Ocean Geology and the Seafloor Explained: Oceanic Crust, Mid-Ocean Ridges, Trenches, Seamounts and the Hidden Landscape Beneath the Sea

What Is Ocean Geology?

Ocean geology, also called marine geology, is the study of the rocks, sediments, structures and
geological processes beneath the oceans. It connects plate tectonics, volcanology, sedimentology, geophysics,
oceanography and paleoclimatology.

Marine geologists investigate how ocean basins open and close, how new crust forms, why underwater volcanoes develop,
how sediments move across continental margins, where earthquakes and submarine landslides occur, and how fluids and
gases escape through the seabed.

The ocean floor also preserves evidence of past climate change, continental breakup, major volcanic events, asteroid
impacts, ancient ocean circulation and biological evolution. Sediment cores retrieved from the seabed can contain
layered records extending back millions of years.

What Does the Ocean Floor Look Like?

Early maps often portrayed the ocean floor as a relatively featureless basin. Modern bathymetric data reveal a landscape
as varied as the continents.

From coastlines toward the deep ocean, a typical continental margin may include:

1. the continental shelf;
2. the shelf break;
3. the continental slope;
4. submarine canyons;
5. the continental rise;
6. deep-ocean basins and abyssal plains;
7. mid-ocean ridges, seamounts or volcanic plateaus;
8. ocean trenches near subduction zones.

Not every margin contains all of these features. Passive continental margins tend to accumulate thick sedimentary
wedges, while active margins may contain trenches, volcanic arcs, fault zones and narrow shelves.

Continental Margins: Where Continents Meet the Deep Ocean

Continental margins form the submerged edges of continents. They represent the transition between relatively thick,
buoyant continental crust and thinner, denser oceanic crust.

Continental Shelves

The continental shelf is the shallow, gently sloping submerged extension of a continent. Shelf width varies greatly.
Some coastlines have broad shelves extending hundreds of kilometers offshore, while tectonically active margins may
have very narrow shelves.

During glacial periods, sea level falls and large portions of continental shelves may become exposed land. When ice
melts, rising seas flood those surfaces again.

Continental Slopes

Beyond the shelf break, the seabed descends more steeply along the continental slope. Submarine canyons, landslide scars,
sediment channels and fault structures commonly dissect this zone.

Continental Rise

At many passive margins, sediments transported downslope accumulate at the base of the slope, forming a broad,
relatively gentle continental rise. Deep-sea fans may spread outward for hundreds of kilometers.

Passive vs Active Margins

Oceanic Crust

Oceanic crust is the thin, dense rocky shell underlying most ocean basins. It is composed mainly of basaltic and
gabbroic rocks formed from magma generated beneath mid-ocean ridges.

Oceanic crust differs fundamentally from continental crust. It is generally thinner, denser, more uniform in composition
and much younger.

Layers of Oceanic Crust

A simplified section of oceanic crust may include:

1. Marine sediments deposited on the seafloor.
2. Pillow basalts formed where lava erupts underwater and cools rapidly.
3. Sheeted dike complexes representing former magma pathways.
4. Gabbroic rocks crystallized more slowly within magma chambers.
5. Upper mantle peridotite beneath the crust.

Complete sections of oceanic crust are difficult to examine because they lie beneath the ocean. However, fragments can
be exposed on land in geological complexes known as ophiolites, where pieces of ancient oceanic crust
and mantle were tectonically placed onto continents.

Why Is Oceanic Crust So Young?

New oceanic crust is continually produced at spreading ridges and destroyed at subduction zones. Older crust becomes
colder and denser as it moves away from a ridge, making it increasingly likely to descend into the mantle where plates
converge.

Seafloor Spreading

Seafloor spreading is the process by which tectonic plates move apart and new oceanic crust forms
between them. It occurs along mid-ocean ridges, where hot mantle rises, partially melts and produces basaltic magma.

How Seafloor Spreading Works

1. Tectonic plates diverge.
2. Hot mantle rises beneath the plate boundary.
3. Pressure decreases as mantle material rises.
4. Partial melting produces basaltic magma.
5. Magma intrudes into fractures or erupts onto the seabed.
6. New crust cools and moves away from the ridge.
7. More magma rises and continues the process.

The process does not necessarily involve two rigid plates simply pulling apart over an empty gap. Plate motion, mantle
flow, magma supply and faulting interact across a broad zone.

