Valleys formed by lithospheric rifting—which cut into the surface of every continent and even the faces of other planets—are distinctive markers of tectonic activity. Rifts form from a structurally weak segment of the brittle upper crust when stress-induced fracturing causes lengthening and spreading over hundreds of thousands of years. Rift valleys, known as graben, can extend for thousands of kilometers and spread at rates up to a few centimeters per year. Where multiple individual rift systems come together, tectonic plates can be cut in two or new crust formation regions can arise. Whether separate rifts in close proximity will lead to one of these more consequential outcomes or stagnate as a series of valleys and peaks depends on the physical properties of the rifts, brittle crust, and viscous lower layers of the lithosphere.