How do we get data about the Earth's crust?

[Manasan3010]

In this video about digging hole into Earth's crust it is said that humans have only reached 12 km depth(Kola Superdeep Borehole). Then how are we able to know the composition of Earth's crust, density of innermost core, physical state of the matter. Where are we getting our data from And how reliable are these predictions?

 

[phyzguy]

Of course we have sampled crustal rocks over much of the surface of the Earth, so we have a good idea of the composition of the surface crust. I think there is no reason to suppose that the deeper crust is hugely different from the surface crust. Most of our knowledge of the interior of the Earth comes from analyzing earthquake data. The propagation of the earthquake waves through the Earth reveals discontinuities, which is why we have the basic model of the interior consisting of crust, mantle, liquid outer core, and solid inner core. Our knowledge of the composition of these regions is uncertain. Volcanic lava gives us some information about the composition of the mantle. The composition of the core is mostly inferred from knowledge of the density and mechanical properties of these layers, guided by a basic knowledge of elemental abundances in the galaxy.

 

[davenn]

hen how are we able to know the composition of Earth's crust, density of innermost core, physical state of the matter. Where are we getting our data from

Specific seismic surveying work and as @physguy said natural seismic signals of earthquakes

And how reliable are these predictions?

Pretty good and improving year by year
They are NOT predictions.

 

[Dragrath]

To add to what others have said months ago there are two other components that allow us to constrain the composition of areas we can't reach.

Earthquakes are the greatest tool with ever increasing precision which work based on the waves response varying between the phase and to a lesser extent composition of material in the crust but also down below.

That said we can actually get some samples of deeper material under circumstances where tectonic and volcanic forces have brought deeper material to the surface via uplift and or volcanic ejection. Such samples are not pristine as the forces that bring them to the surface alter them but by understanding the effects of such forces and the forces of erosion since their arrival on the surface we can gain valuable insight into their parent medium.

There is even a role in simulating temperatures and pressures observed via siesmic waves and mineral formation conditions and the elements expected to form there both using material samples or computational simulations.