The Seafloor and

Continental Margins

Learning Goals

•Introduce main features of the seafloor

•Learn how we study the surface and subsurface

of the seafloor

•Understand what the various features tell us

about Earth processes

•History of the oceans and some smaller seas

10.00.a1

Observe the seafloor around Monterey Bay, California

Monterey Bay

Monterey

submarine

canyon

Fault

cutting

seafloor

10.00.a2

Observe this geologic map

of the Monterey Bay

area

Cenozoic marine

sediments in

yellow, tan,

brown

Deep part of canyon is

hard Mesozoic granite

Faults cut

across seafloor

10.01.a

How We Study the Seafloor

Use submersibles to observe

and collect rocks

Use sound waves

to map depths

Use ships to drill

holes in ocean floor

10.01.b

What We Can Learn from Drilling

Fossils (age and environment)

Type of sediment or rock

Determine isotopic ages and can get rate of deposition =

thickness/time span

10.01.d1

Observe this seismic-reflection profile of the ocean

floor. Can you see each feature that is labeled?

10.02.a

Earth’s Magnetic Field

Normal magnetic field

Reversed magnetic field

Liquid outer core contains

convection currents

10.02.c

Observe how

magnetic

reversals are

recorded by a

mid-ocean ridge

Time 1:

normal

polarity

Time 2:

reversed

polarity

Build series

of magnetic

stripes over

time

10.02.d

Magnetic Stripes on Seafloor

10.03.a1

Observe what features are present in mid-ocean ridges

Asthenosphere

Lithospheric

mantle

Form a consistent

sequence of rocks in

oceanic crust

Fissure eruptions

in rift

Dikes

Magma

chamber

Abyssal plain

Seamount

Mid-ocean

ridge

Accretionary

prism

10.04.a1

Observe important features of the deep seafloor

10.04.a2

Observe sediment thicknesses on the ocean floor (red is

thickest; white is thinnest). What settings have the

thickest or thinnest sediment?

Thinnest near

mid-ocean

ridges

Thickest along

passive continental

margins

Thick

offshore of

large rivers

10.04.a

Observe a map showing depth of the seafloor

Age increases

systematically

out from ridge

Mid-ocean ridges

less deep

because young

Deepest seafloor

is oldest

Age patterns

truncated at

trenches

Depth (dark is deep)

Age (orange is young)

Compare the relationship between depth and age

10.05.a

Observe how flat-topped

seamounts form

Volcano rises above

the sea as

an island

Eruptions of

lava onto seafloor

Top of mountain

beveled off by

waves; island

subsides below sea

10.05.b

Rising mantle

plume at hot

spot

Kerguelen oceanic

plateau

Submarine flood

basalts pour onto

seafloor

Plateau forms

over several

million years

Oceanic Plateaus

Asthenosphere

Lithospheric

mantle

Asthenosphere

10.06.a1

Observe the processes that form island arcs

Melting of

mantle

10.06.b1

Observe what happens in front of

and behind an island arc

Aleutian island arc

Aleutian

trench

Siberia Alaska

10.06.c

More surface

area on

outside than

at depth

Earth is a sphere,

not a flat plane

Why Are Island Arcs Curved?

Philippines

Japan

Mariana

Aleutian

Tonga

Sumatra-

Java

Lesser

Antilles

Scotia

Aegean

10.06.d1

Observe the location of the main island arcs

Bering Sea: New subduction

farther from coast

Sea of Japan:

Back-arc rifting

Philippine Sea:

Back-arc spreading

Asia

Australia

10.07.a

Observe some smaller seas of the Pacific

North Sea: Continental

rifting during formation of

Atlantic

Persian Gulf:

Loading by

thrust sheets

during collision

10.08.a

Observe how smaller seas near Eurasia formed

Red Sea: Rifting then

early stages of

seafloor spreading

Fringing reef

Barrier reef

Atoll

10.09.a

Observe where coral reefs form

Great Barrier Reef

10.09.b1-3

Observe one way an atoll forms

Island cools

and sinks but

reef continues

to build upward

toward light

Volcanic island

forms, followed

by formation of

fringing reef

Volcano sinks

below sea

level, leaving

reef as atoll

10.09.c1

Observe the location of reefs around the world

Hawaii

Polynesia

Yucatan

Florida and

Bahamas

Lesser

Antilles East

Africa Indian

Ocean

Red Sea

Great Barrier

Reef

Indonesia Philippines

Micronesia

Sediment Normal

faults Thick

sediment

Continental

slope

Abyssal

plain

10.10.a

Observe the

features and

structures of

continental

margins

Continental

shelf Continental

slope

Continental

rise

Submarine

canyon

10.10.c

Underwater Slope Failures

Deposition in

submarine fans

Turbidity current Graded beds

Underwater

landslides on

continental slope

and flanks of

islands

10.10.c

Submarine Canyons

River and turbidity

currents eroded into

continental shelf

Turbidity currents erode into

continental slope

10.11.a1

Broad flat areas

adjacent to sea

Bodies of water

with evaporation

Behind barrier that

restricts influx of

seawater

Observe some settings that can deposit salt

10.11.c

What Structures Do Salt Deposits Form?

Salt Domes

Faulting and

folding over

weak salt

layer

10.11.d1

Observe the setting of salt structures along the Gulf

Coast of the United States. Salt is shown in black.

Thick layer of salt deposited

during evaporation of seawater Subsurface flow of

salt disrupts surface

Pressure of overlying

rocks causes salt to

flow up and sideways

10.12.a1

200 m.y. Ago: End of Pangaea

Gondwana

intact

Central Atlantic forms

as NAM-Africa rift

One global

ocean

10.12.a2

150 m.y. Ago: New Oceans Open

Gondwana

starts rifting

Central

Atlantic

open

10.12.a3

120 m.y. Ago: Dispersal of Gondwana

NAM – Europe still joined

Southern

continents

mostly

outlined

10.12.a4

90 m.y. Ago: Atlantic Ocean Open

Gondwana

continents isolated

NAM rifts from

Europe, forming

North Atlantic

10.12.a5

30 m.y. Ago: Closing Tethys Sea

India collides Asia

Tethys nearly

closed

10.12.a6

Present Day

Predict what will happen in the future to each ocean

10.14.a1

Investigation: How Did These Ocean Features and

Continental Margins Form?

10.14.a

Location of Cross Section

Location of

cross section A

A'