Which characteristics indicate relationships between Earth's layers?

• A. Seismic wave speed
• B. Magnetic field
• C. Color and texture
• D. Density, composition, and temperature

Answer: (D)

The main idea is how the actual properties that materials have—how heavy they are (density), what they’re made of (composition), and how hot they are (temperature)—define how Earth's layers relate to each other. As you go deeper, materials change from lighter, silica-rich rocks in the crust to denser rocks in the mantle, and then to metallic iron-nickel in the core. Temperature and pressure rise with depth, influencing whether materials are solid or liquid and how they behave at boundaries. These differences in density, composition, and temperature determine where layers begin and end (the boundaries) and why each layer has its distinct properties, like the liquid outer core driving the geodynamo and the solid inner core remaining solid under immense pressure.

Seismic wave speed can reveal where those boundaries lie, but it’s a measurement that depends on the underlying density and elastic properties rather than a defining trait of the layers themselves. The magnetic field is tied to dynamic processes in the core, not the general relationship between all layers. Color and texture are superficial indicators, not reliable signs of deep interior structure. So density, composition, and temperature best capture how Earth's layers relate to one another.