Mid Ocean Ridges and Process of Plate Tectonics to Continue

Seafloor Geomorphology—Coast, Shelf, and Abyss

Peter T. Harris , in Seafloor Geomorphology as Benthic Habitat, 2012

Mid-Ocean Ridges

Mid-ocean ridges are created by the upwelling of basaltic lava and lateral rifting of ocean crust ( Figure 6.12). They form a rift valley system that encircles the Earth along a total length of over 75,000 km (Figure 6.11). The mid-ocean ridges are the earth's largest volcanic system, accounting for >75% of all volcanic activity on the planet. The heat from this volcanism is dispersed by hydrothermal circulation of seawater. Hot seawater venting from the seafloor supports exotic benthic communities that have evolved to survive by using the hydrogen sulfide dissolved in the hot fluid (discussed later). The mid-ocean ridges are flanked on either side by abyssal hills and hilly abyssal plains (Figure 6.12).

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Seafloor geomorphology—coast, shelf, and abyss

Peter T. Harris , in Seafloor Geomorphology as Benthic Habitat (Second Edition), 2020

Mid-ocean ridges

Mid-ocean ridges are created by the upwelling of basaltic lava and lateral rifting of ocean crust ( Fig. 6.12). They form a rift valley system that encircles the Earth along a total length of over 75,000 km (Fig. 6.11). The mid-ocean spreading ridge covers the largest fraction of abyssal zone in the Arctic Ocean, where it characterizes 4.76% of the area of abyssal zone, and it is absent from the Mediterranean and Black Sea. The greatest area of mid-ocean ridges occurs in the South Pacific Ocean where this feature type covers an area of 1,868,490 km².

The mid-ocean ridges are the Earth's largest volcanic system, accounting for more than 75% of all volcanic activity on the planet. The heat from this volcanism is dispersed by hydrothermal circulation of seawater. Hot seawater venting from the seafloor supports exotic benthic communities that have evolved to survive by using the hydrogen sulfide dissolved in the hot fluid (discussed further below). The mid-ocean ridges are flanked on either side by abyssal hills and hilly abyssal plains (Figs. 6.12 and 6.13).

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Plate Tectonics and Volcanism

Peter C. LaFemina , in The Encyclopedia of Volcanoes (Second Edition), 2015

2.1.1 Mid-Ocean Ridges

Mid-ocean ridges are the most prominent plate boundaries and the most active volcanic features on Earth ( Figure 3.1). In Earth's current lithospheric plate configuration, the mid-ocean ridge system extends connected for over 60,000 km making them also the longest mountain chain and plate boundary type. Mid-ocean ridges are related to heat generation in the mantle by radioactive decay and other thermal sources, which result in mantle convection and upwelling. Decompression melting of upwelling mantle along mid-ocean ridges forms MORB magmas (see Chapter 1—Melting in the Mantle of the Earth and Chapter 4—Origin and Composition of Magmas), which are intruded into the crust and erupted at the surface forming oceanic crust and lithosphere.

Oceanic lithosphere is comprised of four main layers. The upper three layers comprise the crust and are up to 6–7 km thick (Figure 3.4). The bottom layer (layer 4) comprises the upper mantle. Layer 1 is typified by several hundred meters of pelagic sediment. As lithosphere moves away from the ridge axis, and therefore increases in age, the sediment cover thickens. This sediment is deposited on top of layer 2a, a layer of up to 0.5 km of basaltic lava flows erupted as pillow or sheet lava flows within the neovolcanic zone along the axis of the mid-ocean ridge (see Chapters 19—Submarine Lavas and Hyaloclastite and 21—Mid-Ocean Ridge Volcanism Chapter 19 Chapter 21 ). Layer 2b is approximately 1.5 km thick and is comprised of vertically oriented, sheeted diabase dikes. These dikes accommodate the <1 to >12 cm a⁻¹ of relative plate motion at mid-ocean ridges, when they are periodically intruded into the crust. Studies of ophiolite complexes (i.e., fragments of mid-ocean ridges or oceanic crust and lithosphere that have been accreted onto the edge of a continent), rare exposures along oceanic rifts (e.g., the Hess Deep at the propagating tip of the Galapagos Rise), and Tertiary dike swarms in Iceland indicate dike thicknesses ranging from <0.01 to >13.0 m with a mean thickness of ∼1.0 m. Dikes are formed by the injection of MORB magmas from central volcanic crustal or subcrustal magma chambers along the ridge axis. Along one segment of mid-ocean ridge there may be one or more axial magma chamber where magma is stored before it is intruded vertically and laterally along the ridge axis (see Chapter 8—Magma Chambers ). The number of magma chambers is related to the relative plate motion rate, length of the ridge segment, and magma supply to the ridge. These magmatic systems have been well imaged geophysically (e.g., Sinton and Detrick, 1992) and geologically mapped in ophiolites and the Tertiary lava pile of Iceland (Figure 3.5). The gabbroic lower crust, accounting for up to 5 km of crustal thickness and layer 3 (Figure 3.4), forms by lateral flow and cooling at the edges of these magmatic bodies.

