Earth's layers can be defined based on two main characteristics: composition and physical state. Here are two correct answers:
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Composition: Earth's layers can be defined by the materials that make them up. For example, the crust is primarily composed of silicate rocks, the mantle consists of silicate minerals but behaves differently due to higher temperatures and pressures, and the core is composed mainly of iron and nickel.
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Physical State: Earth's layers can also be defined by their physical state. For instance, the lithosphere (which includes the crust and the upper mantle) is rigid, while the asthenosphere (part of the upper mantle) is semi-fluid and allows for the movement of tectonic plates. The outer core is liquid, while the inner core is solid due to immense pressure.
These two classification criteria (composition and physical state) provide a comprehensive understanding of Earth's layered structure.