Question

Describe some differences between the lithosphere beneath a continent and that beneath oceanic crust.

Short answer

Continental lithosphere is thicker, less dense, and older, while oceanic lithosphere is thinner, denser, and younger.

Step-by-step solution

Introduction to Lithosphere

The lithosphere is the outermost layer of the Earth, composed of the crust and the uppermost part of the mantle. It is divided into tectonic plates that float on the semi-fluid asthenosphere beneath. This includes both continental and oceanic regions.

Characteristics of Continental Lithosphere

The continental lithosphere is generally thicker, ranging from about 35 to 100 km. It is primarily composed of less dense granitic rocks, which gives it a buoyant nature. This granitic composition results in an average density of about 2.7 g/cm³.

Characteristics of Oceanic Lithosphere

The oceanic lithosphere is relatively thinner, with a typical thickness of about 5 to 10 km. It is mainly composed of denser basaltic rocks. This composition results in a higher average density of approximately 3.0 g/cm³, making it less buoyant than the continental lithosphere.

Age and Recycling of Oceanic Lithosphere

Oceanic lithosphere is generally younger compared to continental lithosphere. It is constantly being formed at mid-ocean ridges and recycled back into the mantle at subduction zones, typically lasting up to about 200 million years.

Stability of Continental Lithosphere

In contrast, the continental lithosphere can be much older, often existing for billions of years without being recycled in the same manner as the oceanic lithosphere. It remains more stable due to its thicker, buoyant nature and lack of subduction at continental boundaries.

Conclusion

In summary, the continental lithosphere is thicker, less dense, primarily composed of granitic rocks, and is typically much older and more stable. Alternatively, the oceanic lithosphere is thinner, denser, composed mainly of basaltic rocks, and is younger due to active recycling processes.

Key concepts

Continental Lithosphere

The continental lithosphere is the thick and robust outermost layer of the Earth that lies beneath continents. Typically measuring between 35 to 100 kilometers thick, it is composed mainly of granitic rocks. These rocks are less dense than other types, giving the continental lithosphere a buoyant quality. Another important aspect is its density, which averages around 2.7 g/cm³, slightly less than the overall density of the Earth.

• Composed primarily of granitic rocks
• Density: 2.7 g/cm³
• Thickness: 35 to 100 km
• Older and more stable

Due to its composition and thickness, the continental lithosphere is incredibly stable and often exists for billions of years, experiencing less recycling compared to its oceanic counterpart.

Oceanic Lithosphere

The oceanic lithosphere is located beneath the earth's ocean basins. It is markedly thinner than the continental lithosphere, with thickness varying from 5 to 10 kilometers. The oceanic lithosphere is primarily constructed of basaltic rocks, which are much denser and heavier than granitic rocks. This results in a higher average density of about 3.0 g/cm³.

• Composed mainly of basaltic rocks
• Density: 3.0 g/cm³
• Thickness: 5 to 10 km
• Subject to recycling

One interesting feature of the oceanic lithosphere is that it is continuously being regenerated. New oceanic lithosphere is formed at mid-ocean ridges, while older sections are recycled back into the mantle at subduction zones. This recycling process makes oceanic lithosphere relatively young, typically not lasting more than 200 million years.

Tectonic Plates

Tectonic plates are massive slabs of lithosphere that fit together over the Earth's surface like a jigsaw puzzle. These plates can encompass both continental and oceanic lithosphere, floating atop the more ductile asthenosphere. Their interactions are responsible for many geological phenomena, such as earthquakes, volcanic activity, and the formation of mountain ranges.

• Composed of lithosphere
• Float on the asthenosphere
• Interactions cause geological activity

The sizes and shapes of tectonic plates vary greatly, and they move slowly over time. Where two plates meet, different types of boundaries form: divergent, convergent, or transform. Each type of boundary results in unique geological processes, contributing to the dynamic nature of the planet's surface.

Asthenosphere

Beneath the lithosphere lies the asthenosphere, a semi-fluid, ductile region of the upper mantle. It extends to about 700 kilometers below the Earth's surface. Despite being solid, it has the ability to flow slowly, which allows the tectonic plates to move. Think of it as a conveyor belt that drives the movement of the lithospheric plates above.

• Semi-fluid part of the mantle
• Allows tectonic plates to move
• Extends to about 700 km deep

The asthenosphere's malleable nature is due to high temperatures and pressure, making it capable of gradual deformation. This enables it to accommodate the shifting of tectonic plates, playing a crucial role in the dynamic processes of plate tectonics that shape Earth’s surface.