TOPOGRAPHY OF THE OCEAN FLOOR
If the water is removed from the oceans the varied topography of the oceans would be exposed.
The topography of the ocean floor can be divided into four categories:
- 1) Continental Margins
- 2) Ocean Floor
- 3) Ocean Ridges
- 4) Deep Sea Trenches
Continental Margins are classified as either Passive or Active.
Passive Continental Margins: no active tectonic change occurring, East Coast of United States.
A passive margin is characterized by a continental shelf, continental slope and continental rise.
Continental shelf: gently sloping sediment deposit that extends outward from the continent.
Continental shelf deposits extend for 100's of miles and can be cut by canyons.
Continental slope: area of steeper slope, extends from edge of continental shelf to ocean floor.
Sediments at the edge of the shelf tumble down the slope into the deep ocean.
Continental rise: a gently inclined apron of sediment at the base of the slope extending into the
Active Continental Margins: tectonic change actively occurring.
A) Japan type: this type of active margin is produced by a subduction zone. It is characterized by
a narrow shelf with a deep sea trench adjacent.
B) California type: this type of active margin is produced by a transform boundary. It is
characterized by a shelf with deep canyons cut into it. The canyons are parallel to one another
but perpendicular to the coastline.
The ocean floor is also referred to as the abyssal plain. It is an area of VERY low relief, it is the
flattest area on the Earth. The abyssal plain is covered with thick sediments that act to blanket it
smoothing the topography. The sediment on the abyssal plain is derived from four sources.
1) Terrigenous sediment: derived from the continent. Some shelf sediment is carried out
to the abyssal plain. Other sediment is deposited on the plain by ice berg rafting, it is
carried by icebergs and when the ice berg melts it is dropped.
2) Biogenous sediment: derived from biological activity. Microscopic plankton in the
ocean have either calcium carbonate shells or silicates shells. When the plankton dies
these shells drop to the abyssal plain and blanket it in an “ooze” of sediment. This is the
largest source of sediment for the ocean floor.
3) Cosmogenous sediment: sediment derived from outer space, meteors and cosmic dust.
A very insignificant source of sediment for the abyssal plain.
4) Hydrogenous sediment: derived from precipitation of chemicals out of seawater.
Manganese and phosphate will form deposits called nodules that precipitate out of the
seawater itself. Manganese nodules are common on the abyssal plain, phosphate nodules
are more common on the shelf.
The abyssal plain is interrupted by areas of extreme relief- the ridges and trenches.
1) Mid-Ocean Ridge: VERY large feature, underwater mountain range. These ridges are areas of
sea floor spreading. Example: Mid-Atlantic Ridge.
2) Aseismic Ridge: area of volcanic islands, seamounts and guyots that occur in the middle of a
plate. Known also as a hotspot. Example: Hawaiian Islands.
Seamount: undersea mountain of volcanic origin. Seamounts have pointy tops which
suggest that they never were tall enough to break the surface of the ocean-erosion never
Guyot: flat topped undersea mountain. The flat top suggests that the mountain was once
exposed above the ocean surface and that erosion wore the mountain down to a flat top.
Deepest part of the ocean, trenches are areas of subduction. Example: Marianas Trench in the
Pacific is 7 miles deep.
The primary component of a reef is the carbonate structure of the coral. Coral are animals that
secrete a calcium carbonate cup structure in which to live. They begin life as a free floating
polyp and become attached in adulthood. Coral have stinging cells called nematocysts on small
tentacle like structures. They use these to feed by filtering food out of seawater. Coral have a
symbiotic relationship with a photosynthetic organism called Zooxanthellae. The Zooxanthellae
live in the shell of the coral and the coral can utilize the Zooxanthellae for food if needed. Coral
are colonial and a “coral” in the ocean is actually made up of thousands of individuals. When
one individual dies it leaves the calcium carbonate shell behind, a coral polyp will attach itself to
this empty shell thereby building up the reef structure. Only the outer portions of a “coral” is
actually composed of living organisms. Coral need very specific environmental parameters in
which to live.
The water must be clear: if there is too much sediment in the water the coral cannot properly
filter feed and will die off.
The water must also be clear to ensure that the Zooxanthellae have sufficient light for
The water must be warm: calcium carbonate becomes more soluble in cold water, to insure
adequate calcium carbonate to support a large reef structure water must be warm.
Three types of reef structures exist:
1) Fringing Reef: reef grows against a bedrock coast and makes up the shoreline.
2) Barrier Reef: lies offshore and separated from the mainland by a non-coral lagoon.
3) Atoll: an island made of a ring of coral. Forms when a fringing reef continues to build as an
island slowly subsides.