4(1)
Continental Margins & Ocean Floors (Ch 4) I. Shape of ocean floors
II. II. Continental margins
I. Shape of ocean floors A. Simple view: Ocean floors in general
separate continents from continents, act like conveyor belts (what???
you exclaim) pulling continents apart and smashing them together
Fig 4.4
B. Ocean floors not shaped like a giant bathtubs, not deepest in
center (Challenger expedition soundings proved this (Fig 3.19. 4.2)
• Generalized view/main features of ocean floor Flgs 4.2 4.3
-Ridges near centers
-Trenches near edges
C. Ocean floor shape created by plate tectonics processes (which
we'll address next week)
-Bottom line (main driving mechanism) as to why ocean is where it is:
-Oceanic crust is composed of dense basalt, which is pulled
closer to the center of the earth by gravity than lighter, granitic ma
a of granite", continental crust.
Therefore, ocean floors are depressed areas relative to
comments, and can hold water
(Tupperware demonstration here..dense container floats lower in
water than light container.. like continental crust & oceanic crust both
"float" on viscous, molasses-like asthenosphere )
D. In reality, ocean floors can be separated into 2 regions. (Fig 4.3)
1. Continental margins (submerged outer edges of
continents, underlain by granitic crust)
2. Ocean basins (underlain by basaltic
crust)
4(2)
II. Continental margins
A. 2 types of margins (Fig 4.4)
1. Passive quiet, divergent; moving passively away from
oceanic spreading centers; ("Atlantic-type")
2. Active earthquake- and volcano-prone, convergent; moving
toward active plate margins; ("Pacific-type")
B. Margins subdivided into three regions
(examples from passive & active margins sli. different)
Fig 4.3 &-Fig 4.4 also WWM4.1
1 . Continental shelf (shelf is underwater extension of continent)
&Passive margin shelf
features:
-Surface usually quite flat, gently dipping seaward (eg, dip
only 9 feet/mile on Atlantic margin)
-Wice (eg, 220 mi on Atlantic coast) Fig 4.4
-tend out to +/-500' water depth
-Passive margins often thickly blanketed w/ sedimentary rx
(40,000-50,000 ft thick) deposited over tens of millions of years
as continent erodes
•Active margin shelf features: Fig.
4.4 4.4
-Surface steeper than passive shelf
-Narrower than passive shelf (Fig 4.4)
-Topography more varied aside ...Shelves
vary in sensitivity to sea level change
Whan they're relatively flat, small change in sea level makes
big_difference in shoreline position on land
Demonstrate here ..draw picture showing diff slopes of
passive vs active shelves)
-Transition from shelf to slope = "shelf-slope break" Fig WWM 4.1
(+/- 500' water depth) marks relatively abrupt break between
continental shelf & continental slope
4(3)
2. Continental slope (steep slope, offshore; Slope is transition
between continental shelf & deep-ocean floor) ( Fig 4. WW 4.1)
-Relatively steep dip (144 ft/mi)
-Relatively narrow (avg 13 mi wide)
-Start in shallow water (+/- 500'), end in deep water (+/- 12,000')
-Bottom of cont. slope =edge of continent (WWM 4.1)
3. Continental rise (again, Figs 4.3 & VWM 4.1)
-Rise is sediment prism found at base of slope on passive
(quiet) margins, often a result of coalescence of deep-sea submarine
fans (Fm e n t transported through submarine canyons, which are
cut into continental margins starting at river mouths (eg, Hudson R.
canyon or Congo R. canyon) (Fla 4.9)
-Origin of submarine canyons due to:
-River erosion during low sea level stands (draw pic)
-Submarine erosion caused by turbidity currents (Fl a.
4.11), dense sediment/water mixtures that can move at
high velocities w/ great erosive power
•Story of Grand Banks Trans-Atlantic cable breaks Nov 1929 - eqktriggered turbidity current ....breaks at 3:30, then 6:35 (100 mi away),
then next morning, then later the next day; orig velocity calculated as
high as 60 mph. later revised to 17 mph (still pretty fast ..... )
4(4)
Continental Margins & Ocean Floors (Ch 4)
Chap 4 - Deep Ocean Basins
I
Deep-ocean basins - cover > 50% of Earth surface
II. Hypsographlc curve
-Features of basins: A. Oceanic ridge (Figs 4.13. 4.14. 4.15.
-Definition & genesis: Mountain chain located at active
oceanic spreading center
-Extent: 41,000 mi (all) -Height: 6500' +/Width: Depends upon rate of spreading;
Guiding principle here: things contract-except water (become
smaller in volume) as they change state from warm to cold
temperature
FIG WWM 4.2
- warm rx have HIGH VOLUME, LOW DENSITY
-cold rx have LOW VOLUME, HIGH DENSITY
Fast spread = wide ridge - rx stay warm some distance out
from spreading center & thus don't contract right next to
center Slow spread = narrow ridge - rx cool off & contract
close to spreading center
Analogy: Warm loaves of bread coming out of oven on
conveyor belt -(faster the belt goes, the further away from
oven the bread goes before it cools'
4 (5)
Associated features w/ rides
• Transform fault Fig 4.14-4.15
-Vertical plane along which roc masses slide past each other
-Connects offset axis of ridge system
-Accommodates different rates of spreading between ridges
(eg, can connect a 'last" center w/ a "slow" one) -Source
of shallow eqks
(World Seismicity Map?)
-Fracture zone (Fig 4.1
Seismically inactive, past transform faults
•Hydrotharmal vents (Fig 4.15)
-Fissures or chimneys which release hot water which has been
"superheated" by hot rx
-Warm water around vents usu 46°-61 ° F, vs normal 37°-39`
-Can be as hot as 660° F
-Home to unusual community of life
B. Abyssal plains (Fla 4.14
-Lie between ocean ri gas & continental margins
•Deef. (12,000-18,000')
-Flat
Oovered w/ up to 3000' of sediment
c.. Abyssal hills (E 4.1
-Lie between ocean ridges
& continental margins
-Deep (12,000-18,000’)
•May represent initial stag %s of vulcanism that led to
sea-floor spreading
D. Trenches Fig s 4.17, 4.18)
-Location were one oceanic plate slides underneath another
-Very active; locus of most BIG eqks (deep, powerful; can
trigger tsunamis ("tidal waves"))
-Dips of trench walls can show direction of subduction (Fig 3.21)
-Steepest wall is on side of overriding plate
4(6)
e. Island arcs (Fig 3.21)
@Volcanic mountains fed by magmas that are created by
heating & melting of rx and sediments as they descend into hot
asthenosphere
*Especially prevalent around the Pacific Rim ("Ring of
Fire")
F. Saamounts & Guyots (Fig 3.2
•Ssarnounts are submerge pointed islands that are extinct
volcanoes
•Guyots ("gee- ohs") are flat-topped seamounts that have had
their tops planed off, probably due to erosion which was due to
wave action ' '
I. Hypsographic curve (Fi~4.1
*Shows distribution of Earth
in °/ area vs elevation
*interesting statistics: ->50% of Earth surface is under
10,000' b,: ow sea level
1,/g depth of ocean=12,430' below sea level vs avg
continent height=2,760' above sea level –Reason fo r
lower height of ocean (as mentioned at beginning of
Lec 3):
-Basalt dense, granite light; gravity pulls basalt closer to
center
of Earth than granite, so top of basalt crust os lower than top
of
granite crust (remember the Tupperware
experiment??)