How can the center of the earth be solid?

As a geophysicist with a deep interest in the structure and dynamics of the Earth, I can explain the concept of the Earth's core being solid at the center. The discovery you mentioned by Inge Lehmann was indeed pivotal in our understanding of the Earth's core. Here's a detailed explanation: The Earth's core is divided into two parts: the outer core and the inner core. The outer core is primarily composed of liquid iron and nickel, which allows seismic waves to travel through it. However, the inner core is a different story. It is believed to be solid for a few key reasons: 1. Pressure: As you go deeper into the Earth, the pressure increases dramatically. The immense pressure at the center of the Earth is so great that it forces the iron and nickel to exist in a solid state, despite the high temperatures. 2. Temperature: While the temperature at the center of the Earth is extremely high, it is not high enough to keep the iron and nickel in a liquid state under the crushing pressure. 3. Seismic Evidence: Seismic waves behave differently when they pass through the inner core. The way these waves are refracted, or bent, indicates a solid medium. Lehmann's observation of seismic waves bouncing off a boundary deep within the previously thought-to-be liquid center of the Earth confirmed the presence of a solid inner core. 4. Density: The density of the inner core is higher than that of the outer core, which is consistent with a solid state of matter. 5. Crystallization: Over time, the cooling of the Earth has led to the crystallization of the inner core from the outer core's liquid state. In summary, the combination of extreme pressure, high density, and seismic observations all point to the existence of a solid inner core at the center of the Earth.

Danish seismologist Inge Lehmann made the discovery in 1936 when she noticed seismic waves bouncing off a boundary point deep within what was believed to be a liquid center. With her finding, the world learned that Earth's core is solid at the center and liquid on the outside.