The upper mantle is composed of the ultramafic rock peridotite. The boundary between layers 3 and 4, that is the Moho, has been defined in oceanic lithosphere both petrologically and seismically. The thickness of oceanic lithosphere is a function of its age. At the ridge axis, where active accretion of new crust is taking place, lithospheric thickness is approximately 2 km (i.e., the thickness of layer 2). As oceanic crust moves away from the ridge axis it cools by conductive cooling. Oceanic lithosphere forms by cooling of the crust and upper mantle and thickens to between 50 and 140 km thick, following the simplified relation:

(3.1) $h ≅ 2 \sqrt{k t}$

where, h is the thickness of the lithosphere, k is the thermal diffusivity of the lithosphere, and t is the age of the lithosphere. The age (t) can be approximated by the quotient of the distance from the ridge axis (l) and the relative rate of plate motion (v). In addition to becoming thicker with distance from the ridge axis, the lithospheric mantle becomes denser. This is an important factor in global tectonics, because after ∼10 Myr oceanic lithosphere is denser than the asthenospheric mantle and can therefore subduct into the mantle. The oldest oceanic lithosphere on Earth is <280 Ma, because as new oceanic crust and lithosphere are created at mid-ocean ridges, they are consumed at subduction zones (Figure 3.6).

Mid-ocean ridges are defined by the relative rate of plate motion between the diverging plates and are subdivided into ultrafast (>12 cm a⁻¹), fast (<12 to >8 cm a⁻¹), intermediate (<8 to >5 cm a⁻¹), slow (<5 to >2 cm a⁻¹), and ultraslow (<2 cm a⁻¹) spreading ridges (Figure 3.3). These differences in spreading rate give rise to distinctive mid-ocean ridge morphologies (Figure 3.7). At fast-spreading ridges the bathymetry gently increases from the abyssal plain to the ridge or rise with an elevation increase of ∼500 m. The neovolcanic zone or region of active crustal accretion is confined to a roughly 100 m wide by 10 m deep axial depression. This region is bound by a several 100 m wide zone of fissuring and up to 10 km of active normal faulting. The bathymetry is subdued by the volume of lava flows erupted long the ridge axis (Figure 3.8). The classic example of a fast-spreading ridge is the East-Pacific Rise (EPR) at ∼9°N, where the spreading rate is ∼10 cm a⁻¹ (Figure 3.9(A)). The rate of spreading along the entire length of the EPR ranges from 7 to 17 cm a⁻¹. Along the EPR there is a shallow axial trough where lavas are erupted, but topography here is subdued due to the larger volume of magmas injected as dikes and lavas erupted along the ridge axis. The bathymetry clearly shows that eruptive centers also occur off axis. Common features of fast to ultrafast-spreading ridges are overlapping or propagating ridge segments (Figure 3.9(A)). Overlapping ridges occur at a number of scales, but could eventually lead to the formation of microplates; for example, the Easter and Juan Fernandez microplates along the southern EPR.

