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Continental rifts form where large‐scale horizontal extension occurs in continental lithosphere (Figure 1.5). In such regions, the lithosphere is progressively stretched and thinned. A candy bar being very slowly stretched in two is a crude metaphor. This stretching occurs by brittle, normal, and detachment faulting near the cooler surface and by ductile flow at deeper, warmer levels. Long‐term extension is accompanied by uplift of the surface as the hot asthenosphere rises under the thinned lithosphere. Rocks near the surface of the lithosphere that rupture along normal and detachment faults produce continental rift valleys. The East African Rift, the Rio Grande Rift in the United States and the Dead Sea Rift in the Middle East are modern examples of continental rift valleys.

Figure 1.4 Principal types of plate boundaries: (a) divergent; (b) convergent; (c), transform; thick lines represent plate boundaries and black arrows indicate relative motion between plates; (d) hybrid convergent‐transform boundary; red arrows show components of convergent and transform relative motion.

Figure 1.5 Major features of continental rifts include rift valleys, thinned continental crust (blue) and lithosphere (brown) with volcanic‐magmatic activity from melts generated in rising asthenosphere (burnt orange).

If horizontal extension and vertical thinning occur for a sufficient period of time, the continental lithosphere may be completely rifted into two separate continents. Complete continental rifting is the diachronous process by which supercontinents such as Pangea and Rodinia were broken into smaller continents such as those we see on Earth's surface at present. When this happens, a new and growing ocean basin begins to form between the two continental fragments by the process of sea floor spreading (Figure 1.6). The most recent example of this occurred when the Saudi Arabian Peninsula separated from Africa to produce the Red Sea basin some 5 Ma. Older examples include the separation of India from Africa to produce the northwest Indian Ocean basin (c. 115 Ma) and the separation of the Americas from Africa to produce the Atlantic Ocean basin (beginning c. 190 Ma). Once the continental lithosphere has rifted completely, the divergent plate boundary is no longer situated within continental lithosphere. Its position is instead marked by a portion of the oceanic ridge system where oceanic crust is produced and grows by sea floor spreading (Figure 1.6).

Figure 1.6 Model showing the growth of ocean basins by sea floor spreading from the ridge system following the complete rifting of continental lithosphere along a divergent plate boundary. Separated continental crust (light blue, topped with yellow sediments) and recently formed oceanic crust (dark blue striped pattern) are produced.