In some portions of the fast spreading East Pacific Rise, off-axis eruptions appear to be related to syntectonic volcanism and the formation of abyssal hills. They commonly are thin (only about 10 cm [4 inches] thick) and cover a broader area than pillow lavas. These studies provide constraints on seismic layer 2A, the mid-crust magma body, and the Moho which marks the crust-mantle boundary.

Perfit, in Encyclopedia of Ocean Sciences (Second Edition), 2001. Oceanic crust is about 6 km (4 miles) thick. Unlike the continental crust, the oceanic crust is continually recycled by the layer below it, called the mantle.

Carbotte, in Reference Module in Earth Systems and Environmental Sciences, 2013. The Ocean Drilling Programs (today the International Ocean Drilling Program, IODP and its predecessors, ODP, IPOD and DSDP) have provided much information about the ocean crust and its overlying sediment.

The oceanic crust is thin, relatively young and uncomplicated compared to the continental crust, and chemically magnesium-rich compared to continental material. There is evidence that sheet flows are erupted at higher temperatures than those of the pillow variety. Crew members aboard a drilling ship inspecting a rock core during a scientific expedition that succeeded for the first time in drilling through the upper oceanic crust. Based on the modeled fluid flow rates, the calculated rates of oxygen consumption are ≤1 nmol cm−3 rock d−1in young, cool basaltic crust.

The balance between the enriched and depleted portions of the crusts indicates that the average bulk fractionation between seawater and mantle is near the steady-state value. Many ophiolites are much older than the oldest oceanic crust, demonstrating continuity of the formation processes over hundreds of millions of years. Announcing our NEW encyclopedia for Kids! In addition to conventional downhole logging, a newly developed deep ultraviolet (UV) fluorescence-based logging tool called the Deep Exploration Biosphere Investigative tool, was also deployed for the first time on Expedition 336 to detect in situ microbial signatures.

Oceanic crust is about 6 km (4 miles) thick. Oceanic basalts formed at ridges are termed Mid-Ocean Ridge Basalts (MORB) as distinct from basalts added on top of older oceanic crust by volcanism in volcanoes on ocean islands, which are termed Ocean Island Basalts (OIB). Oceanic crust is generated along mid-ocean ridges, and becomes progressively older, colder, and denser with increasing distance from the ridge axis. (2005) model subduction zone thermal structure with a dynamic mantle convection code and include dehydration reactions and a rheology that depends on pressure, temperature, strain rate (i.e., non-Newtonian), and water content. pp.

The average thickness of normal oceanic crust, away from regions considered for various reasons to be anomalous, was found to be about 6.5 km.

When the magma cools to form rock, its magnetic polarity is aligned with the then-current positions of the magnetic poles of the Earth.

Oceanic crust formed at spreading ridges is relatively homogeneous in thickness and composition compared to continental crust. Below the extrusive basalts, there is a transition into feeder dykes. 3). This figure has changed little in more recent compilations, despite the use of a variety of different definitions for what is normal oceanic crust.

These experiments involved measuring the travel times of seismic waves generated by explosions (such as dynamite blasts) set off over distances of several tens of kilometres.

Great strides in understanding the oceanic crust were made by the study of ophiolites. Nature 505, 204-208, harvnb error: no target: CITEREFCogley1984 (. Peacock (2001) suggests that the lower line of earthquakes in double Bennioff zones may be related to dehydration. The initial oceanic crust accretes at the midocean ridge with an isotopic composition typical for midocean ridge basalt, i.e., 5.7 per mil. They can be stronger (“positive”) or weaker (“negative”) than the average total field. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. Li, M., & McNamara, A. Plate Tectonics: Unraveling the Mysteries of the Earth.

Lett. As mantle melts rise to the surface and freeze, they form an internally stratified crust of extrusive basalts and sheeted dykes underlain by layered and massive gabbros. The recovery of large amounts of serpentinized peridotite by dredging, drilling, and submersible diving on the Mid-Atlantic Ridge and other slow-spreading ridges has led some investigators to suggest that layer 3 consists of isolated gabbroic intrusions in serpentinized peridotite and to question the conventional interpretation that the Moho marks a petrological boundary between mafic rocks above and ultramafic rocks beneath. Oceanic crust is created at mid-ocean ridges (see TECTONICS | Mid-Ocean Ridges) as mantle material upwells and undergoes pressure-release melting in response to ongoing seafloor spreading. These gabbro layers are thought to represent the magma chambers, or pockets of lava, that ultimately erupt on the seafloor. The extent to which microbes colonize, alter, and evolve in the crustal habitat remains largely unknown (Edwards et al., 2005).

The process of super-continent formation and destruction via repeated cycles of creation and destruction of oceanic crust is known as the Wilson cycle. Three-dimensional diagram showing crustal generation and destruction according to the theory of plate tectonics; included are the three kinds of plate boundaries—divergent, convergent (or collision), and strike-slip (or transform). Let us know if you have suggestions to improve this article (requires login).

