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Contents of 8.5 How hot is the interior of Earth?

• The geothermal gradient
• Near-surface 25-30oC in continental crust, and 60oC beneath oceans.
• If this continued within Earth (say at 30oC) the core would have to be at 200,000oC. This is unreasonable.
• Therefore, the gradient must decrease with depth.
• The mantle low-velocity zone must be partially molten at depths of 75-100 km, meaning that a temperature of 1300oC is reached.
• Experiments show that diamonds are created at ~1000-1200oC at ~100-150 km depth, meaning that the gradient under continents is less than 10oC/km.
• Lab results show that temperatures of 1450 150oC are needed to transform olivine into denser minerals at 410 km, meaning a gradient of only 3.5oC beneath oceans.

8.5 How hot is the interior of Earth?