Our planet resembles a giant peach. If we could slice through such an enormous fruit, we would see that the peel is like the outer layer of rock on which we live, what geologists call the crust. The Earth’s crust is very thin, extending to depths of 30 to 65 km under the continents, and only 5 to 15 km under the oceans. Beneath the crust (or peel!) lies the Earth’s mantle, which is analogous to the juicy flesh of the peach. The mantle is made of hot rock that can flow slowly, just as you can deform peach flesh as you squeeze it in your fingers. Finally, hidden deep beneath the mantle at a depth of 2,885 km is the outer edge of the Earth’s iron-nickel core – the peach pit if you will! But the similarities end there. The inner core may be solid, but the outer part is hot metallic liquid – the only liquid layer in the planet.

Not all mantle rock can flow easily. The uppermost mantle, along with the thin crust, defines a fairly rigid layer known as the lithosphere. The lithosphere ranges from 10 to 250 km thick and is broken into fragments, called plates, which move around the Earth like pieces of a giant jigsaw puzzle.

Below the lithosphere, enormous heat creates convection cells in the deeper mantle. The stiff lithospheric plates are carried along by these currents of hot flowing rock, much like a conveyor belt. The movement is slow – not much faster than your fingernails grow – but over geologic time, the effect is very dramatic. As the plates move, they carry with them the parts that poke above the water – the continents and islands – and in so doing, constantly reshape the geography of our planet!

This activity, known as plate tectonics, has not been steady over time. Some periods were calmer; others, like the Devonian, much more intense. Where plates moved apart, oceans formed, and where they pushed against each other, continents collided and the towering majestic peaks of new mountain chains emerged from the disappearing Devonian seas...
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