The Nether Spire is a structure from Minecraft: Pocket Edition, first appearing in version Alpha 0.5.0 and later removed in version Alpha 0.12.1 with the introduction of The Nether. The Nether Spire would generate around the Nether Reactor — a player-built structure — upon its activation.
Immediately after activating the Nether Reactor Core by hitting it, the Nether Spire would generate around it. With the Nether Reactor Core initialised the cobblestone of the Nether Reactor would turn into glowing obsidian layer-by-layer from bottom to top. After the cobblestone, the four blocks of gold in the corners would also turn into glowing obsidian. This process would take around 5 seconds.
In the Pocket Edition, version Alpha 0.5.0 the Nether Spire was made out of regular obsidian. With the introduction of netherrack in version Alpha 0.6.0 the structure would generate using it, instead.
The blueprint shows the full Nether Spire made out of netherrack and with the Nether Reactor fully activated.
Extinguished Nether Reactor
After the 45 second cycle of spawning Zombie Pigmen and items otherwise unobtainable in this stage of the development of the game, the Nether Reactor deactivates with the 3×3×3 cube around the initialised Core (including the air blocks) turning into regular obsidian layer-by-layer from top to bottom. The Nether Reactor Core itself turns darker and becomes a finished Nether Reactor Core.
After the deactivation of the Nether Reactor, the Nether Spire instantly deteriorates with a number of random netherrack blocks (obsidian blocks in version Alpha 0.5.0) disappearing, leaving holes in the structure.