A ridge is a geological landform characterized by a long, narrow elevation of the Earth's surface, typically forming a continuous crest or chain of hills, mountains, or submarine features.^[1][2] These structures rise above surrounding terrain or ocean floor, often resulting from tectonic processes such as folding, faulting, or volcanic activity that create elongated uplifts.^[3] Ridges play a crucial role in geomorphology, influencing erosion patterns, water drainage, and biodiversity by dividing landscapes into distinct compartments.^[4] On land, they commonly appear as parallel features within mountain belts, such as the Valley and Ridge province of the Appalachian Mountains, where folded sedimentary rocks form repeating ridges and valleys.^[4][5] In marine environments, mid-ocean ridges represent the most extensive ridge systems, comprising a global network of submarine mountain chains exceeding 65,000 kilometers in length, driven by seafloor spreading at divergent plate boundaries.^[6][7] Notable examples include the Mid-Atlantic Ridge, which bisects the Atlantic Ocean and exemplifies plate tectonics in action through magma upwelling and crustal formation.^[8] Beyond their structural significance, ridges impact human activities, serving as natural barriers for transportation, agriculture, and settlement while hosting unique ecosystems adapted to steep, exposed conditions.^[4] Their formation and evolution provide key insights into Earth's tectonic history, with ongoing geological studies revealing how ridges contribute to continental drift and global seismic patterns.^[9]

Definition and Characteristics

Geological Definition

In geomorphology, the scientific study of landforms and the processes that shape them, ridges represent fundamental features influenced by a combination of endogenous forces—internal Earth processes such as tectonic and volcanic activities—and exogenous forces, which include external climatic actions like weathering and erosion by wind, water, or ice.^[10] These forces interact to create and modify surface elevations, with endogenous processes often providing the structural framework and exogenous ones refining the form through degradation or accumulation.^[11] A ridge is an elongated, narrow elevation of the Earth's surface, typically linear in form and separated from surrounding terrain by steeper slopes on at least one side, resulting from geological processes including uplift, erosion, or deposition.^[10] The term originates from Old English hrycg, meaning "back," "spine," or "ridge," evoking the image of a raised, backbone-like feature.^[1] Unlike rounded and isolated hills or higher, broader mountains, ridges emphasize elongation and linearity, often exhibiting an aspect ratio where length significantly exceeds width, distinguishing them as chain-like rather than compact or massive landforms.^[10] Subaerial ridges vary widely in scale, ranging from small features several meters in length to extensive structures hundreds of kilometers long, with widths typically ranging from hundreds of meters to 10 km and heights from a few meters to several thousand meters, though specific dimensions depend on the dominant forming processes and rock resistance.^[12] For instance, resistant rock layers can produce long, narrow ridges with heights of tens to hundreds of meters, while depositional forms like dunes may reach only meters in height but extend tens of kilometers.^[10] This variability underscores ridges' role as transitional landforms between more prominent relief features like mountains and flatter terrains.

Morphological Features

Ridges typically exhibit asymmetric cross-sectional profiles, characterized by a steep scarp face on one side and a gentler backslope on the other, as seen in homoclinal ridges where the dip slope ranges from 10° to 25° while the scarp exceeds 30°.^[13] Symmetric profiles occur in sharp-crested forms like hogbacks, with steep sides often greater than 45° on both flanks.^[13] Linearity is a defining trait, with aspect ratios of length to width commonly exceeding 5:1, emphasizing their elongated, narrow form over broader hill-like structures.^[13] Surface textures vary widely, from smooth and compact in volcanic or till-covered ridges to rugged and rocky in eroded exposures.^[13] Variations in ridge morphology include crest width, which is narrow and sharp in high-relief forms like hogbacks, contrasting with broader, rounded crests in some lower-relief erosional ridges.^[13] Slope angles on escarpments typically range from 20° to 60°, with free-face segments often surpassing 40° before transitioning to talus-covered lower angles near the angle of repose.^[14] Longitudinal profiles may be straight and linear, following underlying structures, or undulating due to differential weathering along the crest.^[13] Topographic metrics quantify these features, with relief defined as the vertical height difference from base to crest, often significant in hogbacks but low relative to adjacent terrain in some erosional forms.^[13] The elongation index measures the linear extent, calculated as the ratio of length to width or via 2√(area)/perimeter for overall shape, highlighting the pronounced anisotropy in ridge forms like drumlins or dunes.^[13] These metrics aid in visualizing ridges through simple profile diagrams: an asymmetric cross-section shows a near-vertical scarp dropping to a base, with a gradual rise on the opposing slope, while the longitudinal view depicts a narrow, extended crest flanked by converging shoulders.^[13] Lithology strongly influences morphology, as resistant rock types like metavolcanics produce steeper slopes (up to 0.85 gradient) and sharper crests compared to softer granodiorite (around 0.79 gradient), with higher relief and less efficient sediment transport in harder materials.^[15] Climate further shapes these traits: arid conditions preserve sharper ridges and steeper slopes through reduced erosion, while humid environments promote rounding and gentler angles via enhanced weathering and mass movement, often yielding greater overall relief.^[16]

