The mantle is the mostly-solid bulk of Earth’s interior, representing about 82% of Earth’s volume. Located between the crust and the core, the mantle extends to a depth of 2,890 km (1,800 miles) making up about 84% of Earth’s volume. The mantle is divided into upper and lower mantle which are separated by a transition zone between 400-650 km depth.
Composition of the Mantle
The mantle is composed of ultramafic rocks high in magnesium and iron content. The main minerals in Earth’s mantle are silicate minerals such as olivine, pyroxene, and garnet which have high melting points allowing them to remain solid under immense pressure and temperatures. The upper mantle is approximately 64% olivine, 16% pyroxene, 8% garnet with the remainder composed of calcium oxide, aluminum oxide, iron oxide and other minerals. The lower mantle contains a higher proportion of magnesium-rich pyroxene and garnet relative to olivine. The exact composition and structure of the deepest part of the mantle (the D” layer just above the core–mantle boundary) is still poorly constrained.
Temperature and Pressure
The temperature of the mantle increases with depth from about 500–900°C at the upper mantle to over 4000°C near the core–mantle boundary. The enormous pressures in the mantle arise because of the weight of overlying material. Pressures increase from only a few kilobars near the crust-mantle boundary to almost 140 gigapascals (GPa) at the core-mantle boundary.
These extreme temperatures and pressures make the lower mantle behave as a viscous plastic solid that slowly flows over geologic timescales. But the upper mantle can behave as a viscous liquid in some regions.
Convection Cells
The mantle convects slowly over millions of years. Hot buoyant rock rises through upper mantle convection cells while cooler, denser mantle rock sinks. This circulation of mass and heat drives plate tectonics at the surface. Mantle plumes are upward surges of hot material that may impinge on the crust to produce volcanic activity.
There may be whole-mantle convection between the upper and lower mantle or there could be layered convection with separate convection cells in the upper and lower mantle. The 660 km seismic discontinuity at the boundary between the upper and lower mantle may inhibit but not completely prevent material transfer between the upper and lower mantle convection cells.
Seismic Discontinuities
Seismic waves travel through Earth and allow us to probe the deep interior. Changes in seismic wave velocities have revealed transition zones between different layers within the mantle caused by phase changes in minerals. The major mantle seismic discontinuities are:
- Moho – crust-mantle boundary (5-10 km depth)
- 410 km discontinuity – phase change olivine to wadsleyite
- 660 km discontinuity – phase change ringwoodite to perovskite+magnesiowüstite
The Moho represents the transition from basal crustal rocks to the ultramafic peridotite of the upper mantle. The 410 and 660 km discontinuities are caused by mineral phase changes as increasing pressure and temperature modifies the crystal structure of minerals.
Mantle Xenoliths
Mantle xenoliths are rock fragments brought rapidly to Earth’s surface in volcanic eruptions. These samples allow geologists to directly study the composition of the upper mantle. Peridotite and eclogite xenoliths reveal the upper mantle is composed mainly of olivine, pyroxenes and garnets.
Physical Properties
Here is a table summarizing some key physical properties of the mantle:
| Property | Upper Mantle | Transition Zone | Lower Mantle |
|---|---|---|---|
| Depth | 35-410 km | 410-660 km | 660-2890 km |
| Temperature | 500-900°C | 900-1500°C | 1500-4000°C |
| Pressure | 10-24 GPa | 24-135 GPa | 135-136 GPa |
| Density | 3.3 g/cm3 | 3.6-5.6 g/cm3 | 5.6 g/cm3 |
Melting in the Mantle
Partial melting occurs within the asthenosphere of the upper mantle giving rise to basaltic magmas and volcanism. The presence of water and volatiles lowers the mantle solidus facilitating melting. Mantle plumes can produce large igneous provinces and flood basalts when they impinge on the base of the lithosphere. There is likely very limited melting within the lower mantle due to the higher pressures.
Movement in the Mantle
Convection cells driven by heat flow lead to slow overturning circulation in the mantle. Cold dense lithospheric plates subduct back into the mantle, while hot buoyant upwellings rise through the mantle. This circulation drives plate tectonics and causes the continental plates to drift across Earth’s surface.
On a smaller scale, mantle plumes are columns of hot rock that rise through the mantle. The heads of these plumes may impinge on the base of the lithosphere to produce large igneous provinces and flood basalts. The Hawaiian islands formed as the Pacific plate moved over a relatively stationary hotspot fed by a mantle plume.
Evidence for Mantle Composition
The composition of the mantle is inferred from:
- Studies of mantle rocks (xenoliths) brought to the surface in volcanic eruptions
- Seismic tomography models that reveal rock density and seismic velocities
- Experiments that recreate the extreme pressures and temperatures of the deep mantle
- Analysis of basalts that originate from partial mantle melting
These lines of evidence indicate the upper mantle is dominantly composed of olivine, with a transition to higher proportions of pyroxenes and garnet minerals within the lower mantle.
Challenges Studying the Mantle
The mantle poses immense challenges for direct study because of the pressures and temperatures deep underground. Geologists have not directly sampled the deep mantle – only the upper few hundred kilometers from mantle xenoliths. Seismology provides the best means to probe the deep interior remotely.
Challenges include:
- Cannot directly sample the lower mantle – must rely on indirect evidence
- Cannot recreate the exact temperatures, pressures and timescales of the mantle in the laboratory
- Sparse distribution of seismometers on Earth’s surface limits resolution
- Interpretations depend on computational models with inherent uncertainties
Future advances may come from improved seismic imaging, higher resolution geodynamic modeling, experiments under higher pressures and temperatures, and possibly mantle sampling through very deep drilling projects.
Significance of Studying the Mantle
Understanding the mantle is key to unraveling many fundamental questions about our planet such as:
- Driving forces behind plate tectonics
- Heat flow and thermal evolution of the planet
- Mechanisms controlling geochemical reservoirs and cycles
- How mantle convection influences surface topography
- Origins of magmatism and volcanism
Studying the mantle provides insights into geologic hazards, Earth’s history, climate changes, and the dynamics of other rocky planets. Challenging as it may be, mantle research will remain important to understand our planet as a complex and dynamic system.
Conclusion
The mantle represents the vast middle layer of our planet extending from the Moho to the core-mantle boundary nearly 3000 km deep. Composed mainly of high-temperature silicate rocks, the mantle convects slowly over millions of years helping drive plate tectonics at the surface. Studying the inaccessible mantle relies on ingenious indirect techniques combined with computational modeling and laboratory experiments. Further research will provide a clearer picture of Earth’s interior and dynamics while also advancing understanding of planetary systems.
