The lower oceanic crust is the lower part of the

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic c ...

and represents the major part of it (volumetrically biggest part). It is generally located 4–8 km below the

ocean floor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

and the major lithologies are

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks incl ...

(

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are comp ...

and gabbroic rocks) which derive from melts rising from the earth's mantle. This part of the oceanic crust is an important zone for processes such as melt accumulation and melt modification ( fractional crystallisation and crustal assimilation). And the recycling of this part of the oceanic crust, together with the

upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from appro ...

has been suggested as a significant source component for tholeiitic magmas in Hawaiian volcanoes. Although the lower oceanic crust builds the link between the mantle and the MORB, and can't be neglected for the understanding of MORB evolution, the complex processes operating in this zone remain unclear and there is an ongoing debate in

Earth Sciences Earth science or geoscience includes all fields of natural science related to the planet Earth. This is a branch of science dealing with the physical, chemical, and biological complex constitutions and synergistic linkages of Earth's four sphere ...

about this. It is 6KM long.

Processes

The lower oceanic crust connects the earth's mantle with the MORB, where around 60% of the total

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural s ...

production of the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

happens. The three main processes happening in this region of the oceanic crust are

partial melting Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the solid ...

of the earth's mantle, melt accumulation at various depths and the chemical modification of this melts during ascent,.J. Leuthold, J. C. Lissenberg, B. O'Driscoll, O. Karakas; T. Falloon, D.N. Klimentyeva, P. Ulmer (2018); Partial melting of the lower oceanic crust at spreading ridges. Frontiers in Earth Sciences: Petrology: 6(15): 20p; This three processes do not happen in a strict order but occur all simultaneously over a depth range of 4–18 km suggesting that these processes can occur already in the upper mantle. The mantle melts are most commonly modified by fractional crystallisation due to cooling and by assimilation of crustal rocks.

Spreading rates

The most important parameter controlling the processes operating in the lower oceanic crust is the magma supply, this is further controlled by the spreading rate, and therefore, spreading rate is a critical variable in models for the formation of the lower oceanic crust. The rate at which plate divergence occurs at mid-ocean ridges is not the same for all ridge segments. Ridges with a spreading rate less than 3 cm/a are considered slow-spreading ridges, while those with a rate greater than 5 cm/a are considered fast-spreading ridges

Fast-spreading ridges

Intensive search spanning over three decades of seismic imaging have shown that the ridge axis is underlain by a

crystal mush A crystal mush is magma that contains a significant amount of crystals (up to 50% of the volume) suspended in the liquid phase (melt). As the crystal fraction makes up less than half of the volume, there is no rigid large-scale three-dimensional n ...

containing a small percentage of melt, capped by a thin melt lens containing a generally high, but variable melt fraction. The completely liquid body is a thin and narrow sill-like lens ( thick and < wide). The lens is maintained by reinjection of primitive magma. The lack of any detectable large

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it u ...

and the common detection of small lens/mush zone at fast-spreading ridges emphasize the small magma chamber model. Modally and compositionally layered gabbroic rock is often found (or abundant) in the lower crustal sections of

ophiolite An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is found ...

. The layered lower crust is thus one of the key features of all models of fast-spreading lower crust. Nevertheless, distinct modal layering as observed in major

ophiolites An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is found ...

has rarely been observed or sampled on the

. The IODP expedition 345 was one of the first drilling project, which sampled a significant thickness of layered igneous rocks. A shallow melt can erupt through cool crust and produce sheeted dikes and volcanics, but the small chamber seems difficult to resolve with traditional ideas of

fractional crystallization Fractional crystallization may refer to: * Fractional crystallization (chemistry), a process to separate different solutes from a solution * Fractional crystallization (geology) Fractional crystallization, or crystal fractionation, is one of th ...

and

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macr ...

settling to form the thick sequence of layered gabbros and foliated gabbros and ultramafics. One proposed model is the so-called "gabbro glacier", where crystals settle in a shallow melt-dominated lens beneath the ridge axis. The weight of the accumulating crystals settling to the bottom of the magma lens induces a

ductile Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

flow and deformation within the gabbros, just like the ice in a glacier responds to accumulated snow. Nevertheless, the model fails to explain the layered variations in

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ...

types, the correlated layering in mineral compositional variations, and the apparently primary near-vertical fabrics in the upper gabbros that appear to represent subvertical melt conduits. Kelemen and co-workers concluded that most of the lower oceanic crust crystallized in place, and proposed "the sheeted sill" model. In the model the sills form when porous flow of rising basaltic liquids (or small melt-filled fractures) are stopped beneath permeability (earth sciences) barriers of earlier crystallized melts and pond to form the sills. Cooling rates are generally sufficiently slow that crystals and their interstitial liquids are in chemical equilibrium, as long as the liquid is immobile. However,

buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pr ...

and/or

compaction (geology) In sedimentology, compaction is the process by which a sediment progressively loses its porosity due to the effects of pressure from loading. This forms part of the process of lithification. When a layer of sediment is originally deposited, it c ...

may induce liquid migration through the mush, resulting a significant compositional and microstructural modification.

Slow-spreading ridges

Slow- and intermediate-spreading ridges form typically valleys about wide and deep, with step-like inward-facing scarps, similar to rift valleys on land. Compared to fast spreading-ridges, the

supply and therefore the

heat flow Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy ( heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conductio ...

is low and can't maintain a persistent liquid

. Sinton and Detrick (1992) modelled a schematic cross section of an axial magma chamber beneath a slow-spreading ridge such as the

Mid-Atlantic Ridge The Mid-Atlantic Ridge is a mid-ocean ridge (a divergent or constructive plate boundary) located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. In the North Atlantic, the ridge separates the North Ame ...

. Due to the reduced heat and magma supply, a steady-state eruptible magma lens is relinquished in favor of a sill-like mush zone and a smaller transition zone beneath the well-developed rift valley. Convection and mixing in the magma chamber is far less likely than at fast ridges. Thermal constrains led to the development of different models to reconstruct the accretion history. The "infinite leek" model suggests small magma batches, forming small "nested" intrusions. Another model proposed that crystallization could occur at depth, where temperatures are higher, the formed

cumulates Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group ...

are then "dragged" up by mantle flow to form the lower oceanic crust. Today, a model intermediate between these two has become popular. This model is referred to as a "plum pudding", where the lower oceanic crust is constructed from a number of nested

plutons In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and co ...

that crystallize within the mantle or crust. Schwartz et al. (2005) describes another variant. He postulates that the lower crust is constructed both from the nested shallow-level plutons and from the products of deeper-seated

crystallization Crystallization is the process by which solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some ways by which crystals form are precipitating from a solution, freezing, or more rarely dep ...

Schwartz, J. J. (2005). Dating the Growth of Oceanic Crust at a Slow-Spreading Ridge. Science, 310(5748), 654–657.

References

{{reflist Plate tectonics Structure of the Earth Oceanographical terminology