NYCPHP Meetup

[Matt Morgan]

David Mintz wrote:

>
>On Tue, 1 Feb 2005, Hans C. Kaspersetz wrote:
>
>  
>
>>I think "dirt" is pretty old:
>>
>>Dirt: Middle English, variant of drit, /excrement, filth, mud/, from Old
>>Norse.
>>    
>>
>
>No, not the word "dirt," but its referent -- the substance itself! (-:
>
>
>  
>
>>David Mintz wrote:
>>
>>    
>>
>>>I have squandered the last several minutes trying to determine how old
>>>dirt actually is, i.e., how long "dirt" itself has been in existence (on
>>>Earth). I know it's gotta be older than PHP.
>>>
>>>Perhaps one of you clever NYPHPistas you knows some geology can help us
>>>out.
>>>      
>>>
Hey, I'm a geologist (by academic training, anyway). Dirt comes from 
rock. The oldest possible dirt would then be about 3.5 billion years 
old, since the oldest rocks on the planet are about that old; but any 
dirt formed that long ago would have become rock again at some point in 
its history.

Given the periodicity of subduction/orogeny, probably no dirt ever gets 
to be more than a couple hundred million years old before being washed 
out to sea, sedimented to the ocean floor, and either compressed into 
sedimentary rock, or failing that, subducted and either remelted 
(becoming igneous rock upon cooling) or metamorphosed. Of course most 
dirt would turn into sedimentary rock much more quickly than that: 
sedimentary rocks form on scales on the order of hundreds of thousands 
to millions of years, rather than tens or hundreds of millions.

There are places where, theoretically, dirt could last longer--I'm 
thinking of the trailing edges of the continental shields, like the east 
coasts of North and South America (among others). Those places are 
stable, at least until the worldwide pattern of plate tectonics 
collapses into some other pattern. That happens roughly every 500 
million years or so (don't quote me on that--but I bet I'm right within 
a factor of 2). But really most dirt would get buried and turn into 
sedimentary rock relatively quickly, anyway.

Thanks for asking!!

--Matt
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