The first association of rock types forms at the mid-ocean ridges where basaltic lava wells up from the mantle and plates move apart, growing laterally by addition of the solidified lava to their edges. Basaltic magma that cools deeper in the crust is a fine-grained crystalline rock called diabase, which forms vertical sheets or dykes, as well as bodies of gabbro, a coarsely crystalline rock of basaltic composition. At the base of the crust, belonging to the underlying mantle, are heavy rocks with a high percentage of iron and magnesium, such as peridotite. Such an association is forming today at the Juan de Fuca Ridge.

On top of the basalt that has moved away and down from the ridge, at water depths typically of 2-7 km and far from the continents, is deposited fine "rained material such as mud, volcanic ash and ooze composed of the siliceous skeletons of microorganisms called radiolarians. Closer to the continental margins the ocean floors are covered by mud and sand that has slumped and flowed down the continental slope under the influence of gravity. This "oceanic assemblage" of different rock types is almost never seen mountain belts in pristine form; something has to happen to bring it up +10 km from its original location below sea level.