Spreading rate has long been recognized as a fundamental variable governing crustal accretion at ridges, with first-order differences observed in a wide range of ridge properties. King, in Treatise on Geophysics, 2007. It is mainly tholeiitic basaltwhich is dark, fine and volcanic structure. S.M. 83–. The entire thickness of the oceanic crust has not been sampled in situ and therefore the bulk composition has been estimated based on investigations of ophiolites (fragments of oceanic and back-arc crust that have been thrust up on to the continents), comparisons of the seismic structure of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from the ocean floor by dredging, drilling, submersibles, and remotely operated vehicles.

Get exclusive access to content from our 1768 First Edition with your subscription. 182–. Water plays several important roles in arc geochemistry and dynamics by reducing the melting temperature (Hirth and Kohlstedt, 1996; Asimow et al., 2004) and impacting the rheology (Hirth and Kohlstedt, 2003).

Ancient examples include the Franciscan complex in California. While subducted oceanic crust is thought to be dehydrated before reaching the transition zone (e.g., Schmidt and Poli, 1998), peridotite could carry water into the transition zone, particularly by water that has percolated to depth in the slab along extensional faults formed at the outer rise (e.g., Peacock, 2001; Rüpke et al., 2004). S.M. A marine magnetic anomaly is a variation in strength of Earth’s magnetic field caused by magnetism in rocks of the ocean floor. Along the fore-arc region accretionary prisms of stacked thrust sheets can form as a result of scraping off sediments from the down-going plate. The upper gabbro layer is isotropic (uniform) in structure. Knowledge of the structure and composition of the oceanic crust comes from several sources. The top of the pile has open fractures and hence low seismic velocities.

Through comparisons with ophiolites (sections of oceanic or oceanic-like crust exposed on land), exposures of faulted oceanic crust, and laboratory measurements of rock samples, a generalized view of crustal structure has emerged, where seismic layer 2 corresponds with the extrusive rocks and dikes of the upper crust, and where seismic layer 3 is associated with the lower crust of gabbroic rocks.

Within a very short period of time (< 1 Myr) the igneous portion of the oceanic crust becomes zoned in δ18O with the upper portion of the crust becoming enriched in 18O and the lower portion of the crust becoming depleted in 18O with respect to pristine midocean ridge basalt. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL:, URL:, URL:, URL:, URL:, URL:, URL:, URL:, URL:, URL:, Encyclopedia of Physical Science and Technology (Third Edition), The geological interpretation of layer 2 and layer 3 remains controversial because thick sections of, Mid-Ocean Ridge Geochemistry and Petrology*, Encyclopedia of Ocean Sciences (Second Edition), Earth and Life Processes Discovered from Subseafloor Environments, Regional Geology and Tectonics: Principles of Geologic Analysis, Stable Isotopes as Tracers of Global Cycles, TECTONICS | Seismic Structure At Mid-Ocean Ridges, TECTONICS | Seismic Structure At Mid-Ocean Ridges☆, Reference Module in Earth Systems and Environmental Sciences, TECTONICS | Mountain Building and Orogeny, Edwards, Bach, Klaus, & the IODP Expedition 336 Scientific Party, 2014. The layer 2 and layer 3 phases are easily recognized on modern record sections from oceanic crust (Fig. Both float on top of the denser mantle. Robert T. Gregory, in Encyclopedia of Physical Science and Technology (Third Edition), 2003.

Here, we review recent microbiological studies that have been conducted in igneous oceanic crust, starting with analysis of seafloor rocks and minerals, moving to deeper crustal samples collected through the recent phase of the ocean drilling program, and concluding with in situ microbiological experiments conducted with Circulation Obviation Retrofit Kit subseafloor observatories.

This has led many researchers to identify the layer 2/3 boundary with the limit of dike brecciation.

Initially, the magmas coming from the asthenosphere under the spreading center are uniform in their δ18O values, + 5.7.

The Moho marks the seismic boundary between plutonic rocks that are gabbroic in composition and those that are mostly ultramafic but may have formed by crystal accumulation in the crust.

It is possible that hydrated phases may only be stable to the transition zone along unusually cold adiabats (e.g., Staudigel and King, 1992), requiring old ocean lithosphere or very high convergence rates, or both. Although it is clear that ophiolites are of marine origin, there is some controversy as to whether they represent typical oceanic crust or crust formed in settings other than an oceanic spreading centre—behind island arcs, for example.

Oceanic crust formed at spreading ridges is relatively homogeneous in thickness and composition compared to continental crust.

97, 275–294, copywrite by Blackwell Science Ltd.).

The geological interpretation of layer 2 and layer 3 remains controversial because thick sections of oceanic crust are exposed at the seabed only in anomalous areas, such as fracture zones; very few such sections have been recovered by drilling; and key boundaries have yet to be sampled in situ. Mature island arcs can produce significant mountain ranges, such as in Japan, where andesitic volcanoes are formed above a deep subduction zone.

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