Revised 8 / 06 (Monroe 6th ed.)

The Seafloor - Chapter 12




Plate Tectonics and the Seafloor

The Seafloor: an Introduction

The Transition from Continental to Oceanic Crust

The Abyss: Deep Oceanic Crust

Coral Atolls

Seafloor Sediments

Economic Geology of the Seafloor



Origin of all life was in the sea

Covers 71% of the earth's surface

Only recently have we been able to study the deep oceans

Many methods - can get from book (any questions?)

I especially like the "seismic profiler" (Monroe; Fig. 12-13, pg. 371)

Last 30 years or so there's been lots of interest & exploration

Partly due to military & communications

New technology has been developed to make it possible

...and new technology demands to be used

Submersibles (Alvin, Sea Cliff, etc.)

Newly discovered life at black smokers

Economic interests are increasing

Beyond the original fishing interests

Oil and Gas

Mining possibilities

Volcanogenic massive sulfide deposits

Black smokers again

Manganese nodules

Probably a pipe dream

Excessive exploration and development costs


Plate Tectonics and the seafloor

Already touched on this quite a bit (click here for additional information)

In many ways tonight should be a review

Spreading Centers

Ophiolites (Monroe; fig. 2-19, pg. 55)

Volcanic Arcs & Trench Complexes

Common where two oceanic plates collide

Japanese Islands, and many others in western Pacific

Similar volcanism occurs onshore at Oceanic/continental margins

Andes, Cascades

Active continental margin: the leading edge (Monroe; fig. 12-11, pg. 368)

West coast of North & South America

Leading edge of continental plate

Extensive tectonics

Erosional processes common

Subduction Zone/Trench

Passive continental margin: the trailing edge (Monroe; fig. 12-11, pg. 368)

Both sides of the Atlantic

Trailing edge of Continental plate

Minimal tectonic activity

Depositional processes common


The Sea Floor

Primarily basalt

Less than 200 m.y. old

Generated at spreading centers

Consumed at subduction zones

Descends back into mantle where it undergoes further differentiation

On the road to granite

DIGRESS TO: Granitization, and purification of the earth's crust

There are several layers

Fairly straightforward regional model for seafloor processes

Actually quite complex at the local level

Based on the study of ophiolites

Pieces of seafloor shoved up onto the continental margin

Describe in detail

Refer to Monroe; fig. 12-6, pg. 364

The seafloor is far from flat and featureless

Highly irregular locally

Regionally, however, there is a systematic regional pattern to seafloor features

Due to plate tectonics

The seafloor is so young that there hasn't been time for the major changes associated with older continental terrains

Metamorphism, etc.

Also, its below sea level

Not subject to significant erosion

However, deposition is a continuing problem

Put it all together and it's possible to still observe the reality of recent tectonic events


The Transition from Continental to Oceanic Crust

(Refer to Monroe; fig. 12-7 to 12-10, pgs. 365 to 368)

There are several topographic levels to the seafloor

Globally persistent and definable

Continental Shelves

The relatively low-relief platform seaward from the shore

Usually fairly shallow water

Surrounds most of the continents

Not uniformly wide - varies quite a bit

Average width 50 miles

Range from non-existent to areas nearly 1000 miles wide

REVIEW: Active vs. passive continental margins

Local relief can be somewhat steep

Especially in areas subjected to glaciation

Or to stream erosion at times of lower sea level

Shelf Break - Outer edge of the shelf where it starts down to the oceanic depths

Shelf Dams

Often form the outer boundary of the shelf

Some sort of raised topographic feature

Sediments fill in behind

Several varieties have been recognized

Tectonic dam

Faulted blocks of crustal material

L.A. Basin - Palos Verdes to Catalina to 6 more offshore


Salt domes - common in the western Gulf of Mexico

Biological - coral reefs

Anyway, where they exist...

Can lead to thick deposits of marine sediments landward

Continental Slopes

Connect the Shelf with the deep ocean floor (Abyss)

Actually a fairly gentle gradient

Average slope 4 deg.

This is the AVERAGE gradient

Local relief can be quite substantial (or quite flat!)

Connects the two major levels of the earths surface

The major continental land masses at just above sea level (average!)

