The Conrad discontinuity corresponds to the sub-horizontal boundary in

continental crust Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called '' sial'' be ...

at which the

seismic wave A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic ener ...

velocity increases in a discontinuous way. This boundary is observed in various continental regions at a depth of 15 to 20 km, however it is not found in oceanic regions. The Conrad discontinuity (named after the

seismologist Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

Victor Conrad Victor Conrad (1876-1962) 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 ...

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

Mohorovičić discontinuity The Mohorovičić discontinuity ( , ), usually referred to as the Moho discontinuity or the Moho, is the boundary between the Earth's crust and the mantle. It is defined by the distinct change in velocity of seismic waves as they pass through ch ...

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

felsic rock In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted with mafic rocks, whi ...

s such as

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies un ...

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

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

(

sima Sima or SIMA may refer to: People * Sima (Chinese surname) * Sima (given name), a Persian feminine name in use in Iran and Turkey * Sima (surname) Places * Sima, Comoros, on the island of Anjouan, near Madagascar * Sima de los Huesos, a cav ...

, 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, from the 1960s onward this theory was strongly contested among

geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althou ...

s. The exact geological significance of the Conrad discontinuity is still not clarified. The possibility that it represents the transition from amphibolite facies to granulite facies

metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ch ...

has been given some support from observations of the uplifted central part of the Vredefort impact structure and the surrounding

Kaapvaal Craton The Kaapvaal Craton (centred on Limpopo Province in South Africa), along with the Pilbara Craton of Western Australia, are the only remaining areas of pristine 3.6–2.5 Ga (billion years ago) crust on Earth. Similarities of rock records fr ...

References

Plate tectonics Structure of the Earth Earth's crust {{geophysics-stub