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Discovering the Deep - Image Captions


P1 – The deep ocean hosts astonishing life forms that thrive in some of the harshest conditions on Earth – the deep seafloor. At 1500 m depth (4920 ft) beneath the ocean surface a 0.5 m diameter “Dumbo” octopus traverses a lobate lava flow on Axial Seamount, Juan de Fuca Ridge.

P2 – Frozen lava columns mark the drain out and subsequent cataclysmic collapse of lava lake ceilings formed during ponding of 1200 °C (2192 °F) melt at the MOR. At a mile or more beneath the ocean surface, the near freezing sea water rapidly quenches the molten lava, forming glass-covered flows that reflect the dynamic nature of submarine eruptions.

P7 – The discovery of high-temperature hot springs systems in 1979 was made at MOR spreading center, the East Pacific Rise at 21° N. These high-temperature vents issues jets of metal-rich superheated fluids from the seafloor and are one of the most profound scientific discoveries ever made. Within these extreme environments, characterized by darkness and hundreds of atmospheres of pressure, life was found to not only survive, but flourish. The lush and vibrant communities of tubeworms, sprouting bright red plumes, in areas typically devoid of much color was a startling discovery. Equally surprising was the presence of microorganisms thriving, in the absence of sunlight, by metabolizing volcanic gases at temperatures in excess of 90 °C (194 °F). With the advent of genomic sequencing, we are now gaining insight into the vast diversity of microbes that live on and within the seafloor.

P8 – One of the most spectacular events in crustal formation is the release of billions of microbes from the seafloor during and following seafloor volcanic eruptions. The microbes form whitish sulfide-rich particles that are ejected in dense, billowing streams from the seafloor – called snowblowers, such as the one imaged here following the 2011 eruption on Axial Seamount.

Chapter 1

1.1 - The deep-diving submersible Alvin preparing to dive to the axis of the Juan de Fuca Ridge. Alvin carries one pilot and two observers and has made more than 4600 dives to the seafloor since it was built in the 1960s. Alvin currently descends to a maximum depth of 4500 m; however, its titanium personnel sphere was replaced in 2013 with one that is certified to dive in 6500 m. The support ship for Alvin is the research vessel (R/V) Atlantis, operated by the Woods Hole Oceanographic Institution.

1.17 – Maurice “Doc” Ewing (top, left) and Allyn Vine (top, right) on the R/V Atlantis holding one of the first deep-sea cameras in the late 1950s. David Owen deploying a deep-sea camera from R/V Vema in the late 1950s (bottom).

1.25 – The newest deep-diving human-occupied submersible – Alvin, of WHOI – was placed into service and certified in early 2014. It is currently certified to -400 m, but will eventually be able to dive to 6500 m with two observers and one pilot, as it has done throughout its operational career.

1.30 – The AUV ABE (Autonomous Benthis Explorer) developed and operated by WHOI (left) being launched in 2001. The WHOI AUV Sentry (right) has a dive capability of 6 km and includes multibeam, sidescan, and CHIPR sub-bottom sonar, and a variety of water properties sensors, as well as a digital still camera and strobes that enable it to take continuous photographic images of the seafloor. Sentry has now replaced ABE as the AUV system in the National Deep Submergence Facility at WHOI.

1.35 – REMUS (Remote Environmental Monitoring UnitS) are a class of torpedo-shaped AUV’s (left) operated by the Ocean Systems Laboratory (OSL) at WHOI that provide autonomous survey capability over a depth range from 100t o 6000 m. The vehicles have been used for measurement of coastal water properties and currents, high-resolution sidescan and multibeam mapping, and for military applications associated with mine clearing operations in coastal waters. A REMUS 6000 A UV being recovered (right). The OSL Group, operation three REMUS 6000 vehicles simultaneously during several cruises, was responsible for the search and discovery of the black box of Air France Flight 447 (Fig. 1.36) which crashed into themed-Atlantic near 2° N and 32° W in June 2009, on the east flank of the Mid-Atlantic Ridge in water depths of approximately 4 km.

