Asthenosphere | Definition, Density & Temperature

The asthenosphere, from the Greek word asthenes meaning "weak", is a portion of the Earth's mantle that flows like molten plastic despite being solid. Understanding the location, behavior and composition of the asthenosphere is key to understanding plate tectonic theory and earthquakes.

The layers of the Earth can be described in two ways: chemically and physically. There are three chemically distinct layers of the planet, the crust, the mantle, and the core. The crust comprises the outer portion of the Earth, and includes the soil in which life grows. It contains high levels of silicon, aluminum, and oxygen. The mantle comprises the middle portion, where magma resides, and is primarily magnesium, iron, and some silicon. The core is the thickest portion and occupies the center of the planet. It is made up of iron and nickel.

Where, then, is the asthenosphere? When the Earth's interior is separated into chunks based on physical characteristics, three distinct layers emerge: the lithosphere, the asthenosphere, and the lower mantle. The lithosphere occupies the crust and some of the upper mantle, while the asthenosphere occupies part of the mantle below the lithosphere. The lower mantle goes even deeper than the asthenosphere. The lower mantle is sometimes called the mesosphere, but as this is also the name of a layer of the atmosphere, it isn't commonly used in geology.

As part of the mantle, the asthenosphere is composed primarily of magnesium and iron, with some silicon. Though the upper and lower mantle have the same chemical composition, they have different physical characteristics due to changes in temperature.

The depth of the asthenosphere typically ranges from 100km to 250km below the Earth's surface. For comparison, the lithosphere occupies the upper 100km, and the lower mantle extends from 250km to 2900km in depth.

Temperature and Density

Perhaps the most important characteristic of the asthenosphere is its temperature. It is temperature, not depth, that marks the boundary between the lithosphere and asthenosphere. That boundary begins when the temperature reaches 1300 degrees Celsius. At this temperature, rock approaches its melting point and begins to flow. The temperature in the asthenosphere continues to increase with depth, maxing out at around 1700 degrees Celsius.

The density of the asthenosphere is impacted mostly by its depth. The deeper into the asthenosphere, the more dense rock material becomes.

These differences in density and temperature contribute to convection currents within the mantle. Remember that the asthenosphere and mantle in general are not liquid; they form a single solid mass of rock, which may contain small pockets of liquid magma. Despite being solid, the asthenosphere has enough elasticity to flow, creating convection currents, which move rock and magma through the Earth. In plate tectonics theory, these convection currents contribute to the movement of the lithospheric plates, ultimately impacting the position of the continents and the creation of mid-ocean ridges, such as the Mid-Atlantic Ridge.

It's important to note that all this data is based on indirect observation; although scientists can get a good idea of the composition and temperature of the Earth's interior by studying volcanic activity, earthquakes, and other geologic phenomena, it is not currently possible to directly observe the Earth's interior. It's simply too deep, and too hot!

Much of the information scientists have gathered about the chemical and physical composition of the Earth's interior come from seismology, or the study of earthquakes. Earthquakes release seismic waves that travel through rock. The behavior of these waves, including their speed, is impacted by the chemical and physical characteristics of the rock they pass through.

Two types of seismic waves emitted by earthquakes are primary and secondary waves. Primary waves move quickly through rigid, solid rock, slowly through elastic rock, and very slowly through liquids. Secondary waves do not move through liquids at all.

It's by looking at the behavior of primary waves that the boundaries of the asthenosphere can be identified. Primary waves begin to slow down when they cross the boundary from the lithosphere into the asthenosphere due to the asthenosphere's reduced rigidity.

A seismometer measures seismic waves at Lick Observatory in California.

Though the existence of the asthenosphere was theorized decades before, the massive 1960 Chilean Earthquake provided data that confirmed its existence. Seismic waves produced by the earthquake pointed to the existence of an elastic layer beneath the lithosphere, where primary waves would move slowly.