پوسته زمین-Earth crust
Earth crust
The low-density outermost layer of the Earth
above the Mohorovičić discontinuity (the Moho), a global boundary that is
defined as the depth in the Earth where the compressional-wave seismic velocity
increases rapidly or discontinuously to a value in excess of 4.7 mi/s (7.6 km/s;
the upper mantle). The crust is also the cold, upper portion of the Earth's
lithosphere, which in terms of plate tectonics is the mobile, outer layer that
is underlain by the hot, convecting asthenosphere. See also: Asthenosphere; Lithosphere;
Moho (Mohorovi^ ić discontinuity); Plate tectonics
Continental
crust
The Earth's continental crust has evolved
over the past 4 billion years, and is highly variable in geologic composition
and internal structure. The worldwide mean thickness of continental crust is 24
mi (40 km), with a standard deviation of 5.4 mi (9 km; Fig. 1). The thinnest
continental crust (found in the Afar Triangle, northeast Africa) is about 9 mi
(15 km) thick, and the thickest (the
Fig. 1 Mercator projection of crustal thickness (10-km contour interval plus 45-km contour) based on seismic refraction profiles.
Seismic refraction profiles
The internal structure of the crust reveals
much about crustal evolution, and one effective way of describing this structure
is based on the results of seismic refraction profiles that define crustal
layers (that is, layers with characteristic compressional-wave seismic
velocity). This seismological method for investigating the crust is widely used
for two reasons: seismic refraction measurements are made on a global basis, and
the seismically defined layers correspond to geologic layers. There is a
relationship between the compressional-wave seismic velocity, density, and rock
composition for coarse-grained igneous rocks (Fig. 2). Seismic reflection
profiles can be used to relate seismic velocities to crustal composition and
density. A composite crustal section can be deduced from these data. See also: Seismology
Fig. 2 Relationship between compressional-wave
seismic velocity, igneous rock type, silica content, and density. Seismic
refraction profiles provide a measurement of compressional-wave velocity as a
function of depth in the crust.
Despite its geologic complexity, the
continental crust may generally be divided into four layers: an uppermost
sedimentary layer, and an upper, middle, and lower crust composed of crystalline
rocks (Fig. 3). The sedimentary cover of the continental crust is an important
source of natural resources. This cover averages 0.6 mi (1 km) in thickness, and
varies from 0 (for example, on shields) to more than 9 mi (15 km) in deep
basins. In stable continental crust of average thickness (25 mi or 40 km), the
crystalline upper crust is commonly 6–9 mi (10–15 km) thick and has an average
composition equivalent to a granite. The middle crust is 3–9 mi (5–15 km) thick
and has a composition equivalent to a diorite; and the lower crust is 3–12 mi
(5–20 km) thick and has a composition equivalent to a gabbro. Owing to
increasing temperature and pressure with depth, the metamorphic grade of rocks
increases with depth, and the rocks within the deep continental crust generally
are metamorphic rocks, even if they originated as sedimentary or igneous rocks
(Fig. 4). See also: Diorite;
Gabbro; Granite; Metamorphic rocks
Fig. 3 Average crustal columns for seven
geologic provinces, showing variations in crustal thickness (light gray areas
correspond to the mantle). Compressional-wave velocity, which has been divided
into five ranges, is directly related to geologic composition, as indicated in
Fig. 2. (After W. Mooney et al., JGR, 1998)
Fig. 4 Cross section of the Earth's crustal
thickness at 40°N. Felsic rock is found throughout the upper continental crust
above diorite and mafic granulite. The crust is thicker beneath mountain ranges
and other regions of high elevation. Vertical thickness relative to horizontal
distance is exaggerated 100×. (After W. Mooney, IASPEI,
2004)
Crustal properties vary systematically with
geologic setting, which may be divided into seven groups: orogens (mountain
belts), shields and platforms, island arcs (volcanic arcs), continental magmatic
arcs, rifts, extended (stretched) crust, and forearcs (Fig. 3). Orogens are
typified by thick crust [average thickness is 29 mi (46 km), but the maximum
thickness is as much as 47 mi (75 km) in the Himalayas of Asia and Andes of
South America]. The seismic structure of orogens is highly variable and is
related to the nature of the orogen (for example, continent-continent collision
versus ocean-continent collision). However, a thick upper crustal layer is
common, and this is the result of thickening of the upper crustal section by
