Final answer:
The middle layer of the Earth is called the mantle, which is a large semi-solid layer between the crust and the core that can slowly flow due to high temperatures and pressures.
Step-by-step explanation:
The middle layer of the Earth is known as the mantle. This layer is enormously large, extending from the underside of the Earth's crust down to a depth of about 2900 kilometers. The mantle is composed primarily of solid rock matter with a semi-plastic consistency, enabling it to deform and flow slowly under high temperatures and pressures.
Due to the intense pressure from overlying materials, the density within this layer increases from about 3.5 g/cm3 at its top to more than 5 g/cm3 near the bottom. Occasionally, pieces of the mantle are brought to the surface through volcanic activity, allowing scientists to analyze this layer directly.