Earth consists of four major layers: the crust, mantle, outer core and inner core. While most of the layers are made of solid material, there are several pieces of evidence suggesting that the outer core is indeed liquid. Density, seismic-wave data and Earth’s magnetic field provide insight into not only the structure but also the composition of Earth’s core.
Structure of the Core
National Geographic notes that the core as a whole is Earth’s deepest and hottest layer. It is made nearly entirely of metal. The outer core is composed of an alloy of iron and nickel. These are two of the most common metals on the planet. At the surface, nickel and iron are almost always found in solid form. The outer core is approximately 2,300 kilometers (1,430 miles) in depth and ranges in temperature between 4,000 and 5,000 degrees Celsius (7,200 and 9,000 degrees Fahrenheit). The inner core, by contrast, is made almost entirely of iron and is only 1,200 kilometers (750 miles) thick. This layer is extremely hot, between 5,000 and 7,000 degrees Celsius (9,000 and 13,000 degrees Fahrenheit), but the pressure exerted by the mass of the rest of the planet prevents this layer from melting.
Density and Gravity
Sir Isaac Newton made the first observation regarding the density of Earth’s core more than three centuries ago. According to the U.S. Geological Survey, Newton, an English scientist, hypothesized that based on his observations of other planets and other data he had collected from his studies on the force of gravity and gravitational pull, Earth’s average density was twice that of the rocks found on its surface, and thus Earth’s core must be composed of much denser material such as metal.
Earthquake data provide more insight into the composition of Earth’s center. During an earthquake, energy is released in waves that travel throughout Earth’s layers. The two types of waves that are released are primary waves, or P waves, and secondary (shear) waves, or S waves. Both P waves and S waves can travel through solids, but the only P waves can travel through liquids. Seismic wave data shows that S waves do not pass through the outer core, and thus this part of the planet's interior must be liquid.
Earth’s Magnetic Field
That Earth has a strong magnetic field that can also be attributed to a liquid outer core. According to PBS.org, the outer core, together with the inner core, forms a Coriolis force that perpetually sustains Earth’s geomagnetic structure. Earth’s rotation causes the liquid outer core to rotate in a countering direction. The liquid metal of the outer core passes through a magnetic field, which generates an electrical current. As the current continues to flow, a stronger magnetic force is generated. This creates a self-sustaining cycle of magnetic force.