• The earth's interior can be divided into three layers of contrasting composition: crust, mantle, and core
• The interior of the earth has been determined using the characteristics of seismic waves
• Two distinct mechanical layers, lithosphere and asthenosphere, can be identified in the outer 300 km of the earth
• Processes that operate on the surface of the earth are driven by energy from the sun (radiation)
• Processes that operate within the earth are driven by heat energy from the radioactive decay of elements in the earth’s interior

Structure of the Earth
he interior of the earth can be divided into three layers of different composition and thickness. On earth, the three layers are the crust, mantle and core. These layers may be further subdivided based upon physical or compositional variations. For example, the composition and thickness of the earth’s crust varies below oceans (crust ~8 km thick) and continents (crust ~40 km thick).

The core is divided into two parts, a solid inner core and a liquid outer core. Scientists realized that the outer core is liquid because seismic waves will not travel through it. The earth’s magnetic field originates from convection currents in the liquid outer core. (For more on how we determine the character of the Earth's interior, see Understanding Earth's Interior).

The upper part of the mantle and the crust together form two layers identified by their relative strength and physical properties. The asthenosphere represents a weak layer in the upper mantle composed of partially melted rock. The lithosphere, a relatively strong rigid layer that includes both the crust (oceanic and continental) and the uppermost mantle overlies the asthenosphere.

The relative positions of two mechanical layers of the crust and upper mantle (lithosphere, asthenosphere).

The different physical properties of the lithosphere and asthenosphere are the result of the interplay between pressure and temperature which both increase with depth. Depending upon which increases most rapidly with depth, rocks may become weaker (temperature dominant, hotter rocks are weaker) or stronger (pressure dominant, increasing pressure results in increasing rock strength).

Energy and the Earth System
The processes that operate on the surface of the earth and within the planet’s interior are driven by energy from different sources. External processes derive energy from solar radiation whereas internal processes are associated with heat generated from the radioactive decay of elements in the earth’s interior.

Atmospheric convection cells generated by contrasts in solar radiation on a rotating Earth.

A fraction of the sun’s energy reaches the earth as solar radiation – the process by which heat passes through a gas, liquid or vacuum. Most solar radiation reaching the earth is absorbed by the land or oceans. Air masses are warmed or cooled by the land or ocean below. Warm tropical air rises over the equatorial oceans. As the air rises it gradually cools and releases its moisture as rain. Cooler air eventually sinks, returning to the surface to repeat the cyclical journey that represents convection – the movement within materials driven by different temperature conditions. For example, when a saucepan of water is heated, the warmest water at the bottom of the pan expands and rises and cooler water at the top sinks forming a rotating convection cell. Heat is distributed throughout the pot as the process continues. Convection cells in the atmosphere and oceans redistribute the heat that is unevenly distributed by solar radiation.

The earth’s geothermal gradient – the change in temperature with depth – illustrates that the planet's temperature increases with depth. The temperature gradient in the crust averages approximately 25^oC per kilometer. The geothermal gradient varies with location (higher in areas of volcanic activity) and depth and illustrates that the interior of the planet is much hotter than the exteriro. Processes such as volcanism are an indication that heat is being transferred from Earth’s interior toward the surface. Heat transfer occurs by convection and conduction.

Convection cells in the mantle are associated with oceanic ridges, regions of high heat flow on the ocean floor.

Convection is also thought to occur within the uppermost layers of the Earth’s interior and drives the process known as plate tectonics that explains the distribution of volcanoes and earthquakes around the world. Heat flow is greatest where these convection cells come to the surface, typically at zones of continuous volcanic activity such as oceanic ridges. However, heat is escaping from all parts of the surface, though at such low rates to be undetectable to only the most sensitive instruments. Such low heat flow is the result of conduction – the movement of heat through a solid body. For exmaple, the handle of a metal saucepan becomes hot when left on the stove as heat is transferred from the stove through the pan to the handle by conduction. Rocks are generally poor heat conductors (good insulators) so even though temperatures near the center of Earth are measured in thousands of degrees, heat loss at the surface is relatively modest.