CONTINENTAL CONTINENTAL CONTINENTAL CONTINENTAL

MARGINS & OCEAN MARGINS & OCEAN MARGINS & OCEAN MARGINS & OCEAN

BASINSBASINSBASINSBASINS

CH. 4CH. 4CH. 4CH. 4

Bathymetry

•Study of Ocean floor

•Historically by soundings

•Lower weighted ropes to touch bottom

•(85 BCE – 1914)

•Laborious

•Time consuming

•HMS Challenger

•Steam-power winch•492 soundings = real view of sea floor

1870’s

Multibeam Sonar:

•~121 beams from ship’s hull

•Continuous “swath” of ocean bottom

•200 vessels = 125 yrs. to chart seafloor

Florida

Coast

details

Sidescan Sonar:

•Towed in water

•Closer to seafloor

•Better images

Satellite Altimetry:

•Can’t measure ocean depth directly

•Gravitational pull of seafloor features warp surface

•Estimate topography from sea surface deformations

Pulls H2O

toward it

from sides

Ocean Floor Topography

As varied as continents

Mid-Atlantic Ridge

Ocean Divided Into 2 Zones:

1. Continental margins (CM): submerged, outer edge of continents

• Shelf, Slope, & rise

2. Ocean Basins: seafloor beyond CM

• Abyssal Plain, Trenches, & Ridges

Continental Margin Ocean Basin Continental Margin

Continental Margins Can Be:

• Passive (Atlantic): Divergent plate boundaries, few volcanoes & earthquakes

• Active (Pacific): Convergent & Transform plate boundaries, many volcanoes & earthquakes

West East

S. American Plate

Continental Margin Types:

a. Continental Shelf: shallow, gently sloping extension of continents

• Formed by continental erosion & sediment accumulation (river transport)

• Wide at passive margins (up to 1280 km = 800 mi.)

• Narrow at Active Margins (few km)

Continental Margin Types:

b. Continental Slope: steeply sloping transition from shelf to seafloor

• Structurally part of continents

Shelf break (~140 m)

Submarine Canyons:

• Can cut into Continental Slope & Shelf:

• Earthquakes shake loose mass of sediment that plunges down shelf

OR

•Continuous small amount of debris erodes shelf

Hudson Canyon

(New Jersey)

Earthquake

generated (1929)

“turbidity current”

Continuous sediment

cascade

Some fed by large

rivers

Turbulence mixes

sediments into water

Continental Margin Types:

c. Continental Rise: accumulated sediment at base of continental slope

• Created by:

• Turbidity current deposits at mouths of submarine canyons

•Descending sediment from slope

Deep-Ocean Basins:

• Accumulated sediment (5 km/3 mi deep) over basaltic rock

• Variable topography

Ocean Basin Features:

a.Abyssal Plains: broad flat regions built by sediment deposition

• Depth = 3,700 – 5,500 meters (2.5-3.6 mi.)

•Covers seafloor basaltic rock

•Common in Atlantic

Abyssal Plains:

• That lack a cover of sediments, expose small hills (Abyssal Hills)

• Formed by seafloor spreading

•New crust (magma) coats hills

•Usually < 200 m. (650 ft.) high

Ocean Basin Features:

b. Seamounts: submerged volcanoes

•Form at hotspots or spreading centers

•Guyot: flat-topped seamount

• Wave erosion

Alone or in chains (10-100)

Ocean Basin Features:

c. Oceanic Ridges: mountain chain at active spreading centers

• Where new ocean lithosphere is made

Ocean Ridges:

• Fracture zones (Transform Faults): occur where plate segments are offset

Youngest Rock= slide horizontally

Iceland’s Rift

Valley

Diverging plates

Ocean Basin Features:

d.Hydrothermal Vents: Seafloor Hot spring

•Continuously spews HOT (350°C = 660°F), mineral-rich water

•Most at depths ≥≥≥≥ 2,100 m. (7,000 ft)

•Areas of seafloor spreading (Mid-Ocean Ridges)

•All seawater circulates through seafloor crust

• ~10 million yrs. “Black Smoker”

Central portion of

Ocean Ridge

Cool water

1

seafloor

•Chemically reacts with minerals

3. Hot H2O & minerals rises to surface

4. Hydrothermal fluid exits chimney & mixes with cold seawater

Metals combine

with sulfur = black

metal sulfides

Cold H2O seeps through seafloor cracks

2. H2O is heated (350°) by molten rock below

Hydrothermal Vents Around World

• 1st discovered (1977) in Alvin

•By Ballard & Grassle (Wood’s Hole OI)

1111stststst Vent Vent Vent Vent DiscoveredDiscoveredDiscoveredDiscovered

Ocean Basin Features:

e. Ocean Trenches: Long, narrow, steep–sided depressions in deep seafloor

•Occur where converging plates are subducted

•Deepest part of ocean

•Where many earthquakes & tsunamis originate

Ocean Trenches:

• Hot rising magma creates volcanic island arcs (Japan) adjacent to trench

Where are Oceanic Trenches?

• Most occur in W. Pacific

• Deepest spot in ocean: Challenger Deep (Marianas Trench) = 11,022 m (36,163 ft.)

Shinkai 6500

Alvin (Challenger Deep)

Deep Sea Research & Diving

•Deeper you dive = more H2O is on top of you

• Greater pressure on body

•Weight of H2O above you: Hydrostatic Pressure

•Deep end of swimming pool

•Hydrostatic pressure against ear drums

Shinkai

6500 Deepest Diver

Alvin (Challenger Deep): 16,000 lbs/in2

7 mi.

of

H2O