Lecture: The Theory of Seafloor SpreadingAuthor: WeChat-Nelly: Narrator: Listen to part of a lecture in an Earth science class. Professor: Up to relatively recently, scientists who claim that Earth land and sea were things, that ... that moved and evolved, were condemned for their ideas, which, uh, I mean, these ideas were seen as scandalous, even if today we know these scientists were just ahead of their time, but nowadays there's been no shortage of research conducted that supports theories of continental movement. Now, a lot of evidences collected comes from a very close study of how the ocean floor is distributed and created. So, to start, can anyone tell me, what the ocean floor is actually made of? Cathy: Isn't is mostly made of sand and other land-based material? Oh, just like that one of cliffs and coast, by the weather. Professor: That's right, Cathy. There's sand along with other small chunks of land-based material created by the natural wear and tear of wind and water flow, that's what we called sediment. The ocean floor is to an extent, made up of land-based sediment, but oceanic sediment is also partly made up of the remains of ocean-dwelling animals and plants. For example, uh, some sediments, they are made up of microscopic shells, the king algae and various single-celled organisms that live in the water. This layer of chalky sediment can be up to a kilometer thick, believed it or not. Male Student: Excuse me, professor, but does, I mean, do all these deep-sea sediment stop the ocean floor from being a perfectly flat plain? Like, is that why it's so uneven down there? Professor: Yes, and that's a great observation, that layer of sediment is the spread across the ocean floor with different levels of thickness, and it's continently moving too, because of, well, because of things like earthquakes, volcanic activity and currents, because sediments shifted from one area to another, there's rapid accumulation in some places, while others have barely any. And can you guess where the areas with the least sediments are? Cathy: En ... far away from land? Away from where sand and rocks drop into the sea? Professor: Hm, yes, you've got the right area, but it's not just for that reason. There are actually a few different forces that work. What I'm getting here, that is, well actually, can anybody think of another reason that sediment coverage might be thinner towards the centers of the oceans? One that might be closely connected to continental movement? Male Student: The land, I mean the fore of the ocean is newer, isn't it? Because that's where the spreading starts. Professor: That's it! But I should apologize, cuz that might not be a very fair question, not yet, at least. Let's go back and look at the middle land of ridge to illustrate. Ridges like this one, this basically consists of chains of mountains with deep valleys right down the center, where you'd expected peaks to be. This one is the region in almost every ocean, now over time, scientists got that this region were actually generating new seafloor. And that thin layer of sediment, well, is evidence for that very idea, but there's more, of course, consider the rocks found near these regions. Cathy: They were formed more recently? Professor: precisely, the vast majority of the rocks found near middle ocean ridges are significantly younger than continental rocks, and even various settlements. What's more, the edge of rocks on the ocean floor increases the farther you move away from the mid-ocean ridge. So new rocks are actually created by volcanic activity at mid-ocean ridges. And as new sea floor is generated, the areas between ridges shift, dragging whole continent along with them. Male Student: I guess the continents are just going along for the ride. Professor: Yes, you could definitely say it like that. Continents are protrusions from the large pieces of the Earth crust, what you know all these tectonic plates, and as new crust is created at mid-ocean ridges, the ocean floor is also being destroyed at certain areas. So that Earth crust remains the same overall size. Those areas are what we called subduction zones, where a tectonic plate, a whole slab of the Earth crust, slides down at even other plate, causing earthquake in forming volcanoes as its dust shore. The push and pull of tectonic plates literally make land masses move as the ocean floor is created, destroyed and created all over again.