The morphology of intermediate to slow-spreading ridges is dramatically different to that of fast-spreading ridges (Figures 3.7 and 3.10). The neovolcanic zone is defined by a >10 km wide by 0.5–2.5 km deep axial graben, bound by inward dipping normal faults. The region of active faulting is up to 50 km wide. The Mid-Atlantic Ridge is a classic example of a slow-spreading ridge with rates in the North Atlantic of ∼2 cm a⁻¹ (Figure 3.9(B)). Lavas are erupted within the neovolcanic zone, and in some cases, erupted outside of the axial trough. Normal faulting at the ridge axis is important for several reasons. First, normal faulting accommodates a fraction of relative plate motion across the ridge axis. Relative plate motion along the ridge axis, however, is mostly accommodated by the intrusion of sheeted dikes. Second, normal faulting provides pathways for ocean water to penetrate the crust where it is heated by the high thermal gradient near the ridge axis. Hydrothermal circulation of ocean water through the oceanic crust causes alteration (i.e., serpentinization) and cooling of the crust. Hydrothermal circulation is dramatically displayed at hydrothermal vents or "black smokers" observed within the axial trough (see Chapter 47—Deep Ocean Hydrothermal Vents). The hydration of oceanic crust and lithosphere at mid-ocean ridges is also important for the cycling of fluids from the hydrosphere into the mantle at convergent margins. In addition to the steeply dipping normal faults found along ridge axes, low-angle normal faults and oceanic core complexes are found along slow- to ultra-slow spreading ridge segments and at segment ends adjacent to oceanic transform faults and nontransform accommodation zones (Figure 3.10). Oceanic core complexes or megamullions are formed by low-angle detachment faulting, and expose peridotite upper mantle and serpentinized shear zones and crust.

Ultraslow mid-ocean ridges accommodate relative plate motions by magmatic and amagmatic accretion processes. The spreading ridges are well defined in bathymetric data (Figure 3.9(C)), sharing some characteristics with slow-spreading ridges; however, the mode of accommodation is different from either fast- or slow-spreading ridges. Magmatic centers occur along the spreading axis, punctuated by regions of amagmatic accretion, where mantle peridotites are emplaced directly to the ridge axis through low-angle normal faulting and exhumation (i.e., formation of oceanic core complexes) (Figure 3.10). Amagmatic accretion is especially prevalent along highly oblique, ultraslow-spreading ridge segments. The Southwest Indian Ridge (Figure 3.9(C)) and Gakkel Ridge are the type localities for ultraslow-spreading ridges. The Gakkel Ridge has one of the slowest spreading rates for a mid-ocean ridge on Earth at 1.1 cm a⁻¹. The spreading rate across the Western Volcanic Zone, Iceland, a dying ridge segment in a propagating ridge system, is <0.8 cm a⁻¹, and the spreading rate across the Red Sea is as low as 0.7 cm a⁻¹.

Magmatic and tectonic processes at mid-ocean ridges form oceanic crust and accommodate relative plate motions. Oceanic lithosphere forms and becomes denser as the plate moves away from the mid-ocean ridge and cools. As we continue to explore and make new geophysical and geochemical observations of the vast ocean basins that comprise approximately 70% of the surface of the Earth, we will develop new insights into the magmatic and tectonic processes that lead to the formation of oceanic crust and lithosphere, and accommodation of relative plate motions at mid-ocean ridges.

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Oceanic Crust☆

T.A. Minshull , in Reference Module in Earth Systems and Environmental Sciences, 2013

Variation with Spreading Rate

Mid-ocean ridge spreading rates vary over an order of magnitude, from around 10 mm yr^{− 1} to around 150 mm yr^{− 1}. The gross thickness of the oceanic crust varies remarkably little with spreading rate due to the balance between mantle upwelling rate, which is the dominant control on the rate of magma production at a mid-ocean ridge, and plate separation rate, which controls the surface area of crust that must be created. Passive upwelling models of magma generation at ridges predict a reduction in crustal thickness at slow spreading rates due to conductive heat loss from the upwelling mantle, which reduces the degree of melting. Hence, at the Southwest Indian Ridge, where the spreading rate is only ∼ 12–16 mm yr^{− 1}, the seismically determined crustal thickness drops to ∼ 3–4 km. Recent observations of broad areas of serpentinized mantle at the seabed on the flanks of the Southwest Indian Ridge suggest that in some settings little or no crust is generated. Crustal thicknesses are generally more variable at slow spreading rates, but most of this variability can be attributed to the effects of ridge segmentation. Despite indications from seismic reflection profiles of substantial differences in the internal architecture of oceanic crust formed at slow spreading rates and crust formed at fast spreading rates, and differences in composition indicated by geological studies of mid-ocean ridge axes, the seismic velocity structure appears to vary little with spreading rate. Where reduced crustal thicknesses are observed, the reduction appears to be predominantly at the expense of layer 3.

