Several months ago, I wrote about the fossil my son found and what it most likely was. What I didn’t talk about in either article was the trip we took to the Trammel Fossil Park here on the north side of Cincinnati. It’s really just the exposed rocky side of a hill, with signs posting the various stratigraphic layers so you know where you’re looking and other signs showing you the fossils you’re likely to find at each level. There’s no cost to go, and you’re allowed to keep any fossil-bearing stones you find that you care to haul down the hill and back to your car. I found some brachiopods.
My son was extremely disappointed with the trip, at least for the first ten or fifteen minutes we were there. We’d told him we were going fossil hunting, after all, and he wanted to find a Tyrannosaurus rex skeleton. Which, lets be honest, would have been extremely unlikely even if the park had exposed strata from the Albian. But he was six at the time, and he wanted a dinosaur. So I reminded him that the layers we were looking at were from an ocean, because Ohio was underwater at the time.
I don’t think he asked today’s question at that point, but it helped inspire it. Because, eventually, he asked me this: “Why was the United States underwater?”
Uhm. Something to do with plate tectonics, I guess? And maybe changes in climate?
Can you do better than that?
Of course I can. This’d be a pretty lame blog post, otherwise.
What are plate tectonics?
That’s a great question, and to understand it we’ll need to cover the structure of the Earth itself. The Earth is comprised of multiple layers, rather like an onion. These layers are the:
- Lithosphere: the outermost rocky shell of a rocky planet (our own, for instance).
- Asthenosphere: the hot, viscous layer that the lithosphere floats on.
- Mesosphere (or mantle). Geologists have an explanation for why this is distinct from the asthenosphere and the outer core, and it has something to do with temperature and pressure causing one type of mineral to decompose into another type of mineral. I didn’t quite follow the explanation, and I think I’ll save trying to understand it for the day when my son asks “what is the mesosphere?”
- Outer Core, a sea of liquid iron and nickel.
- Inner Core, an extremely hot ball of (mostly) iron and nickel kept solid by pressure.
The lithosphere is the layer we live on – the high parts are the continents and the lower parts are covered with water. And it isn’t a solid shell. It’s broken up into (depending on who you ask and the definitions they use) seven or eight major tectonic plates and a bunch of minor ones. And the plates move.
Why do they move?
The tectonic plates move because the Earth is hot.
Let’s start with an analogy. When you boil water, you get an uneven distribution of heat Heat rises, after all, but the source of the heat is at the bottom. So the hot water rises and the cool water sinks. But then the hot water at the surface cools and sinks, and the cool water at the bottom heats up and rises. This gives rise to something called convection currents. this effect isn’t limited to water, though. All liquids do it – our atmosphere, for instance (which functions a lot like a liquid).
The Earth, when you get below the lithosphere, is pretty much a liquid as well. The mesosphere has convection currents in it, and the tectonic plates can be thought of as the “cool water” part of the current in the boiling water analogy. Magma pushes up from the mesosphere into the lithosphere at the Ocean Ridge (a planet-circling chain of mid-ocean ridges), pushing and expanding the plates. The plates then sink back down towards the mesosphere at subduction zones. These currents also push around the solid chunks of the lithosphere, in much the same way that ice cubes floating in boiling water will be pushed and shoved around.
Now, even the “minor” tectonic plates are massive structures. So, when they get moving, there’s a lot of force built up. When they collide, something has to give. And frequently, what gives is the structure of the plate itself – it will buckle and crumple, throwing up mountain ranges and pushing parts of the plate below sea level. If water, in the form of the oceans, gets access to that portion of the plate below sea level, it will begin to fill the depression. That’s what happened in the theorized Zanclean Deluge, for instance. 5.33 million years ago, the Mediterranean was a depression in the Eurasian plate (bordered by the African and Arabian plates) that was below sea level. It had been a sea previously, until shifting plates cut off access to the Atlantic and the waters dried out. Then the plates shifted further, access to the Atlantic reopened, and the basin refilled in a period of approximately 2 years (with water gushing in at a flow rate 1,000 times greater than that of the Amazon River).
So. Plate tectonics is the answer?
Really? What else is there?
There’s changing climates. See, the Earth was – on average – a whole lot warmer back before the continents had moved into the form we’d recognize today. At present, our average global temperature is about 60 degrees Fahrenheit. During the Paleocene-Eocene Thermal Maximum (55-56 million years ago) the average got up to about 73 degrees F – there were no ice caps at the poles then, and there were palm trees and crocodiles above the attic circle.
Now, estimates are that if the ice caps melted then global sea levels would rise about 70 meters. So that’s not really enough to make an ocean out of (say) the Great Plains, although it would completely reshape the coast and drown Houston and New Orleans. But since the plates were buckled differently back then, the extra water would have increased the odds of flooding taking place.
But, ultimately, North America being underwater had far more to do with plate tectonics than changes in climate.
Oh, as a bonus, the Paleomap Project has a series of great maps of the Earth in different geologic epochs. Here’s what the Earth looked like during the age of the dinosaurs:
Yep. It was a different world, back then.