1.41 – Ocean-bottom seismometers (OBSs) on the back deck of the R/V Wecoma of Oregon State University (top) in preparation for deployment for the CASCADIA experiment, an onshore and offshore seismic experiment designed to study large magnitude (>9) megathrust earthquakes along the Washington and Oregon coast ( An OBS being deployed from R/V Oceanus (bottom left) and the electronics of an OBS housed in a glass pressure sphere (bottom right).

1.48 – Illustration showing the ROV Jason2 and a deep-sea light system used for illuminating the seafloor for high-definition video imaging during a University of Washington experiment on Juan de Fuca Ridge in 2005, in preparation for the installation of NEPTUNE.

1.49 – Illustration depicting the main components of a telepresence system – the ROV at the seafloor making observations (lower left and center), and the fiber-optic cable transmitting data and imagery from the ROV up the cable to the support ship and the scientists onboard (upper right). Via the Internet, these data and imagery are now streamed live to shore-based laboratories, educators and to the general public, who experience real-time interactions with scientists and engineers while getting information from the ROV as it explores the ocean depths (right).

1.50 – A high-definition still camera (foreground) deployed in front of “Mushroom Vent” on Axial Seamount on the Juan de Fuca Ridge as part of the Ocean Observing Initiative experiment, and a 3D thermistor array (blue rods in background) positioned over a diffuse flow vent. The orange coil of cable is the power and data “extension” cord, a fiber-optic cable that transmits the imagery and thermistor data from the instruments back to shore-based laboratories for analysis. Field of view – 6 m.

Chapter 2

2.14 – A step-like pattern of increasing compressional wave velocity with depth is typical of the layered structure of the oceanic crust (left). Contacts between layers and the internal structure of layers are best modeled as velocity gradients corresponding to gradational compositional changes (White et al, 1992). Laboratory measurements of compressional wave velocities of oceanic and ophiolite rocks link the internal structure of ophiolite complexes to the velocity structure of oceanic crust (right).

2.15 – The first samples obtained from the seafloor were recovered by dredging – dragging a strong, steel mesh bag across steep, rugged areas of the seafloor. The dredge is connected to the ship by -1-2 cm thick steel wire rope able to withstand 10-25 tons of tension. Dredging continues to be an important means of collecting large volumes of oceanic rocks and for reconnaissance petrological and geochemical studies of the crust.

2.18 – Investigations of the axial regions of active spreading centers (left) and major escarpment “tectonic windows” (right) provide different but complementary perspectives on the oceanic crust and seafloor spreading processes. Numbered black lines showing locations of near-bottom studies – both submersible dives and camera tows. White stars show locations of ODP/IODP drill holes.

2.22 – Drilling into exposures of plutonic rocks on the seafloor has recovered extensive cores of deep crustal gabbroic rocks with diverse textures and mineralogies (left) and serpentinized upper mantle peridotites (right). ODP Leg 154, Holes 924 and 920. Cores are about 5 cm wide.

2.26 – Summary of the layered structure seen in many ophiolite complexes correlated with the oceanic crust and upper mantle. The recurring sequence of rock units in ophiolite complexes inspired the views adopted by the 1972 Geological Society of America Penrose Conference (Table 2.2)

Chapter 3

3.2 – The global MOR system is a continuous network of volcanic and tectonic features marking divergent lithosphere plate boundaries. This global perspective highlights the MOR (relatively shallow, light-green area on the seafloor) as it encircles the globe. Differences in morphology of these different parts of the MOR reflect the very different structural, petrological, and morphological characteristics as a function of spreading rate and magma budget. The East Pacific Rise (left globe) is a fast- to superfast-spreading ridge, while the Mid-Atlantic Ridge (middle) is a slow-spreading ridge. MORs in the floor of the Indian Ocean (right) include the Southeast Indian Ridge and Central Indian Ridge spreading at intermediate rates, and the ultraslow-spreading Southwest Indian Ridge.

3.7 – The morphology of spreading centers and adjacent abyssal hills varies dramatically with spreading rate. These images are all at the same scale and have the same color-coded depth scale (lower right). Smooth axial highs with small axial summit depressions occur at high spreading rates, such as the East Pacific Rise. Both axial highs and rift valleys occur at ridges with intermediate rates, such as the Juan de Fuca Ridge. The slow-spreading MAR has a broad rift valley and rugged terrain including dome-like oceanic core complexes. Ultraslow-spreading ridges, such as the Southwest Indian Ridge, have segmented rift valleys and complex, rugged terrain with abyssal hills that are much more irregular and varied than the others shown here.