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Tectonics and Volcanic and Igneous Plumbing Systems

Benjamin van Wyk de Vries , Maximillian van Wyk de Vries , in Volcanic and Igneous Plumbing Systems, 2018

7.2.1 Volcanic and Igneous Plumbing Systems at Mid-Ocean Ridges

Mid-ocean ridges are the longest, largest and most voluminous magmatic environment on Earth. Ridges are the site of new lithospheric and crustal production that may be subsequently subducted into the mantle and recycled, or involved in magma-producing dehydration reactions that slowly build up continental crust ( Fig. 7.2).

The mid-ocean ridge system is made up of diverging lithosphere over rising asthenosphere (Fig. 7.2A). The latter decompresses and melts with volumes related to its ascension rate. At fast spreading ridges, such as the East Pacific Rise, the rapid spreading rate (up to 20 cm/year) produces large amounts of magma as decompression is fast in relation to cooling, whereas at slow spreading ridges, magma production is very low as cooling dominates over decompression.

In ocean-ridge plumbing systems, this means that at fast spreading ridges, magma is the dominant material added to make new crust (typical ophiolite sequence), whereas at slow spreading ridges the asthenosphere may rise to the surface without any magmatic crust forming (serpentinitic crust; Fig. 7.2C). One consequence of this is that at fast spreading ridges, the crustal structure is controlled by magma and volcanics, whereas at very slow spreading ridges, faulting and stretching dominate, allowing exhumation of the asthenosphere. The oceanic crust at slow spreading ridges is thus thinner than fast ones and includes exhumed mantle. The thickness relation between the fast and slow ridges can be seen on Moho depth maps. Departures from this pattern happen where other influences, such as mantle plumes, add additional magmatic input. For example, in Iceland, the magmatic production is high compared to the spreading rate, leading to unusually thick crust (see maps in Artemieva et al., 2017).

The magma plumbing system at a mid-ocean ridge starts with an area of partial melt generation and migration within the rising asthenosphere. When enough magma is collected, it can move upwards as a body, through buoyancy (see Chapter 2). Magma may continue to rise as a dyke and either erupt or cool in the crust, or it may feed into a magma chamber. A conceptual magmatic system is show in Fig. 7.2, which shows that often the whole oceanic crust is made up of magma plumbing elements, topped by lavas and sediments. The thickness and the extent of the oceanic crust relate to the melt flux and the spreading rate, such that at slow spreading ridges some crust will be composed of serpentinised mantle, whereas at fast ridges (or ones with excess melting, such as Iceland), the crust would be exclusively built out of plumbing elements.

Along a mid-ocean ridge, magma collects into discrete centres, each of which concentrates magma and distributes it outwards via a plumbing system of dykes and sills (Fig. 7.2B). The Axial seamount on the Juan de Fuca Ridge is a good example of this (Sigmundsson, 2016) (see also Chapter 11).

Consequently, the intrusive complex can be considered to extend from the axial magma chamber throughout the entire plate, making up the largest intrusive systems on Earth (Fig. 7.2B).

Two types of field sites provide evidence of mid-ocean ridge plumbing. The older parts of Iceland represent exposed sections of magmatic systems, where analogues to mid-ocean ridges can be seen. These have central intrusive systems with cone-sheet intrusions and dyke swarms (see Chapter 4). There are generally many more dykes below 2 km depth than above (80% are below this level). In rifting episodes, such as at Krafla volcano in the 1970s and 1980s (Einarsson and Brandsdóttir, 1980), the majority of dykes identified in seismic data did not reach the surface. While exposures such as those in the east and west of Iceland provide a view into the upper few kilometres of the oceanic crust, deeper levels remain unexposed. However, in ophiolites (obducted oceanic crust), entire sections from the mantle upwards can be seen. In these ophiolites, a complex plutonic sequence of coarse gabbros that transition up into sills and dykes, and finally into lavas (Moores, 2003) is exposed, representing typical fast spreading mid-ocean ridge segments.

The mid-ocean ridge system is driven by several forces, in particular, from asthenospheric flow as the subducting oceanic lithosphere sinks. Known as slab pull, this force is thought to be the main driving force for ocean spreading. The expansion from intruded dykes, and the gravitational force from the topography and magmatism and the slope of the crust–mantle boundary also contribute. All the forces acting from around a mid-ocean ridge can be grouped together and termed 'ridge push', with restrictive forces being the rheology of the lithosphere/asthenosphere, friction and inertia.

