Fig. 2. (a) Extensional tectonics. When forces in the interior of a planet cause a section of crust to be extended (or pulled apart), the motion along faults in the crust causes parts of the crust to be lowered relative to their surroundings. Geologists refer to the resulting topographic valleys as graben. Such deformation is important in the Tharsis region of Mars (slides #6 and #8). In this figure, the large arrows show the orientation of the forces applied to the crust and the small arrows show the motion along the faults. (b) Compressional tectonics. When forces in the interior of a planet compress (or push together) a section of crust, the motion along faults in the crust causes parts of the crust to be uplifted relative to their surroundings. Examples of ridges produced on Mars in this way are shown in slide #9. In this figure, the large arrows show the orientation of the forces applied to the crust and the small arrows show the motion along the fault.