Exploring Geology
Chapter 10
The Seafloor and
Continental Margins
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Observe the seafloor around Monterey Bay, California
Monterey Bay
Monterey
submarine
canyon
10.00.a1
Fault
cutting
seafloor
Observe this geologic map
of the Monterey Bay
area
Cenozoic marine
sediments in
yellow, tan,
brown
Faults
interpreted to
cross seafloor
Deep part of canyon is
hard Mesozoic granite
10.00.a2
How We Study the Seafloor
Use sound waves
to map depths
Use submersibles to observe
and collect rocks
Use ships to drill
holes in ocean floor
10.01.a
*What We Can Learn from Drilling
Type of sediment or rock
Fossils (age and environment)
Determine isotopic ages and can get
rate of deposition = thickness/time span
10.01.c
Observe this seismic-reflection profile of the ocean
floor and find each feature that is labeled
10.01.d1
*Earth’s Magnetic Field
Normal magnetic field
Liquid outer core contains
convection currents
Reversed magnetic field
10.02.a
Time 1:
normal
polarity
*Observe how
magnetic
reversals are
recorded by a
mid-ocean ridge
during seafloor
spreading
Time 2:
reversed
polarity
Build series
of magnetic
stripes over
time
10.02.c
*Magnetic Stripes on Seafloor
Calculate spreading rate in km/million years (keep it simple)
Example: the area shown below is 50 km wide and the oldest crust
age is 2.5 million years (50km/2.5 million=20 km per million years)
10.02.c
Observe features that are present in mid-ocean ridges
Fissure eruptions
in rift
Fissures that form dikes
Form a consistent
sequence of rocks in
oceanic crust
Magma
chamber
Lithospheric
mantle
Asthenosphere
10.03.a1
*Observe important features of the deep seafloor
Seamount
Abyssal plain
Mid-ocean
ridge
Accretionary
prism
10.04.a1
*Observe sediment thicknesses on the ocean floor (red is
thickest; white is thinnest). What settings have the
thickest or thinnest sediment?
Thickest along
passive continental
margins
Thinnest near
mid-ocean
ridges
10.04.a2
Thick
offshore of
large rivers
*Observe a map showing depth of the seafloor
Compare the relationship between depth and age
10.04.a
Deepest seafloor
is oldest
Mid-ocean ridges
less deep
because young
Depth (dark is deep)
Age increases
systematically
out from ridge
Age patterns
truncated at
trenches because
of subduction
Age (orange is young)
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; crust cools
and island subsides
below sea
10.05.a
Oceanic Plateaus
Kerguelen oceanic
plateau
Rising mantle
plume at hot
spot
Submarine flood
basalts pour onto
seafloor
Plateau forms
over several
million years
10.05.b
Observe the location of hot spots, linear island chains,
and oceanic plateaus
Iceland
Azores
Ontong Java
Plateau
Hawaii
Canary
Islands
Afar
Galapagos
Tahiti
Easter
Island
Tristan
da Cunha
Kerguelen
Plateau
10.05.c1
*Observe the processes that form island arcs
Asthenosphere
Melting of
mantle
Lithospheric
mantle
Asthenosphere
10.06.a1
*Observe what happens in front of
and behind an island arc
10.06.b1
*Why Are Island Arcs Curved?
Siberia
Alaska
Aleutian island arc
Aleutian
trench
Earth is a sphere,
not a flat plane
More surface area on
outside than at depth so a
slab bends as it subducts
10.06.c
*Observe the location of the main island arcs
Aleutian
Aegean
Japan
Mariana
Philippines
Lesser
Antilles
SumatraJava
Tonga
Scotia
10.06.d1
Observe some smaller seas of the Pacific
Bering Sea: New subduction
farther from coast
Philippine Sea:
Back-arc spreading
Asia
*Sea of Japan:
Back-arc rifting
10.07.a
Australia
*How the Gulf of California Formed
Stage 1:
Subduction
along west
coast, to
later
become a
coastal
transform
fault
Stage 2:
Transform
boundary
and
spreading
centers
jump
inland,
carving off
Baja
10.07.a
Observe how smaller seas near Eurasia formed
North Sea: Continental
rifting during formation
of Atlantic
Persian Gulf:
Loading by
thrust sheets
during collision
*Red Sea: Rifting then
10.08.a
early stages of
seafloor spreading
Observe where coral reefs form
Barrier reef
Fringing reef
Atoll
10.09.a
*Observe one way an atoll forms
10.09.b
Volcanic island
forms, followed
by formation of
fringing reef
Island cools
and sinks but
reef continues
to build upward
toward light
Volcano sinks
below sea
level, leaving
reef as atoll
*Observe the location of reefs around the world
Florida and
Bahamas
Red Sea
Indonesia
Hawaii
Yucatan
Polynesia
Lesser
Antilles
East
Africa
Philippines
Micronesia
Indian
Ocean
Great Barrier
Reef
10.09.c1
*Observe the
features and
structures of
continental
margins
Sediment
Continental
shelf
Submarine
canyon
Normal
faults
Thick
sediment
Continental
slope
Continental
slope
Continental
rise
Abyssal
plain
10.10.a
*Underwater Slope Failures
Turbidity current
Deposition in
submarine fans
Graded beds
Underwater
landslides on
continental slope
and flanks of
islands
10.10.b
Submarine Canyons
Turbidity currents erode into
continental slope
River (due to drop
in sea level during
ice age) and
turbidity currents
eroded into
continental shelf
10.10.b
Observe some settings that can deposit salt
Broad flat areas
adjacent to sea
Bodies of water
with evaporation
Behind barrier that
restricts influx of
seawater
10.11.b1
What Structures Do Salt Deposits Form?
Salt domes
Folded salt layers
in salt dome
Faulting and
folding over
weak salt
layer
10.11.c
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
10.11.d1
200 m.y. Ago: End of Pangaea
Central Atlantic forms
as NAM-Africa rift
One global
ocean
Gondwana
intact
10.12.a1
150 m.y. Ago: New Oceans Open
Central
Atlantic
open
Gondwana
starts rifting
10.12.a2
120 m.y. Ago: Dispersal of Gondwana
NAM – Europe still joined
Southern
continents
mostly
outlined
10.12.a3
90 m.y. Ago: Atlantic Ocean Open
NAM rifts from
Europe, forming
North Atlantic
Gondwana
continents isolated
10.12.a4
*30 m.y. Ago: Closing Tethys Sea
(Atlantic opening; Pacific Shrinking)
Tethys nearly
closed
India collides Asia
10.12.a5
*Present Day
10.12.a6
Predict what will happen in the future to each ocean
Observe the present setting of the Gulf of Mexico
and the Caribbean region
Subduction
beneath
Central Am.
Subduction
beneath
Lesser Antilles
Transform
Complex area
10.13.a1
Jurassic History
(175 m.y. to
130 m.y. ago)
Early rifting
Gulf
opens
Gulf stops
growing
10.13.a
Cretaceous to
Neogene History
(84 to 5 m.y. ago)
Plateau
enters
Caribbean
Cuban
collision
Panama
connection
10.13.a
Investigation: How Did These Ocean Features and
Continental Margins Form?
10.14.a1
Location of Cross Section
A
Location of
cross section
A'
10.14.a