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Seafloor Processes

DelWayne R. Bohnenstiehl , Robert P. Dziak , in Encyclopedia of Ocean Sciences (Third Edition), 2019

Abstract

Earthquakes at mid-ocean ridges and oceanic transforms are used to understand tectonic, magmatic, and hydrothermal processes along these plate boundaries. Global patterns in mid-ocean ridge seismicity largely reflect differences in the rate of plate motion, the thickness of the brittle lithosphere, and the supply of magma. Earthquake monitoring, using a combination of seismic and hydroacoustic sensors, is critical in identifying sites of active seafloor volcanism, and mid-ocean ridge seismicity can provide information on the pattern of faulting and the circulation of fluids beneath the seafloor.

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Mid-Ocean Ridge Volcanism

S. Adam Soule , in The Encyclopedia of Volcanoes (Second Edition), 2015

Abstract

Mid-ocean ridges are considered the planet's largest magmatic system. At divergent plate boundaries, magma is generated by decompression melting of upwelling mantle. Melts are focused as they ascend through the upper mantle and lower crust and collect beneath the ridge axis in elongate melt lenses. Plate spreading is accommodated by episodic faulting and magma injection into dikes, some of which reach the seafloor and produce basaltic lava flows. The geometry of the magmatic systems, dynamics of seafloor eruptions, lava geochemistry, and ridge morphology are among a host of ridge properties controlled by the rate of magma supply to the crust, which is strongly influenced by the spreading rate of the ridge. This chapter describes the magmatic and volcanic processes operating at divergent plate boundaries in the context of their spreading rate.

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Volume 3

Weiwei Wu , ... Huichao Rui , in Encyclopedia of Geology (Second Edition), 2021

MOR-Type Ophiolites

Definition

Mid-ocean ridge (MOR) ophiolites represent oceanic lithosphere formed in the sea-floor spreading centers of ocean basin ( Fig. 2; Pearce et al., 1984). The overall mechanism by which ophiolites are produced is reasonably well understood, although details continue to be controversial. Beneath ocean ridges, upwelling asthenosphere gives rise to decompression partial melting, producing basaltic magma, and eventually, the formation of oceanic crust. The oceanic lithosphere mantle represents melting residue left after the extraction of oceanic crust.

Geochemical Features

In order to avoid influenced by alteration, immobile element geochemistry of volcanic rocks in ophiolite complexes has developed over the past 50 years to decipher their most probable tectonic setting (Cann, 1970; Pearce and Cann, 1971; Pearce, 2014). The trace elements Y, Zr, Nb, Ti, V, Cr, Co, Ni, rare earth elements (REEs), Th, and Ta are generally relatively immobile during metamorphism and alteration; therefore, they are reliable as indicators to trace the origin of the ophiolites (Cann, 1970; Pearce and Cann, 1971; Shervais, 1982; Dilek and Furnes, 2011, 2014; Pearce, 2014; Saccani, 2015; Furnes and Dilek, 2017). For example, in the Nb/Yb versus Th/Yb diagram (Pearce, 2008), the lavas and dikes of the subduction-unrelated ophiolites plot within the mantle array (Dilek and Furnes, 2011). Moreover, the subduction-unrelated ophiolites display large variations in the Nb/Yb versus Th/Yb discrimination diagram, which may be related to the degree of partial melting and the mantle temperature and fertility (Dilek and Furnes, 2014). In a Ti-V discrimination diagram (Shervais, 1982), subduction-unrelated ophiolites straddle the field defined by the ratios between 20 and 50, typical of mid-ocean-ridge basalts (Dilek and Furnes, 2011).

Formation Setting and Process

It has been evident that sea-floor spreading could take place in a number of settings, resulting from non-plate tectonic process (e.g., plume) and/or the Wilson cycle, not just at ridges in the centers of mature ocean basin (Pearce et al., 1984; Dilek and Furnes, 2014; Pearce, 2014). Thus, Pearce (2014) recognized six types of ridges that might responsible for different MOR-type ophiolites, including "normal" mid-ocean ridges, plume-related mid-ocean ridges, continental margin ridges, subduction-initiation ridges, back-arc basin ridges and subducted ridges.

