This is the beginning and end of it really. We can wipe out life on earth with a combination of phased biological, chemical, and nuclear attacks. As you say though, Earth is a big (I'd say massive) rock... or rather molten metals and the magma. If some detonation (and it couldn't be fission, there's a limit) "cracked" the planet, it would just do what it did when it formed; collsapse back to a roughly spherical shape under its own gravity.
I think in this discussion, that consideration has been missing: to destroy a planet you really would need to "sandblast" it bit by bit (as Sol will do eventually), tear it apart with a black hole (not going to happen), or somehow disrupt its orbit and send it into another massive body. Despite some comments here, the LHC has no capacity to create a black hole of the type and mass to cause ANY destruction. Far more energetic collisions occur in the upper atmosphere, and have for billions of years without incident. This also tends to rule out the notion of conversion to "strange" matter, which is highly theoretical.
Destruction of a planet through explosive force would have to eject the bits of planet so far apart that gravity wouldn't cause the mass to "clump" together again and reform. Earth really is incredibly massive, and composed of everything from diamond and iron, to gasses in the upper atmosphere.
Log in to comment