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149:, or the CMB. It is a narrow, uneven zone, and contains undulations that may be up to 5–8 km (3–5 mi) wide. These undulations are affected by the heat-driven convection activity within the overlying mantle, which may be the driving force of plate tectonics-motion of sections of Earth's brittle exterior. These undulations in the core–mantle boundary are also affected by the underlying eddies and currents within the outer core's iron-rich fluids, which are ultimately responsible for Earth's magnetic field.
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The boundary between the core and the mantle does not remain constant. As the heat of the earth's interior is constantly but slowly dissipated, the molten core within Earth gradually solidifies and shrinks, causing the core–mantle boundary to slowly move deeper and deeper within Earth's core.
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occurs within Earth's interior at a depth of about 2,900 km (1,800 mi) below the surface, where there is an abrupt change in the
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137:(generated by earthquakes or explosions) that travel through Earth. At this depth, primary seismic waves (
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