And the abyssal depths at 12,000' below sea level

Seems like it should take most of the way to England

12,000 feet X sin (4 deg.) = 172,027 feet

172,027 feet / 5280 feet/mile = 32.58 miles

Surprise! DIGRESS TO: Topographic profiles and vertical exaggeration

Looks steep on most X-section due to vertical exaggeration

Refer to Monroe; fig. 12-9, pg. 367

Submarine Canyons (Monroe; Fig. 12-8, pg. 366)

Characteristic features of the continental slopes

Canyons can have quite a bit of relief - "rival the Grand Canyon"

Origin of the submarine canyons

The formation of these is difficult to explain

Hard to imagine surface erosional processes under the sea

Relatively poorly studied - as is much of the ocean floor

Lots of theories proposed in the past

Early Favorites :

Probably started by sub-aerial erosion processes

During glacial events when sea level was lower

This only accounts for the upper portions of the canyons

Sea level was never low enough to expose the entire length of the longer and deeper canyons

Structural origin (for at least some of the canyons)

Mendocino Fracture Zone & Canyon - off the California coast

Strong bottom currents may help in the erosion of the canyons

Most now agree that Turbidity Currents are primarily responsible

"Density currents of debris-laden water" - DEFINE

Quite viscous

Like a Mud- or Earth-flow

Can move fast and far

Up to 100 kph for distances of up to 700 km

Can be set off by seismic or other disturbances

Example: Grand Banks - off Newfoundland 1929 (Monroe; fig. 12-9, pg. 367)

Earthquake set off a large turbidity current

Severed Trans-Atlantic phone lines

Many cables over 13 hours

Speed of the current 66 ft/sec. (75 kph)

Anyway, all this debris piles up at the mouth of the canyons

Submarine fans

Like alluvial fans in an arid landscape (Monroe; Fig. 18-23, pg. 588)

Continental rise

Coalesced fans (like a bajada in an arid landscape) (Monroe; Fig. 18-24, pg. 588)

Forms the boundary between the slope and the abyssal plain


The Abyss: Deep Oceanic Crust

The basic oceanic depths

Quite a bit of relief (DIGRESS TO: low vs. high relief)

Abyssal Plains

Generally fairly low-relief

Cover large portions of the ocean floor

Most of them probably have at least a thin veneer of sediments (or oozes) covering them

Increasing depth of sediments with distance from spreading center

Abyssal Hills

Topographic mounds on the abyssal plain

Remain well below sea level

Mere "pimples" on the sea floor

Oceanic Ridge systems (Monroe; fig. 2-14, pg. 46) (Monroe; fig. 12-6, pg. 364) (Monroe; fig. 12-12, pg. 370)

Spreading centers for the earth's tectonic plates

Formation of mafic basaltic crust

Can rise above sea level

Iceland, the Azores, Ascension Island

Up to 7500' above sea level (Pico Island in the Azores)

Median valley (Monroe; fig. 12-14, pg. 371)

The actual rift at the crest of the ridge/rise system

General features

High heat flow

High level of seismic activity

Thin crust so generally a lot of small quakes

Extensional environment

Normal faulting

Fluid penetration and hydrothermal vents


Increased depths below the main level of the abyssal plains

Generally long and narrow features - like the ridges

Monroe; Fig. 12-12, pg. 370 and relief map

Associated with subduction zones and volcanic arcs

Represent zones of oceanic plate subduction

Review subduction processes

These are the lowest elevations on earth!

Mariana Trench @ -35,785'

Tonga Trench @ -35,326'

General features

Lower heat flow (relative to spreading centers)

High level of seismic activity

Thicker crust so generally less frequent but potentially greater quakes

Compressional environment

Reverse and thrust faulting


Volcanic mounds on the ocean floor which don't extend above sea level

"Thousands of them"

10,000 alone in the Pacific Basin

Basaltic composition - no surprise here

Commonly occur in clusters or linear arrangements

Related to faulting at spreading centers?

Guyots - originally described by Hess (what a guy)

More puzzling than seamounts

Nearly level submarine plateaus

3000' to 5000' below sea level!

Volcanic origin, but

Some are covered with rounded boulders

Evidence of fossil corals

also fossils of Globigerina

Single-celled surface dwelling organism

Origin is VERY unclear

Almost certainly the result of wave action

Clearly represent erosional surfaces at great depth

Would require remarkable sea level fluctuations, or

Extreme regional subsidence of the sea floor

This is the more likely explanation

Plate tectonics may supply an answer

They were originally islands associated with a spreading center

Like Iceland, Azores

Near enough to the surface to be eroded by wave action

Plate motions transport these ridge features off the topographic high associated with the ridge

And down into the mid-ocean depths

Submarine volcanoes - can be very large

Like the Hawaiian/Emperor chain

Composed of more fluid basalt than their terrestrial counterparts

Mafic vs. intermediate vs. felsic

Called "aseismic" islands

Means "without seismic activity"

Not actually true

Lots of activity, just not as strong as at plate boundaries

Origin of these "mantle plumes" or "hot spots" is unclear

Impacts sites?


