I'm curious about this. So I'm in the Frontier and not in the safe area and a creeper blew up in a built-up base we have and it did damage to me but no blocks were broken. This is far underground near the bedrock. At the surface though I've seen creepers create craters many times. So I'm curious what the mechanics are and when creepers do or don't do physical damage (aside from player damage)
Creepers will damage basic blocks like dirt, sand, grass etc but will not damage general building blocks - stone, cobble, bricks etc. This is what I've found at my outpost, but you'll probably get a more detailed answer from "those in the know"
Every block has a different certain number of resistance to explosions. Hence dirt and sand are likely more found as craters than stone underground.
Although yes, stone has a higher resistance to explosions than dirt/sand, a creeper will still break a few blocks of it, blowing up in a cave, in Vanilla mechanics. On EMC, any blocks other than the very common worldgen terrain blocks are immune to TNT/creeper explosion damage. You said it blew up "in a built-up base"? Well there you go - it was probably surrounded by stone bricks or other crafted blocks, which would blow up normally, but not on EMC.
Explosions are a little bit random, and the powder drops off quick from the center. Each block also has it's own blast resistance: http://minecraft.gamepedia.com/Explosion#Blast_resistance So if a creeper is on sand, you'll see this giant 5x5 crater, but if they are on cobble/stone you might see as little as 1 block gone. Half Slabs of stone or cobble placed on the bottom half of a slab will prevent spawning on that block and give a high level of protection against explosion damage. Ghasts in the nether can't break stone (or nether brick) at all, so knowing this will keep your favorite pathway from turning into a firey crater.
ah okay, yeah we're using stone bricks and slabs so they must be immune to creepers that's VERY useful info to know!
Creepers will not damage any block that you make, they damage all natural blocks. That is what I have seen from testing it in our mob farm. We made it all out of obsidian, but have slowly replaced it with other material to test. Even stuff like wooden fence, gates and glass take zero damage because you crafted them.
I believe the feature was implemented to keep people from griefing other's bases by luring creepers in.
