Although a complete section of oceanic crust has not yet been drilled, geologists have several pieces of evidence that help them understand the ocean floor. Estimations of composition are based on analyses of
ophiolites (sections of oceanic crust that are thrust onto and preserved on the continents), comparisons of the seismic structure of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from the ocean floor by
submersibles, dredging (especially from
ridge crests and
fracture zones) and drilling.
 Oceanic crust is significantly simpler than continental crust and generally can be divided in three layers. According to mineral physics experiments, at lower mantle pressures, oceanic crust becomes denser than the surrounding mantle.
- Layer 1 is on an average 0.4 km thick. It consists of unconsolidated or semiconsolidated
sediments, usually thin or even not present near the
mid-ocean ridges but thickens farther away from the ridge.
 Near the continental margins sediment is terrigenous, meaning derived from the land, unlike deep sea sediments which are made of tiny shells of marine organisms, usually calcareous and siliceous, or it can be made of volcanic ash and terrigenous
sediments transported by
- Layer 2 could be divided into two parts: layer 2A – 0.5 km thick uppermost volcanic layer of glassy to finely crystalline
basalt usually in the form of
pillow basalt, and layer 2B – 1.5 km thick layer composed of
- Layer 3 is formed by slow cooling of
magma beneath the surface and consists of coarse grained
ultramafic rocks. It constitutes over two-thirds of oceanic crust volume with almost 5 km thickness.
The most voluminous volcanic rocks of the ocean floor are the mid-oceanic ridge basalts, which are derived from low-
potassium tholeiitic magmas. These rocks have low concentrations of large ion lithophile elements (LILE), light rare earth elements (LREE), volatile elements and other highly
incompatible elements. There can be found basalts enriched with incompatible elements, but they are rare and associated with mid-ocean ridge
hot spots such as surroundings of
Galapagos Islands, the
Oceanic crust is continuously being created at mid-ocean ridges. As plates diverge at these ridges, magma rises into the upper mantle and crust. As it moves away from the ridge, the lithosphere becomes cooler and denser, and sediment gradually builds on top of it. The youngest oceanic lithosphere is at the oceanic ridges, and it gets progressively older away from the ridges.
As the mantle rises it cools and melts, as the pressure decreases and it crosses the
solidus. The amount of melt produced depends only on the temperature of the mantle as it rises. Hence most oceanic crust is the same thickness (7±1 km). Very slow spreading ridges (<1 cm·yr−1 half-rate) produce thinner crust (4–5 km thick) as the mantle has a chance to cool on upwelling and so it crosses the solidus and melts at lesser depth, thereby producing less melt and thinner crust. An example of this is the
Gakkel Ridge under the
Arctic Ocean. Thicker than average crust is found above
plumes as the mantle is hotter and hence it crosses the solidus and melts at a greater depth, creating more melt and a thicker crust. An example of this is
Iceland which has crust of thickness ~20 km.
The age of the oceanic crust can be used to estimate the (thermal) thickness of the lithosphere, where young oceanic crust has not had enough time to cool the mantle beneath it, while older oceanic crust has thicker mantle lithosphere beneath it.
 The oceanic lithosphere
subducts at what are known as
convergent boundaries. These boundaries can exist between oceanic lithosphere on one plate and oceanic lithosphere on another, or between oceanic lithosphere on one plate and continental lithosphere on another. In the second situation, the oceanic lithosphere always subducts because the continental lithosphere is less dense. The subduction process consumes older oceanic lithosphere, so oceanic crust is seldom more than 200 million years old.
 The process of super-continent formation and destruction via repeated cycles of creation and destruction of oceanic crust is known as the
The oldest large scale oceanic crust is in the west Pacific and north-west Atlantic - both are about up to 180-200 million years old. However, parts of the eastern
Mediterranean Sea are remnants of the much older
Tethys ocean, at about 270 and up to 340 million years old.