OCEAN BASINS and CONTINENTAL MARGINS
the Challenger Expedition, the view of the Ocean Basins has become known as
the Bathtub View. That is, the center of the ocean basins is the deepest
area, and it is shallower at the margins or edges. But when the Challenger discovered
the mountains in the mid-Atlantic, that view was abandoned and modified to basins
with the deepest areas around the edges - closer to the continents.
discovery that contributed to unravel the nature of the ocean basins came from
Gravimeter measurements. A gravimeter is
an instrument that measures the relative density of rocks that make up the earth's
surface. The results indicates that the continents are partly covered up by
the ocean water column a short distance seaward. At a certain distance the rock
density increases above the rock density of continental rocks. Later sampling
confirmed that the continents have an average composition of the rock Granite,
whereas the ocean basins have an average rock composition of the rock Basalt.
Because basalt is denser (3.0 g/cubic cm), it stands lower that the less
dense granitic rocks (2.6 g/cubic cm). That is partly why the true edge of the
continent, known as the Continental Margin,
is submerged. Seaward of the continental margin is the Ocean
from the introductory chapter that the average ocean depth is 4.0 Km below mean
sea level (msl). Because the ocean covers a significant amount of the Earth
surface, this "elevation" (or negative elevation) is therefore the
most dominant (or common) elevation on the solid surface of the earth.
are two types of continental margins:
Passive margins, which
are common around the Atlantic Ocean, consist of:
- Continental Shelves,
- Continental Slopes,
- Continental Rises.
margins, which are common around the Pacific Ocean, consist of:
- Continental Slopes,
- Ocean or Submarine
The Continental Shelf
continental shelf (CS) is a gently-sloping submerged edge of the continents
that makes up 18% of the continental surface area. The slope is about 2m per
km. The CS is covered by a thick blanket of sediment, eroded from the land and
trapped behind natural dams of ancient coral reefs, or fractured ridges of granite.
This sediment in some areas is up to 15 km thick. A result of such a massive
accumulation of sediment, the seaward edge of the CS has been depressed slowly
into a gently inclined slope. Passive CS are generally much wider (up to 1200
km in some areas), and active margins CS are narrower. Typically, the shelf
width depends on marine processes such as currents, and sea level fluctuations.
Break represents the seaward edge of the CS, which is
consistently located at ~ 140 m depth below msl. The
only exception is the shelf break around Antarctica which are at depths greater
than 140 m (in some areas, at 600 m below msl) because of the massive ice sheets
that has depressed the CS around Antartica.
The Continental Slope
slope (CSL) begins seaward of the CS break, and it is identified by an abrupt
increase in slope from an average of 2m/km to 70 m/km (or 4 degrees slope).
Slopes as steep as 25 degrees has been identified at active margins. The average
width is 20 km. Geologically, the seaward edge of the CSL represents the boundary
between the continents and the ocean basins.
with the CS and the CSL are a major feature cutting through the CS/CSL at right
angle to the shorelines. This feature is known as SUBMARINE
CANYONS. Along some passive margins, they appear to be seaward extensions
of major river valleys ( examples are the Hudson River, Congo River, and Chesapeake
Bay) that deepened during low sea level periods. However, the canyons are generally
explained as erosion caused by turbidity currents.
Turbidity currents are a consistent slurry of seawater/sediment mixture (like
freshly mixed concrete). These currents are occasionally activated by submarine
earthquakes, which mobilizes the slurry to move downslope at speeds that can
attain up to 17 mph.
are present only along passive margins. In other words,they are present only
where ocean trenches are absent.
They typically form at the base of the CSL, where gradient decreases
sharply to an average of 6m/km. The feature is formed by accumulating sediment
that is delivered downslope by turbidity currents. The CR materials may be moved
around by deep ocean currents particularly around western ocean margins where
the currents are relatively stronger. The width of CRs can vary from 0.1 to
typically include the following features:
- Island Arcs,
- Abyssal Plains and
- Seamounts and Guyots
- Oceanic Ridge Systems,
- Hydrothermal Vents.
trenches are long, arc-shaped troughs at the foot of the CSLs, along Active
Margins. Trenches also mark the locations of convergent
plate boundaries (see Plate Tectonic
Chapter), where older oceanic plates plunge into the Asthenosphere.