Consensus exists that ophiolites are fragments of ancient oceanic lithosphere obducted onto continental crust irrespective to the tectonic setting in which they form (Coleman, 1971; Dewey and Bird, 1971; Dewey, 1976, 1977; Stern, 2004; Dilek and Furnes, 2011). Ophiolite obduction is a complex process involving several mechanisms and has been discussing for several decades (Coleman, 1971; Dewey and Bird, 1971; Dewey, 1976, 1977). No single mechanism can be responsible for the emplacement of all obducted ophiolites worldwide (Dewey, 1976). Recently, Condie (2016) summarized three major mechanisms for emplacement of ophiolites in arcs or collisional orogens, including: (i) obduction or overthrusting of oceanic lithosphere onto passive continental margin during continental collision, (ii) splitting off and obduction of the upper section of the subducting slab onto a former arc, and (iii) underthrusting of oceanic lithosphere into an accretionary prism in subduction zone. Irrespective the variety of geodynamic settings requiring for subduction, ophiolites are emplaced when their plate margin settings switch from tension or strike-slip to compression (Shervais, 2001; Stern, 2002, 2004). Stern (2004) showed that it is almost impossible to emplace true MORB crust at a convergent plate boundary, giving rise to the paucity of the MOR ophiolites around the world.

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TECTONICS | Mid-Ocean Ridges

K.C. Macdonald , in Encyclopedia of Geology, 2005

Introduction

The mid-ocean ridge system is the largest mountain chain and the most active system of volcanoes in the solar system. In plate-tectonic theory, the ridge is located between plates of the Earth's rigid outer shell that are separating at speeds of approximately 10–170 mm year⁻¹ (up to 220 mm year⁻¹ in the past). The ascent of molten rock from deep within the Earth (ca. 30–60 km) to fill the void between the plates creates new seafloor and a volcanically active ridge. This ridge system wraps around the globe like the seam of a baseball and is approximately 70 000 km long (including the lengths of ridge offsets, such as transform faults). Yet the ridge itself is only about 5–30 km wide, very small compared with the plates, which can be thousands of kilometres across (Figure 1).

Early exploration showed that the gross morphology of spreading centres varies with the rate of plate separation. At slow spreading rates (10–40 mm year⁻¹) a rift valley 1–3 km deep marks the axis, while for fast spreading rates (more than 90 mm year⁻¹) the axis is characterized by an elevation of the seafloor of several hundred metres called an axial high (Figure 2). The rate of magma supply is a second factor that may influence the morphology of mid-ocean ridges. For example, a very high rate of magma supply can produce an axial high even where the spreading rate is slow; the Reykjanes Ridge south of Iceland is a good example. Also, for intermediate spreading rates (40–90 mm year⁻¹) the ridge crest may have either an axial high or a rift valley depending on the rate of magma supply. The depth to the seafloor increases from a global average of approximately 2600 m at the spreading centre to more than 5000 m beyond the ridge flanks. The rate of deepening is proportional to the square root of the age of the seafloor because it is caused by the thermal contraction of the lithosphere. Early mapping efforts also showed that the mid-ocean ridge is a discontinuous structure, which is offset at right angles to its length by numerous transform faults that are tens to hundreds of kilometres long.

Maps are powerful: they inform, excite, and stimulate. Just as the earliest maps of the world in the sixteenth century ushered in a vigorous age of exploration, so the first high-resolution continuous-coverage maps of the mid-ocean ridge system stimulated investigators from a wide range of fields, including petrologists, geochemists, volcanologists, seismologists, tectonicists, and practitioners of marine magnetics and gravity, as well as researchers outside the Earth sciences, including marine ecologists, chemists, and biochemists. Marine geologists have found that many of the most revealing variations are observed by exploring along the axis of the active ridge. This along-strike perspective has revealed the architecture of the global rift system. The ridge axis undulates up and down in a systematic way, defining a fundamental partitioning of the ridge into segments bounded by a variety of discontinuities. These segments behave like giant cracks in the seafloor, which can lengthen or shorten and have episodes of increased volcanic and tectonic activity. In fact, elementary fracture mechanics can be used to explain the interaction between neighbouring ridge segments.

