Friday, February 15, 2013

Meteorite Hit Indiana!

I posted this image earlier today on twitter as a tease, asking if anyone knew what it is. It is proof that a meteorite struck Indiana! Before my passion for the planet turned green, I studied exploration geophysics. A science-y background is serving me well as a BathroomFarmer because I have a science-y vocabulary and can read serious science-y publications without dying of boredom and I can fail forward on the BathroomFarm in a systematic science-y way so no one in the future has to make the same mistakes when growing an indoor garden of his own!

If you have ever been to Newton or Benton Counties in Indiana, you know it has some of the best farmland in the world. It's nice and flat with a thick topsoil of glacial till washed all the way down to Central Indiana from the glaciers that carved the Great Lakes. There's nothing but farm for miles around and that also makes it an ideal place for a giant wind farm spanning several counties, too. Well, except around Kentland, IN,  where it gets a little weird.

Here's the weird. This is called the Kentland Structure. Around 97 million years ago, a really big meteorite impacted this spot. It is one of the biggest impact features in the world and the 4th largest known such structure in the United States. I won't bore you with all of the ho-hum science, but this area is a quarry today. The "money rock" at this operation is called St. Peter sandstone. It's a white sandstone that is almost pure quartz and is used in glass manufacturing.

This is a hand sample of what St. Peter sandstone normally looks like and what the grains look like under magnification. It is glistening and gorgeous in large outcrops. There's plenty of this rock at Kentland, but there's also something amiss about some of the sandstone, too. My rock sample at the top of this post is called a shatter cone and is pretty well recognized today as a result of meteorite impact. I visited this quarry in 1979 with a class. We toted tripods and theodolites all over that quarry and measured locations and angles of the St. Peter sandstone that was altered by catastophe and now looked like dull, gray cones. Some of these cones were more than 3 feet long. We were trying to replicate the study behind a hugely controversial theory from one of the most famous geophysicists ever. In the old days, when we carved our class notes into the same tablets of stone we studied, people thought Robert Sinclair Dietz was a little, well, "out there".


I still think it's pretty cool that his original scientific theory was formulated and, now agreed, proven, right here in the Great State of Indiana. What makes the Kentland Structure the location for the study is the almost complete flatness of the area, which has always been flat throughout the history of the planet. There is, therefore, no necessity to reconstruct the position of the rocks at the exact time of impact. When we say that nothing changes in Indiana, we truly do mean EVER. Once we measured angles of the cones, it was possible to calculate per Dietz's idea, where the points of the cones intersected above the ground. That's the actual point of the impact. Almost no very large meteorites ever hit the ground. Friction from our atmosphere cause the explosion to occur well above the ground, which is the case here.

It still packed a giant PUNCH, though. It blew a dome 12.5 kilometers in diameter and created faults in the process. It exposed some really valuable rock that was deeply buried. It pulvarized the quartz particles in some of the St. Peter sandstone into rock flour. Some of this rock flour was instantaneously rehardened by intense, fast heat and pressure into cones making a record of the event. In case you ever need something smart to say at a cocktail party, the shock metamorphism that made the shatter cones is technically called cryptoexplosion deformation. How big was the blast that made the Kentland Structure? It would be the equivalent of the force of an earthquake measuring XII+ in the Mercalli Scale, but concentrated in a tiny area.

Persons settling "these parts" in Benton County, IN had no idea what was hidden beneath their crops. As erosion thinned the topsoil, revealing rock with economic use and quarrying began, people observed the big clues to a catastrophic past. Studies conducted in this site have contributed to a better understanding meteorite impacts that are studied all over the world. No doubt, geophysicists will be heading to Russia with their tripods and theodolites to look for the new clues. What makes the Valentine's Day Impact very exciting to study is that there are videos and eyewitness accounts to add to the information in the rocks! Who'd a thunk all this ruckus started back home on Indiana farmland?


  1. I've been to this site, and I have a shattercone from there too. Pretty cool stuff.