1 The central part of the Earth, the core is the point within the Earth where S-waves cannot penetrate, marked by an abrupt increase in pressure. It is believed to be composed primarily of a nickel-iron alloy (along with abundant platinum-group elements), with a liquid outer zone, 2 900–5 000 km deep, and a solid inner zone, 5 000–6 370 km deep.
2 See core region.
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