Earth is a complex system composed of solids, liquids, and gases. While the visible outer layer includes a thin, gaseous atmosphere, the planet’s mass is overwhelmingly composed of dense, non-gaseous materials, primarily metals and silicate rock. Understanding Earth’s substance requires exploring the distinct physical states and chemical compositions of its internal structure.
Earth’s Outer Shell and Atmosphere
The crust, the part of Earth we live on, is made of rock. Continental crust is largely composed of silica-rich rocks like granite, which is relatively low in density. Oceanic crust is thinner and denser, consisting mainly of dark, silica-poor rocks such as basalt. These rocks are primarily silicates, compounds built around oxygen and silicon.
The solid crust is encased by the atmosphere, the gaseous component of the planet. The atmosphere’s total mass is less than one millionth of the Earth’s total mass, but it is responsible for weather and climate. The air is predominantly nitrogen (about 78%) and oxygen (about 21%). Other components include argon, carbon dioxide, and trace amounts of other gases.
The Vast Mantle and Outer Core
Beneath the rocky crust lies the mantle, which is approximately 2,900 kilometers thick and constitutes about 68% of the Earth’s total mass. It is composed of hot, dense, silicate rock rich in iron and magnesium. Although described as solid rock, high temperatures and pressures allow the silicate material to behave plastically, enabling it to flow slowly through convection over geologic timescales.
Below the mantle is the outer core, about 2,300 kilometers thick. Its liquid state is due to intense heat (4,000 to 6,000 degrees Celsius) overcoming the pressure at this depth. The outer core is primarily an alloy of iron and nickel, along with lighter elements like sulfur and oxygen. The movement of this molten iron creates convection currents that generate Earth’s magnetic field.
Composition of the Inner Core
The deepest part of the planet is the inner core, with a radius of about 1,220 kilometers. It is composed of the same iron and nickel alloy as the outer core, but its physical state is different. The temperature of the inner core is estimated to be around 5,700°C.
Despite this immense heat, the inner core remains solid due to the overwhelming pressure from the weight of the material above it. The core overall makes up about 31% of the Earth’s mass, with the inner core being the final, densest layer.