Why doesn't Earth explode from the great pressure inside? The inside of Earth is very hot, molten, and under incredible pressure. When small cracks form, the pressurized contents spill out and we get volcanoes and eruptions. So does that mean that with a large enough crack Earth could explode like a balloon? Is the outer shell really so sturdy to contain all that pressure? It feels like there's something wrong with this reasoning but I don't know what...
 A: Under gravity the heaviest material - mostly iron - sinks to the core, the middling stuff - heavy minerals - forms the mantle and the lighter stuff - lighter minerals - forms the outer layer which freezes into a solid crust. Overall, the pressure is created by the weight of the stuff on top and near the surface it is minimal (unlike the surface of a balloon).
But like a pot of boiling jam, everything is seething about, the crust is like the scum on the jam, it is not sturdy at all and bits get carried all which ways (continental drift). Large cracks are always present where continental or seafloor plates are moving apart. Among other things, a "bloop" of molten stuff sometimes breaks through the crust elsewhere, creating a volcano.
In other words, the Earth is a dynamic system which is stable on a large scale. It will no more explode than a pot of boiling jam will.
A: Gravity is not strong enough to create pressure enough for explosive forces in the center of the earth. One needs much larger masses, as happens with the  Supernova explosions:

A supernova (/ˌsuːpərˈnoʊvə/ plural: supernovae /ˌsuːpərˈnoʊviː/ or supernovas, abbreviations: SN and SNe) is a powerful and luminous stellar explosion. This transient astronomical event occurs during the last evolutionary stages of a massive star or when a white dwarf is triggered into runaway nuclear fusion. 

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heoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star; or the sudden gravitational collapse of a massive star's core

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