3.14 – Shaded relief image of the Cleft Segment of the Juan de Fuca Ridge, one of the most intensively studied intermediate-rate ridges, looking from the south (- 44°33’ N) to the north (- 45°05’ N). The elevated summit area (gray to pink) is cut by a distinct -2 km wide rift valley, bounded by fault scarps, which bisects the linear ridges to either side. Young lavas occur in the axial valley (neovolcanic zone) but also occur as isolated flows on the flanks of the bounding crestal ridges. Note the AST or “cleft” is apparent as a thin linear fissure that bisects the axis in the middle of the axial valley. Light gray color is depth range from 2100 to 2200 l’ dark green > 2600 m (modified from Stakes et al, 2006).

3.32 – Tectonic windows into intermediate- to fast – spreading crust of the Pacific provide extensive exposures of upper crustal rock units. Exposures of pillow lavas, sheeted dike complexes, faulted lavas and dikes, and gabbroic rocks occur in a regular layered sequence as found in many ophiolite complexes. Field of view -2 m, except for the gabbroic rock (lower right), which is about 20 cm long.

3.41 – Locations of tectonic windows (escarpments) and deep crustal drill holes in slow- and ultraslow- spreading crust (above): ATM, Atlantis Massif; GOR, Gorringe Bank (Gettysburg and Ormond Seamounts); KTR, King’s Trough; MARK, MAR at Kane Transform; MCSC, Mid-Cayman Spreading Center; OFZ, Oceanographer Transform Fault; SMARK, Southern MARK Area; TAG, TransAtlantic Geotraverse Area; VTF, VemaTransform Fault. Note that the Atlantis II Transform is on the Southwest Indian Ridge. Columnar sections (below) summarizing geology in tectonic windows and deep crustal drill holes in ultraslow- to slow-spreading crust reveal complex geological structures. Only at the Vema Transform does there appear to be a “normal” crustal structure with the rock units anticipated from ophiolites. Other areas show lava lying directly over variably deformed and metamorphosed gabbroic and serpentinized ultramafic rocks. (Data from OTTER, 1984; Karson, 1998; Blackman et al, 2002; Godard et al, 2003; Dick et al, 2008.)

3.44 – Schematic diagram of a fast-spreading ridge and crustal accretion processes creating a uniform crustal structure from a continuous axial magma chamber.

3.45 – Schematic diagram of a slow-spreading ridge and crustal accretion processes creating heterogeneous and discontinuous crustal structure.


Chapter 4

4.1 – This 1 m tall “chimlet” at the top of the black smoker chimney called Sully is at a depth of 2200 m on the Juan de Fuca Ridge. It billows 360 °C, metal-rich fluids and is built on a -5 m tall mound of sulfide debris. It hosts a vibrant colony of tubeworms with bright red plumes.

4.2 – Sully, in 2006, hosted multiple black smoker orifices and extinct oxidized sulfide debris. Areas of robust, low- to moderate-temperature diffuse flow support “fat” healthy tubeworms (Ridgeia piscesae) reaching – 0.75 m in length.

4.29 – A community of tubeworms (rifita rachyptila), brachyuran crabs, and vent fish (Thermarces Cerberus) thrive in nutrient-rich, warm fluids issuing from Tubeworm Pillar on the EPR at 9° N. Crabs are ~5 cm across. In 2004, these animals perished at this site due to a rapid cessation of venting, and the 11 m tall sulfide pillar was subsequently destroyed during eruptive events in 2005-2006.

4.30 – The TAG Hydrothermal Field supports dense aggregations of shrimp (Rimicaris exoculata). These animals are of interest because, even though they have no eyestalks, they have a dorsal organ hosting a visual pigment that absorbs maximally near 500 nm. High-temperature chimneys emit light in the non-visible spectrum, supporting the idea that shrimp detect dim light with their non-image-forming “eyes.”