Magnetic Stripes on the Seafloor

Basalt contains iron-bearing minerals that align with Earth’s magnetic field as lava cools. Because the planet’s
magnetic polarity has reversed many times, alternating bands of normal and reversed magnetization are preserved on
opposite sides of mid-ocean ridges.

These symmetrical magnetic stripes provided powerful evidence for seafloor spreading and plate tectonics.

Age Patterns Across Ocean Basins

The youngest oceanic crust lies near spreading centers. Crust becomes progressively older with distance from the ridge.
Sediment cover also tends to thicken with age because older crust has had more time to accumulate marine deposits.

Fast and Slow Spreading Ridges

Spreading rates influence ridge shape. Fast-spreading ridges commonly have smoother axial highs and a more continuous
magma supply. Slow-spreading ridges tend to have rugged terrain, deeper central valleys and more extensive faulting.

Mid-Ocean Ridges

Mid-ocean ridges are enormous submarine mountain systems marking divergent plate boundaries. Together they form a
connected volcanic and tectonic network extending through the Atlantic, Pacific, Indian and Southern oceans.

How Mid-Ocean Ridges Form

As tectonic plates separate, hot mantle material rises beneath the boundary. Partial melting supplies magma, while
faulting and volcanic eruptions construct new crust.

Although called ridges, these structures are not simple narrow crests. They can include axial valleys, volcanic cones,
lava fields, fissures, faults, hydrothermal vents and uplifted flanks.

The Mid-Atlantic Ridge

The Mid-Atlantic Ridge separates tectonic plates in the Atlantic Ocean. It includes slow-spreading segments, transform
offsets and volcanic islands where the ridge rises above sea level.

The East Pacific Rise

The East Pacific Rise is a relatively fast-spreading ridge system. Its greater magma supply generally produces smoother
topography near the ridge axis than many slow-spreading Atlantic segments.

Ridge Segmentation

Mid-ocean ridges are divided into segments by transform faults and overlapping spreading centers. Individual segments
can differ in magma supply, crustal thickness, volcanic activity and hydrothermal circulation.

Hydrothermal Vents

Seawater enters fractures in newly formed oceanic crust, becomes heated near magma and reacts chemically with the rock.
The altered fluid may return to the seafloor through hydrothermal vents, carrying dissolved metals and minerals.

Explore these environments in:
Hydrothermal Vents Explained.

Abyssal Plains

Abyssal plains are broad, remarkably flat regions of the deep-ocean floor. They commonly occur between continental
margins and mid-ocean ridges.

Why Are Abyssal Plains So Flat?

Newly formed oceanic crust is rough and volcanic. Over time, fine marine particles, biological remains, windblown dust,
volcanic ash and sediment transported from continents accumulate across the seafloor.

Turbidity currents and deep-sea sediment flows fill depressions and bury irregular volcanic terrain, producing extensive
smooth surfaces.

What Covers an Abyssal Plain?

Sediment may include:

• clay minerals transported from continents;
• microscopic shells of plankton;
• volcanic ash;
• windblown desert dust;
• material carried by turbidity currents;
• chemical precipitates;
• meteorite and cosmic-dust particles.

Are Abyssal Plains Lifeless?

No. They support diverse communities of worms, crustaceans, sea cucumbers, microorganisms and other deep-sea organisms.
Food is limited, however, because most organic matter must descend from surface waters or arrive through localized
transport processes.

Abyssal Hills

Beneath or between sedimented plains are smaller ridges and hills produced by faulting and volcanic construction near
spreading centers. These abyssal hills are among the most widespread landforms on Earth.

Ocean Trenches

Ocean trenches are long, narrow depressions marking locations where a tectonic plate bends downward into a subduction
zone. They contain the deepest known parts of the ocean.

How Ocean Trenches Form

When two plates converge, the denser oceanic plate may begin descending beneath another plate. As it bends downward,
a deep trench develops near the plate boundary.

Trenches are part of larger subduction systems that may also include:

• an outer-rise region;
• normal faults in the bending plate;
• an accretionary wedge;
• a forearc basin;
• a volcanic island arc or continental volcanic chain;
• deep and shallow earthquake zones.

The Mariana Trench

The Mariana Trench in the western Pacific contains the deepest known oceanic depressions. It formed where the Pacific
Plate subducts beneath the smaller Mariana Plate.

Why Are Trenches Not Filled With Sediment?

Some trenches contain thick sediment, while others remain relatively sediment-starved. Sediment supply, nearby rivers,
ocean currents, tectonic deformation and the rate of subduction all influence trench fill.

