"The study of extensional tectonics harkens to the early days of plate tectonics, as the search for mechanisms driving large continental blocks to drift apart led to advances in paleomagnetism and geochronology. Divergent plate boundaries form extensive and continuous volcanic systems covering large portions of the Earth and are fundamental to understanding how plates form. The study of extensional dynamics transects traditional geoscience discipline boundaries and is critical in the search for the origin and evolution of life. Extensional and normal faults are not restricted to divergent plate boundaries but are also located in regions of plate convergence and within plates. Failed rifts pose significant seismic hazards. This chapter aims to identify the importance of the study of extension and dispel common misconceptions regarding the process. Stress is a critical factor in explaining why extension occurs in the lithosphere and how magma focuses in regions of extensional tectonics."-- Provided by publisher.