are a few ways PBRs can form. One of the most common causes is glacier action. The balanced rock might be what scientists call a "glacial erratic." This means that the huge rock got lifted up and carried away by a glacier sliding over it. When the ice melted away, the rock was left balancing on top of rocks in a new location. This action explains landforms like the Kummakivi Balancing Rock in Finland.
PBRs can also be caused by weathering and erosion. When a rock outcropping is a different makeup than the surrounding landscape, weathering and erosion has a different effect on the two materials. The Devil's Marbles in Australia, shown in this antique photo, are clear examples of this type of geological change. Hard granite structures broke through the local sandstone due to tectonic plate movement. The exposed granite was then shaped by wind and water into rounded shapes.
Sometimes what appears to be a boulder balanced on top of others is actually just one big rock that's been carved by wind erosion. These are what scientists call "hoodoos," and there's quite a few of them in Big Bend, Texas. The part of the rock that's exposed to the most intense wind will wear away, leaving behind tall, top-heavy structures. Balanced Rock in Arches National Park in Utah is an iconic example.
PBRs can tell us about more than glaciers and weathering; PBRs can tell scientists a lot about the seismic activity in a region, too. Precariously balanced rocks are exactly that - precarious. If they are shaken by a strong earthquake, for example, they fall down. If we can estimate how long a PBR has been in its current position, we can tell the strength of the largest earthquake that has occurred in that area during that time span.
Here's a graph showing some of the more famous precariously balancing rocks and their impressive approximate weights.
If I brought this graph to class, here are some questions I'd have to go along with it:
💡Which PBR weighs the most according to this graph? Which PBR weighs the least? The Balanced Rock in Utah has the greatest weight with 3,577 tons. Bubble Rock in Maine has the least amount of weight at 100 tons.
💡One ton is equal to 2,000 pounds. What is the weight of Krishna's Butterball in pounds? Krishna's Butterball weighs 250 tons. 250 x 2000 = 500,000 pounds.
💡Tripod Rock in New Jersey weighs 163,260 kilograms. How could you find out how many kilograms are in one ton? Tripod Rock has a weight of 180 tons. If you divide 163,260 by 180, that will give you the amount of kilograms in one ton (907).
