fast-spreading rates (>80 mm/year full rate), where
magmatism is the dominant mode of lithospheric accretion. However, it is becoming increasingly accepted that
the Penrose model fails to explain many features observed
at MORs where plates spread apart at slower rates. Along
slow- and ultra-slow-spreading MORs, the lithosphere is
generally thicker and colder than at fast-spreading MORS,
tectonic activity is a dominant process accommodating
plate separation, and magmatism is highly variable both
in time and space (e.g., Dick et al., 2010; Michael et al.,
2003; Schroeder et al., 2007). In these settings, the interplay between magmatic and tectonic extension and their
space-time variability result in oceanic lithosphere in
which the crust is no longer stratified but disrupted,
forming a heterogeneous mixture of magmatic and mantle
rocks (Cannat, 1993).
Bibliography
Cannat, M., 1993. Emplacement of mantle rocks in the seafloor at
mid-ocean ridges. Journal of Geophysical Research, 98,
4163–4172.
Christeson, G. L., McIntosh, K. D., and Karson, J. A., 2007. Inconsistent correlation of seismic layer 2a and lava layer thickness in
oceanic crust. Nature, 445, 418–421.
Detrick, R. S., Collins, J. A., Stephen, R. A., and Swift, S. A., 1994.
In situ evidence for the nature of the seismic layer 2/3 boundary
in oceanic crust. Nature, 370, 288–290.
Dick, H. J. B., Lissenberg, C. J., and Warren, J. M., 2010. Mantle
melting, melt transport, and delivery beneath a slow-spreading
ridge: the paleo-MAR from 23
15'N to 23
45'N. Journal of
Petrology, 51(1–2), 425–467.
Forsyth, D. W., 1992. Geophysical constraints on mantle flow and
melt migration beneath mid-ocean ridges. In Phipps Morgan,
J., Blackman, D. K., and Sinton, J. M. (eds.), Mantle flow and
melt generation at mid-ocean ridges. Washington, DC: AGU,
pp. 1–65.
Michael, P. J., Langmuir, C. H., Dick, H. J. B., Snow, J., Goldstein,
S. L., Graham, D. W., Lenhert, K., Kurras, G. J., Jokat, W.,
Mühe, R., and Edmonds, H. N., 2003. Magmatic and amagmatic
seafloor generation at the ultraslow-spreading Gakkel ridge,
Arctic Ocean. Nature, 423, 956–961.
Penrose Conference participants, 1972. GSA Penrose Field Conference on Ophiolites. Geotimes, 17, 24–25.
Schroeder, T., Cheadle, M. J., Dick, H. J. B., Faul, U. H., Casey,
J. F., and Kelemen, P. B., 2007. Nonvolcanic seafloor spreading
and corner-flow rotation accommodated by extensional faulting
at 15
N on the Mid-Atlantic Ridge: A structural synthesis of
ODP Leg 209. Geochemistry, Geophysics, Geosystems, 8(6),
Q06015, doi:10.1029/2006GC001567.
Sinton, J. M., and Detrick, R. S., 1992. Mid-ocean ridge magma
chambers. Journal of Geophysical Research, 97, 197–216.
White, R. S., McKenzie, D., and O'Nions, R. K., 1992. Oceanic
crustal thickness from seismic measurements and rare earth element inversions. Journal of Geophysical Research, 97,
19683–19715.
Wilson, D. S., Teagle, D. A. H., Alt, J. C., Banarjee, N. R., Umino,
S., Miyashita, S., Acton, G. D., Anma, R., Barr, S. R., Belghoul,
A., Carlut, J., Christie, D. M., Coggon, R., Cooper, K. M.,
Cordier, C., Crispini, L., Durand, S. R., Einaudi, F., Galli, L.,
Gao, Y., Geldmacher, J., Gilbert, L. A., Hayman, N. W.,
Herrero-Bervera, E., Hirano, N., Holter, S., Ingle, S., Jiang, S.,
Kalberkamp, U., Kerneklian, M., Koepke, J., Laverne, C.,
Lledo-Vasquez, H. L., Maclennan, J., Morgan, S., Neo, N.,
Nichols, H. J., Park, S.-H., Reichow, M. K., Sakuyama, T., Sano,
T., Sandwell, R., Scheibner, B., Smith-Duque, C. E., Swift,
S. A., Tartarotti, P., Tikku, A. A., Tominaga, M., Veloso, E. A.,
Yamasaki, T., Yamazaki, S., and Ziegler, C., 2006. Drilling to
gabbro in intact ocean crust. Science, 312, 1016–1020.
