Structured Geological Glossary: The Earth

differentiated planet	One that is chemically zoned because heavy materials have sunk to the center and light materials have accumulated in a crust.
crust	"The outermost layer or shell of the Earth, defined according to various criteria, including seismic velocity, density and composition; that part of the Earth above the Mohorovicic discontinuity, made up of the sial, or the sial and the sima. It represents less than 0.1% of the Earth's total volume. Crustal thickness is approximately 50 km and outer radius is approximately 6371 km."
crust	The outermost rock hell. Outer part of the earth composed essentially of crystalline rocks. Crustal thickness is approximately 50 km and outer radius is approximately 6371 km.
ocean crust	The relatively thin, dense crust that form the ocean basin. The crust is mostly basalt with an average depth of 3.7 km below sea level.
continental crust	The layer of the Earth that lies under continent and the continental shelves. It ranges in thickness from 35 to 60 km. Its upper layer has a density of ~2.7 g/cm3 and is composed of rocks that are rich in silica and alumina. The average elevation of the Earth's continental crust is 0.84 km above sea level.
brittle-ductile transition zone	The location at depth within the earth's crust where the temperature and pressure have risen to such a high level that directed stress results in plastic deformation as opposed to fracturing and faulting.
mantle	The main bulk of the Earth, between the crust and core, ranging from depths of about 40 to 3480 kilometers. It is composed of dense mafic silicate and divided into concentric layer by phase changes that are caused by the increase in pressure with depth.The mantle is made up of dense, iron and magnesium rich (ultramafic) rock uch as dunite and peridotite. The uppermost part of the mantle is rigid and, along with the crust, form the 'plates' of plate tectonics.
670-km discontinuity	Boundary that separates the upper mantle (35-670 km) from the lower mantle (670-2900 km). It could be a natural barrier to whole-mantle convection.
perovskite	Structure of the main mineral phase of the lower mantle. (MgFe)SiO3 perovskite represents approximately 40% by volume of the entire earth.
core	The innermost layer of the Earth. The inner core is solid and has a radius of about 1300 kilometers. (The radius of the Earth is about 6371 kilometers.) The outer core is fluid and is about 2300 kilometers thick. S-waves cannot travel through the outer core.
outer core	The liquid outer layer of the core that lies directly beneath the mantle.
lithosphere	The component of the Earth's surface comprising the rock, soil and sediment. It is a relatively passive component of the climate system and its physical characteristics are treated as fixed elements in the determination of climate. Semirigid upper platelike layer of the earth. Its thickness is near zero at oceanic ridge axes, but it increases with age to about 100 km. Oceanic lithosphere consists of a crustal layer ~5 km thick of basaltic composition underlain by mantle rocks. Continental lithosphere consists of a layer of crustal rock 30-60 km thick of low density and variable composition underlain by mantle rocks. In plate tectonics, an outer layer of great strength relative to the underlying asthenosphere for deformation at geologic rates. It includes the crust and part of the upper mantle and is about 100 km thick.
exogenic system	Earth's interacting envelopes: the hydrosphere (mostly the ocean), atmosphere, biosphere, reactive lithosphere (that part of the rocky crust within reach of the processes of weathering). The processes of deep seawater circulation at spreading ridges overlap with endogenic (within the earth) processes. Also relevant are extraterrestrial processes. Refractory problems such as mass extinction usually become arenas for competing theories drawn from the three sources.
asthenosphere	The layer below the lithosphere which is marked by low seismic wave velocities and high seismic wave attenuation and probably partially molten. The layer or shell of Earth below the lithosphere which is weak and in which large-scale isostatic adjustment take place, magma may be generated, and seismic waves are strongly attenuated. This zone of soft, easily deformed rock exists at depths of 100 kilometers to as deep as 700 kilometers.
mesophere	The lower mantle.
atmosphere	The gaseous envelope surrounding the Earth. The mixture of gases that surrounds the Earth, being held thereto by gravity. It consists by volume of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and minute quantities of helium, krypton, neon, and xenon. The atmosphere is so compressed by its own weight that half is within 5.5 km of the Earth's surface.
surface of the earth	The value given is the depth below the surface of the mean spheroid. The mean spheroid is a uniform approximation to the true shape of the Earth. No adjustment is made to the depth due to any differences between the true Earth and the mean spheroid. For example, the minimum depth that will be given is 0 kilometers even though a quake directly under Mount Everest (elevation 8848 meters) could legitimately have a depth of -6 kilometers and still be 2 kilometers underground. On the other hand, a depth of 10 kilometers would actually be more than 1 kilometer above the ocean floor of Challenger Deep (elevation -11,033 meters) in the Marianas Trench of the Pacific Ocean.
sial	"A petrologic name for the upper layer of the Earth's crust, composed of rocks that are rich in silica and alumina; it may be the source of granitic magma. It is characteristic of the upper continental crust. Etymol: an acronym for silica + alumina."
sima	A term used to describe the more magnesium-rich igneous rocks (basalt, gabbro, and peridotite) of the ocean crust. Sima rocks are denser than the continental (sial) rocks.
Mohorovicic discontinuity	"The boundary surface or sharp seismic-velocity discontinuity that separates the Earth's crust from the subjacent mantle. It marks the level in the Earth at which P-wave velocities change abruptly from 6.7 to 7.2 km/s (in the lower crust) to 7.6 to 8.6 km/s or average 8.1 km/s (at the top of the upper mantle); its depth ranges from about 5 km beneath the ocean floor to about 35 km below the continent, although it may reach 60 km or more under some mountain ranges. The discontinuity probably represents a chemical change from basaltic or simatic materials above to peridotitic or dunitic materials below, rather than a phase change (basalt to eclogite); however, the discontinuity should be defined by seismic velocities alone. It is variously estimated to be between 0.2 and 3 km thick. It is named in honor of its discoverer, Andrija Mohorovicic (1857-1936), Croatian seismologist. Abbrev. Moho"
moho	The boundary separating the base of the Earth’s crust and the top of the mantle. The Moho occurs at a depth of 5-10 kilometers beneath oceanic crust and about 35-65 kilometers below continental crust. The term moho is an abbreviation for Mohorovicic discontinuity, named for Andrija Mohorovicic, a Croatian seismologist.
Milankovitch theory	An astronomical theory formulated by the Yugoslav mathematician Milutin Milankovitch that associates climate change with fluctuations in the seasonal and geographic distribution of insolation determined by periodic variation of the Earth's eccentricity and obliquity and the longitude of perihelion.