4.31 – Global distribution of distinct biogeographic regions currently recognized for MOR hydrothermal vent fauna [including some non-ridge sites; after Rogers et al. (2012)]. The regional suites of species characteristics of hydrothermal vents can be considerably different in various parts of the world ocean as a consequence of the evolutionary history of the different ocean basins. Vicariant events and physical oceanographic process, species-specific variation in the life histories of individual species, rates of seafloor spreading, and the frequency and spacing of active venting along the MOR system may impact faunal distribution.

4.68 – The actively venting Nature Tower, on the east side of Lost City, rises 30 m above the surrounding seafloor. Significant limestone talus at the base₄ of the structure attests to the collapse of spires and rebuilding as part of the evolution of these complex chimneys. The ROV Hercules and support vehicle Argus are embedded in the photo for scale.

4.83 – The outsides of the active chimneys at Lost City are covered in dense strands of filamentous bacteria that thrive in the mix zones of high-pH, CH₄- and H₂-rich hydrothermal fluids and seawater. The carbonate interstices shrouded by these filamentous microbes serve as microhabitats for numerous species of small gastropods, polychaetes, and amphipods.

4.85 – Geryonid crabs are common features among the ledges and outcrops in the periphery of the main field. Males of this genus will hold the female underneath for long periods of time (e.g., weeks) prior to mating.

4.117 – This black smoker edifice called Bastille, in the Main Endeavour Field, rises more than 15 m above the seafloor and is composed of multiple towers. It hosts dense colonies of tubeworms (ridgeia piscesae), palm worms, and limpets. Scale bar is equal to 1 m.

4.138 – Tubesworm- and limpet-covered sulfide chimney in the ASHES Vent Field near the southwestern caldera wall on Axial Seamount. The small, black chimney is very young (<1 year) and composed of fine-grained, highly friable sulfide minerals that include pyrite and zinc sulfide phases. Tubeworm casings are ~1 cm across.

4.153 – Octopi are relatively rare in the caldera floor of Axial Seamount. They likely feed on mollusks, such as clams, that thrive in sites of diffuse flow. This octopus (Graneledone sp.) is ~1 m long.

4.155 – A typical assemblage of Guaymas Basin Riftia tubeworms near a small mound of hydrothermal sediment and minerals that can be 1-20 m high and hundreds of meters in area. The system is rich in hydrocarbons due to high organic concentrations in the sediments.

4.179 – Tubeworm communities on the 11 m tall Tubeworm Pillar show the spatial zonation of EPR vent habitats with Tevnia tubeworm (yellower tubes) occupying areas of more sulfide-rich fluids, temporally mixed with lower oxygen concentrations. The larger Riftia tubeworms (from middle to right side of image) displace Tevnia as vent flux and H₂S/temperature decrease over time. White brachyuran crabs and pink zoarcid fish are active in communities. These sites can host more than a dozen species, including amphipods, gastropods, polychaetes, mussels, and shrimp. Brachyuran crabs graze on tubeworm tubes. An eelpout fish, known for eating tubeworm plumes and other polychaetes, is positioned amongst the tubes. Field of view ~1.5m.

4.182 – Close-up of the Choo Choo Train Riftia community (shown in the center of Fig. 4.181) being outcompeted by bathymodiolin mussels for access to vent fluids. A cluster of amphipods (whitish haze) just above the Riftia plumes form dense swarms (~1000 individuals/liter). These amphipods, Halice hesmonectes, are pelagic crustaceans in the deep sea. Polypro rope holding the red and white marker is 0.6 cm wide. Field of view ~2 m.

4.185 – Dense thickets of Riftia pachyptila along the EPR at 12°50’ N are commonly covered in iron oxide, the product of microbial activity in these vent systems. Note the spatial distribution of live versus dead tubeworms reflecting the spatial distribution of the venting activity. Live, densely packed tubeworms in the center and vacant tubes along the periphery suggest a recent change in the distribution of subsurface vent fluid supply and chemistry. Field of view ~2 m.

4.194 – The majority of low-temperature sites of Galápagos Rift are expansive and contain both live and dead Calytogena clams and Bathymodiolin mussels (e.g., Clambake and Oysterbed). Perhaps the largest active bivalve field is also the shallowest – the Calyfield Vent Field at 1670 m, near 89.5 W. The Calyfield hosts a large vesicomyid clam (Calyptogena magnifica), mussel (Bathymodiolus thermophiles), and endemic sponge (n .sp.) community that covers an approximate area of 60 m x 70 m. Extinct high-temperature sulfide chimneys were also discovered several hundred meters northeast of Calyfield. Field of view ~1 m.