Formation Processes

Erosional Mechanisms

Erosional mechanisms play a central role in ridge formation by selectively wearing away landscapes, exposing or carving linear elevated features through the removal of material. Differential erosion is the primary process, where more resistant rock layers, such as sandstone caps, protect underlying softer materials from rapid breakdown, leading to the development of inverted topography as the less resistant strata are preferentially eroded. This subtractive action creates prominent ridges by highlighting durable layers that stand out against surrounding terrain lowered by erosion.^[17] River incision contributes by deepening valleys adjacent to these resistant layers, while mass wasting, including rockfalls and landslides, accelerates the removal of loosened debris from slopes.^[18] Key examples of ridges formed through these mechanisms include cuestas, which arise from the erosion of gently dipping resistant strata, resulting in asymmetric ridges with a steep scarp face and a gentler dip slope.^[19] Hogback ridges, in contrast, form from steeply dipping beds where differential erosion produces narrow, symmetric crests with near-vertical flanks on both sides, often along faulted or tilted structures.^[20] These features emerge as softer interbedded layers are stripped away, leaving the harder caps as elevated spines. The development of such ridges typically unfolds in stages, beginning with initial tectonic uplift that exposes stratified rocks to surface processes. Progressive scarp retreat follows, where the escarpment face erodes laterally at rates of approximately 1-10 mm per year, varying with rock strength and climate; for instance, semiarid settings may see rates around 1 mm/year due to episodic weathering.^[21] Base-level lowering by streams further drives incision, promoting undercutting and retreat as valleys deepen and expose more of the resistant layers to erosion.^[18] Several factors influence these erosional processes, notably the presence of joints and fractures, which guide erosion along predefined lines by facilitating water infiltration and mechanical breakdown of rock blocks.^[22] In arid climates, formation accelerates through intense flash floods that deliver high-energy erosion, rapidly stripping unconsolidated material and enhancing differential wear on fractured surfaces. These steep scarps and asymmetric profiles often result directly from such targeted erosional action.^[20]

Depositional and Constructive Mechanisms

Depositional and constructive mechanisms for ridge formation involve the accumulation and buildup of sediments or volcanic materials in linear or elongated patterns, driven by various geological agents such as wind, ice, water, and magmatic activity. In sedimentary environments, wind-blown sand in aeolian settings accumulates into linear dunes through the transport and deposition of grains along prevailing wind directions, creating elongated ridges that can extend for kilometers. Similarly, glacial till deposited at the margins of retreating ice sheets forms moraines as unsorted debris is pushed or dropped in ridge-like formations. Volcanic extrusion occurs when basaltic lava erupts along linear fissures, building shield-like ridges through successive flows that solidify into elongated volcanic constructs. The key stages of ridge formation begin with the initial transport of materials, followed by selective accumulation and eventual stabilization. In aeolian systems, sand is transported by saltation and suspension, with rates reaching 10-50 meters per year in active desert environments, concentrating coarser grains on the windward slopes and crests while finer particles settle in leeward troughs. This leads to the buildup of elongated mounds as deposited layers migrate and accrete, often aligning with unidirectional winds to form longitudinal or transverse ridges. Accumulation in glacial settings involves the bulldozing of till by advancing ice fronts, creating push moraines through compressive deformation and dumping of debris during stasis. For volcanic ridges, magma ascends through fissures, extruding as low-viscosity lava that spreads linearly before cooling. Stabilization occurs via biogenic or chemical processes; vegetation colonizes aeolian dunes to anchor sands, while cementation by carbonates or silica binds glacial deposits, preventing further dispersal. Unique factors influencing these depositional ridges include grain size sorting and biogenic contributions. In aeolian and fluvial depositional ridges, wind or water selectively sorts sediments, resulting in coarser grains dominating crests due to higher energy deposition zones, while fines accumulate in adjacent lows. Biogenic processes play a significant role in coastal settings, where coral polyps and algae construct reef ridges through skeletal growth and storm-induced deposition of carbonate debris, forming emergent linear features up to several meters high in tropical environments. Ridge formation timescales vary widely depending on the process, ranging from years for active aeolian dunes, which can reach heights of 1-30 meters through rapid sand accretion, to millennia for glacial moraines, which build to 10-50 meters over extended periods of ice margin stability. Volcanic ridges may form over decades via repeated fissure eruptions. Subsequent erosional processes can modify these deposits by sculpting their profiles, but the primary architecture remains depositional in origin.

