Grabens, the Mid-Atlantic Ridge, and the Forces That Pull Continents Apart

The tilted rock layers visible in mountain formations record compressional forces - the slow collision of tectonic plates that buckles and folds the crust upward. But the Earth's interior is also capable of producing the opposite effect, and Randall Carlson traces both ends of that spectrum.

Where plates are pulled apart rather than pushed together, the result is extension. Magma plumes rising through the mantle push upward and outward at spreading ridges like the Mid-Atlantic Ridge, forcing the plates on either side progressively further apart. Where that extensional force acts on continental crust rather than oceanic crust, the result can be a graben - a block of land caught between two parallel fault lines that drops downward as the surrounding crust pulls away from it. The structure is essentially the inverse of a mountain range - instead of rock being thrust upward by compression, a section of crust subsides between fractures. Ring-shaped down-faulted grabens represent one of the more complex expressions of this process, and Randall connects the underlying mechanics back to the broader picture of a planet whose surface is being continuously remade by forces operating from deep within.