Cross-references
Crustal Accretion
Cumulates
Gabbro
Mid-ocean Ridge Magmatism and Volcanism
Ophiolites
Submarine Lava Types
LITHOSPHERE: STRUCTURE AND COMPOSITION
Martin Meschede
Institute of Geography and Geology, Ernst-Moritz-Arndt
University, Greifswald, Germany
Definition
The lithosphere (from the Greek words lithos [lίyοB]
meaning rocky and sphaira [sFaῖra]meaning sphere) is
defined as the outermost rigid shell of the Earth (Skinner
et al., 2003). It comprises the crust and the upper portion
of the mantle which is stiff and rigid and behaves elastically over millions of years. The mantle part of the lithosphere is named the “lithospheric mantle.” Beneath the
lithospheric mantle follows the asthenosphere (from the
Greek words asthenḗs [ἀsyenήB] meaning weak and
sphaira [sFaῖra] meaning sphere). Both the lithospheric
mantle and the upper part of the asthenosphere are mainly
composed of peridotitic rocks.
Introduction
The crust overlying the lithospheric mantle is either of
oceanic type which is mainly composed of gabbroic and
basaltic rocks resulting in a mean density of 3.0 g/cm
3
,
or of continental type with a gabbroic/granulitic layer at
its base and a variety of rock types in the upper part formed
by magmatic differentiation, metamorphism, and/or sedimentation over long periods of the Earth’s history
(Figure 1). The mean density of the continental crust is
2.8–2.9 g/cm
3 in the deeper portion and 2.6–2.7 g/cm
3
in the upper portion. The boundary between the Earth’s
crust and the lithospheric mantle is known as the
Mohorovičić discontinuity (short form: Moho; Figure 1),
named after the Croatian geophysicist Andrija
S. Mohorovičić (1857–1936).
Oceanic lithosphere is continuously formed at
mid-oceanic ridges and returned into the mantle at subduction zones. Oceanic lithosphere is, therefore, much younger than continental lithosphere. The oldest oceanic
lithosphere is about 185 myrs old (Müller et al. 2008),
whereas continental lithosphere comprises remnants of
all periods of the Earth’s history. The oldest parts of
LITHOSPHERE: STRUCTURE AND COMPOSITION
393
magmatism is the dominant mode of lithospheric accretion. However, it is becoming increasingly accepted that
the Penrose model fails to explain many features observed
at MORs where plates spread apart at slower rates. Along
slow- and ultra-slow-spreading MORs, the lithosphere is
generally thicker and colder than at fast-spreading MORS,
tectonic activity is a dominant process accommodating
plate separation, and magmatism is highly variable both
in time and space (e.g., Dick et al., 2010; Michael et al.,
2003; Schroeder et al., 2007). In these settings, the interplay between magmatic and tectonic extension and their
space-time variability result in oceanic lithosphere in
which the crust is no longer stratified but disrupted,
forming a heterogeneous mixture of magmatic and mantle
rocks (Cannat, 1993).
Bibliography
Cannat, M., 1993. Emplacement of mantle rocks in the seafloor at
mid-ocean ridges. Journal of Geophysical Research, 98,
4163–4172.
Christeson, G. L., McIntosh, K. D., and Karson, J. A., 2007. Inconsistent correlation of seismic layer 2a and lava layer thickness in
oceanic crust. Nature, 445, 418–421.
Detrick, R. S., Collins, J. A., Stephen, R. A., and Swift, S. A., 1994.
In situ evidence for the nature of the seismic layer 2/3 boundary
in oceanic crust. Nature, 370, 288–290.
Dick, H. J. B., Lissenberg, C. J., and Warren, J. M., 2010. Mantle
melting, melt transport, and delivery beneath a slow-spreading
ridge: the paleo-MAR from 23
15'N to 23
45'N. Journal of
Petrology, 51(1–2), 425–467.
Forsyth, D. W., 1992. Geophysical constraints on mantle flow and
melt migration beneath mid-ocean ridges. In Phipps Morgan,
J., Blackman, D. K., and Sinton, J. M. (eds.), Mantle flow and
melt generation at mid-ocean ridges. Washington, DC: AGU,
pp. 1–65.
Michael, P. J., Langmuir, C. H., Dick, H. J. B., Snow, J., Goldstein,
S. L., Graham, D. W., Lenhert, K., Kurras, G. J., Jokat, W.,
Mühe, R., and Edmonds, H. N., 2003. Magmatic and amagmatic
seafloor generation at the ultraslow-spreading Gakkel ridge,
Arctic Ocean. Nature, 423, 956–961.
