Earth’s crustgeology

...entirely produced by the internal dynamo. Radially outward from the Earth’s core, the next major source of magnetic field is crustal magnetization. The temperature of the materials constituting the crust is cool enough for them to exist in solid form. The solids may become magnetized by the Earth’s main field and cause detectable anomalies.

The average thickness of the terrestrial crust for both East and West Antarctica approximates that of other continents. Although it has been postulated that West Antarctica might be an oceanic island archipelago if the ice were to melt, its crustal thickness of about 20 miles indicates an absence of oceanic structure. This thickness is similar to that of coastal parts of other continents. The...

chemical elements

Direct observations of chemical composition can be made for the Earth, the Moon, and meteorites, although there are some problems of interpretation. The chemical composition of the Earth’s crust, oceans, and atmosphere can be studied, but this is only a minute fraction of the mass of the Earth, and there are many composition differences even within this small sample. Some information about the...

Direct information on the composition of the Earth’s crust is available in the form of thousands of analyses of individual rocks, the average of which provides a reasonably precise estimate of the bulk composition. For the mantle and the core the information is indirect and thus much less precise. The origin of the Earth by the accretion of planetesimals is a well-founded hypothesis, however,...

• alkali metals ( in alkali metal )

  ...forms alkalies (i.e., strong bases capable of neutralizing acids). Sodium and potassium are the sixth and seventh most abundant of the elements, constituting, respectively, 2.6 and 2.4 percent of Earth’s crust. The other alkali metals are considerably more rare, with rubidium, lithium, and cesium, respectively, forming 0.03, 0.007, and 0.0007 percent of Earth’s crust. Francium, a natural...

• carbon group elements ( in carbon group element: Occurrence and distribution )

  On a weight basis, carbon is 19th in order of elemental abundance in the crust of the Earth, and there are estimated to be 3.5 times as many carbon atoms as silicon atoms in the universe. Only hydrogen, helium, oxygen, neon, and nitrogen are atomically more abundant in the Cosmos than carbon. Carbon is the cosmic product of the “burning” of helium in which three helium nuclei,...

• oxygen group elements ( in oxygen group element )

  ...within a few kilometres of the surface of the Earth—oxygen is the most abundant element: in mass, it makes up about 20 percent of the air, about 46 percent of the solid crust of the Earth, and about 89 percent of the water.

• phosphorus ( in phosphorus: Properties, occurrence, and uses. )

  ...in nature except in a few meteorites, phosphorus occurs in compounds that are widely distributed in many rocks, minerals, plants, and animals. Ranking 12th in abundance among the elements in the Earth’s crust, phosphorus constitutes approximately 0.10 percent of the crust in the form of minerals such as apatite, wavellite, and vivianite; it always occurs as the phosphate ion. The chief...

Earth’s outermost, rigid, rocky layer is called the crust. It is composed of low-density, easily melted rocks; the continental crust is predominantly granitic rock (see granite), while composition of the oceanic crust corresponds mainly to that of basalt and gabbro. Analyses of seismic waves, generated by earthquakes within Earth’s interior, show that the crust extends about 50 km (30 miles)...

The outermost layer of the lithosphere is called the crust. It is composed of low-density material crystallized from molten rock (magma) produced by partial melting of the lithosphere or asthenosphere. The average thickness of the oceanic crust is about 4 mi (6.4 km). Oceanic plateaus and seamounts are localized areas of abnormally thick oceanic...

petrology

• feldspar ( in feldspar )

  any of a group of aluminosilicate minerals that contain calcium, sodium, or potassium. Feldspars make up more than half the Earth’s crust, and professional literature about them constitutes a large percentage of the literature of mineralogy.

• metamorphic rocks ( in metamorphic rock: Distribution of metamorphic rocks )

  ...is one that formed at a depth of tens of kilometres and later returned to the surface. Hence, metamorphic regions are also regions of former or recent intense orogeny. More stable regions of the Earth’s crusttend to be covered with sediments, and only deep drilling will reveal the metamorphic rocks below.

• sedimentary rocks ( in Grand Canyon: Geologic history )

  Although its awesome grandeur and beauty are the major attractions of the Grand Canyon, perhaps its most vital and valuable aspect lies in the time scale of Earth history that is revealed in the exposed rocks of the canyon walls. No other place on Earth compares with the Grand Canyon for its extensive and profound record of geologic events. The canyon’s record, however, is far from continuous...

  in sedimentary rock )

  Sediments and sedimentary rocks are confined to the Earth’s crust, which is the thin, light outer solid skin of the Earth ranging in thickness from 40–100 kilometres (25 to 62 miles) in the continental blocks to 4–10 kilometres in the ocean basins. Igneous and metamorphic rocks constitute the bulk of the crust. The total volume of sediment and sedimentary rocks can be either...

• silica mineral ( in silica mineral: General considerations )

  Silica minerals make up approximately 12 percent of the Earth’s crust and are second only to the feldspars in mineral abundance. Free silica occurs in many crystalline forms with a composition very close to that of silicon dioxide, 46.75 percent by weight being silicon and 53.25 percent oxygen. Quartz is by far the most commonly occurring form. Tridymite, cristobalite, and the hydrous silica...

