Earth is a layered sphere, much like an onion. Beneath its crust and mantle lies a dense, hot core, divided into a solid inner core and a surrounding liquid outer core. Understanding these deep-seated layers provides insights into our planet’s dynamics and the phenomena observed on its surface.
Defining Earth’s Outer Core
Earth’s outer core is a fluid layer positioned between the solid mantle and inner core. It begins approximately 2,890 kilometers (1,800 miles) beneath the surface and extends to about 5,150 kilometers (3,200 miles) deep, with an estimated thickness of 2,200 to 2,300 kilometers (1,367 to 1,400 miles). Temperatures within the outer core are immense, ranging from 4,000°C (7,200°F) to 6,100°C (11,000°F) near the inner core boundary. Despite these extreme temperatures, the outer core remains liquid due to immense pressure, which prevents it from solidifying.
The Outer Core’s Unique Composition
The primary constituents of the outer core are molten iron and nickel. Geophysical measurements indicate it is 5 to 10 percent less dense than pure iron at prevailing temperatures and pressures. This density deficit suggests the presence of lighter elements dissolved within the iron-nickel melt.
Scientists propose these lighter elements include sulfur, oxygen, silicon, carbon, and potentially hydrogen. Their presence contributes to the outer core’s lower density and influences its fluid dynamics.
Generating Earth’s Magnetic Field
The liquid outer core is the source of Earth’s magnetic field, explained by the “geodynamo” theory. Heat escaping from the inner core drives convection currents within the molten iron and nickel of the outer core. As this electrically conductive fluid moves, it generates electric currents.
Earth’s rotation, through the Coriolis effect, organizes these turbulent flows into spiraling eddies. This motion creates a self-sustaining feedback loop where electric currents induce magnetic fields, amplifying the field. The magnetic field within the outer core is estimated to be 50 times stronger than at the surface. This field extends into space, forming the magnetosphere, which deflects harmful solar winds and cosmic radiation, preventing them from stripping away Earth’s atmosphere. It also provides a directional guide for navigation.
Probing the Planet’s Deep Interior
Direct observation of Earth’s deep interior is impossible due to extreme conditions. Scientists primarily rely on seismic waves generated by earthquakes to infer the outer core’s properties. Two main types of body waves, P-waves (compressional) and S-waves (shear), travel through Earth’s layers.
P-waves travel through both solids and liquids, though their speed changes and they refract when passing into the liquid outer core. S-waves cannot propagate through liquids because their shearing motion requires strong bonds between particles. The observation that S-waves are not transmitted through the outer core provides evidence for its liquid state. Analyzing these waves allows scientists to map boundaries and determine the outer core’s physical properties. Additional indirect evidence, such as gravitational measurements, also supports these findings.