The Ocean Floor and Its Evolution

I. Introduction

A. The 3 1/2 year voyage of the HMS Challenger out of Great Britain in the 1870's represented a major turning point in Ocean Science. The voyage generated 50 large volumes of data on every aspect of the oceans.

1. Many techniques developed on this voyage are still used today by oceanographers.

2. Included in the data were hundreds of soundings taken by lowering a weighted line overboard.

3. Provided depth data, but it was very slow. If not for the echo sounder we would still not have a clear picture of the topography of the ocean floor.

B. Echo sounder ("sub detector" adapted from WWII use)

II. Major topographic divisions of ocean floor

A. By the late 1940's and 1950's use of the echo sounder had allowed oceanographers to put together a pretty picture of the topography of the ocean floor.

B. Three major divisions

1. Continental Margins/Trenches

2. Ocean basins

3. Mid-oceanic ridges

III. Oceanic Ridge Systems

A. Volcanically active linear submarine rifts and ridges covering 23% of the Earth's surface. Averages 2 km high and extends throughout the world's oceans.

1. Examples

a. Mid-Atlantic Ridge

b. East Pacific Rise

c. Pacific-Antarctic Ridge

B. Largely confined to ocean bottom, but does encroach upon the land surface on the island of Iceland. Pillow lavas are erupted along these ridge systems.

C. Divided into two topographic provinces:

1. Ridge crest -

a. Central rift valley - steep-walled, in center of ridge, floor lies 1000 to 3000 m below mtns.

b. Rift valley is blocked or discontinuous at places because of displacement along fracture zones. Adjacent segments of the rift valley are offset along transform faults

2. High fractured plateau on flanks of ridge

a. Away from the ridge crest the volcanic and seismic activity and the elevation decreases. The relief also becomes less rugged as sediment covers the oceanic crust to increasingly greater depths.

IV. Ocean Basins

A. Portion of ocean floor extending from the ridge on one side to a trench or continental margin on the other. Cover 30% of the Earth's surface, more than either the ridges or continents. These are fairly flat, but have a variety of topographic forms within them.

B. Abyssal plains - nearly flat region of the ocean located between the continental margin and the mid-oceanic ridge. Sediments directly from the continents and from settling of suspended sediment in the ocean fill in the irregularities of the ocean bottom and give this region its flat character.

C. Volcanic features found in numerous locations throughout ocean basins not associated with ridge or subduction volcanism.

1.Seamount - undersea volcanic mountain that extends more than 1 km above the abyssal floor -- formed by passage of the plate over a hot spot.

2. Guyot - flat-topped seamount that was once exposed at the sea surface and whose top was leveled by erosion

3. Seamount chains - When hot spots are long-lived features in terms of geologic time a chain of volcanic mountains can result.

a. Hawaiian Island - Emperor Seamount Chain

b. Ninety East Ridge

c. Kelvin Seamounts - off of NE USA shelf

d. The ultimate fate of the present Hawaiian Islands will be to vanish beneath the waves and become guyots.

4. Atoll - small ring-shaped reef found growing in what would otherwise be deep ocean waters. Formed when a volcanic island sinks below the sea surface at a slow enough rate for coral reef growth to keep up.

a. Darwin (1842) first developed the theory explaining their origin.

b. Fringing and barrier reefs form first.

5. Plateaus-subcontinental-size, continent-like (granitic) fragments that occur in the ocean basin. Rise about 1-2 km above the ocean floor but don't reach the surface. When they are carried into a Subduction zone they are usually too buoyant to be carried very far down the zone and instead are accreted to the continental

V.Continental Margins

A. Continental shelf- gently sloping submerged edge of a continent, extending commmonly to a depth of 200 m.

1. Extension of continent that has alternately been submerged and exposed as sea level has fluctuated during geologic time.

2. The slope of the shelf is 1:1000 and the width varies between 100 meters and 480 kilometers.

3. Outer edge is called the shelf break.

B. Continental slope-steeply sloping region between shelf & rise

1. Marks the outer edge of the continent and beginning of the ocean basin.

2. Slope = 1:40

3. Extends to depths of a few thousand meters

C. Continental rise - broad, gently sloping ramp that rises from the abyssal plain to the continental slope.

1. Formed by deposition of land-derived sediments.

2. Lots of "alluvial fan" - like deposits where canyons and gullies in the slope empty out onto the ocean floor.

3. Slope = 1:300

4. Somewhere beneath the thick sediments of the rise, the underlying rock makes the transition from granitic continental crust to basaltic oceanic crust

D. The types of continental margins relate, in a general way to the various types of plate boundaries: converging, diverging, and transform. So, we will consider their characteristics based on the nature of the plate tectonic boundary on which they are found.

E. Diverging boundary margin - The Red Sea region is an example of one in the very early stages of development.

1. Shelf-slope-rise sequence has yet to develop

2. Only the steep slopes of the facing escarpments on either side of the rift are present.

3. The Red Sea and its adjacent borders in Africa and Arabia illustrate what happens when spreading centers develop within a continental area. The bordering highlands slope away from their crests towards continental interiors. The highlands undergo extensive erosion as they rise.

F. Trailing boundary margin (Passive Margin) - forms on back side of continent as it moves away from the spreading center. Only these passive margins exhibit a well-developed shelf-slope-rise system.

1. Usually very broad, well-developed continental shelves.

2. East coasts of North and South America, and west coasts of Europe & Africa are good examples.

3. As these margins move away from the rift zone sediments shed from the adjacent highlands and the steep escarpment accumulate in thick piles to form the continental shelf.

4. Submarine canyons are common on continental shelves and slopes such as these. They are typically curved and have tributary channels leading into them, much like river valleys on land. Some are as large as the Grand Canyon. Act as sediment transport pathways for much of the sediment deposited on the continental rise and the abyssal plains.

a. Fast-moving, sediment-laden masses of shelf water called turbidity currents move down the submarine canyons carrying sediment out onto the rise and abyssal plains. Currents also excavate the canyon walls.

b. Sediments deposited by turbidity currents show a distinctive feature called graded bedding which makes them very easy to recognize in the rock record. These are layers of sediment containing grains of different sizes, with the largest grains on bottom.

G. Converging boundary or "leading edge" continental margins. (Also called active margins)

1. These are composed mainly of an extremely steep continental slope. The shelf, if it exists at all, is very narrow and the rise is absent

a. The most spectacular slopes of all the continental margins are developed here.

2. The continental slope leads down directly into a deep oceanic trench. The western margin of South America is a typical leading edge margin. Here the westward-moving American plate is riding over the eastward-moving Nazca plate which is being subducted. From the coast the surface of the continental block plunges precipitously beneath the sea. The towering Andean mountain range appears to rise abruptly out of the Pacific.

H. Transform boundary continental margin

1. These are by far the rarest of the margins, but since we have one along the San Andreas fault, I'll mentions it.

2. Sequences of mountain ranges and valleys form along such coasts due to the strong tectonic forces at work as plates slide past one another.

a. Parallel coast and continue offshore from Southern Calif. where their highest parts project above sea level they form the Channel Islands.