4.198Riftia tubeworms, anemones, and small (<4 cm long) mussels (center lower right) recruiting along cracks adjacent to diffuse flow vent openings surrounded by large aggregations of live and dead clams at the Tempus Fugit Vent Field discovered in 2011 using Little Hercules and the Okeanos Explorer. Field of view ~1 m.


Chapter 5

5.2 – Pillow lava mound on the summit of Axial Seamount, Juan de Fuca Ridge, with well-developed bread-crust textures on pillows at the left side of the image. Lobate lava flows (darker lava) emplaced during the 2011 eruption on the right side of the image are flowing over and between the older pillows. Field of view ~3 m.

5.12 – Ancient (roughly 95 Ma) pillow lavas surrounded by red cherty sedimentary rocks crop out in an ophiolite in Oman. Exposures like this are remarkably similar to those seen along faults in modern oceanic crust.

5.26 – A cross section (left) of a lava pillar collected from the EPR axis near 9°50’ N at 2505 m depth. The pillar formed during the 1991-1992 eruption. It shows a double channel configuration where two streams of vaporized seawater provided the nucleating cylinders of cold water around which the lava cooled as the axial lava pond inflated. The glass-lined interior of the two channels attests to the formation process. The distance across the long axis of the pillar is ~20 cm. Lava pillar (right) in the caldera of Axial Seamount that formed during the 2011 eruption showing the well-developed selvages on its side. These may represent still-stands of the ponded lava upper surface as it deflated during magma withdrawal that accompanied the waning phase of the eruption (pillar diameter ~20-30 cm).

5.29 – Folded sheet flow (top) in the axial valley of the eastern Galápagos Spreading Center at the Rosebud Hydrothermal Vent Field near 86CH4 W (Shank et al., 2003). The folds in the lava surface are nearly entirely glass; field of view ~0.75 m. The eruption that created this flow is believed to have occurred sometime between 1992 and 2003, based on observations during Alvin diving programs to the area during that decade. Rat-tail fish is about 25 cm long. Folded sheet flow on the summit of Axial Seamount (middle); field of view ~1.5m m. Close-up on a hackly sheet flow surface on the summit of Axial Seamount, part of the 2011 eruption (bottom). All of the folds in this image are thin (under 0.5 cm) ribbons of glass that cooled on contact with seawater as the fast-moving flow traversed the seafloor. Note that the folds are well preserved, suggesting that the upper flow surface still behaved plastically during emplacement; field of view ~0.5m.

5.35 – A distinct transition between lava flows; a tubular pillow lava (0.75 m diameter) with well-developed bread crust surface texture flowed across an older lobate and pillow flow on the axis of the Galápagos Spreading Center. The site is near 91° W at the Caly Hydrothermal Vent Field site at a depth of 1950 m. Hydrothermal venting at this site is low temperature (only 1 or 2 °C above ambient) with low levels of H₂S supplying sufficient nutrients to sustain vesicomyid clam (white shells in the image) and bathgrid mussel communities. Open cavities within lobate flows and along the margins between lobes can provide pathways for hydrothermal fluids.

5.36 – Abrupt transitions between flat sheet flow surface and ropy, folded, or crenulated sheet flow can occur frequently in submarine flows, and attest to the dynamic and fast movement of lava on the seafloor despite rapid chilling of the magma by the near-freezing water. Structures such as those observed in this image indicate that the lava remains plastic for a short period of time after extrusion in order to form the complex structures that typify these types of flows. This image is from the 2010 eruption of Axial Seamount. Field of view ~ 2 m.

5.51Alvin photo taken in 1991 of small pillars at the margin of the EPR axial summit trough near 9°50’ N coated with microbial mat. During the eruption, hydrothermal fluids were seen emanating from the tops of pillars and many of the pillars were covered with microbial mat. This supports the supposition that pillars are formed in locations where hydrothermal fluids exit the seafloor below lava ponds and lakes. Field of view ~4 m.

