Every once in awhile, a question makes it to my inbox that’s too good to ignore.
A friend and I were joking about being “older than dirt” and he asked a question I thought you might enjoy: “Hey, ask Ethan how old dirt is and how it got here.”
You did the smart thing by coming to me, because the alternative is to ask yahoo. (Shudder!) Well, right away, we’ve got an upper limit, because the entire Solar System is only about 4.5 billion years old. So, there’s no way that this:
is any older than this:
Now, you can definitely go looking for the oldest rocks on Earth, and if you look very hard, you’ll find rocks that are 4.4 billion years old, or nearly as old as the Earth itself!
These rocks come courtesy of the Jack Hills in Western Australia. But, that’s rock, not the dirt you’re used to seeing. Because the Earth is an active planet, with oceans, rains, winds, and mobile tectonic plates, what we commonly know as “dirt” gets recycled over and over again.
How does that work? Well, let’s go through a brief version of the history of dirt on Earth. The interior of the Earth is molten, and this heat comes up from somewhere. Right now, the sea floor is spreading in the middle of the Atlantic Ocean:
This causes the plates to spread away from the ridge. As these plates tend to move around, two of them will, at some point, collide together. What happens when we get a collision of plates? We form mountain ranges!
These are the Himalayas, not only the tallest mountain range in the world, but also one of the youngest mountain ranges in the world. Compare this to the Appalachian mountain range, which is less than one-quarter of the height of the Himalayas, but which is the oldest mountain range in the world, with parts dating back as far as 480 million years!
Well, even 480 million years ago, the world was teeming with plants, animals, bacteria, rain, winds, and lots of other things that erode rock and produce dirt. But, this dirt, especially sand and topsoil, is subject to the most relentless force of all on Earth: gravity. Over time, this dirt finds its way –one way or another — to the lowest elevation possible, which is to say, the bottom of the ocean.
So, although rocks may be over 4 billion years old, dirt isn’t. In fact, after the end of the last ice age (around 12,000 years ago), glaciers moved much of the topsoil around North America, leaving many regions scarred and soil-less. And yet, plants, wind, rain, ice, animals, and other factors immediately began creating soil again. While there is most definitely “old” dirt that is many tens of thousands of years old, most of the dirt that’s out there, as best as I’ve been able to find, is just a few thousand years old. Or, you know, as old as some of the oldest trees we have:
So, the next time someone tells you you’re older than dirt, you’ll be able to smile to yourself, knowing that dirt on Earth isn’t nearly as old as they think it is. It’s just rock that’s been reduced to a dusty pulp over the last few thousand years. Not so old, at least, geologically speaking.
That is, until someone tells you that you’re older than Moon dirt. That is billions of years old, and that’s old any way you slice it. So watch out; those are fightin’ words.