The inner rift valley of the Mid-Atlantic Ridge at 36°48′ N. is 1.5 to 3 km wide and 100 to 400 m deep. It is symmetrical in profile with a discontinuous medial ridge 100 to 240 m high and 800 to 1,300 m wide along its axis. The medial ridge is replaced every 1 to 3 km with a central trough 200 to 600 m wide.
The medial ridge is apparently built by eruptions of pillow basalt recurring at intervals of roughly 14,000 years at a given point. Between eruptions (and possibly during them), the ridge splits and divides along its axis and subsides, which produces the central trough. As the trough widens and deepens, it eventually taps magma in a shallow reservoir, initiating a new eruption that rebuilds the medial ridge.
Outward spreading of the inward-dipping shingled halves of the former medial ridge produces a layer of pillowed basalts about 400 m thick (oceanic layer 2A), in which resides the bulk of the remanant magnetization of the ocean floor. This layer overlies a layer of intrusive rock (layer 2B) composed of a dike complex that feeds eruptions building the medial ridge as well as the outward moving, solidified shells of a shallow magma chamber.