Another important change in perspective came from the discovery of hydrothermal vents by marine geologists and geophysicists. It became clear that, in studies of mid-ocean ridge tectonics, volcanism, and hydrothermal activity, the greatest excitement is in the linkages between these different fields. For example, geophysicists searched for hydrothermal activity at mid-ocean ridges for many years by towing arrays of thermistors near the seafloor. However, hydrothermal activity was eventually documented more effectively by photographing the distribution of exotic vent animals. Even now, the best indicators of the recency of volcanic eruptions and the duration of hydrothermal activity are found by studying the characteristics of benthic faunal communities. For example, during the first deep-sea mid-ocean-ridge eruption witnessed from a submersible, divers did not see a slow lumbering cascade of pillow lavas, as observed by divers off the coast of Hawaii. What they saw was completely unexpected: white bacterial matting billowing out of the seafloor, creating a scene much like a mid-winter blizzard in Iceland, covering the freshly erupted glassy black lava with a thick blanket of white bacterial 'snow'.

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Deep Earth Seismology

R.A. Dunn , in Treatise on Geophysics (Second Edition), 2015

1.13.1 Introduction

Mid-ocean ridges mark the boundaries where oceanic plates separate from one another ( Figure 1 ) and thus lie above upwelling regions of mantle circulation. The upwelling mantle undergoes pressure-release partial melting because the temperature of the mantle solidus decreases with decreasing pressure. The melt, being less viscous and less dense than the surrounding solid, segregates from the solid mantle and buoyantly rises to the surface, where it forms new, principally basaltic, oceanic crust. The crust and mantle cool at the surface by thermal conduction and hydrothermal circulation, which generates a strong thermal boundary layer, the lithosphere. As the lithosphere travels away from the ridge, it thickens via additional cooling, becomes denser, and subsides deeper into the underlying ductile asthenosphere. This aging process causes the oceans to double in depth toward continental margins and subduction zones. Mid-ocean ridges represent one of the most important geologic processes shaping the Earth: Over the last 200 million years, two-thirds of the Earth has been resurfaced through this process of seafloor spreading, either at conventional mid-ocean ridge spreading centers or at spreading centers in back-arc basins behind subduction zones; they are a primary means of geochemical differentiation in the Earth; and they drive vast hydrothermal systems that influence ocean water chemistry and support enormous ecosystems.

Around the global ridge system, new crust and lithosphere form along 55 000 total kilometers of ridge at rates that differ by more than a factor of 10 ( Figures 1 and 2 ). This causes large differences in the nature of the magmatic, tectonic, and hydrothermal processes occurring along these ridges. For example, fast-spreading ridges, such as the East Pacific Rise (EPR), exhibit smooth topographic rises with more uniform magmatism, whereas slowly spreading ridges, such as the Mid-Atlantic Ridge (MAR) and Southwest Indian Ridge (SWIR), exhibit heavily faulted deep axial valleys and large variations in volcanic output with time and space. Such observations can be largely understood in the context of two basic processes: asthenospheric flow, which controls temperatures and melt production rates, and the balance between heat delivered to the ridge axis from below and heat extracted by conduction and hydrothermal circulation from above. Additional influences include global variations in mantle background temperature and volatile content, such as near hot spots. In back-arc settings, subducted lithosphere releases water and other elements into the mantle wedge above the subducted plate and below back-arc spreading centers. Consequently, as the back-arc spreading centers open over time, they sweep across strongly varying mantle compositional domains. This influences the degree of melting and the ultimate composition of the crust.

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Mid Ocean Ridges and Process of Plate Tectonics to Continue

Seafloor Geomorphology—Coast, Shelf, and Abyss

Mid-Ocean Ridges

Seafloor geomorphology—coast, shelf, and abyss

Mid-ocean ridges

Plate Tectonics and Volcanism

2.1.1 Mid-Ocean Ridges

Oceanic Crust☆

Variation with Spreading Rate

Tectonics and Volcanic and Igneous Plumbing Systems

7.2.1 Volcanic and Igneous Plumbing Systems at Mid-Ocean Ridges

Seafloor Processes

Abstract

Mid-Ocean Ridge Volcanism

Abstract

Volume 3

MOR-Type Ophiolites

Definition

Geochemical Features

Formation Setting and Process

TECTONICS | Mid-Ocean Ridges

Introduction

Deep Earth Seismology

1.13.1 Introduction

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