Earth’s layers — Earth science, 14–17 years
Earth is not the same all the way down. Its layers differ in material, temperature and behaviour, and scientists use earthquakes to infer what cannot be reached by drilling.
What lies beneath
Earth has a thin crust, a thick mantle and a metallic core. The outer core is liquid, while the inner core is solid because immense pressure changes how iron behaves. These layers are defined by their properties, not just by their depth.
Why scientists infer them
No borehole reaches the mantle, let alone the core. Scientists needed a way to study the deep Earth, so they compared how seismic waves travel, speed up, slow down or disappear inside it. Changes in the waves reveal boundaries and materials.
Reading a seismic clue
Imagine a wave that travels through a solid layer at 8 km/s, then enters a liquid layer where that wave type cannot pass. A station on the far side records no such wave. Step by step: identify the missing wave, locate a hidden liquid boundary, then use many stations to map the outer core.
The common mix-up
A reasonable mistake is to think the mantle must be liquid because it is extremely hot. Temperature alone does not decide whether rock flows: pressure and the timescale matter too. Mantle rock is mostly solid, yet it can deform and move very slowly over millions of years.
Why it matters
This model helps scientists understand Earth’s magnetic field, which is linked to motion in the liquid outer core. It also helps interpret seismic records after an earthquake and compare Earth with rocky planets such as Mars. It is a model based on evidence, not a direct photograph.
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