What Is the Hottest Layer of Earth and Is the Core Cooling?

The distance from the Earth's surface to its center is almost 4,000 miles—but our planet's structure is not uniform and is actually made up of several different layers, some of which are hotter than others.

The Earth's cool, thin outer layer, known as the crust, mostly consists of solid rock and is generally around 20-30 miles thick in continental areas, although in oceanic regions the average thickness is roughly four miles.

Below the crust lies the mantle, which extends downwards for roughly 1,800 miles, making up 84 percent of the Earth's total volume.

This layer consists of rock material that is more dense than in the crust and mostly solid, although melting can occur in some localized regions due to high pressures. While the mantle is mostly solid, on geologic timescales, it essentially behaves like a viscous fluid.

The pressure and heat in the mantle generally increases the deeper you go. In regions closer to the crust, the temperature may be around 1,800 degrees Fahrenheit. Temperatures at the bottom of this layer, meanwhile, hover around the 6,700 degrees Fahrenheit mark.

What Is the Hottest Layer of the Earth?

Underneath the mantle is the Earth's core, which is comprised of an inner and outer section. The outer core extends for around 1,400 miles and mostly consists of liquid iron and nickel. Temperatures here are thought to range between around 7,200 and 9,000 degrees Fahrenheit.

Below this is the dense inner core, which is around 750 miles thick and consists mostly of iron and small amounts of nickel, among other elements. Scientists estimate that the inner core is extremely hot—anywhere between around 9,000 degrees Fahrenheit and 13,000 degrees Fahrenheit. This is comparable to temperatures on the surface of the sun.

"This incredibly high temperature is due to energy released as radioactive materials decay, the initial temperature when the Earth formed, and the incredible pressure at the innermost reaches of the Earth," Lisa Wald, a geophysicist and science communicator with the the United States Geological Survey, told Newsweek.

While the inner core is incredibly hot, scientists have traditionally thought that the material here is solid due to the effects of the extreme pressures in this region. However, one recent study indicated that the inner core could consist of an exotic state of matter that is neither solid nor liquid.

Is the Earth's Core Cooling?

The entire Earth has been cooling since it formed around 4.5 billion years ago, not just the core.

"As the outer liquid core circulates above the inner core, it removes heat and causes the iron to crystallize on the surface of the inner core, making it about a millimeter greater in diameter each year," Wald said.

"The rate of cooling has been constant, but scientists recently discovered that there is uneven cooling, resulting in lumps and bumps on the surface of the inner core."

What Happens When the Core Cools Significantly?

According to Wald, it will take tens of billions of years for the inner core to cool significantly.

"As it does so, the convection in the mantle, powered by the heat from the outer core, will slow down, ultimately causing the end of plate tectonics—the motion of the Earth's crust that causes earthquakes and volcanoes," she said.

"The Earth would lose its magnetic field, since that is also created by the convection outer core and rotation of the Earth. This in turn would make the surface of our planet more vulnerable to solar storms from the sun, and it would cause problems with any living being or technological component that uses magnetism. For example, birds would no longer be able to migrate, and compasses would no longer point north."

How Do Scientists Know About the Earth's Structure?

Scientists have mapped the Earth's internal structure by looking at seismic waves produced by large earthquakes that travel through and around the entirety of our planet.

"Different structures within the planet have different effects on the seismic waves—bending them, slowing them down, making them larger or smaller," Wald said.

"Seismologists can 'read' the seismic data recorded at each seismic station on the surface of the Earth and learn what the energy travelled through to create the 'signature' on the recording. The data from thousands of earthquakes and their seismic waves has enabled us to 'image' the entire planet like an X-ray can image the bones inside your body."