The Conrad discontinuity corresponds to the sub-horizontal boundary in the

continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary 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 '' si ...

at which the

seismic wave A seismic wave is a mechanical wave of acoustic energy that travels through the Earth or another planetary body. It can result from an earthquake (or generally, a quake), volcanic eruption, magma movement, a large landslide and a large ma ...

velocity increases in a discontinuous way. This boundary is observed in various continental regions at a depth of 15 to 20 km, but 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 (or generally, quakes) and the generation and propagation of elastic ...

Victor Conrad) is considered to be the border between the upper continental (sial, for silica-aluminium) crust and the lower one (

sima Sima or SIMA may refer to: People * Sima (Chinese surname) * Sima (Persian given name), a Persian feminine name in use in Iran and Turkey * Sima (Indian given name), an Indian feminine name used in South Asia * Sima (surname) * Sima (born 1 ...

, for silica-magnesium). It is not as pronounced as the

Mohorovičić discontinuity The Mohorovičić discontinuity ( ; )usually called the Moho discontinuity, Moho boundary, or just Mohois the boundary between the Earth's crust, crust and the Earth's mantle, mantle of Earth. It is defined by the distinct change in velocity of s ...

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 ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, 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 coo ...

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

basalt Basalt (; ) is an aphanite, 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 planetary surface, surface of a terrestrial ...

(

). 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. Despite the fact that sial and sima are two solid layers, the lighter sial is thought to "float" on top of the denser sima layer. This forms the basis of

Alfred Wegener Alfred Lothar Wegener (; ; 1 November 1880 – November 1930) was a German climatologist, geologist, geophysicist, meteorologist, and polar researcher. During his lifetime he was primarily known for his achievements in meteorology and ...

's 'Continental Drift Theory.' The area of contact during the movement of the Continental plates is on the Conrad discontinuity. However, from the 1960s onward, this theory was strongly contested among

geologist A geologist is a scientist who studies the structure, composition, and History of Earth, history of Earth. Geologists incorporate techniques from physics, chemistry, biology, mathematics, and geography to perform research in the Field research, ...

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

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose ...

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with distinctive characteristics. The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or con ...

to granulite facies

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

has been given some support from observations of the uplifted central part of the

Vredefort impact structure The Vredefort impact structure is one of the largest impact structures on Earth. The crater, which has since been eroded away, has been estimated at across when it was formed, the latter estimate suggesting the initial crater was larger than ...

and the surrounding Kaapvaal craton.

References

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