Divergent Boundaries and Continental Rifting

Divergent boundaries don't just appear in the ocean; they often begin beneath continents. The process starts with upwarping, where hot mantle material rises, stretching the lithosphere until it fractures. This creates a Rift Valley, characterized by a series of down-dropped blocks called grabens and uplifted blocks called horsts. The East African Rift is the world's premier laboratory for this. As the crust thins, the pressure on the underlying mantle decreases, leading to volcanic activity. If the stretching continues, the valley drops below sea level, forming a linear sea, much like the modern-day Red Sea.

A common misconception is that the mantle melts because it gets hotter. In reality, the mantle is mostly solid. At divergent boundaries, magma forms through decompression melting. As plates pull apart, the underlying asthenosphere rises to fill the gap. Because rock is a poor conductor of heat, it stays hot as it rises, but the pressure $P$ decreases significantly. When the pressure drops below the rock's solidus (the temperature where melting begins), the rock begins to melt into basaltic magma. This relationship can be expressed as a function of depth $z$, where pressure $P \approx ho g z$.

Once the continent has fully split, a Mid-Ocean Ridge (MOR) forms. These are the longest mountain ranges on Earth. However, they are not straight lines. Because the Earth is a sphere, plates cannot move apart in a perfectly linear fashion. To accommodate this geometry, the ridge is broken into segments by transform offsets. Between two ridge segments, plates slide past each other along transform faults, creating intense shallow-focus earthquakes. Beyond the ridge segments, these become inactive fracture zones, which stand as scars on the ocean floor marking the direction of past plate motion.