Discovered by Danish seismologist Inge Lehmann in 1936, this discontinuity is the boundary between the liquid outer core and the solid inner core. She proved the inner core’s existence by showing that P-waves were being reflected and refracted by a solid center. ANSWER: (C) Outer Core and Inner Core
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Before Lehmann, scientists thought the entire core was liquid. She noticed that some P-waves were showing up in the “shadow zone” where they shouldn’t be. She theorized that these waves were bouncing off a solid inner sphere. Her discovery changed our entire model of the Earth’s interior. We now know the inner core is a solid ball of iron and nickel, kept solid by immense pressure despite temperatures exceeding 5,000°C. This solid inner core is essential for the Earth’s rotation and the stabilization of our magnetic field.