Marine Provinces

General Shape of the Ocean (Bathymetry)

Physiography of the Oceans

General shape and features of the seafloor

Land/ocean

Continental/oceanic lithosphere

Ocean lithosphere is dense, floats lower in the asthenosphere

Distribution of Elevations and Depths on Earth

70% of earth’s surface below sealevel

Highest pt Mt Everest (8km)

Lowest pt Mariana Trench (11km)

Overall average = -2440m

Ave. depth of ocean = -3794 m

Ave. elevation of land = 840m

sealevel

Continental Margins

Passive vs. Active Margins

Passive

Not seismically active

No subduction

Land/sea does NOT = plate boundary

Remnant of rifting

Typical of the Atlantic margins

Active
Seismically active
Subduction zone
Land/sea = plate boundary

Typical of the Pacific margins

Passive margins

Continental Shelves

Submerged outer edge of a continent

Extension of the continent

Underlain by continental crust

Nearly flat- gentle seaward slope (parking lot)

Width and depth vary with sea level
Typical water depth at edge of shelf = 140m

Shelf width (average ~70 km)

Passive margin = 350 km

Active margin = narrower (trench)

During last ice age (18, 000 yrs ago)

Water stored as ice-

Sealevel fell ~120m

Shelves almost complete exposed

- course sediment transported to the shelf edge

- sediments reworked by rivers and waves

* features- wave cut terraces

drowned river valleys

Active shelves

Narrower

Faulted, volcanism

Shelves = continuation of continents

Continental crust overlain by-

Continental sediments

Oil and gas
Mineral deposits

Drilling

Continental Slope

Transition from nearly flat shelf to deep ocean

Steep portion of the margin

Built up of sediments that

Have been transported over the shelf/slope break (passive)
Scraped off the descending slab

Submarine Canyons

Common feature

Cut into shelf and slope

V-shaped canyons

End in fan of sediment

Found in all ocean basins

Passive and active margins

As large as the Grand Canyon

How do they form?

Erosion:

Carved by rivers when sea level was lower

extend from modern rivers

Found too deep

2. Carved by turbidity currents

Turbidity Currents

Avalanches of water + sediment

Denser than water

Flows down slope

Scours the canyon as it flows (underwater)

Can be triggered by earthquakes

Natural turbidity currents have never been observed, but:

Recorded in sediment as turbidites

Grand Banks Cable Break 1929

Earthquake triggered current
Timed by breaks in cables = 30 mph
Deposited 1 m of sediment over 100,000 km2

Continental Rise

Accumulation of sediment at the base of the slope

Sediment from turbidity currents

reworked by currents

Deep Ocean Basins

Trenches

Found at the base of the slope along active margins

Deepest places on the earth
Areas where the ocean crust is being subducted
Tend to be arc shaped- intersection of a plate on a sphere
Trenches trap the sediment that would normally form the rise, so no rises along active margins

Oceanic Ridges and Rises

Longest continuous mountain chain in the world

Runs through every ocean
Encircles the globe
65,000 km long (1.5 x circumference)
Stands 1-2 km high about seafloor (hot and buoyant)

Areas where new seafloor is forming

Can rise above sea level (Iceland, Azores…)

Segments of the ridge are offset by transform faults (offset created during initial rifting)

Separates the ocean basins, can disrupt flow of water

Ridges = steep

Slower spreading (~2cm /yr)
Faulted axial valley

Example: Mid Atlantic Ridge

equidistant b/w Afr and So Am

Rises = gentle slopes

Faster spreading (~10 cm/yr)
Little to no axial valley

Example: East Pacific Rise

Hydrothermal vents

Underwater hot springs

Sea water that is circulated through the crust rises to the surface

Warm water vents- below 30^o C emit water as a clear cloud

White Smokers- 30-350^oC water appears white because of light colored compounds

Black smokers above 350^oC

Water appears black because of dark colored metal sulfides

Form chimney like structures up to 100ft high

Support biological communities

Abyssal Plains

Flat, deep portions of the oceans

3.7 — 5.5 km

Featureless- sediments bury any topography

Flattest places on Earth (<1m/km)

Between the continental margins and the ridges

Most common in the Atlantic and Indian - most sediment

Sediments

Mostly terrestrial (turbidites)

Abyssal Hills

Regions where the basaltic basement is not completely covered by abyssal plain sediments

Made up of faulted basement or volcanoes

Very common along the margins of the mid ocean ridges

Most common topographic feature on Earth

Seamounts

Steep sided volcanoes

Do not extend above sea level

vs. volcanic islands

Circular to elliptical
Individual or in groups (hotspots)

Built by volcanism (often near the ridge), then subside due to isostacy

Guyots- flat topped seamounts

Flattened by wave activity