5.57 – Three ages of lava on the EPR axis near 9°50’ N (left). Left lobe shows black, glassy lava emplaced during the 2005-2006 eruption, while the middle lobe of sediment-dusted lava is the oldest. Tubular pillow on right may be 1991-1992 lava; its age, based on the character of the outer crust and slight devitrification of the outer class, is intermediate between the other two lava forms; field of view ~2 m. TowCam photo (right) of the contact between 2005-2006 lava ponded against the first inward facing fault east of the EPR axis at 9°53’ N. Fault height is about 3 m; field of view ~4 m.

5.58 – Collapse pit in a lineated flow that transitions to both pillow and lobate forms along the margin of the flow. This flow is believed to be part of the early stages of the 2011 eruption at the summit of Axial Seamount. Field of view ~4 m.

5.59 – A phase of the West Mata eruption imaged and sampled in 2009 using the ROV Jason. Showing extensive water-magma interaction and resulting fragmental material caused by the explosive nature of the eruption. Yellow color in plume is sulfur produced during the eruption. Nozzle at right is the tip of a titanium gas-tight water sampler. Depth of the eruption site is about 1200 m. Field of view ~1 m.


Chapter 6

6.1 – Sheeted dike complexes are composed of parallel arrays of tabular basaltic intrusions, each about 1 m wide. In this outcrop from the Pito Deep Right in the Southeast Pacific, several well-defined dikes dip moderately to the southeast (right), away from the East Pacific Rise (EPR) spreading center where they were formed. Field of view ~ m.

6.5 – Sheeted dikes (dikes intruded side-by-side, making up more than 90% of the rock unit) are important components of ophiolites and oceanic crust. Vertical sheeted dikes in the Oman Ophiolite (left) are each about 1 m wide. Vertical sheeted dikes are also exposed on steep seafloor escarpments which reveal the structure of the upper oceanic crust (right), as in this example from the Pito Deep Right. Field of view ~3 m in both images.

6.15 – Fault breccia from the Pito Deep Rift has blocks of massive material surrounded by highly fractured and altered dike rock (left). Horizontal field of view ~3 m. Sample of fault gouge and fault breccia from a discrete fault zone from the Pito Deep Rift (middle). The block is bounded by smooth, curved, striated fault surfaces on either side. The block is ~20 cm long. Photomicrograph (shown in cross-polarized light) of fault breccia (right) with a clast of basaltic material (left side) in fine-grained fault gouge (dark material). Horizontal field of ~4 mm.

6.20 – Dike interiors can exhibit relatively coarse diabasic or subophitic textures where plagioclase (white and gray laths) is partially or wholly enclosed in clinopyroxene (green on left, gold and blue on right) as in this sample from the Mid-Cayman Spreading Center. Image on left show in plane-polarized light and on the right in cross-polarized light. Field of view ~6 mm.

6.25 – Exhumed crustal sections near magmatic centers expose dikes and other shallow-level intrusions. Several basaltic dikes, each 2-3 m wide (dark gray to brown), cut light-colored granitic gneiss (left) showing dilational offsets where they intersect. Dense swarms of dikes create a tangle of intrusions on a cliff face more than 500 m high (right). Both images are from near the continent-ocean transition on the East Greenland Volcanic Rifted Margin.

Chapter 7

7.1 – Coarse-grained gabbroic rocks make up most of the middle to lower oceanic crust. The structure and composition of this unit are critical to understanding magmatic processes and accretion at MOR spreading centers. Complex structures such as the cross-cutting volumes of layered gabbro from the Oman Ophiolite shown here reveal relationships at a scale and resolution that is not yet possible on the seafloor.

7.8 – Gabbroic xenoliths enclosed in MORB from the Cleft Segment of the Juan de Fuca Ridge provide rare glimpses of the plutonic portions of the oceanic crust beneath an active spreading center. Aphyric host lava with medium-grained olivine gabbro (top left) and photomicrograph showing equigranular olivine and plagioclase with lesser clinopyroxene (top right). Photomicrograph of olivine-bearing gabbro xenolith (bottom left and right) showing small plagioclase crystals enclosed in a single large clinopyroxene oikocryst with minor quantities of interstitial glass. Images shown in cross-polarized light (right) and in plain-polarized light (left).

