Short answers, pulled from the story.
The solid inner core of Earth has a radius of about 1,220 kilometers. This massive iron-nickel sphere was confirmed through experiments where samples were subjected to core-like pressures between diamond tips and heated to approximately 4,000 Kelvin. X-ray observations revealed that these giant crystals run north to south, creating a unique structure that has remained stable for billions of years.
Inge Lehmann discovered the inner core of Earth in 1936 by analyzing seismic waves. She noted that some waves were reflected back to the surface in a way that suggested a solid inner sphere within the liquid outer layer. This discovery proved that the Earth is not a uniform ball of rock but a complex system of distinct layers.
The mantle extends to a depth of approximately 2,900 kilometers, making it the thickest layer of the planet. It accounts for 83.7 percent of Earth's volume. Although solid, the mantle's extremely hot silicate material can flow over very long timescales, a property that drives the motion of tectonic plates in the crust.
Earth's outer core is a fluid layer about 2,200 kilometers in height, composed mostly of iron and nickel. This layer generates the Earth's magnetic field through a process known as the geodynamo, where convection combined with the Coriolis effect creates a magnetic field that is 50 times stronger than the field at the surface. The average magnetic field in the outer core is estimated to measure 25 microteslas.
The oldest known mineral grains on Earth are approximately 4.4 billion years old. These ancient fragments, found within younger rocks, serve as time capsules from the Hadean Eon, a period when the surface was likely molten and hostile. They indicate that the planet has possessed a solid crust for nearly its entire existence.
One estimate suggests that the core of Earth would not be expected to freeze up for approximately 91 billion years. This timeline is well after the Sun is expected to expand, sterilize the surface of the planet, and then burn out. The solid inner core is too hot to hold a permanent magnetic field, as the temperature exceeds the Curie temperature.