Answer to Question 1
Answer: The shape of Earth's magnetic field resembles that produced by a laboratory bar magnet. Geologists know from the lines of magnetic force that the field originates deep within Earth, where temperatures are too high for the existence of permanently magnetized material. The magnetic field must be continually generated by the geodynamo- rapid convective motion of electrically conductive, iron-rich liquid in the outer core. Complexities in rapid outer-core convection caused by electrical conductance through the inner core and transfer of heat from the outer core to the mantle may account for rapid fluctuations in the strength and orientation of the magnetic field and even reversals of the field.
Answer to Question 2
Answer: Mantle convection explains why heat flow is greater through the seafloor than through continents and why decompression occurs that melts rock to form magma. Mantle convection occurs because upward-moving rock expands more by decompression than it contracts by cooling, allowing it to remain hotter and less dense than its surroundings, so it continues to move up. Likewise, sinking mantle compresses more because of high pressure than it expands by warming, so that it remains cooler and denser than its surroundings, and it continues to move down. Seismic tomography shows regions of the mantle where seismic waves travel slightly slower or slightly faster than in adjacent rock. The slower regions are interpreted to represent warmer, upwelling mantle, and the faster regions are interpreted to represent cooler, downward-flowing rock. Computer simulations of mantle convection produce patterns similar to those inferred by seismic-tomographic data.
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