Final answer:
Lower mantle rocks remain solid due to the extreme pressure from overlying materials and the increased density with depth, which prevents them from melting even at high temperatures.
Step-by-step explanation:
The lower mantle rocks remain solid despite the high temperatures because they are subject to extreme pressures. As the depth increases, the density in the mantle increases from about 3.5 g/cm³ to more than 5 g/cm³.
This compression coupled with the weight of overlying material prevents the rocks from melting. In addition, the presence of a slow-moving convective flow allows for the transportation of heat from the interior of the Earth to the upper mantle, maintaining the mantle's solid state.
It is also important to note that although solid, mantle rocks are capable of slow deformation and flow due to the temperatures and pressures at that depth.
The lower mantle rocks remain solid due to the extremely high temperatures and pressures found in that region. Although the mantle is more or less solid, the rocks can deform and flow slowly under these intense conditions. The weight of the overlying material compresses the lower mantle, increasing its density and keeping the rocks in a solid state.