Classification by Origin

Aeolian Ridges

Aeolian ridges are depositional or erosional landforms shaped primarily by wind action in arid environments, encompassing features such as linear dunes and yardangs. Linear dunes, also known as longitudinal dunes, form through the accumulation of sand under persistent unidirectional winds, where sand grains are transported and deposited parallel to the wind direction, often merging from smaller barchan dunes—crescent-shaped forms that evolve into elongated ridges when winds are consistent. Yardangs, in contrast, arise from wind erosion, where prevailing winds sculpt softer bedrock or consolidated sediments into streamlined, ridge-like structures aligned with the wind flow, leaving resistant materials as elongated hills.^[23][24] These ridges exhibit distinct orientations relative to wind patterns: longitudinal dunes align parallel to the prevailing wind, extending for tens to hundreds of kilometers, while transverse ridges, such as those formed by coalescing barchans, orient perpendicular to the wind. Migration rates vary but can reach up to 20 meters per year for active dunes in strong wind regimes, driven by sand transport on the windward side and avalanching on the leeward slope. In semi-arid zones, vegetation like grasses or shrubs can stabilize these ridges, reducing mobility and promoting fixed forms that support local ecosystems.^[23][25][23] Prominent examples include the longitudinal dunes of the Namib Sand Sea in Namibia, where ridges stretch up to 100 kilometers in length, formed over millions of years from sand sourced from distant river systems. Adjacent to yardang ridges, deflation hollows—shallow basins scoured by wind removal of loose particles—often develop, enhancing the ridged topography in areas like the Lut Desert in Iran.^[26][24] Aeolian ridges predominate in arid regions receiving less than 250 millimeters of annual rainfall, where sparse vegetation and frequent high winds facilitate sediment transport. Globally, these features play a key role in dust mobilization and transport, contributing to atmospheric dust loads that influence climate, soil fertility, and ocean nutrient cycles.^[24][27]

Coastal Ridges

Coastal ridges form primarily through wave-driven depositional processes along shorelines, where sediment dynamics shape distinct geomorphic features in coastal environments. Beach ridges arise when storm waves transport and deposit sand in parallel lines perpendicular to the prevailing wave approach, building successive berms that become stabilized ridges as the coastline progrades.^[28] Chenier ridges, a variant typical of low-energy, mud-dominated coasts, develop on expansive mudflats as waves selectively winnow and concentrate shell hash and coarser clastic material into elevated, linear caps overlying finer sediments.^[29] These formations highlight the interplay of marine hydrodynamics and sediment sorting in constructing shoreline topography. Sediment transport by waves and currents provides the material for these depositional ridges, often in response to episodic high-energy events.^[30] Key characteristics of coastal ridges include their rhythmic spacing, modest elevation, and variable growth rates, which reflect local wave energy and sediment availability. Individual beach ridges are typically spaced 30 to 50 meters apart, though this can extend to 100 meters in slower-prograding systems, creating a series of subparallel features that record shoreline migration.^[31] Elevations generally range from 1 to 5 meters above high tide, providing limited protection against inundation while marking former shoreline positions.^[32] In active coastal settings, progradation rates average 0.5 to 5 meters per year, enabling the buildup of extensive ridge plains over centuries to millennia.^[33] Prominent examples illustrate the diversity and historical significance of coastal ridges. In the Gulf of Mexico, Holocene beach ridges form expansive plains, such as the chenier plain along the Louisiana coast, where successive ridges document progradation since approximately 3,000 years ago amid fluctuating deltaic sediment inputs.^[34] Storm berms, initially transient mounds of overwash sediment from intense wave events, often evolve into permanent ridges through vegetation stabilization and reduced wave reworking, preserving records of past storms in systems like those on the U.S. Gulf Coast.^[35] The development and stability of coastal ridges are profoundly influenced by sea-level fluctuations and sediment supply, which dictate their formation and long-term preservation. Relative sea-level stability or fall promotes ridge building by allowing progradation, while ample fluvial or littoral sediment sustains deposition; conversely, deficits in supply lead to erosion and ridge truncation.^[36] Since 1900, accelerated sea-level rise driven by anthropogenic climate change—averaging 1.7 mm per year globally but higher in subsiding regions like the Gulf of Mexico—has heightened vulnerability, exacerbating erosion rates and inundating low-lying ridges at paces exceeding natural variability.^[37] This ongoing threat underscores the sensitivity of coastal ridges to modern environmental shifts, potentially altering their role in habitat protection and shoreline defense.^[38]

