Seafloor Features:Continental Margins

& Deep Ocean Basins

Shape of the Ocean Floor• The ocean is not shaped like a

bath tub (ex. Shallow edges, deep center)

• continental margin- Submerged edge of the continent

• ocean basin- Sea floor

Continental Margins•Diverging plates have passive

margins that have little (volcanic/seismic) activity

•Converging plates have active margins with high (volcanic/seismic) activity

A. Continental Shelf-• Shallow submerged edge of

continent• 220 miles out from shore with an

avg. depth of 250 ft (gentle incline)• Shelf was exposed 18,000 years

ago during ice age, and during late Cretacous sea level was 1,000 ft higher and flooded 35% of the land

• Many natural resources are found here

B. Continental Slopes-• The transition between the shelf

and ocean floor• 4 degree slope or descends 144

ft/mile (greatest- 25 degree slope)• Depth of 12,000 ft• Bottom of the slope is the true

edge of the continent

C. Continental Rise-• The base of the continental slope

that is covered by accumulating sediment

• Sediments can be carried there by turbidity currents rushing down the slope like an avalanche

D. Submarine Canyons-• V-shaped indentations incised into the

continental shelf and slope, often terminating on the deep sea floor in a fan of sediment

• Turbidity currents cause sediments to fall into the canyons

• Can be as large as the Grande Canyon• Navy submarines can hide within

these

Deep Ocean Basins• The seafloor is blanketed by

sediments overlying basaltic rocks

• Deep ocean basins account for ½ of the Earth’s surface

• The deep ocean floor consists of ocean ridges, plains, trenches, and masses of sediment

A. Ocean Ridges-• A mountain chain of young basaltic rocks

at an active spreading center (they cover the earth like the seams of a baseball)

• When they project out of the seaislands

• Rift valleys form as new ocean emerges between lithospheric plates (young rock at the center, oldest at edges) and ridges become steeper

1. Hydrothermal Vents on Ridges• Hot springs that have upwellings of mineral

laden water at temperatures of 660 F• Water descends in cracks on the ridge,

comes in contact with super heated hot rocks

• The superheated water dissolves minerals & gases escape upward through vent by convection

• Plays a major role in ocean chemistry

B. Abyssal Plains & Hills• Abyssal means “without

bottom”• 25% of the earth’s surface is covered by abyssal plainsand hills• Abyssal plains-are flat, cold

featureless expanses of sediment between cont. margin & ridges at 12,000-18,000 ft

• Abyssal hills-small hills of sediment covering extinct volcanoes (> 650 ft tall)

C. Trenches• Arc shaped depression

where ocean plates subduct

• Most active geologic feature on earth (earthquakes/ tsunamis)

• Marianas Trench is 36,163 ft (7 miles) below sea level, 20% deeper than Mt. Everest is tall, & 44 miles wide by 1,600 miles long

D. Island Arcs• Curving chains of

volcanic islands and seamounts that parallel trenches

• trenches and island arcs formed by subducting plates

• Descending plates melt as they subduct, magma rises and lava forms a chain of islands behind the trench

E. Seamounts• Circular or elliptical projections about .6 of

a mile high with a steep slope (25 degree)• Usually numbering 10-100 in any given

area and thought to be submerged inactive volcanoes formed at spreading centers

• Movement of plates away from spreading center moves seamounts out and down

F. Guyots• Seamounts with flat peak due to

wave erosion • guyots also form near spreading

centers and are transported out and down as the plates move