To the Earth scientist, the crust includes not only the top layer of solid material (soil and rocks to a depth of 6 to 70 km [4 to 44 miles], separated from the underlying mantle by differences in density and by susceptibility to surficial geologic processes) but also the hydrosphere (oceans, surface waters on land, and groundwater beneath the land surface) and the atmosphere. Interactions...

Taken in perspective, it is evident that many parts of the Earth’s crust have experienced reheating temperatures above 300° C—i.e., reset mica ages are very common in rocks formed at deep crustal levels. Vast areas within the Precambrian shield, which have identical ages reflecting a common cooling history, have been identified. These are called geologic provinces. By contrast,...

...evolution but has displayed only limited potential for isotopic dating. Technical difficulties have yet to be overcome. Osmium is strongly concentrated in the mantle and extremely depleted in the crust, so that crustal osmium must have exceedingly high radiogenic-to-stable ratios while the mantle values are low. In fact, crustal levels are so low that they are extremely difficult to measure...

...to form the secondary atmosphere and the oceans. This chemical process of melting, separation of material, and outgassing is referred to as the differentiation of the Earth. The earliest thin crust was probably unstable and so foundered and collapsed to depth. This in turn generated more gravitational energy, which enabled a thicker, more stable, longer-lasting crust to form. Once the...

The chemical history of the oceans has been divided into three stages. The first is an early stage in which the Earth’s crust was cooling and reacting with volatile or highly reactive gases of an acidic, reducing nature to produce the oceans and an initial sedimentary rock mass. This stage lasted until about 3.5 billion years ago. The second stage was a period of transition from the initial to...

The vast majority of tectonic basins and valleys is produced by an extension of the Earth’s crust and the subsequent dropping of a block of crust into the space created by the divergence of large crustal blocks or lithospheric plates. The extension of the brittle crust causes it to fracture, and as the adjoining crustal blocks or plates move apart, a smaller block slides down into the resulting...

ideal theoretical balance of all large portions of Earth’s lithosphere as though they were floating on the denser underlying layer, the asthenosphere, a section of the upper mantle composed of weak, plastic rock that is about 110 km (70 miles) below the surface. Isostasy controls the regional elevations of continents and ocean floors in accordance with the densities of their underlying rocks....

In terms of chemical composition, and therefore density, the Earth’s crust is lighter than the underlying mantle. Beneath the oceans, the typical thickness of the crust is only six to seven kilometres. Beneath the continental regions, the average thickness is about 35 kilometres, but it can reach 60 or 70 kilometres beneath high mountain ranges and plateaus. Thus, most ranges and plateaus are...

The shape of the oceans and seas of the world has changed significantly throughout the past 600 million years. According to the theory of plate tectonics, the crust of the Earth is made up of many dynamic plates (see plate tectonics; and earth: The surface of the Earth as a mosaic of plates). There are two types of plates—oceanic and continental—which float on the surface of the...

The lithosphere is subdivided on the basis of the presence of rock types with different chemical compositions. The outermost layer of the lithosphere is called the crust, of which there are two types, continental and oceanic, which differ in their composition and thickness. The distribution of these crustal types closely coincides with the division into continents and ocean basins. The...

Croatian meteorologist and geophysicist who discovered the boundary between the Earth’s crust and mantle—a boundary subsequently named the Mohorovičić discontinuity.

The Earth’s crust is generally richer in oxygen-18 (¹⁸O) than is the mantle, as a result of the reaction of these upper-layer rocks with the hydrosphere and atmosphere. This fact allows oxygen-18 to be used to assess the degree to which ascending magmas have incorporated crustal rocks as they rise to the surface. The use of isotopes has proved...

Observations of earthquake waves by the mid-1900s had led to a spherically symmetrical crust–mantle–core picture of the Earth. The crust–mantle boundary is marked by a fairly large increase in velocity at the Mohorovičić discontinuity at depths on the order of 25–40 kilometres on the continents and five–eight kilometres on the seafloor. The...

The ocean floor has the same general character as the land areas of the world: mountains, plains, channels, canyons, exposed rocks, and sediment-covered areas. The lack of weathering and erosion in most areas, however, allows geological processes to be seen more clearly on the seafloor than on land. Undisturbed sediments, for example, contain a historical record of past climates and the state...

...has made rapid progress and is now used for oil and gas exploration. The improvement in the instruments and techniques achieved after World War II made it possible to determine the structure of the Earth’s crust to a depth of 40 to 50 km (about 25 to 30 miles) by detonation of a small amount of explosive.

The thin surface rock layer surrounding the mantle is the crust, whose lower boundary is called the Mohorovičić discontinuity. In normal continental regions the crust is about 30 to 40 km thick; there is usually a superficial low-velocity sedimentary layer underlain by a zone in which seismic velocity increases with depth. Beneath this zone there is a layer in which P-wave...