The lithosphere is Earth's rigid outer layer, composed of the crust and the uppermost part of the mantle. This strong, solid layer is crucial for understanding plate tectonics and Earth's surface processes.
Composition
The lithosphere isn't uniform; its composition varies depending on whether it's oceanic or continental.
- Oceanic Lithosphere: Primarily composed of dense, mafic rocks like basalt and gabbro. It's relatively thin, typically ranging from 50 to 100 kilometers thick.
- Continental Lithosphere: Composed of less dense, felsic rocks like granite. It's significantly thicker, ranging from 100 to 200 kilometers thick, including a substantial crustal component.
Structure
The lithosphere is mechanically distinct from the underlying asthenosphere, a more ductile and viscous layer in the upper mantle. The lithosphere-asthenosphere boundary (LAB) marks the transition between these two layers.
- Crust: The outermost solid layer of the Earth.
- Oceanic Crust: Approximately 5-10 km thick, composed mainly of basalt.
- Continental Crust: Approximately 30-70 km thick, composed mainly of granite.
- Uppermost Mantle: This portion of the mantle is rigid and fused to the crust, forming the lithospheric plate. It's primarily composed of peridotite.
The lithosphere is broken into large and small pieces called tectonic plates. These plates "float" on the asthenosphere and move relative to each other, driven by convection currents in the mantle. Plate interactions are responsible for many geological phenomena, including earthquakes, volcanoes, and mountain building.
Key Differences: Oceanic vs. Continental Lithosphere
| Feature | Oceanic Lithosphere | Continental Lithosphere |
|---|---|---|
| Composition | Basalt, Gabbro | Granite |
| Density | Higher | Lower |
| Thickness | 50-100 km | 100-200 km |
| Age | Younger (typically < 200 million years) | Older (can be billions of years) |
Interaction with Other Spheres
The lithosphere interacts extensively with the atmosphere, hydrosphere, and biosphere. This interaction leads to weathering, erosion, and the formation of soil in a process called pedogenesis, creating the pedosphere – the outermost layer of the Earth that is composed of soil.
In summary, the lithosphere's composition and structure are fundamental to understanding plate tectonics, geological processes, and the interaction between the Earth's different spheres. It consists of the rigid crust and the uppermost mantle, with distinct characteristics between oceanic and continental regions.
