Both of these geologic units consist of igneous rocks, but are characterized by different compositions. Minerals that contain a lot of silicon and aluminum weigh less than minerals containing iron and magnesium. Many types of sedimentary rocks tend to be enriched in felsic minerals and, therefore, tend not to be very dense. In addition, because of the way metamorphic and igneous rocks form, they tend to have very little pore space and, thus , they usually are fairly dense rocks. An isostatic residual gravity map of the Bay area Shaded areas on Figure 3 on GP-1006 show some of the prominent gravity highs in the Bay area.
Plutonic rocks are rocks that solidified from molten rock beneath the surface of the Earth.
Most of the gravity lows occur over Cenozoic sedimentary rocks that have formed in valleys and former seas that once covered parts of the San Francisco Bay area. The magnitude of the gravity low is proportional to the thickness of the low-density sedimentary rocks. In many cases, the earthquakes tend to follow closely spaced contour lines. Because the gravity values reflect rock densities, this means that the density of the rocks varies spatially. In areas where geologists cannot map or observe faults at the surface because they are covered by buildings or unconsolidated sediments or where seismologists cannot map the faults because of the lack of earthquakes, one can sometimes use gravity gradients to locate faults.