wedging and stacking of the compressed upper crust. Shields and platforms, such
as the Canadian Shield and the Russian Platform, commonly have an approximately
26–mi-thick (42-km) crust, including a 3–6-mi-thick (5–10-km) high-velocity
(4.4–4.7-mi/s or 7.1– 7.5-km/s) lower crust. In comparison with shields, island
arcs (such as Japan) have thinner crusts and significantly shallower middle and
lower crustal layers due to the intrusion of mafic (that is, low silica content)
plutons. Continental magmatic arcs, such as the Cascades volcanoes of the
northwestern United States, intrude preexisting continental crust, and therefore
they are generally 3–9 mi (10–15 km) thicker than island arcs. Continental
rifts, such as the East African and Rio Grande rifts, have an average crustal
thickness of about 22 mi (36 km), but show great variability in seismic
structure. Extended continental crust, such as the Basin and Range Province of
the western United States, averages 18 mi (30 km) in thickness, and has
relatively little high-velocity (greater than 4.2 mi/s or 6.8 km/s) crustal
material. Forearcs are regions that were formed by the coast, near volcanic
arcs, including much of the west coast of North America. They typically have
thin crust, about 15 mi (25 km), and have a thick (9 mi or 15 km) upper crustal
section that consists of relatively low-density metasedimentary rocks. See also: North America; Oceanic
islands; Pluton; Rift valley; Sedimentary rocks; Volcano
Seismic reflection
profiles
Seismic reflection profiles use vertically
traveling waves to obtain a high-resolution image of the crust, whereas
refraction profiles use horizontally traveling waves to obtain seismic velocity
information. Reflection profiles provide images of the deep crust that may be
interrupted in terms of lithologic, metamorphic, and porosity layering, dipping
fault zones, and (more rarely) the presence of crustal melts. Distinct
reflection patterns that correlate with geologic setting can also be identified.
These patterns provide important information on how the crust has been formed
and modified. In many regions, particularly in extended crust, the lower crust
contains numerous horizontal reflections that terminate at the depth of the
Moho. These reflections are most likely the result of igneous intrusions into
the lower crust, combined with horizontal stretching of the warm and ductile
lower crust. See also: Igneous
rocks
Seismic reflection profiles across mountain
belts (orogens) show near-horizontal sheets of rock (nappes) in zones of crustal
thickening and deformation. These upper crustal nappes can be followed to
midcrustal levels in the Alps, and what has been termed thin-skin tectonics [the
horizontal transport of 3–6 mi-thick (5–10-km) sheets of upper crustal material
over large distances] has been confirmed in the Appalachians of the eastern
United States and in western Europe.
See also: Cordilleran belt; Mountain systems
Seismic reflection profiles in Precambrian
crust show pronounced, widespread structural features dating from that period,
indicating long-term thermal and mechanical stability. It is possible that the
geometry of these structures is qualitatively consistent with plate tectonic
processes operating 2 billion years ago.
See also: Precambrian; Seismology
Geoelectrical
measurements
Measurements of electrical conductivity
within the deep continental crust have revealed high conductivities at
midcrustal depths that provide important information on crustal properties. High
crustal conductivity can be caused by several factors, including the presence of
saline fluids, partial melts, sulfide minerals, or carbon films. In regions of
high heat flow, such as actively extending crust, high crustal conductivities
are often found at midcrustal depths (6–12 mi or 10–20 km), and may indicate the
melting of wet granitic rocks or the presence of fluids produced by metamorphic
dehydration reactions. In young mountain belts, such as the Carpathian Mountains
of eastern Europe and the Alaska Range of central Alaska, dipping
high-conductivity zones can be correlated with large carbon-rich flysch and
black shale basins that have been crushed and deeply buried during the
mountain-building process. See
also: Black shale; Geoelectricity; Orogeny; Turbidite
Continental crustal
evolution
The properties of the continental crust
provide important constraints on the process of crustal evolution. The
continents coalesce and disperse as ocean basins open and close, and continental
crust is deformed and modified during rifting and at ocean-continent and
continent-continent collisions. A net increase in the total volume of
continental crust requires extraction of crustal material from the mantle and
the stabilization of this material within a continental block.
