When something hard is under pressure, it can bend or deform, or it can break or shatter.
Imagine a hammer striking a sheet of metal and the metal denting. For a brief moment, the pressure and heat of the hammer strike causes the metal to deform. Conversely, if the hammer hit something like a sheet of glass, it would shatter.
Plastic in this sense is not like Tupperware plastic at room temperature, but plastic heated when it is first being molded into shape.
Explosive deformation is again like the hammer hitting a sheet of glass- if the glass were already under pressure, breaking it would relieve the pressure causing shards to fly everywhere.
Rock at the surface is relatively cold and under little pressure- it is hard, but brittle, and simply breaks. As you get deeper, it is under more pressure and as it breaks, that pressure can get relieved unpredictably (fault lines cause formerly stable walls of already drilled hole to collapse).
With even more depth comes more pressure but also more heat, which relaxes the crystalline structure of the rock. It can start deforming rather than breaking. Drill bits become less effective as the rock flows around it rather than getting broken up and pumped away.
Edit: if you haven't watched videos of hydrologic presses destroying things on YouTube, you see something like this in reverse; sometimes things can deform a bit, but eventually explode under pressure. The most surprising to be to watch was a deck of playing cards literally shatter.
Imagine a hammer striking a sheet of metal and the metal denting. For a brief moment, the pressure and heat of the hammer strike causes the metal to deform. Conversely, if the hammer hit something like a sheet of glass, it would shatter.
Plastic in this sense is not like Tupperware plastic at room temperature, but plastic heated when it is first being molded into shape.
Explosive deformation is again like the hammer hitting a sheet of glass- if the glass were already under pressure, breaking it would relieve the pressure causing shards to fly everywhere.
Rock at the surface is relatively cold and under little pressure- it is hard, but brittle, and simply breaks. As you get deeper, it is under more pressure and as it breaks, that pressure can get relieved unpredictably (fault lines cause formerly stable walls of already drilled hole to collapse).
With even more depth comes more pressure but also more heat, which relaxes the crystalline structure of the rock. It can start deforming rather than breaking. Drill bits become less effective as the rock flows around it rather than getting broken up and pumped away.
Edit: if you haven't watched videos of hydrologic presses destroying things on YouTube, you see something like this in reverse; sometimes things can deform a bit, but eventually explode under pressure. The most surprising to be to watch was a deck of playing cards literally shatter.