Happy Thought for 28 March 2025

Have a Happy Thought: 

 

Does anyone remember when a Department of Transportation was warning people about a partial road closure by telling people there was a “large boulder the size of a small boulder” on the road? 

San Miguel Sheriff @SheriffAlert Large boulder the size of a small boulder is completely blocking east-bound lane Highway 145 mm78 at Silverpick Rd. Please use caution and watch for emergency vehicles in the area. 2:45 PM · Jan 27, 2020 · Twitter Web App 

Image: screen capture of Twitter post from January 2020 

 

So, this is actually an issue of crossing common (or “colloquial” if you want to be fancy) usage with technical usage of the word “boulder”. You see, most of us humans see a large enough rock and think “boulder”, until it gets even larger and it probably gets to “hill” size. 

 

Geologists, though, need to be a bit more precise, and so have fully defined grain sizes from eensy (not a technical term) all the way to ginormous (again, non-technical). 

 

To a geologist, a “boulder” is anywhere from 0.25 to 4 meters in length / diameter. So that boulder shown in the picture above? Yeah, that’s on the larger size of this “boulder” class of particles. 

 

Figure from https://pubs.geoscienceworld.org/sepm/jsedres/article-abstract/69/1/6/99030/Grain-size-and-textural-classification-of-coarse 

 

I personally really love the fact that these “grain” sizes go up so high – so we can even talk about many asteroids and even small moons in these terms. For example, 99942 Apophis (the asteroid that is very much likely to not hit the Earth in 2036) would be a coarse slab – not even a monolith or megalith! 

 

Uluru, the beautiful and sacred sandstone rock in Australia’s Northern Territory, is indeed a monolith, as described on its Wikipedia page.

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Uluru Rock. By Ek2030372672 - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=105272234  

 

Meanwhile the oldest crater yet found on Earth – dated to 3.47 Billion years ago – was possibly formed by a megalithic asteroid (and possibly led to the inception of life on Earth? But that... is a story for another Friday.) 

Conical red rocks in a ruddy landscape with hills in the distance. 

Evidence of asteroid impact in Very Old Rocks in Australia. An approximately one metre tall shatter cone ‘hut’, with the rolling hills of the Pilbara in the background. Image: Chris Kirkland 


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