Marine geophysicists on the Department's faculty have traditionally focused on the acoustic, gravity, magnetic and seismic measurements relevant to shallow structure and to understanding near surface processes including those forming the seafloor. In recent years, collaboration with earthquake seismologists has resulted in major contributions to the field of plate tectonics and the dynamics of seafloor spreading and its consequences. Consequently, the boundary between these two groups has largely blurred as is reflected by an overlap in the scopes of their respective research. A number of the faculty study the way in which the crust and upper mantle of the earth are formed at local to global scales; a subject that has both oceanic and terrestrial elements. Other areas of expertise include earthquakes and faulting, and petroleum exploration and extraction. Work in monitoring nuclear explosions and verification of nuclear test ban treaties has drawn some members of the faculty into the national debate of these issues in the policy arena.