Geography · Ch 9 — Interior of the Earth
Structure of the Earth
Structure of the Earth
The diagram is a wedge cutaway — imagine slicing a single pie-slice out of the Earth so you can see both the surface and the deep interior at once. The outer edge of the wedge shows the surface drawn with a jagged, mountain-range profile, and from that surface down to the point of the wedge (the centre of the Earth) the layers are laid out concentrically, each with its own label and depth marking.
Starting at the outside, the Crust is shown as the thin outermost shell, with its thickness noted as 0–100 km. The jagged profile at the surface is not decoration — it signals that the crust is not uniform: it is thinner under the oceans and thicker under continents, and thickest of all beneath major mountain systems. Just below the crust, a bracket groups the crust together with the uppermost solid part of the mantle and labels the combined unit the Lithosphere. This bracket is the key teaching point: the lithosphere is not a separate physical layer but a mechanical one — the rigid, brittle shell that includes the crust and the top of the mantle.
Beneath the lithosphere sits the Asthenosphere, the weak, partially molten zone of the upper mantle. The diagram places it directly under the lithosphere, and the textbook text adds that it extends up to about 400 km depth and is the main source of magma that rises during volcanic eruptions. Below the asthenosphere, the rest of the Mantle is shown continuing down to the 2,900 km mark — this is the lower mantle, which is in a solid state. The mantle as a whole is the thickest of the Earth's layers, running from the Moho discontinuity at the base of the crust all the way to the core–mantle boundary.
At 2,900 km, the diagram marks the boundary where the mantle ends and the Core begins. The core is split into two distinct zones: an outer Liquid zone and an inner Solid zone, with the inner core boundary drawn at 5,100 km. This liquid-outer/solid-inner distinction is the single most important fact the diagram conveys, because it is exactly what earthquake wave behaviour revealed — S-waves cannot pass through the liquid outer core, which is how geophysicists first inferred the core's existence and state. Finally, at the very point of the wedge, the total radius of the Earth is labelled 6,378 km, giving the reader a sense of scale: the crust is a mere skin on a sphere whose radius is over six thousand kilometres. …
The Earth is not a uniform ball of rock. It is built in concentric layers, each with its own physical state, composition and behaviour. The three great divisions from the outside in are the crust, the mantle and the core.
The Crust
The crust is the outermost solid part of the Earth. It is brittle in nature, which is why it fractures and breaks rather than bending under stress. Its thickness is not uniform — it differs sharply between the oceanic and continental areas.
The oceanic crust is thinner than the continental crust. The mean thickness of the oceanic crust is about 5 km, while the continental crust averages around 30 km. The continental crust grows thicker beneath major mountain systems; in the Himalayan region it reaches as much as 70 km in thickness.
The Mantle
Beyond the crust lies the mantle. It extends from the Moho's discontinuity — the boundary between crust and mantle — down to a depth of 2,900 km. The mantle is not a single uniform layer; it has distinct parts.
The upper portion of the mantle is called the asthenosphere. The word astheno means weak, and the name reflects the physical character of this layer. It is considered to extend up to about 400 km in depth. The asthenosphere is the main source of magma that finds its way to the surface during volcanic eruptions.
The crust together with the uppermost part of the mantle is called the lithosphere. Its thickness ranges from about 10 to 200 km. Below the asthenosphere lies the lower mantle, which extends deeper into the Earth and is in a solid state.
The Core
The existence of the core was understood with the help of earthquake wave velocities, which behave differently as they pass through materials of different states. The core–mantle boundary is located at a depth of 2,900 km.
The core itself is divided into two parts with different physical states: the outer core is in a liquid state, while the inner core is in a solid state. The core is made up of very heavy material, mostly constituted by nickel and iron. Because of this composition, it is sometimes referred to as the nife layer — ni for nickel and fe for iron.
| Layer | State | Key feature |
|---|---|---| …