At mid-ocean ridges, oceanic plates are moving away from each other at a rate of ~2cm/year. As the plates diverge, they create space in which new crust forms. This new crust cools rapidly and forms the newest part of each plate. The oceans are filling topographic low points (think about this like a HUGE valley) that have water in it because water flows to the lowest topographic point. In the northern hemisphere, the Atlantic Ocean is underlain by both the North American and Eurasian plates, which are pulling apart from each other as we speak. To see the trace of where they are pulling apart, go to google.com. Click on Maps and click the "satellite" button. Now look at the Atlantic Ocean. Along the middle of it, you can trace a mountain range (the mid-ocean ridge where new crust is forming because the plates are moving apart and creating space for the crust). This is also the boundary between the North American and Eurasian plates.

Well, you are right that plates move on AVERAGE a few centimeters a year. Now the Earth's surface is made up of about 12 major plates. The relative motion between two plates can be one of three types. The plates can move apart, they can move together such that one plate dives beneath the other or they can slip side by side.

In Santa Barbara we are very close to a plate boundary between the NORTH AMERICAN plate and the PACIFIC plate. The motion between these two plates is side by side or slips.If you imagine keeping the North American plate fixed (motionless) then the pacific plate is moving North Northwest.In about 40 million years, Los Angeles will be up by the Aleutian Islands!!!

The continents do not drift, but they ride on top of the plates. Imagine that the Earth is like a huge ball, with layers. The top layer is brittle and breakable, like hard candy; underneath this layer of brittle candy there is a mushy layer, kind of like unbaked cake batter - really gooey,and thick. Beneath this gooey unbaked cake layer, there are other layers, all the way down to a very dense iron core. It is hot down inside the earth, both from the pressure of everything above and also because there are radioactive elements down there that are always decaying, and giving off heat. So, heat is constantly rising to the surface from inside the earth.

Think about what happens when you heat oatmeal on the stove - it starts to boil and bubble, right? Well, this sort of heating and boiling action (we call this convection) happens in the earth, only much more slowly than on your stove. And this heating from below makes the mushy gooey layer that is thick like unbaked cake batter (called the "aesthenosphere") go into aslowly rolling boiling action - kind of like what happens to your oatmeal,only on a very big scale, and very slowly. Now the mushy gooey layer has been doing this slow boiling action for millions and millions of years, so that caused the brittle surface of the earth to break up into sections -those are what we call the PLATES.

So, now the surface of the earth is broken up into plates, which are moving all the time, very slowly, as they are being dragged along because of the slowly boiling action inside the mushy gooey layer. But, the plates stick together (like sticky candy!) and resist moving, until they have to break - and that is why we have earthquakes!

At some places along the earth's surface, two plates slip past each other,like in California, where we get shallow earthquakes; at other places, one plate is pulled down under another one (we call these subduction zones),like under Alaska, or under Japan, and there they get very deep earthquakes. In other places, the plates are pulling apart from each other and in those places the hot mushy gooey stuff - now let's call it LAVA - comes up through the cracks! That is happening along the big mountain chains that are underneath the oceans, like in the middle of the Atlantic Ocean, and in the Pacific Ocean. We call those areas MID OCEAN RIDGES, or SPREADING CENTERS.

So, along the subduction zones the brittle top layer of the earth gets pulled down inside the hot earth. What happens when you heat up a lollipop over the stove? It melts! And that is just what happens to the brittle rock layer of the earth when it gets pulled down into a subduction zone: it melts, and eventually gets recycled. So, at the mid ocean ridges, lava comes out molten, and cools; at the subduction zones, the surface layer goes down back inside the earth and melts.

So, to summarize: Plates separate along plate boundaries; along subduction zones the surface layer is pulled down inside the earth and melts, and gets recycled; along the mid ocean ridges lava comes out from the cracks in the earth, where the plates are being pulled apart, and new crust is formed. Plates don't "split" at 2 cm per year; the Atlantic Ocean plates are moving apart from each other at about 2 cm per year, but the Pacific plate is moving toward Alaska - the plates don't all go in the same direction. The earth is a big ball, so as the plates slide along the surface of the earth, they are actually rotating, so in different areas the motion is not going to be the same. For example: North America is moving westward, but Europe is moving eastward, because of the spreading in the middle of the Atlantic Ocean.

I hope this answers your question. For further information, you can look at the home page of Professor Tanya At water, of the UCSB Geology Department. She has lots of cool animations which explain all this stuff about Plate Tectonics Here is the address:

Great question about the sea floor. You were able to take information about something (the plates) and make a prediction about what would happen (separating the ocean). The surface of the earth is made up of plates. North America sits on one of them. You can see the plates at this site, just click on the green V.

It's hard to believe, but the ocean floor actually is splitting, it's just being filled in with melted (molten) rock. There are huge cracks that run through places on the ocean floor where new rock is slowly, but continuously, coming to the surface and spreading the sea floor. This means that the sea floor also has to "tuck" under the continents at the edge of the sea. To see the places where it's spreading, click on the pink triangle at the same site where you saw the plates.

What do you think happens when the sea floor tucks under the plates that the continents are on? Hint: If you get stuck, look up "subduction."

Volcanoes along the boundaries between plates that are splitting apart, erupt lava into the fissure between them, thereby creating new crust. Thus, although a given rock on one plate is continually moving away from the boundary, there is always crust.