The Conrad discontinuity corresponds to the sub-horizontal plane that takes place in the continental crust

Continental crust

The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...

 in which the seismic wave

Seismic wave

Seismic waves are waves of energy that travel through the earth, and are a result of an earthquake, explosion, or a volcano that imparts low-frequency acoustic energy. Many other natural and anthropogenic sources create low amplitude waves commonly referred to as ambient vibrations. Seismic waves...

 velocity increases in discontinuous mode. This plane is observed in various continental regions in a depth of 15 to 20 km, however it is not found in oceanic regions.

The Conrad discontinuity (named after the seismologist Victor Conrad

Victor Conrad

Victor Conrad was an Austrian-American physicist, seismologist and meteorologist. He was the first director of the Austrian seismological service, and a reputed academician of international accomplishment. He was politically victimized twice, in 1919 for his ethnicity and in 1934 as a socialist...

) is considered to be the border between the upper continental crust and the lower one. It is not as pronounced as the Mohorovičić discontinuity

Mohorovičić discontinuity

The Mohorovičić discontinuity , usually referred to as the Moho, is the boundary between the Earth's crust and the mantle. Named after the pioneering Croatian seismologist Andrija Mohorovičić, the Moho separates both the oceanic crust and continental crust from underlying mantle...

, and absent in some continental regions. Up to the middle 20th Century the upper crust in continental regions was seen to consist of felsic rocks such as granite

Granite

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

 (SiAl

Sial

In geology, the sial is the upper layer of the Earth's crust made of rocks rich in silicates and aluminium minerals. It is sometimes equated with the continental crust because it is absent in the wide oceanic basins, but "sial" is a geochemical term rather than a plate tectonic term.Geologists...

, for silica-aluminium), and the lower one to consist of more magnesium-rich mafic rocks like basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

 (SiMa

Sima (geology)

Sima is the name for the lower layer of the Earth's crust. This layer is made of rocks rich in magnesium silicate minerals. Typically when the sima comes to the surface it is basalt, so sometimes this layer is called the 'basalt layer' of the crust. The sima layer is also called the 'basal crust'...

, for silica-magnesium). Therefore, the seismologists of that time considered that the Conrad discontinuity should correspond to a sharply defined contact between the chemically distinct two layers, SiAl and SiMa.

However, form the 1960s onward this theory was strongly contested among geologist

Geologist

A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...

s. The exact geological significance of the Conrad discontinuity is still not clarified. The partial melting

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....

zones that are scattered in the continental crust might provide an explanation.