Friday Sprog Blogging: where does land come from?

Dr. Free-Ride: So, where do you think land comes from?

Younger offspring: Land comes from ... I don't know.

Dr. Free-Ride: If you had to guess ...

Younger offspring: Land comes from a whole giant island. And then it breaks apart since the dinosaur times.

Dr. Free-Ride: So, like one big continent that sort of all used to be together?

Younger offspring: Yeah. They broke up. Even Australia and Antarctica.

Dr. Free-Ride: That's interesting. Do you have any ideas about how or why it broke up from one big land mass?

Younger offspring: Volcano rocks? I mean, volcanoes?

Dr. Free-Ride: Well, I think volcanoes are part of the story of where land comes from and how we got the land masses we've got today. But, there's another part of the story, I think, that has more to do with how the one big land mass broke up to give us continents. And I think it's related to what sometimes happens here in California.

Younger offspring: Earthquake!

Dr. Free-Ride: Yeah. Do you remember why earthquakes happen?

Younger offspring: Earthquakes happen because there might be a big eruption far away.

Dr. Free-Ride: You're all about the volcanoes.

Younger offspring: No! I just heard it and I thought it was true.

Dr. Free-Ride: I understand. I mean, I don't think this is a problem that you find the most fascinating mechanisms for land creation and land movement to be related to volcanoes. That just means volcanoes are the bit of it you pay attention to. But there are other explanatory mechanisms I think we could turn to. Do you know what causes earthquakes?

Elder offspring: Plates moving up against each other or suddenly colliding.

Dr. Free-Ride: Plates! Tell me more about these plates. What are plates?

Elder offspring: Well, they're the continental plates, and they're separated by trenches. They hold up the land and they make up the crust of the earth.

Dr. Free-Ride: OK. And, they don't stay stuck in place then, huh?

Elder offspring: No. Watch my demonstration. (Two flattened hands are used to represent plates as they brush against each other, occasionally thrusting upward or downward.)

Dr. Free-Ride: Oh those finger edges of the plates sort of got caught with each other and there was pressure!

Elder offspring: Or ...

Dr. Free-Ride: So sometimes they thrust up, and sometimes they thrust past each other --

Elder offspring: Or sometimes they rub up against each other like this. That lets lava out, creating some volcanoes.

Dr. Free-Ride: See, it does come back to volcanoes, which you think are the bees' knees!

Younger offspring: Yay!

Dr. Free-Ride: And how do volcanoes make land, while we're on that?

Younger offspring: Volcanoes make land because I think the lava rocks dry up in the ocean.

Dr. Free-Ride: Uh huh. Or cool down, at least, right? It's maybe not so much drying up, because drying up when you're in the ocean is a hard thing. But, if you've got basically melted rock and it hits something nice and cold like ocean water, it cools down and it's no longer a liquid rock but it's solid rock. Right?

Younger offspring: Yes.

Dr. Free-Ride: Did you know that back in the 1950s, people did not believe that the continents could have moved?

Elder offspring: No.

Dr. Free-Ride: It's true! It used to be the case that people thought there must be -- you know, that land is land, that land is sort of permanently fixed where it is, that it takes something pretty serious to move even little chunks of it. So, the observation that some of you alluded to looking at the shapes of the continents, and saying gosh, don't they look like puzzle pieces that kind of fit together, I think scientists would have said --

Elder offspring: Pangea.

Dr. Free-Ride: Well, the whole Pangea thing seemed a little crazy to some scientists. They were like, well how could that be? How could land move? Land doesn't float on the ocean or anything like that.

Elder offspring: Hawaii didn't move. Hawaii was made by volcanoes.

Dr. Free-Ride: That part was -- oh, are you saying that Hawaii left sort of a cookie crumb trail that should have told them that stuff was moving?

Elder offspring: Mmm hmm.

Dr. Free-Ride: Awesome.

Younger offspring: Oh, I get it, 'cause it's so far away from the continent!

Dr. Free-Ride: And also, Hawaii isn't one big island, right?

Younger offspring: I know.

Dr. Free-Ride: It's a chain of islands.

Younger offspring: It's a million tiny little islands.

Dr. Free-Ride: I think a million might be estimating a little high.

Younger offspring: Oh, um, a thousand!

Elder offspring: Too high.

Younger offspring: A hundred and ninety-nine!

Dr. Free-Ride: I think there's a handful of main islands in Hawaii.

Elder offspring: Most islands in Japan aren't populated because ... it's impossible for humans to populate them.

Dr. Free-Ride: Why is it impossible?

Younger offspring: Someone told me that there were more than a hundred islands. Oh wait, that was the Aleutian islands!

Dr. Free-Ride: So, OK, the fact that these island chains come in chains is maybe a clue that things were moving around. But it was a big deal when they figured out -- and I don't know the exact when that the scientific community kind of came over, but I think it was around the 1960s -- when this whole idea of the plates and plate tectonics was finally accepted as not crazy.

Younger offspring: I think the islands were made -- the small islands that maybe weren't in crumbs or were in chains -- I think they were made out of smaller cooled down lava rock.

Dr. Free-Ride: So again, your favorite mechanism comes to the rescue. You need more land? Shoot hot liquid land out from the center of the earth and cool it down, and you got land. It's like frozen cookie dough in some ways, isn't it? And you like moving things around, sliding the plates?

Elder offspring: Yeah.

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wiki says one hundred thirty seven.
But I bet we can fix that with more volcanoes!!!

'Course, that sidesteps the issue of where Pangea came from; why there was one single landmass (as opposed to lots of small platelets for instance, or no plates at all) in the first place.

When I was a couple years older than Elder Offspring, puffy fabric paints were in all the rage with the grade school set.
So I made myself a sweatshirt with a sparkly globe on it and the words "Reunite Pangea." No wonder the other kids thought I was weird.

The land was formed 2.5 to 4.4 billion years ago when the Earth was much hotter and it differentiated so that denser material sank and lighter material formed the buoyant, stable continental cratons that mostly still exist.

Of course, it's more complicated than that. But don't lose sight of the fact that the basic principle of land formation is tremendous heat, then separation by relative density and buoyancy.

I thought land was initially created by the aglomeration of island arcs...and that when ocean crust was subducted, it mixed with water to form less dense material that, once ejected again, was too buoyant to subduct again, gradually accumulating into continents.