People often think of the planet Earth as being mostly water because most of Earth's surface is water. However Earth's interior composition and structure is quite different from its surface. Earth's interior is mostly rocky or metallic and is layered into the crust, mantle, inner core, and outer core with the denser metallic materials concentrated towards the center.
Deducing Earth's Interior Structure
Geophysicists cannot study Earth's interior directly, so they have a variety of clues to help them deduce Earth's interior structure and composition. These clues include Earth's density, seismic waves traveling through Earth's interior, and Earth's magnetic field. These same clues can also be applied to other terrestrial planets.
Earth's Crust
The crust is Earth's outermost layer, and it is the only layer that scientists can directly study. Earth's crust ranges from 8 to 70 kilometers (5 to 40 miles) thick, according to Michael Zeilik's Astronomy: The Evolving Universe (Cambridge, 2002). It is solid and has a rocky composition. The top portion of the crust is the only portion of Earth's interior that geologists can study directly.
The crust is divided into tectonic plates that float on the mantle. The slow drifting of these tectonic plates causes most of Earth's geological processes.
Earth's Mantle
Like the crust, Earth's mantle has a rocky composition. Most people would describe the rocky mantle as being solid, however technically it is not really solid. The mantle is in a putty-like semi-solid state that can flow very slowly.
Heat sources in the core cause convection currents in the slowly flowing mantle. These convection currents cause the continental plates in Earth's crust to drift and transfer heat energy from Earth's core to the upper mantle and crust.
Earth's mantle is about 2900 kilometers (1800 miles) thick.
Earth's Core
The core of the Earth is divided into two portions: the 2300 kilometer (1400 mile) thick outer core and the 1200 kilometer (750 mile) radius inner core.
Both the inner and outer cores have a nickel-iron metallic composition. The outer core is liquid nickel-iron but most geophysicists think that the inner core is solid. Because the temperature increases with depth in Earth's interior, it seems paradoxical that the hotter inner core should be solid while the outer core is liquid. However the pressure in the inner core is greater than in the outer core. Melting points of most materials increase as the pressure increases. The melting point of the nickel-iron inner core is therefore higher than that of the outer core because of this increased pressure.
Implications to Science of Earth's Structure
Earth's interior is divided into layers with the more dense nickel-iron metallic materials concentrated in the core and the less dense rocky materials concentrated in the mantle and crust. Scientists say that the Earth's interior is differentiated.
This differentiation tells astronomers and geophysicists that the planet Earth was liquid when it first formed. To understand why set a nail on a solid ice cube and another nail in a glass of water. Dense materials will sink to the bottom of a liquid, but not to the bottom of a solid. When the entire Earth was liquid just after it formed, most of the dense nickel-iron sank to the core, leaving the less dense rocky materials in the crust and mantle.
As the planet Earth cooled, the crust solidified first, just as the top of a pond freezes first in winter. Earth's outer core still has not solidified.
The science of Earth's interior structure helps scientists understand both Earth's current geology and Earth's geologic history.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
