Final answer:
The temperature and pressure change with depth within the Earth's layers, and scientists use the study of seismic waves from earthquakes to understand these changes and the characteristics of different layers.
Step-by-step explanation:
Within the layers of the Earth, two things change with depth: the temperature and pressure. As one moves from the surface down through the crust, into the mantle, and eventually to the core, both temperature and pressure increase steadily. The increment in temperature is due to the heat from the Earth's inner core and from radioactive decay within the Earth. The pressure increases due to the weight of the layers above pressing down. Scientists can discern these changes and the depths of the different layers through the study of seismic waves. Seismic waves are produced by earthquakes and their speed, path, and the time they take to travel through the Earth vary depending on the properties of the material they pass through. By analyzing seismic wave data, we can create models of Earth's interior and make accurate deductions about the composition and state of each layer.