Glacial Ridges

Glacial ridges form through the depositional processes of glacial activity, where ice masses transport and deposit sediment known as till, an unsorted mixture of clay, silt, sand, gravel, and boulders derived from bedrock erosion.^[39] These landforms include moraines, drumlins, and eskers, each shaped by distinct interactions between ice flow, subglacial water, and sediment deformation. Till accumulation occurs as glaciers advance, bulldozing debris into ridges or streamlining it beneath the ice.^[39] Terminal and lateral moraines represent primary types of glacial ridges formed by debris pushed by advancing ice. Terminal moraines develop at the glacier's front where the ice margin stabilizes, allowing sediment to accumulate in arcuate ridges as the ice overrides and thrusts material forward.^[39] Lateral moraines form along glacier sides in the ablation zone, consisting of rockfall debris from valley walls that builds sharp-crested piles marking the ice edge.^[40] Eskers arise from subglacial meltwater tunnels, where sediment-laden streams deposit stratified gravel and sand in meandering, steep-sided ridges as water flow diminishes during glacier retreat.^[41] Drumlins, elongated streamlined ridges, emerge from subglacial deformation of till, sculpted into teardrop shapes with steeper stoss (up-ice) ends and tapering lee sides, often occurring in vast fields.^[42] These ridges typically exhibit heights of 10-50 meters, though drumlins and eskers may reach only 5-30 meters, with compositions dominated by till's heterogeneous mix and sinuous or linear patterns aligned with former ice flow directions.^[42][43] Notable examples include the moraines of the Laurentide Ice Sheet in North America, such as the Bemis and Altamaha end moraines marking major advances of the Des Moines Lobe across Iowa and Minnesota around 14,000-12,000 years ago.^[44] In Ireland, drumlin fields like those in Clew Bay feature thousands of elongated hills, up to 1 kilometer long, formed beneath the Irish Ice Sheet during the last glacial maximum.^[45] Following ice melt approximately 10,000 years ago in many mid-latitude regions, these ridges were exposed, initiating post-glacial evolution through weathering and fluvial erosion.^[46] Current erosion rates on moraines remain low, typically around 0.0008 mm per year in arid settings like Antarctica's dry valleys, allowing preservation of these landforms while soils develop over millennia.^[47]

