A thought experiment

What would happen if we sliced the earth in half? You see it in movies and cartoons all the time. HUEG fucking laser comes down and swipes a planet in half. Lots of explosion, billions of people die, shit gets fucked. But what would really happen?

For simplicities sake lets make a few assumptions. Firstly, the cut has no width. Second, it happens instantly, there is no drag time. Third, it passes directly through the center of the earth in a straight line.

So let us begin to analyse the effects on our beloved earth. Starting from the inside out.

Section 1: The inner core.
This is the enormous ball of solid iron at the very center of the earth. It contains a large proportion of the mass of the earth. It sits at temperatures several thousand degrees kelvin, yet remains solid because of the immense gravity crushing it together. And in all it's bad-ass-itude, it would be completely unaffected by our precision laser cut. Literally, nothing would happen to it. It's very mass would simply weld it back together the moment the cut is finished. Bad. Ass.

Section 2:  The outer core.
Huge volume of melted and semi-solid rock. At temperatures that Megan Fox look like an iceberg (And she's fuckin HOT). Due to the fact that it's a liquid, it doesn't give a rats ass about the cut. Just keeps on flowin. Like the juices of a good hooker. Yeah, i went there.

Section 3: The mantle.
Pretty bad ass name right there. Just rolls off the tip of your tongue. Mantle... *shiver*. This is where shit starts to get interesting. We all know what a volcano is right? No-one who reads my blog is a bitch-ass 5 year old. Or, they shouldn't be. Given my usage of the phrase bitch-ass. Anyway. For the faggots among you, they're caused by the fucking ridiculous amount of pressure that the outer core is under. Huge fountains of liquid rock shoot out, bitches start complaining, and whole islands sink under the sea. They form at weak points of the earths crust, that solid bit all us puny humans live on.
Now, if you slice down the middle of the planet, what you do, is create a single line down which the mantle can no-longer contain this immense amount of pressure. What would end up happening is that lava would force the two halves of the mantle apart, probably by no more than a meter at its worst. Through this gap, a ring of hot burny shit would come flowing out, until the pressure of the inner core is no longer enough to hold the world apart, and it would crash back together. This would set off earthquakes, and probably more eruptions, around the globe, as the world suddenly adjusted to the massively reduced pressure that it was under. This would reduce the diameter of the earth by an amount i can't estimate, and the rearrangement of earths mass (from the lava flowing out) would probably throw our orbit and rotation out a bit. Also, people would not be very happy. Haters gonna hate. A final effect would be the areas over which large amounts of lava pooled would be more massive than average, causin them to slowly submerge back into the ground, where they would melt back into lava. But the temperature at the core of the earth would be significantly reduced due to radiation leaking out into space. So there wouldn't be as much lava, and the mantle would be thicker than before.

Section 4: The Atmosphere.
This bitch would be all "Yo nigga, what the fuck you think yo been doin?" as ash from all the volcanic eruptions slowly filled the air. All those vaporised trees, and screaming women and children would fill up the atmosphere until the sky was black 24 hours a day. This would have the effect of slowing down the rate of heat-loss into space, trapping the heat in between the mantle and cloud layer, raising the temperature of the surface of the earth to quite uncomfortable temperatures. And if that didn't kill us, we'd either choke to death on the poisonous gasses released during the eruptions, or starve as the lack of sunlight prevents us from growing food crops.

In summary, humanity gets rectally plowed, the earth gets munt-faced, and the inner core is badass.