Question

why is the crust and mantle and inner a solid. is it because of hight pressure

Answer 1

Short answer: partly. Whether a layer is solid or liquid depends on temperature, pressure, and composition together — not just pressure alone.

Why that matters
- Melting is controlled by both temperature and pressure (and what the material is made of). Higher pressure generally raises the melting temperature of most materials, so material that would melt at low pressure can stay solid at very high pressure.
- Temperature also increases with depth (the geotherm). If the temperature at some depth is higher than the melting temperature for that pressure and composition, you get melt (liquid); if it is lower, the material stays solid.

How that plays out in Earth’s layers
- Crust: relatively cool compared with its melting point, so mostly solid rock. Local melting produces magma (volcanoes, magma chambers).
- Mantle: mostly solid rock (silicate minerals) even though it’s very hot, because pressures are high and mantle minerals have high melting points. The mantle behaves like a very slow-moving solid (creeps and convects) rather than a fluid on human timescales. Small amounts of partial melt occur in places (mid-ocean ridges, hotspots, subduction zones) where conditions lower the melting point (decompression or water addition).
- Outer core: mainly liquid iron–nickel alloy because at those pressures and temperatures the material is above its melting point.
- Inner core: solid iron–nickel because pressure is so high that the melting temperature of iron exceeds the actual temperature there, so it freezes despite the very high temperature.

Evidence
- Seismic waves show S-waves do not travel through the outer core (signature of a liquid), while P-wave behavior and seismic wave conversions indicate a solid inner core. Laboratory experiments and theory on melting curves of iron and mantle minerals support the same conclusions.

So yes, high pressure is a major reason the deep interior can be solid, but it’s the balance between pressure, temperature, and composition that determines whether a layer is solid or liquid.