The history of how Earth's interior evolved, and how it accounts for many aspects of our planet's behaviour, remains largely unwritten, Taking water into account could well help to explain a great deal more. The basic divisions of Earth's internal structure (crust, mantle and core) have I been known for a long time. But the evolutionary path that gave us this structure, and that provides the dynamics of plate tectonics, volcanism and magnetic-field generation, remains poorly understood, Why do we have plate tectonics? What is the nature and extent of melting deep within Earth? How does the core manage to keep generating such a richly complex magnetic field?
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