Earthquakes and Tsunamis

Large subduction-zone earthquakes can suddenly displace the seafloor and generate tsunamis. However, not every trench
earthquake produces a tsunami, and not every tsunami originates at a trench.

Learn more in:
Subduction Zones Explained.

Fracture Zones and Transform Faults

Fracture zones are long linear features crossing the ocean floor. They form because mid-ocean ridge segments are offset
from one another.

Transform Fault vs Fracture Zone

The terms are related but not identical.

• A transform fault is the active plate boundary between two offset ridge segments. Earthquakes occur
  because plates slide past each other.
• A fracture zone is the inactive continuation of that structure beyond the ridge segments. Crust on
  both sides generally moves in the same direction, but differences in age and elevation remain preserved.

Why Fracture Zones Form Long Seafloor Scars

Oceanic crust on opposite sides of a fracture zone may differ in age, temperature and thickness. Older crust is colder
and lies deeper than younger crust, creating long steps or ridges across the seafloor.

Fracture Zones and Ocean Circulation

Deep gaps along fracture zones can guide bottom-water flow across mid-ocean ridges. They therefore influence not only
geology but also deep-ocean circulation and sediment transport.

Seamounts

Seamounts are underwater mountains, most commonly formed by volcanic activity. They may rise thousands of meters above
the surrounding seafloor without reaching the ocean surface.

How Seamounts Form

Seamounts can develop in several tectonic settings:

• above mantle hotspots;
• along mid-ocean ridges;
• near subduction-related volcanic arcs;
• along fractures and zones of crustal weakness;
• within large submarine volcanic provinces.

Seamount Chains

As a tectonic plate moves over a relatively persistent magma source, a chain of volcanoes may form. Younger volcanoes
occur near the active source, while older seamounts are carried away, cool and subside.

Can Seamounts Become Islands?

Yes. If repeated eruptions build a volcanic edifice high enough, it may emerge above sea level as an island. Erosion,
landslides, wave action and subsidence may later reduce it below the surface again.

Seamount Ecology

Seamounts alter ocean currents and provide hard surfaces in otherwise sediment-covered regions. Their slopes may support
corals, sponges, fish and other marine communities.

Seamount Hazards

Potential hazards include submarine eruptions, flank collapses, earthquakes and landslides. However, only a small
proportion of known seamounts are actively erupting at any given time.

Explore their volcanic origins in:
Submarine Volcanoes and Seamounts Explained.

Guyots

Guyots are flat-topped submarine mountains. They are generally interpreted as former volcanic islands or seamounts whose
summits were eroded near sea level before the structures subsided beneath the ocean.

How Guyots Form

1. A submarine volcano grows upward.
2. The volcano may emerge as an island.
3. Waves erode and flatten its summit.
4. The underlying oceanic plate cools and subsides.
5. The eroded platform sinks below sea level.
6. Marine sediments or coral deposits may accumulate on top.

Why Do Oceanic Islands Sink?

Oceanic crust cools, contracts and becomes denser as it moves away from a mid-ocean ridge. The plate gradually lies
deeper, carrying old volcanoes downward with it.

Guyots and Coral Atolls

In warm, shallow water, coral reefs may grow around a subsiding volcanic island. If reef growth keeps pace with
subsidence, a ring-shaped atoll may remain after the volcanic island disappears beneath the sea.

What Guyots Reveal

Guyots record former sea levels, plate movement, volcanic histories, subsidence and the changing position of tectonic
plates through time.

Submarine Canyons

Submarine canyons are deep, steep-sided valleys cut into continental shelves and slopes. Some resemble river canyons on
land but occur entirely beneath the sea.

How Submarine Canyons Form

Several processes may contribute:

• erosion by turbidity currents;
• submarine landslides;
• sediment-laden density flows;
• river incision during periods of lower sea level;
• groundwater discharge;
• faulting and tectonic deformation;
• repeated collapse of canyon walls.

Turbidity Currents

A turbidity current is a dense mixture of water and suspended sediment flowing downslope under gravity. Such currents
can move rapidly, erode channels and transport sediment far into the deep ocean.

As a turbidity current slows, it deposits layered sediment known as a turbidite. Repeated events can
build enormous deep-sea fans.

Can Submarine Canyons Extend From Rivers?

Yes. Some begin near river mouths and channel sediment from continental sources into deep basins. Others are disconnected
from modern rivers and may reflect older sea levels, tectonic structures or entirely submarine erosion.

