Planetary crust

The planetary crust is the outer compositional rock layer of a differentiated rocky planet. Its thickness need not equal either the lithosphere's thermal thickness or its effective elastic thickness.

The distance from a body's surface to its crust-mantle boundary, under a stated geometric convention. It differs from lithosphere thickness and effective elastic thickness. Converting a vertical column thickness to a layer thickness normal to a dipping interface requires its geometry.

Crust dominated by mafic rocks, commonly basalt and gabbro or their metamorphosed equivalents.

The seismic boundary between crust and mantle, commonly identified by a marked increase in elastic wave speeds. It is a compositional boundary and need not coincide with the base of the lithosphere.

The compositionally diverse, commonly thicker and more felsic crust of continental regions.

Horizontal contraction of continental crust, commonly accommodated by folding and thrust faults and contributing to tectonic thickening.

A long-lived stable part of a continent, commonly with a thick, depleted lithospheric mantle keel; that keel is distinct from the continental crust.

Baltica is an ancient continental block associated with the core of northeastern Europe. In paleogeographic reconstructions it is treated as a coherent unit over specified intervals, using the surviving rock record rather than present political boundaries to define its identity.

Paleomagnetic poles, ages of deformation and correlations between rock sequences constrain its past position and relation to Laurentia and other continental blocks. Agreement between several independent indicators is stronger than a visual fit of coastlines. Reconstructed arrangements remain hypotheses with stated age ranges and uncertainties; the modern position of Baltica does not determine its ancient orientation.

Laurentia is the ancient continental core underlying much of present North America and Greenland. Its old, mechanically persistent crust allows geological observations from different regions to be combined into a history of continental assembly and motion.

Paleomagnetism constrains past latitude and orientation, while matching geological ages and structures helps test connections to other blocks such as Baltica. A paleocontinent is therefore a reconstruction from preserved physical records, rather than a claim that every part of a modern continent shared one uninterrupted history. Younger accreted margins must be distinguished from the older core.

The Siberian craton is an ancient, long-lived continental block in northern Asia. Its comparatively stable interior preserves a record that can be used to reconstruct earlier continental configurations, distinct from younger tectonic belts around its margins.

A reconstruction combines dated rock units, structural relationships and paleomagnetic poles. The poles constrain latitude and orientation within a magnetic-field model but generally leave a longitude ambiguity. Consequently, placing the Siberian craton relative to Laurentia or Baltica requires additional geological evidence; a single magnetic direction cannot uniquely determine a complete ancient world map.

Increase in crust or lithosphere thickness through tectonic deformation, including crustal shortening and stacking by thrust faults.

A near-surface layer of sedimentary rocks over crystalline basement; its thickness can be zero and is not a fixed fraction of continental crust.

The principally mafic compositional outer layer of an oceanic plate; it is only part of the thicker oceanic lithosphere.

An assemblage of many intrusive dikes, representing conduits feeding lavas at a spreading ridge.

Crust dominated by basalt and compositionally related intrusive rocks.