Coral Atolls

Ring-shaped islands composed of the skeletons of corals

Original central island is below sea level

Three major stages of development (Monroe; "Reefs," pg. 380)

As defined by Darwin during his voyage on the Beagle

Fringing reefs

Initial development of coral reef near shore

Barrier Reefs

Near complete barrier of coral around an island


The final reef-enclosed lagoon

The island is essentially gone due to subsidence (and erosion)

The 3 types are related to each other in a gradational sequence

Fringe will evolve, with time, through a barrier reef into an atoll

Islands commonly begin on or near ridge axis

Combination of upward coral development and subsidence of the island

The rate of coral growth must keep pace with the rate of sinking of the island

Physical characteristics of atolls

Can get quite large

Kwajelein - 15 miles X 75 miles

Most are far smaller

Generally low relief

The island mass has essentially been eroded to below sea level

Fringing reef made of solid coral

Doesn't extend above sea level

Very exposed to ocean storms

Waves can easily override them

Especially storm waves

Encloses the lagoon and Reef Island

Drilling to test Darwin's Atoll theory

Hypothesis: "If the islands are sinking, then the coral reefs should be extremely thick"

A-Bomb testing required some deep holes

Marshall Islands

Deepest hole was 2516'

In coral all the way

Eniwetok - nearby atoll

Deepest holes were 4152' and 4456'

Penetrated 3936' coral and then seafloor basalt

Dated as Eocene (60 m.y.)

Rate of subsidence not constant

Diminished with time

Average 50 to 170 ft per million years

This testing certainly supports Darwin's theories

At least for the two islands which were drilled

It also introduced several problems, at least as far as the indigenous population was concerned

Extensive atmospheric and underground testing

Possible sites for long-term storage of nuclear wastes

There's another real problem with the assumed origin of the Pacific Atolls

Fluctuating sea level due to glaciation

Complicates the upward growth of the coral reefs

By and large, though, the Darwin model is generally accepted


Seafloor Sediments

Different types of sediments cover most of the ocean floor

Near shore

Terrigenous sediments

Sand and silt predominate on the beaches and Continental Shelf

Facies changes with distance from shore

Dependent on depositional energy


Deep ocean sediments

Initial work by the H.M.S. Challenger (1872-1876)

Earliest (?) work on seafloor (abyssal plains)

Defined the broad picture of seafloor sedimentation (Monroe; fig. 12-20, pg. 377)


Descriptive term which characterizes the majority of deep ocean sediments

Represents accumulations of debris which settles to the bottom

Called Pelagic sediments

Usually microscopic marine organisms

Lack of terrestrial sediments causes them to be concentrated in the deep ocean

Pelagic sediments also occur near shore but are masked by the overwhelming volume of terrestrial debris

Ooze composition varies systematically across the ocean floor

Calcareous oozes

Tropical and temperate seas <15,000' deep

FORAMINIFERA - common source

Single-celled calcium based creature

Like many single-celled organisms, they divide into two individual creatures

The vacated shells sink to the bottom

Forms Calcareous oozes

In cold and/or deep water, the calcium can re-dissolve

Calcium Compensation Depth (CCD)

Siliceous oozes

Single-celled silica based organisms

Radiolaria - animals

Diatoms - plants

Form in deep water where calcium can't remain stable

Or in localized areas where excess silica increases their production

Black and White smokers

Siliceous ooze tends to concentrate in colder/deeper waters

But also have a temperature/pressure where they become unstable and re-dissolve (SCD)

Red/Brown clays - occur in the deepest oceanic basins

Most widespread of all sedimentary deposits on the earth

Almost totally inorganic

Actually terrigenous in origin

Very fine grain

Accumulate at a very slow rate

The only thing which can survive the extreme pressure of the deepest basins

Sediment thickness and rate of deposition varies throughout the ocean

Thickness generally increases away from the spreading ridges

Rate of deposition varies with:

Proximity to the continental land masses

Erosion rate on land

Local proliferation of marine organisms

And the solubility of calcium and silica

Water depth

Calcium to silica to clays

Ocean currents

Some coarse-grained sediments do occur

Usually near the base of the continental slope

Probably the result of turbidity currents

Submarine boulder fields

Probably carried offshore by icebergs (glacial erratics)

These are called Glacial Marine deposits

The deep ocean basins provide an excellent, unbroken record of the recent geologic past

Lots of recent studies of sediment cores taken throughout the ocean depths

The type of cold- or warm-water organisms indicate the relative temperatures

Can be used to date glacial and interglacial events

Indications of at least 6 major glacial periods in the last 400,000 years


Economic geology of the seafloor

Oil & gas

Continental shelf

Manganese nodules (Monroe; fig. 12-19, pg. 376)

Feature of the abyssal depths

Golf-ball to bowling ball sized nodules

Grow very slowly

"Ten layers of atoms per year"

Approx. 3 mm per million years

Contain Mn, Fe, Cu, Ni

Estimated reserves: 1 trillion tons

Not currently economic

Tough mining and political problems

Volcanogenic sulfide deposits

Associated with hydrothermal vents at spreading centers (Monroe; fig. 12-15, pg. 372)

Turner-Albright Massive Sulfide Deposit, Josephine County, Oregon


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