At least three processes provide new
continental crust. The first is the accretion and consolidation of island arcs,
such as Japan or the Aleutian Islands, onto a continental margin. The second
process is the tectonic underplating of oceanic crust at active subduction
zones. In this process, the continental crust grows from below as oceanic crust
is welded to the base of the continental margin, either when subduction stops or
when subduction steps oceanward and a new trench is formed. This process has
been identified in western Canada and southern Alaska. The third process is the
magmatic inflation of the crust at continental arcs, rifts, and regions of
crustal extension. This process has been identified in many regions. See also: Geodynamics
Although both the accretion of island arcs
and tectonic underplating of oceanic crust can be documented as stages of
continental growth, these processes by themselves would result in a bulk crustal
composition that is more mafic (low silica content) than the actual bulk
composition of continental crust. This apparent discrepancy can be resolved in
at least two ways. First, the mafic lower crust of some island arcs may be
delaminated (peeled off from below) and subducted back into the mantle, thereby
leaving the remaining crust less mafic. Second, an originally thick mafic lower
crust may undergo secondary differentiation that produces an intermediate
composition melt and a mafic-ultramafic residue (restite), which ponds at the
base of the crust. After cooling, the restite would have a seismic velocity of
about 5 mi/s (8 km/s) and therefore would constitute a new, shallower seismic
Moho with a less dense, less mafic overlying crust.
Walter D. Mooney
Oceanic
crust
The surface of the ocean crust, except for
some locally high volcanoes and plateaus, resides some 1–3 mi (2–5 km) below sea
level, approximately 3–6 km below the average level of the continents. While the
continental crust consists mainly of ancient mountain belts, with an average age
of 1500 Ma, oceanic crust is relatively young and dynamic, having a maximum age
of only 200 Ma. Most of it was produced at mid-ocean ridges during sea-floor
spreading, where ridges define the accreting boundaries of major lithospheric
plates moving about the surface of the Earth. Thus, the oldest rocks of the
ocean crust date back no earlier than the rifting episodes that initiated the
most recent phase of continental drift (the Pangaean breakup) in Late Jurassic
times. See also: Jurassic;
Mid-Oceanic Ridge
Sea-floor
spreading
The sea-floor spreading mechanism holds that
the ocean basins, for example the Atlantic and Indian oceans, developed after
continental rifting. During sea-floor spreading, magmatic upwelling at the
central mid-ocean ridge repeatedly introduces new basaltic crust along a narrow
median rift, in the center of a basin within a global system of underwater
mountains. This system of mountains, which is over 12,000 mi (20,000 km) long,
is interconnected through all the major ocean basins, including the Pacific and
Arctic basins. It has been determined that this process of rifting, basaltic
magmatism, and continued sea-floor spreading has produced the vast majority of
ocean crust (Fig. 5). The rates of plate motions are deduced from the records of
Earth's magnetic reversals preserved in the basaltic rocks surrounding ocean
ridges. While magnetic domains in liquid magma will rotate according to the
Earth's magnetic field, once lava solidifies, the orientation of material is no
longer flexible and the basalt will “freeze,” reflecting the direction of the
Earth's magnetic field at its time of emplacement. The anomalies, which are
mapped with magnetometers towed from ships, document the shifting patterns of
plate motion in the ocean basins, as well as the history of sea-floor spreading.
They demonstrate that the ocean crust is one vast, ∼6-km- thick (4-mi) basaltic lava field covering nearly
70% of the Earth's surface. See
also: Basalt; Basin; Continental drift; Earth, convection in; Lava; Magma;
Magnetometer; Rock magnetism
Fig. 5 The sea-floor spreading mechanism begins
with continental rifting initiated by magmatic processes (a). Once a basin has
rifted apart significantly (b), just as in the East African Rift Valley, it may
fill with water, eventually forming a lake or sea (c). Over time, the extrusion
of magma forms new crust and the ridge continues to spread, creating a vast
ocean (d).
The extent of this lava field and its
fundamental basaltic composition have been documented by extensive dredge
sampling programs along active ridges, the transform faults that offset them,
and the fracture-zone scars that trail far off onto the flanks of the ridges.