Why Submarine Canyons Matter

They transport sediment, nutrients, organic carbon and human-made debris into the deep sea. They can also guide powerful
flows capable of damaging seafloor cables and infrastructure.

Seafloor Pockmarks

Pockmarks are crater-like depressions formed when fluids or gases escape through seabed sediments. They range from small
pits to large structures hundreds of meters across.

What Causes Pockmarks?

Possible causes include:

• methane migration;
• groundwater discharge;
• pore-water expulsion during sediment compaction;
• gas-hydrate dissociation;
• hydrothermal fluid flow;
• buried organic matter generating gas;
• leakage along faults and fractures.

How Does Fluid Escape Create a Crater?

Upward-moving gas or water can disturb sediment, remove fine particles and weaken the seabed. Repeated seepage or a
sudden release may excavate a depression.

Are All Pockmarks Active?

No. Many are relic structures produced during earlier episodes of fluid escape. Determining whether a pockmark is active
requires measurements of gas bubbles, chemical anomalies, temperature, microbial communities or sediment movement.

Pockmarks and Methane Seeps

Some pockmarks host cold seeps where methane-rich fluids reach the seafloor. Specialized microorganisms consume methane,
supporting communities of clams, tubeworms and other organisms.

Explore this process in:
Methane Seeps Explained.

Seafloor Scars, Grooves and Unusual Tracks

Sonar maps often reveal long grooves, furrows, scrape marks, trenches and scars crossing the seabed. These features can
have many different origins.

Iceberg Plough Marks

In polar and formerly glaciated regions, the deep keels of floating icebergs can scrape across shallow seabeds. The
resulting grooves may extend for kilometers and persist long after the iceberg has melted.

Submarine Landslide Scars

When sediment masses fail and move downslope, they leave headwall scarps, evacuation zones, channels and hummocky
deposits. Some slides involve enormous volumes of material.

Trawl Marks and Human Disturbance

Fishing gear dragged across the seabed can produce linear scars that resemble geological grooves. Pipelines, anchors,
dredging and cable installation also modify the ocean floor.

Fault Scarps

Tectonic movement can displace the seabed along faults, producing steps or scarps. Fresh fault traces may reveal recent
earthquakes or active deformation.

Current-Generated Furrows

Strong bottom currents can erode sediment into parallel grooves, moats and channels. These features may record persistent
deep-water circulation.

Sediment Waves

Sediment waves are large, wave-like structures on the seafloor. They may resemble underwater dunes, but their size,
composition and origin can vary significantly.

How Sediment Waves Form

Sediment waves may develop through:

• bottom-current transport;
• turbidity currents;
• internal ocean waves;
• repeated sediment deposition;
• contour-following deep-water currents;
• instability within sediment flows.

Contourite Deposits

Deep currents flowing along continental slopes can erode, transport and redeposit sediment. The resulting deposits,
called contourites, may form drifts, moats and wave fields.

Do Sediment Waves Move?

Some migrate gradually as sediment is eroded from one side and deposited on the other. Others remain largely fixed or
record ancient current conditions.

Why Sediment Waves Matter

Their geometry can help reconstruct past ocean circulation, sediment supply and deep-water flow. They may also affect
the stability of pipelines, cables and offshore structures.

Marine Sediments

Much of the ocean floor is covered by sediment rather than exposed rock. These deposits come from continents, marine
organisms, volcanoes, chemical reactions and material from space.

Terrigenous Sediment

Terrigenous sediment originates on land. Rivers, wind, glaciers and coastal erosion deliver sand, silt and clay into the
ocean.

Biogenous Sediment

Biogenous sediment consists largely of shells and skeletal remains from microscopic marine organisms. Calcium-carbonate
and silica-rich particles can accumulate as deep-sea ooze.

Hydrogenous Sediment

Hydrogenous deposits precipitate directly from seawater or pore fluids. Examples include manganese nodules, mineral
crusts, evaporites and some hydrothermal deposits.

Volcanogenic Sediment

Submarine and subaerial eruptions supply ash, glass fragments, pumice and other volcanic material to the oceans.

Cosmogenous Material

Tiny meteorite fragments and cosmic dust continuously settle through the atmosphere and water column, forming a very
small but scientifically valuable component of marine sediment.

Why Seafloor Sediments Matter

Sediments preserve fossils, chemical signals, volcanic ash layers, magnetic records and evidence of past climate. Core
samples allow scientists to reconstruct changing ocean temperatures, ice-sheet activity and biological productivity.

