Mechanical Layers: How Earth Moves
Beyond just chemical composition, geologists also classify Earth by how its layers behave mechanically. The lithosphere combines the crust with the uppermost rigid mantle - this is the solid, rocky shell we live on.
Just below that sits the asthenosphere, a weaker, more flexible layer in the upper mantle. This is where the rock is hot enough to flow slowly, allowing the lithosphere above to move around like puzzle pieces.
The mesosphere (not to be confused with the atmospheric layer) is the stronger, more solid part of the mantle below the asthenosphere. Here, even though it's extremely hot, the increased pressure makes the rock more rigid.
This mechanical layering system explains how plate tectonics actually work. The rigid lithospheric plates can move because they're floating on the softer asthenosphere beneath them, creating the continental drift that shapes our planet's surface over millions of years.
Study Tip: Remember that mechanical layers focus on strength and movement, while compositional layers focus on what materials make up each part.