Correct Response: D. The magnetic orientation of ferromagnetic minerals in ocean-floor basalts demonstrated that oceanic lithosphere was moving apart as new basalt was generated at ocean spreading centers. This provided a mechanism to help explain two important components of plate tectonic theory; the movement of tectonic plates and the formation of new oceanic crust. Astronomical evidence and not geologic evidence indicates that, as a result of gravitational interactions, Earth's rotational velocity is decreasing very slowly over time (A) and is not stable. Because ocean crust is recycled at subduction zones, ocean-floor basalt is typically younger than 200 million years old and therefore does not provide evidence of Earth's age (B). The physical character of the upper mantle was not determined through analysis of the magnetism of ocean-floor basalt (C). The upper mantle is solid and together with oceanic crust constitute oceanic lithosphere.