7.11 – Gabbroic rocks on OCCs show a range of outcrop-scale deformation structures. Intensely jointed gabbroic rocks on the wall of the Kane Transform Fault (cross section of an OCC) (upper left); complex pattern of foliation on horizontal surface of the Atlantis Bank OCC (upper right); coarse foliation and slabby form of metagabbros on the right valley wall of the Kane RTI OCC (lower right); very platy, mylonitic metagabbros on the Kane RTI OCC (lower left). Note light-colored pelagic ooze highlights the deformation features. Field of view in all images is ~4 m. The inclined camera orientation makes the structures in the lower images appear much steeper than they are (~30° dip).

7.17 – Photomicrographs of gabbroic material deformed at high temperatures (>800 °C) show evidence of crystal-plastic deformation and recrystallization. Mylonitic gabbro (left) with large, lens-shaped, porphyroclasts of clinopyroxene surrounded by finely recrystallized plagioclase and hornblende, white and green, respectively (upper left). Upper image shown in plane-polarized light; lower image in cross-polarized light. Photomicrographs on right show similar features. Finely recrystallized plagioclase enclosing stronger, and in some cases fractured and internally deformed, clinopyroxene (partially replaced by green amphibole) shown in plane-polarized light (top). Lower two images show details of crystallized plagioclase (gray) in deformed gabbroic rocks. Both show in cross-polarized light. All images are from the MARK area except upper right from the Vema Fracture Zone. Scale bars on all images are 1 cm.

7.30 – Layered wehrlite (brown) and clinopyroxenite (green-gray) cumulate rocks are common near the base of the plutonic section of many ophiolites, for example the Oman Ophiolite (top). Photomicrograph in cross-polarized light shows typical texture in clinopyroxenite layer from Bay of Islands Ophiolite. Scale bar is 1 mm.

Chapter 8

8.1 – Earth’s mantle is dominantly peridotite, a rock composed mostly of the mineral olivine. Ductile flow and partial melting of mantle peridotite produces the magmas from which the oceanic crust is constructed. The mosaic texture of the thin section of peridotite, as shown here in cross-polarized light, is the result of solid-state flow and recrystallization beneath a MOR spreading center. Field of view ~4 cm.

8.5 – Hydration of mantle peridotites results in the conversion of olivine to serpentine minerals and related processes collectively referred to as serpentinization. Serpentinized peridotite from ODP Leg 153 (left) shows the evidence of mantle deformation in deformed pyroxene grains (pale green). Most of the olivine is replaced by serpentine minerals (black) and serpentine veins (white). In thin section (right) the colorful mosaic texture of anhydrous peridotite (Fig. 8.1) is replaced by gray “mesh-textured” serpentine.

8.11 – The crust along slow- to ultraslow-spreading ridges is highly heterogeneous and variable in thickness along spreading centers. Deep crustal and upper mantle rocks are exposed by extreme tectonic extension in oceanic core complexes and also in relatively cool areas such as transform faults and non-transform offsets.

8.15 – Sedimentary rocks atop the Atlantis Massif OCC. Lower images show serpentinite clast sedimentary breccias directly on top of the detachment shear zone on the south wall (left) and in large, jumbled outcrops on top of the central dome of the massif. The breccias are overlain by white pelagic limestone (upper left) through a gradational unit with dark serpentinite clasts in pink carbonate matrix (upper right). Photomnicrograph (upper right) shown in plane-polarized light. Field of view ~2 cm. Outcrop images have fields of view of ~3 m.

8.20 – Deformed serpentinites and related rocks appear to be common along detachment faults in oceanic core complexes. These examples show mylonitic fault rocks from the Atlantis Massif OCC (top pair) with dark serpentinized peridotite enclosed in finely recrystallized serpentine and talc (left) and talc schist (right); fault rocks from the MARK Area (middle pair) with cataclastic serpentinite fault rock with colorful remnants of pyroxene (left) and serpentinite shear zones (right and lower left); and a detail of multiple cross-cutting serpentine veins (black and gray) (lower right). All images shown in cross-polarized light. Field of view is 4 cm for all images except the lower right, which is 4 mm.

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