This is due to the different properties of the depth of the earth. The interior of the earth is made of heavier metals such as nickel and is hotter than the upper layers. The great pressure in these deep depths keep the elements from melting even at very high temperatures. The upper layers are made up of lighter elements such as iron and magnesium - and are less dense. These layers experience less pressure and temperatures than the deep interiors. Therefore their properties is a bit different. This also allows these layers to be semi-plastic than the soil core.