The older portions of the ocean crust have been sampled in dozens of localities
by drilling through a carapace of mainly biogenic marine sediments. In all
cases, the continuity of the spreading process has been confirmed. Thus,
emplacement of basalt at mid-ocean ridges is the universal mechanism by which
ocean crust is formed. See also:
Marine sediments; Transform fault
Characteristics
The base of the ocean crust is defined by
the location of the seismic Moho. The Moho is usually an abrupt transition
between rocks with average seismic velocities of about 4.2 mi/s (6.7 km/s) to
rocks with velocities of about 5 mi/s (8 km/s). The former velocities agree with
experimental determinations on varieties of nonporous mafic rocks including
gabbros and amphibolites (rocks that have significant proportions of plagioclase
and little olivine), whereas the latter velocities can represent only nearly
fresh (nonserpentinized) peridotite, consisting mainly of olivine and pyroxenes
with virtually no plagioclase. In the oceans, explosion seismology and seismic
reflection carried out from ships are used to determine both the location of the
Moho and the structure of the overlying crust. Both gabbros and serpentinized
peridotites are present at various tectonic exposures in the ocean crust, such
as along fracture zones, the walls of median rifts, and low-angle detachment
surfaces in the floors of rifts. Thus, it is likely that gabbros are the rocks
of the lower ocean crust that overlie mantle peridotites. However, there is no
known place in the ocean basins where a coherent crust-mantle transition is
exposed in outcrop. See also:
Amphibolite; Peridotite
There are fault slices of former ocean crust
on land, known as ophiolites, where nearly complete cross sections through the
crust can be mapped and sampled (Fig. 6). These are robust indications that the
ocean crust consists (from top to bottom) of submarine extrusives (usually
pillow basalts), feeder dikes (often vertically sheeted), or sills, gabbros, and
peridotites. There is much uncertainty, however, about the extent to which
typical ophiolites, most of which formed in island-arc or backarc environments,
can represent abyssal ocean crust, which is produced at the major accreting
plate boundaries. Moreover, due to the extent of alteration and metamorphism,
particularly in ultramafic sections, the rocks in ophiolites are sometimes not
well correlated with those in ocean crust. Thus, seismic models of the ocean
crust and the nature of the Moho transition therein cannot be determined
directly. See also: Ophiolite
Fig. 6 Interpretation of Bay of Islands
ophiolite crust as oceanic crust and mantle, with the various thicknesses
estimated from geologic mapping. (After R. G. Coleman, Ophiolites, Springer,
1977).
Nevertheless, the general seismic model of
the ocean crust and the basic ophiolite model of crustal stratigraphy have
generally been confirmed both by direct observation and by drilling. Sheeted
dikes, for example, have been observed from submersibles on faulted exposures in
both the equatorial Atlantic and eastern Pacific. Seismic velocities in the
upper part of the ocean crust are consistent with the presence of pillow lavas
or flows with many cracks and cavities. Velocities increase systematically with
depth, indicating that this porosity structure diminishes, primarily because of
compaction, filling of veins with alteration minerals, and transition to compact
dikes. The deepest hole drilled in ocean crust, on the flank of the Costa Rica
Rift in the eastern equatorial Pacific, reflects these relationships in downhole
logging measurements through 1 mi (1.8 km) of igneous material. The transition
to rocks with velocities of about 4.2 mi/s (6.7 km/s) is actually within the
compacted dike sequence, and does not formally correspond to the lithologic
transition to gabbros, which still has not been identified. See also: Petrology; Stratigraphy
Crustal
formation
The composite lithology of gabbros, dikes,
and extrusives forms ocean crust above the mantle in the following way. Gabbros
comprise the frozen walls, roof, and floor of a spreading ridge's magma-chamber
complex, which is more or less continuously supplied with magma from the upper
mantle. The flux of magma, and thus the details of this process, differ
according to whether the ridge is spreading rapidly or slowly, or is near an
unusually hot region of mantle (that is, Iceland). The magma is supplied because
of decompression melting of hot peridotite as it convects upward or rises
diapirically into the region beneath the spreading ridge. Melts are driven out
of the mantle rocks because they are under stress. While they are still molten,
they coalesce, flow into fissures, and feed into the lower crust, where they
cool and begin to crystallize. See
also: Diapir; Hot spots (geology)
The steady-state process of sea-floor
spreading, however, requires that magmas be supplied to the lower crust nearly
continuously. This forces magmas already residing in the crust out toward the
surface. Elastic materials will initially respond to stress by bending or
stretching, but not by breaking (similar to taffy or putty), in a process called
ductile deformation. If the rate of stress is fast enough, then rocks will
fracture under brittle deformation. At high temperatures (perhaps 1100–1300°F or
600–700°C), rocks will tend to experience only ductile deformation. At lower
temperatures, they will more likely experience brittle failure. Earthquakes, in
particular, are evidence for large-scale brittle failure at some depth in the
crust or upper mantle.
