Land formation

It only looks that way because you're seeing it from above the surface. The earth is a sphere (more or less) 8,000 miles in diameter (more or less). I haven't got the formula handy, but that's billions of cubic miles of volume.

Most of that is what we'd call "land," in the sense that it sure ain't water. A lot of that "land" is molten (help me out guys, I think that actually most of it is molten), but in terms of the molecules that comprise it it is definitely molten rock and stuff like that, not water.

The outside of the sphere that's exposed to the much cooler external temperature has solidified into big chunks of solid land that float on the huge sphere of liquid land underneath. I don't think any of them are even 500 miles thick (again, help me out, dudes). Those are called tectonic plates. They keep bumping into each other; where they bump they either crumple or one rides up on top of the other. Those rather insignificant wrinkles in the earth's surface, in a cosmic sense, are what we call mountain ranges. The biggest ones only stick up about five miles above the surface of the tectonic plates, so when I say insignificant I'm not kidding.

Similarly, there are areas where the plates have drifted apart, leaving some of the softer, hotter land underneath exposed to the elements, so it solidifies too. We call those the ocean floor. Also pretty insignificant in the grand scheme of the universe, the deepest ones go about five miles down. (Am I right, guys?)

So, we've got a sphere 8,000 miles in diameter that has irregularities on its surface of plus or minus 5 miles -- one tenth of one percent of the diameter. The distortion of the earth's shape at the equator from centrifugal force causes a larger discrepancy than that!

Still, the difference between the highest and lowest points on the earth's surface is a full ten miles. You start covering the surface with water, at first it all follows gravity down into those depressions. Eventually it covers some pretty wide areas and they start to look like lakes. If you've got enough water, the level keeps rising and the lakes get connected into seas and the seas get connected into oceans.

If we're in a period of relatively high average temperature so there's not a lot of water trapped in the polar ice caps, the water level gets so high that instead of a giant land mass peppered with big seas, the earth starts to look like a giant ocean dotted with big but separated land masses. That's where we are right now. The oceans are all connected, but Afro-Eurasia, Australia, Antarctica and America are four separate land masses.

If it gets colder and the ice caps grow, sea level falls. Siberia gets connected to Alaska and Tierra del Fuego hooks up with Antarctica. One big supercontinent and one little Australia. If it gets hotter and the sea level rises, the Red Sea connects to the Mediterranean and Africa gets separated from Eurasia. Five continents.

What you're asking is: how much more water needs to be dumped on earth before none of the land sticks up out of the water at all? If you really mean "none," then we need enough water to cover the Himalayas. Sea level has to rise another five miles -- 25,000 feet.

Right now it looks like the earth's surface is more than half water, but those bodies of water are shaped like bowls. Just peeling off the top five miles of the earth's surface so we get all the water, we'll find that most of that five mile rind is land, because the cross sections of the oceans get much smaller the deeper you go.

But if you start pouring more water on the surface, you're starting at a level where the cross sections of the oceans cover more area than all the land. Once you cover it to a height of a mile or so, no more land will be visible except the big mountain ranges. The Andes, Alps, Himalayas, Sierra Nevada, Appalachians, Carpathians, etc. And those are going to look like the most pathetic little island chains surrounded by all that ocean, probably not one percent of the ocean surface. You've still got four more miles to go if you want to cover Mount Everest and make the earth into a water planet.

My back-of-an-envelope calculations suggest that you would need approximately five times as much water as there currently is in all the oceans to be able to raise the sea level by five miles.

There isn't that much water trapped in the polar caps. I don't know how much water is floating around in the atmosphere, but the only way I can think of to make it all precipitate out is to lower the thermostat, and then a whole bunch of it starts to freeze again and we lose it to the polar caps.

The only way to get that much liquid water would probably be to get rammed by a comet or asteroid that was all water. Using the same back of the envelope, I figure the asteroid would have to be about five hundred miles in diameter to deliver enough water to do the job.

It would probably work out OK. The only lifeforms that could survive the energy blast of that collision would be one-celled forms that prefer to live in water anyway.

 

Wait a minute, why doesn't solid rock sink?

I seem to have forgotten that water is the only common substance that is denser in its liquid form than solid. That's why ice floats on top of bodies of water, insulating the liquid trapped below from the cold temperature of the air, allowing it to remain liquid and continue supporting life during winter.

Rock isn't like that. Solid rock is denser than molten rock. Therefore, it ought to sink and leave the earth's surface covered with magma.

How come that doesn't happen?