Penrose Conference participants, 1972. GSA Penrose Field Conference on Ophiolites. Geotimes, 17, 24–25.
Schroeder, T., Cheadle, M. J., Dick, H. J. B., Faul, U. H., Casey,
J. F., and Kelemen, P. B., 2007. Nonvolcanic seafloor spreading
and corner-flow rotation accommodated by extensional faulting
at 15
N on the Mid-Atlantic Ridge: A structural synthesis of
ODP Leg 209. Geochemistry, Geophysics, Geosystems, 8(6),
Q06015, doi:10.1029/2006GC001567.
Sinton, J. M., and Detrick, R. S., 1992. Mid-ocean ridge magma
chambers. Journal of Geophysical Research, 97, 197–216.
White, R. S., McKenzie, D., and O'Nions, R. K., 1992. Oceanic
crustal thickness from seismic measurements and rare earth element inversions. Journal of Geophysical Research, 97,
19683–19715.
Wilson, D. S., Teagle, D. A. H., Alt, J. C., Banarjee, N. R., Umino,
S., Miyashita, S., Acton, G. D., Anma, R., Barr, S. R., Belghoul,
A., Carlut, J., Christie, D. M., Coggon, R., Cooper, K. M.,
Cordier, C., Crispini, L., Durand, S. R., Einaudi, F., Galli, L.,
Gao, Y., Geldmacher, J., Gilbert, L. A., Hayman, N. W.,
Herrero-Bervera, E., Hirano, N., Holter, S., Ingle, S., Jiang, S.,
Kalberkamp, U., Kerneklian, M., Koepke, J., Laverne, C.,
Lledo-Vasquez, H. L., Maclennan, J., Morgan, S., Neo, N.,
Nichols, H. J., Park, S.-H., Reichow, M. K., Sakuyama, T., Sano,
T., Sandwell, R., Scheibner, B., Smith-Duque, C. E., Swift,
S. A., Tartarotti, P., Tikku, A. A., Tominaga, M., Veloso, E. A.,
Yamasaki, T., Yamazaki, S., and Ziegler, C., 2006. Drilling to
gabbro in intact ocean crust. Science, 312, 1016–1020.
Cross-references
Crustal Accretion
Cumulates
Gabbro
Mid-ocean Ridge Magmatism and Volcanism
Ophiolites
Submarine Lava Types
LITHOSPHERE: STRUCTURE AND COMPOSITION
Martin Meschede
Institute of Geography and Geology, Ernst-Moritz-Arndt
University, Greifswald, Germany
Definition
The lithosphere (from the Greek words lithos [lίyοB]
meaning rocky and sphaira [sFaῖra]meaning sphere) is
defined as the outermost rigid shell of the Earth (Skinner
et al., 2003). It comprises the crust and the upper portion
of the mantle which is stiff and rigid and behaves elastically over millions of years. The mantle part of the lithosphere is named the “lithospheric mantle.” Beneath the
lithospheric mantle follows the asthenosphere (from the
Greek words asthenḗs [ἀsyenήB] meaning weak and
sphaira [sFaῖra] meaning sphere). Both the lithospheric
mantle and the upper part of the asthenosphere are mainly
composed of peridotitic rocks.
Introduction
The crust overlying the lithospheric mantle is either of
oceanic type which is mainly composed of gabbroic and
basaltic rocks resulting in a mean density of 3.0 g/cm
3
,
or of continental type with a gabbroic/granulitic layer at
its base and a variety of rock types in the upper part formed
by magmatic differentiation, metamorphism, and/or sedimentation over long periods of the Earth’s history
(Figure 1). The mean density of the continental crust is
2.8–2.9 g/cm
3 in the deeper portion and 2.6–2.7 g/cm
3
in the upper portion. The boundary between the Earth’s
crust and the lithospheric mantle is known as the
Mohorovičić discontinuity (short form: Moho; Figure 1),
named after the Croatian geophysicist Andrija
S. Mohorovičić (1857–1936).
Oceanic lithosphere is continuously formed at
mid-oceanic ridges and returned into the mantle at subduction zones. Oceanic lithosphere is, therefore, much younger than continental lithosphere. The oldest oceanic
lithosphere is about 185 myrs old (Müller et al. 2008),
whereas continental lithosphere comprises remnants of
all periods of the Earth’s history. The oldest parts of
LITHOSPHERE: STRUCTURE AND COMPOSITION
393