Happy Thought for 1 November 2024

Have a Happy Thought:    

Sometimes when the Earth moves under your feet, it's for really, really cool reasons! 

 

The example I’ve known about for quite some time is a devastating earthquake in the midwestern United States of America just over a century ago. This massive 8.2-magnitude earthquake in December 1811 known as the New Madrid earthquake (and subsequent aftershocks) happened in basically the middle of a continental plate – this is weird because most earthquakes happen at the edges of and interactions between tectonic plates.  

 

Image: approximate epicentre of New Madrid earthquake, nowhere near a plate edge. Source: https://en.wikipedia.org/wiki/File:Usa_edcp_relief_location_map.png  

 

There are two major competing theories for why there could have been such a big earthquake at this location, and both relate to isostatic rebound. Basically, this is a fancy term for the seemingly-solid tectonic plate actually having a little bit of bend and flex under massive loads. And when those loads come off, the ground – very slowly – returns to its un-loaded shape. 

 

What loads could possibly bend an entire continent, you ask?  

One theory is: glaciers. Yes, they’re “just ice”, but... well, not really. They’re lots and lots of ice, and often lots of rocks and other material caught up in the ice. And when that ice-and-other-stuff is more than a kilometre thick, well, that’s a LOT of pressure on the ground beneath it. So when those glaciers retreat, and that pressure comes off, you might expect some isostatic rebound to happen over the following millennia. 

 

The other theory about what could change so much pressure on a piece of earth is, well, earth itself. Some geologists have looked at the sheer amount of earth, in the form of sediment, that is carried by the Mississippi River system every year, and have theorised that over the millennia (yes, we’re still working on geologic timescales here), enough weight can come off the continental plate to allow for isostatic rebound.  

 

Just a couple of months ago, a much smaller (thankfully) earthquake happened in another place it was again very much not expected – about halfway in between Quebec City and Montreal, Canada. 

Image: marker showing approximate epicentre of earthquake on 1 September 2024. 

 

The possible cause of this 3.8-magnitude earthquake is even crazier than continent-size tectonic plates slowly bending under pressure (or relief). Because this was caused by... 

 

An asteroid! 

 

Sometime around 450 million years ago, a huge asteroid (although smaller than the one that killed the non-avian dinosaurs) landed on the middle of what was at the time an active fault system. This left a giant divot in the ground and a central peak that can still be seen today: 

Image: Annotated photo of the central peak and surrounding rings of the impact structure, from https://craterexplorer.ca/charlevoix-impact-structure/  

 

Now, the fault zone that the asteroid landed on is generally not active any more – hundreds of millions of years have passed since this fault zone laid on a tectonic boundary (for geology buffs: check this out because it’s a really complex history that sent me down several very fun rabbit holes). But this recent earthquake, as well as several other anomalously large (for the area) earthquakes have led some geologists to propose that the asteroid’s impact weakened the crust in a way that channels any stresses in the surrounding bedrock into this one small area. 

 

Long story short: massive ice sheets, rivers, extinct faults, and asteroids can all leave an impression on the landscape, and definitely leave an impression on people experiencing earthquakes, millions of years later! 

 

All of these things, along with the many many other ways that land is shaped and morphed, lead to the natural beauty of our landscape. 

And the best way to celebrate that landscape is to write your name with photos of geologic features. Which you can actually do, thanks to NASA: Here is your Landsat-image “Happy Thought” 

Image: various landsat images of features spelling out ’happy thought’, including images from Brazil, Alaska, Kyrgysztan and the UAE among others. 

 

Write your own name on the earth, here: https://landsat.gsfc.nasa.gov/apps/YourNameInLandsat-main/index.html  

  

If you have been forwarded this and wish to receive these each Friday, email zizabeph@gmail.com and ask to subscribe.    

If you get too many emails, email zizabeph@gmail.com and ask to un-subscribe. (no hard feelings, I promise!)   

    

Also posted to:    

cymittw.blogspot.com