Submarine Landslides

Submarine landslides occur when sediment or rock moves downslope beneath the ocean. They can range from small local
failures to events involving hundreds or thousands of cubic kilometers of material.

What Triggers Submarine Landslides?

• earthquakes;
• rapid sediment accumulation;
• gas or fluid overpressure;
• volcanic growth and flank instability;
• erosion at the base of a slope;
• gas-hydrate changes;
• storm-wave loading in shallow water;
• weak sediment layers.

Can Submarine Landslides Cause Tsunamis?

Yes. A rapidly moving mass can displace water and generate a tsunami. Landslide-generated waves may be especially large
near the source but often behave differently from tsunamis produced by major megathrust earthquakes.

Debris Flows and Turbidity Currents

A failing sediment mass may transform into a debris flow or turbidity current as it moves. These flows can travel long
distances across the ocean floor.

Infrastructure Risks

Submarine landslides and sediment flows can break telecommunications cables, damage pipelines and threaten offshore
platforms.

Hydrothermal Vents, Cold Seeps and Fluid Flow

Fluids circulate through fractures and sediments across the ocean floor. Their temperature and chemical composition
vary depending on the geological setting.

Hydrothermal Vents

Hydrothermal vents are generally associated with heat from magma or hot crust. Seawater penetrates fractured rock,
becomes heated, reacts with minerals and returns to the seabed carrying dissolved chemicals.

Cold Seeps

Cold seeps release fluids near ambient seawater temperature. They commonly occur where methane, sulfide-rich water,
hydrocarbons or groundwater migrate through sediment.

Mud Volcanoes

Submarine mud volcanoes form where pressurized fluids, gas and fine sediment rise through the seabed. They can build
cones, domes and flow deposits without molten magma.

Gas Hydrates

Gas hydrates are ice-like solids containing gas molecules, commonly methane, trapped within water-crystal structures.
They remain stable under suitable pressure and temperature conditions within marine sediments and beneath permafrost.

How Scientists Map the Ocean Floor

Directly seeing the entire seafloor is impossible because most of it lies beneath deep, dark water. Scientists combine
several technologies to create bathymetric maps and investigate seabed geology.

Single-Beam Sonar

A single-beam echo sounder sends a sound pulse downward and measures how long the echo takes to return. Water depth is
calculated from travel time and sound velocity.

Multibeam Sonar

Multibeam systems send out a fan of sound pulses, measuring a wide strip of seafloor beneath a vessel. This produces
detailed three-dimensional maps of ridges, trenches, faults, canyons, landslides and volcanic features.

Side-Scan Sonar

Side-scan sonar images the texture and reflectivity of the seabed. Hard surfaces, objects, sediment patterns and
geological structures produce distinctive acoustic shadows and reflections.

Satellite Altimetry

Satellites measure subtle variations in sea-surface height caused by gravitational differences. Large underwater
mountains and trenches slightly alter the local gravity field and therefore the shape of the ocean surface.

Satellite data provide broad global coverage but do not match the detail of ship-based multibeam mapping.

Seismic Reflection

Seismic systems send sound energy into the seabed and record reflections from buried layers. The method reveals sediment
thickness, faults, fluid pathways, buried channels and structural traps.

Seismic Refraction

Refraction studies use the speed and path of seismic waves to investigate deeper crustal structure and estimate the
properties of oceanic crust and upper mantle.

Remotely Operated Vehicles

Remotely operated vehicles, or ROVs, carry cameras, lights, sampling tools and scientific sensors. They allow direct
observation of vents, seeps, lava flows, biological communities and seafloor structures.

Autonomous Underwater Vehicles

Autonomous underwater vehicles can travel close to the seabed without a continuous physical connection to a ship. They
produce high-resolution sonar maps and collect chemical, magnetic and photographic data.

Deep-Sea Drilling

Scientific drilling retrieves rock and sediment cores from beneath the seabed. These samples provide direct evidence of
crustal composition, climate history, microbial life and tectonic processes.

Why Is So Much of the Seafloor Still Poorly Mapped?

The oceans cover most of Earth’s surface, and detailed sonar mapping requires ships to travel systematically across
immense areas. Deep water, rough seas, remote locations and high operating costs slow the process.

Satellite gravity data provide a broad approximation of global seafloor topography, but they cannot reveal every small
canyon, volcanic cone, fault scarp or pockmark.