Ocean trenches are also associated with earthquakes, high heat flow, and volcanic
activity. Trenches are indeed the deepest areas of the earth surface ( 3 to
6 km below the average ocean floor). The Marianas Trench is 7 km below the adjacent
Pacific Ocean floor. Typically, the deepest part of a trench is given a separate
name. Ex. the Challenger Deep is the deepest part of the Marianas Trench.
volcanic activity associated with trenches build up a chain of bow-shaped submarine
volcanoes directly above the ocean floor, and landward of the particular trench.
These chain of volcanoes become islands over million of years. They also follow
the orientation of the trench by producing a set of arc-shaped islands. So the
origins of these two oceanic features (trenches, and island arcs) are intimately
associated. Typically the trenches that produce a particular island arc is inclined
at an angle beneath the island arc. Examples include Japan, Indonesia, Phillipines,
and the Aleutian Islands.
abyssal plains are the flatest, or the most level areas on the Earth's surface.
The levelling is a result of fine sediments from the continents covering up
most of the volcanic irregularities on the ocean floor. Overall abyssal plains
cover 25% of the earth's surface. They are virtually featureless, except near
the oceanic ridge areas, where small volcanic hills protruding through
the sedimentary layers become common. These abyssal hills
are on average less than 200 meters high. The abyssal
hills have a complementary relationship to the abyssal plains. Oceans with few
abyssal hills tend to have more extensive abyssal plains. Hence abyssal hills
are less common in the Atlantic, and Indian Oceans, but common in the Pacific
SEAMOUNTS AND GUYOTS
are small, relatively steep-sided volcanoes rising to heights of at least 1
km above the average ocean floor. In general, they are submerged volcanoes that
may be isolated or organized in chains. In a few places, they are sufficiently
elevated to become islands. Many of them form over areas that have highly concentrated
heat energy known as hotspots. But
a few may form near the ocean ridges. Hence, both types are carried along in
the direction of moving tectonic plates. Seamounts are most common in the Pacific
Ocean, or any plate with rapid seafloor spreading rates. The Hawaiian Islands
are the classic example of a chain of seamounts. A chained set of seamounts
indicate the direction of plate movement.
are flat-topped seamounts whose tops were eroded by surface
ocean waves in the past. Because seamounts sink below the ocean water surface
as they become older, they are basement or foundations for coral islands in
are volcanic chimney rocks on the ocean floor where hot, steamy, dark water
discharges after being heated below the ocean floor. In a sense, they are submarine
hotsprings that were first discovered in 1977 on
the East Pacific Rise. Since then, several more have been discovered including
one in a deep water lake called Lake Baikal, Russia. Typically, most of them
are on or near the ocean ridges. Water temperatures around the vents average
8 - 16 degrees Celcius, which is 100 to 200% higher than the sorrounding deep
ocean water (averages at 4 degrees Celcius) away from hydrothermal vents.
as they are sometimes referred to, originate from circulating seawater, superheated
by very hot volcanic rocks, in active areas of seafloor spreading. The superheated
water dissloves minerals from the hot rocks, rises upwards and escape through
vents or cracks. The minerals are then deposited as chimneys similar to stalagmites
seen in limestone caves.
MID-OCEAN RIDGES (MOR)
are linear, elevated parts of the ocean floor that typically mark the divergent
plate boundaries. The MORs are known as the most extensive
features on the Earth's surface. Indeed, they are present in all ocean basins.
Their total length is estimated worldwide to be 65,000 km or about 40,000 miles,
and they can be as wide as 500 km in some areas. Their elevation may vary from
2.5 km above the average ocean floor, to elevations above the ocean water surface.
Islands such as Iceland, Easter Island, the Azores, are all examples of MORs
built up above msl (into islands).
active areas of MORs are the mid-section known as the Rift
Zone. Within the rift zone are deep, narrow valleys offset
in many places by perpendicular faults known as Transform
Faults. Also, the following properties are typical of
the rift zone:
flow is high.
walls are elevated. Why?
are frequent. Why?
- volcanic eruptions
are frequent. Why?
As this elevated, hot
part of the lithosphere cools, it shrinks and sinks. Over millions of years,
it becomes more like the flatter, gently sloping abyssal plains, and hills.