Rocks in the uppermost crust, chilled by
hydrothermal circulation of seawater, often experience brittle failure along
vertical fractures under the combined impetus of upward magma injection and
sea-floor spreading. The tensional stress regime at spreading ridges determines
that most fracturing will occur in a narrow zone parallel to the ridge (Fig. 7).
Thus, dikes tend to be sheeted, and when these breach the sea floor, eruptions
occur. Lava flows through dikes will be interrupted by more dikes as the ridge
continues to spread, and repeatedly buried by younger flows after being carried
to one side of the ridge or the other. Observations made from drilling and
submersible instruments indicate that this process takes place perhaps 10–20
times in the course of building a given segment of crust at rifted, slowly
spreading ridges such as the Mid-Atlantic Ridge. Lava flows are often thinner
along the much faster spreading East Pacific Rise and, therefore, more eruptive
events may be required to build the same length of crust there. This
speculation, though, has not yet been confirmed by drilling.
Fig. 7 Open-system magma chamber that may exist
beneath all active mid-oceanic ridges, except those that are spreading extremely
slowly. (After M. Wilson, Igneous Petrogenesis, Routledge, Chapman, and Hall,
1988).
Coarse-grained gabbros crystallized from the
magma chamber complex comprise approximately two-thirds of ocean crust. They may
exist as solids in the deep crust due to the hydrothermal processes acting at
the top of the crust that cool the steady-state crystallizing mass to nearly
90°F (50°C) below the average temperature of magmas arriving from the mantle.
This cooling mechanism will crystallize at least 50–70% of molten material at a
fast-spreading ridge before it can erupt. At slowly spreading ridges, where
magma is supplied more gradually, hydrothermal cooling drives crystallization in
the deep crust almost to completion. At both rapidly and slowly spreading
ridges, crystallization occurs by the plating out of minerals (olivine,
plagioclase, and clinopyroxene) along the walls of intrusive bodies in the
gabbroic mass, all within close range (a few kilometers) of the center of
spreading. The crystallization forces changes in the compositions of melts and
thus drives magmatic differentiation in the ocean crust. The sequence of liquid
compositions produced by this process changes from olivine basalt, to
titanium-rich ferrobasalt, to rarely erupted ferroandesites and dacites. The
latter liquids often remain in the lower crust, crystallizing to oxide-rich
gabbro and plagiogranite. Occasionally, at slowly spreading ridges, rift
dynamics will allow normal faults to penetrate the bodies of crystallizing
magma-chamber complexes. This produces shear zones along which melts may
migrate. Thus, dredged and drilled gabbroic rocks from these environments are
often lithologically complex. While the deformed rocks may be highly sheared,
with gneissic or porphyroclastic textures, they are often cut by younger veins
of oxide gabbro or granitic material.
See also: Dacite; Gabbro; Gneiss; Olivine
The fundamental melt product at mid-ocean
ridges is variously termed abyssal tholeiite or mid-ocean- ridge basalt. In most
places, this parental material is remarkably uniform in its fundamental
petrologic and geochemical properties. Thus, it has a consistent crystallization
sequence from place to place, and in most areas possesses depleted isotopic and
trace-element signatures. The source of most abyssal tholeiites is a vast,
globally distributed shallow mantle reservoir that previously experienced
partial melting (thus losing radioactive parents to the radiogenic isotopes) at
some distant time in the geological past. Currently, most tholeiites are
experiencing further partially melting. There are places along spreading ridges,
however, where less depleted or even enriched basaltic lavas are erupting. Most
of these are in elevated, hot-spot regions such as
James H. Natland
Walter D. Mooney
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(eds.), Lower Continental Crust, 1992
I. G. Gass, S. J. Lippard, and A. W. Shelton
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R. Meissner, The Continental Crust: A
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Alifazeli=egeology.blogfa.com
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