Mapping quality also varies. Some regions are surveyed at very high resolution, while nearby areas may rely on sparse
ship tracks or indirect satellite estimates.

Mapping Artifacts

Online bathymetric maps sometimes display straight lines, rectangular patches or abrupt changes in detail. These are
often artifacts created where high-resolution ship surveys are combined with lower-resolution background data.

Such patterns do not necessarily represent real seafloor structures.

Ocean Geology and Natural Hazards

Geological processes beneath the ocean can produce earthquakes, tsunamis, volcanic eruptions, landslides and sudden
changes in seabed elevation.

Subduction-Zone Earthquakes

Many of the world’s largest earthquakes occur beneath the ocean along subduction zones. Sudden fault movement can
displace the seafloor and generate basin-wide tsunamis.

Submarine Volcanic Eruptions

Underwater eruptions may produce lava flows, pumice rafts, ash clouds, explosions, hydrothermal plumes and temporary or
permanent islands.

Submarine Landslides

Slope failures can damage infrastructure and generate local tsunamis. They may also transform into long-traveling
turbidity currents.

Gas and Fluid Release

Methane seepage and gas-hydrate destabilization can weaken sediments locally. However, dramatic claims about sudden
global methane explosions should be evaluated cautiously and against geological evidence.

Seafloor Infrastructure

Telecommunications cables, pipelines, offshore energy systems and scientific observatories can be threatened by
earthquakes, currents, volcanic activity, landslides and sediment flows.

How Ocean Geology Influences Climate and Ocean Circulation

Seafloor topography guides deep currents, controls passages between ocean basins and influences the movement of heat,
nutrients and dissolved gases.

Ridges and Deep-Water Passages

Mid-ocean ridges obstruct deep circulation, while fracture-zone valleys provide passages through which dense bottom
water can flow.

Volcanic and Hydrothermal Inputs

Underwater volcanism and hydrothermal systems exchange heat and chemicals with seawater. They influence local ocean
chemistry and support ecosystems but do not heat the global ocean uniformly.

Sediment Records of Climate

Marine sediments preserve changes in ocean temperature, ice volume, productivity, dust transport and atmospheric
circulation. Researchers analyze microfossils, isotopes, magnetic properties and chemical markers within sediment cores.

Tectonic Gateways

The opening or closing of ocean passages can reorganize global circulation. Continental drift therefore influences
climate over geological timescales.

Geological Resources on the Ocean Floor

Marine geological environments contain hydrocarbons, mineral deposits, sand, gravel, phosphorites and metal-rich crusts.
Their extraction presents technological, environmental and legal challenges.

Polymetallic Nodules

Polymetallic nodules are mineral-rich concretions that grow slowly on parts of the deep seafloor. They may contain
manganese, nickel, cobalt, copper and other metals.

Seafloor Massive Sulfides

Hydrothermal vents can deposit copper-, zinc-, iron- and sulfide-rich minerals around vent fields.

Cobalt-Rich Crusts

Metal-rich mineral coatings can grow on exposed rocks along seamounts and ridges.

Oil and Natural Gas

Thick sedimentary basins along continental margins may contain buried organic matter transformed into oil and natural
gas.

Environmental Concerns

Deep-sea mining could disturb habitats, create sediment plumes, generate noise and affect organisms that recover very
slowly. Scientific understanding of many deep-sea ecosystems remains incomplete.

Ocean-Floor Mysteries: Myths vs Geological Evidence

“The Ocean Floor Is Mostly Flat”

False. Sedimented abyssal plains can be flat, but the global seafloor also contains immense ridges, volcanoes, trenches,
fault scarps and canyons.

“Straight Lines on Bathymetric Maps Are Ancient Roads”

Usually false. Many straight lines are ship-survey tracks or boundaries between datasets of different resolution.

“Every Circular Hole Is an Impact Crater”

False. Circular seafloor depressions may be pockmarks, volcanic craters, collapse structures, sinkholes or mapping
artifacts.

“All Underwater Mountains Are Active Volcanoes”

False. Many seamounts are extinct, heavily eroded, buried or far from active magma systems.

“Submarine Canyons Were All Cut by Ancient Rivers”

False. Some were influenced by rivers and lower sea levels, but turbidity currents, landslides, groundwater flow and
tectonics also create and deepen submarine canyons.

“We Have Never Mapped the Seafloor”

False. The broad shape of the global seafloor is known, but detailed high-resolution mapping remains incomplete.

Famous Ocean-Floor Features