Volcanic and Hydrothermal Ridges

Volcanic ridges form primarily through fissure eruptions where magma ascends along fractures in the Earth's crust, leading to the extrusion of basaltic lava flows that build linear topographic features.^[48] These eruptions often produce pahoehoe lava, a smooth, ropy-textured type that spreads and cools to create elongated swells or ridges aligned with the controlling fissures.^[49] In subaerial settings, such as those in Iceland, repeated flows from these fissures construct low-relief ridges up to several kilometers long and tens of meters high.^[50] Hydrothermal ridges, in contrast, develop from the precipitation of minerals dissolved in superheated fluids emanating from seafloor vents, often associated with volcanic activity at spreading centers.^[51] These fluids, heated by underlying magma, mix with cold seawater, causing sulfides and other minerals to deposit as chimneys or mounds that form ridge-like structures.^[52] Black smoker chimneys, a prominent feature of these systems, can reach heights of up to 50 meters on the seafloor, though subaerial hydrothermal features, such as sinter terraces, provide analogous precipitation-driven ridges on land.^[53] Characteristics of volcanic and hydrothermal ridges include a dominant basaltic composition for the igneous components and fracture-controlled linear alignment, reflecting the tectonic stresses that guide magma ascent and fluid flow.^[54] Volcanic ridges typically exhibit rough, blocky surfaces from cooled lava and may host fumaroles or hot springs, while hydrothermal ridges feature porous, sulfide-rich deposits that support unique chemosynthetic ecosystems.^[55] Notable examples include segments of Iceland's Reykjanes Ridge, where en echelon axial volcanic ridges have formed through episodic fissure eruptions, creating bow-shaped features that evolve from active construction to erosional stages.^[56] Similarly, the 1963 eruption at Surtsey Island off Iceland's coast began with submarine fissure activity that built an initial ridge approximately 100 meters long before emerging as a volcanic island through pahoehoe and ash flows.^[57] Post-2000 discoveries have expanded understanding of hydrothermal systems in ultraslow-spreading environments, such as the 2002 identification of active vents along the Southwest Indian Ridge, revealing widespread mineral precipitation despite minimal magmatism.^[58] More recent findings, including the 2024 Jøtul field on the Knipovich Ridge, highlight persistent high-temperature venting in these settings.^[59] These systems play a key role in forming polymetallic sulfide deposits, rich in copper, zinc, and gold, which represent potential deep-sea mineral resources but raise environmental concerns for extraction.^[60]

Tectonic and Structural Ridges

Tectonic and structural ridges arise primarily from crustal deformation driven by plate tectonic forces, manifesting as linear topographic highs aligned with regional stress fields. These features form through two main mechanisms: compressional folding that produces anticline ridges and faulting that creates fault-block ridges. Anticline ridges develop when horizontal compressional stresses cause rock layers to buckle upward into arch-like structures, with the crest of the anticline forming the ridge axis while the limbs dip away from it.^[61] In contrast, fault-block ridges result from brittle fracturing under extensional or compressional regimes, where normal faulting in rift settings elevates horst blocks—upthrown slabs between parallel faults—into prominent ridges, or reverse faulting along convergent margins thrusts older rocks upward to form escarpments.^[62] Key characteristics of these ridges include their strong alignment parallel to the direction of maximum tectonic stress, reflecting the orientation of compressive or extensional forces that initiated deformation. Folded anticline ridges often exhibit dip slopes on their flanks, where the inclined bedding planes of the limbs create gentler, outward-sloping surfaces that parallel the rock dip angles, typically 10–30 degrees. Fault-block ridges, meanwhile, are defined by steep fault scarps on their margins, with cumulative scarp heights reaching up to 1 km in highly extended terrains, though individual earthquake displacements are usually meters to tens of meters.^[63][64] These scarps result from repeated slip along active faults, contributing to the ridges' rugged profiles. Prominent examples illustrate these processes in diverse tectonic settings. The Appalachian Valley and Ridge Province in eastern North America exemplifies compressional folding, formed during the Alleghanian orogeny approximately 300–250 million years ago, when the collision of ancestral North America and Africa produced a series of parallel anticline ridges separated by synclinal valleys, extending over 1,500 km from New York to Alabama.^[65] In the U.S. Southwest, the Basin and Range Province showcases fault-block ridges through Miocene-to-present extensional tectonics, where normal faulting has uplifted narrow mountain ranges, such as the Sierra Nevada front, creating a mosaic of horsts rising 1–2 km above adjacent basins.^[66] The formation and evolution of tectonic ridges are influenced by plate convergence rates, which typically range from 2–10 cm per year at collisional boundaries, accumulating strain over millions of years to drive folding and thrusting. Seismic activity is particularly pronounced along ridge scarps, where active faults accommodate ongoing deformation; for instance, in the Basin and Range, moderate-to-high seismicity (magnitudes up to 7) occurs along these scarps due to crustal extension rates of 1–2 cm per year. Erosional mechanisms can further enhance the structural relief of these ridges by preferentially wearing down intervening valleys.^[67][68]

