In the two previous sections, we discussed gravity and magnetism separately. However, geophysicists often study both of these data sets together in order to further pinpoint the type of rocks present beneath the Earth's surface. For example, where is the largest isostatic gravity value on map GP-1006? It is located over Mt. Diablo. Looking at Table 1, what rock types have average densities greater than 2.67 g/cm3? At least two rock types on average are dense enough to cause a gravity high. Looking at GP-1007, are the rocks forming Mt. Diablo magnetic? A large magnetic anomaly coincides with the location of the gravity anomaly. The rock under Mt. Diablo must be both magnetic and dense. Only one rock type listed in Table 1 (gabbro; unit ci) fits both of these qualifications and, indeed, gabbro is exposed locally at the top of the mountain.

Study of the gravity and magnetic fields in conjunction with the geology exposed at the surface gives geophysicists a way to "see" into the Earth. With this information and some assumptions, we can help locate faults, mineral and petroleum resources, and groundwater. This information can also give insight into the processes that help shape our world.

Box 4.1 The Chicxulub Crater (or how gravity and magnetic data helped find the smoking gun that killed the dinosaurs).

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