Submarine Ridges

Mid-Ocean Ridges

Mid-ocean ridges are underwater mountain ranges that form at divergent plate boundaries, where tectonic plates pull apart and new oceanic crust is continuously created through seafloor spreading. These ridges constitute the longest mountain chain on Earth, encircling the globe like a seam on a baseball, with a total length of approximately 65,000 kilometers. The structure typically features a central rift valley, or axial valley, that can reach depths of 1 to 2 kilometers below the surrounding seafloor, flanked by steeper slopes and broader plateaus formed by accumulated volcanic material. Mid-ocean ridges vary significantly based on spreading rates, which influence their morphology and geological characteristics. Fast-spreading ridges, such as the East Pacific Rise, exhibit spreading rates of 10 to 15 centimeters per year and display relatively smooth, dome-like topography with minimal faulting due to robust magma supply. In contrast, slow-spreading ridges like the Mid-Atlantic Ridge spread at rates of 2 to 5 centimeters per year, resulting in rugged terrain characterized by extensive normal faulting, elongated valleys, and exposed mantle rocks in some segments. These differences arise from variations in the rate of plate divergence and the efficiency of magmatic processes along the ridge axis. The formation of mid-ocean ridges involves the upwelling of hot mantle material beneath the diverging plates, leading to partial melting and the generation of basaltic magma that intrudes as dikes and erupts to form new crust. As the crust moves away from the ridge axis, it cools and contracts, while periodic reversals in Earth's magnetic field—occurring every 0.1 to 1 million years—imprint symmetric magnetic stripes on the seafloor, providing key evidence for seafloor spreading. This process, first proposed by Harry Hess in 1962, underscores the ridges' central role in plate tectonics. Mid-ocean ridges are geologically significant, hosting about 70% of Earth's volcanic activity and serving as primary sites for submarine earthquakes due to the stresses of plate separation. Recent bathymetric mapping efforts since 2010, including data from initiatives like Seabed 2030, have revealed diverse microbial ecosystems thriving in the extreme conditions of ridge hydrothermal vents, supported by chemosynthetic bacteria that form the base of unique food webs.^[69] These findings highlight the ridges' importance for understanding global biogeochemical cycles and potential analogs for extraterrestrial life.

Other Submarine Ridge Types

Other submarine ridge types encompass aseismic ridges and sediment-derived features that form without ongoing plate spreading, distinguishing them from active mid-ocean systems. These structures arise primarily from intraplate volcanic activity or sedimentary processes on the ocean floor, contributing to the topographic diversity of abyssal regions. Aseismic ridges, in particular, represent linear volcanic chains generated by mantle plumes, while depositional ridges emerge from the accumulation of turbidites along submarine channels.^[70][71] Aseismic ridges originate from hotspot plumes that pierce the oceanic lithosphere, producing chains of volcanoes as the plate moves over a fixed mantle anomaly. For instance, the Ninetyeast Ridge in the Indian Ocean, extending over 5,000 km, formed as the Indian plate transited the Kerguelen hotspot between 43 and 83 million years ago. Similarly, the Walvis Ridge in the South Atlantic traces back to the Tristan da Cunha hotspot, with volcanism initiating around 130-135 million years ago and continuing as an age-progressive track. Guyots and seamount chains, often associated with these hotspots, evolve into ridge-like features through subaerial and marine erosion; guyots are flat-topped remnants of extinct volcanoes beveled by wave action before subsidence, and their linear arrangements in chains can mimic elongated ridges. Turbidite deposition contributes to linear mounds on abyssal plains via the construction of natural levees, where overspilling sediment-laden currents from submarine channels build asymmetrical ridges parallel to flow paths.^{[72][73][74][71]} These ridges typically exhibit depths of 2-4 km, shallower than surrounding abyssal plains due to isostatic uplift from excess volcanism or sediment loading, with widths ranging from 50-200 km. Unlike mid-ocean ridges, they lack significant seismicity, reflecting their intraplate, non-tectonic origins. Recent surveys in the 2020s have highlighted their role as biodiversity hotspots; for example, the Salas y Gómez and Nazca ridges in the southeastern Pacific support exceptionally high deep-sea diversity, including unique coral assemblages and crustacean populations, with escarpments and submarine ridges hosting over 73% of recorded vulnerable marine ecosystem indicators in areas beyond national jurisdiction. Such features underscore the ecological importance of these static submarine structures, often serving as refugia for endemic species amid broader ocean floor uniformity.^{[75][76][77][78][79]}