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 Risk management
 Prolongation of the Land Mass
 Ridges and submarine elevations
 Accretion and suturing

Oceanic Ridges, Submarine Ridges, and Natural Components of the Margin

Article 76 paragraphs 3 and 6 distinguish among three types of sea-floor highs:

• oceanic ridges of the deep ocean floor,
• submarine elevations that are natural components of the margin, and
• submarine ridges.


The distinction among these types of sea-floor highs is important because it directly influences the area of the extended continental shelf. The continental shelf of coastal States can extend to 200 nautical miles on oceanic ridges, to 350 nautical miles on submarine ridges, and to either 350 nautical miles or 100 nautical miles beyond the 2,500 metre isobath on submarine elevations.

At a fundamental level, the CLCS Guidelines (1999) (7.1.8) state that:

The distinction between the “submarine elevations” and “submarine ridges” or “oceanic ridges” shall not be based on their geographical denominations and names used so far in the preparation of the published maps and charts and other relevant literature. Such a distinction for the purpose of article 76 shall be made on the basis of scientific evidence taking into account the appropriate provisions of these Guidelines.


The guidelines summarise the evidence necessary to distinguish among the sea-floor highs in paragraph 7.2.10:

Therefore the Commission feels that in cases of ridges its view shall be based on such scientific and legal considerations as natural prolongation of land territory and land mass, morphology of ridges and their relation to the continental margin as defined in paragraph 4, and continuity of ridges.

The three types of sea-floor highs are distinguished on the basis of their morphological, geological and tectonic continuity with the land mass and continental margin.

Also at a fundamental level, the term “ridge” evokes the concept of an elevated, narrow, and elongated body with steep sides. Some of the other technical terms of article 76 have different meanings when used in legal, geomorphological or geological contexts, but there appears to be agreement on the concept of a ridge.

Oceanic ridges
Oceanic ridges are part of the deep ocean floor and are not part of the continental shelf. According to article 76 paragraph 3, oceanic ridges must lie beyond the shelf, slope and rise. These ridges have not been tectonically accreted or sutured to the continental margin or be in any way an integral part of the land mass; otherwise they are part of the continental shelf.

There is no mention of rock type of oceanic ridges, though they will include ridges formed by sea-floor spreading and associated processes. Two common examples of oceanic ridges are mid-ocean spreading ridges, and ridges formed along transform faults perpendicular to sea-floor spreading ridges. The mechanics of continental break up and sea-floor spreading mean that transform ridges can impinge on the continental margin, although they are often buried by sediments. Spreading ridges do not usually impinge on the continental margin, but can do so in some tectonic settings.

Submarine elevations that are natural components of the margin
Article 76 paragraph 6 includes “plateaux, rises, caps, banks and spurs” among submarine elevations that are natural components of the margin. The CLCS Guidelines (1999) (7.3.1) recognise at least two origins of submarine elevations that are natural components of the margin:

(a) In active margins, continents grow by the accretion of sediments and crustal material of oceanic, island arc or continental origin onto the continental margin. Therefore, any crustal fragment or sedimentary wedge that is accreted to the continental margin should be regarded as a natural component of that continental margin; and

(b) In passive margins, continental break up involves thinning, extension and rifting of the continental crust and extensive intrusion of magma into and extensive extrusion of magma through that crust. This process adds to the growth of the continents. Therefore, seafloor highs that are formed by this breakup process should be regarded as natural components of the continental margin where such highs constitute an integral part of the prolongation of the land mass.

Sea-floor highs that are “an integral part of the prolongation of the land mass” are features that have a continuous morphological, geological and tectonic connection with the land mass (see Prolongation of the land mass and Accretion and suturing on this website). 

Article 76 does not mention the oceanic or continental affinity of the rocks of the continental shelf. A submarine elevation that is a natural component of the margin can be either oceanic or continental in origin, providing natural prolongation (continuity of morphology, geologic origin and history) can be established to the coastal State’s land mass. 

Based on article 76 and the CLCS Guidelines (1999), a volcanic seamount, rocks of a mid-ocean spreading ridge and even normal oceanic crust can be natural components of the margin if they have been accreted or sutured to a continent by tectonic activity and no longer reside in their original deep ocean setting. 

Island arcs contribute significantly to continental growth, particularly in the Pacific region, and are natural components of the margin. 


Figure 1. White Island is an active volcano located in the Bay of Plenty, about 50 kilometres north of the North Island. It is part of the Kermadec Ridge system, a volcanic island arc associated with the subduction of the Pacific Plate beneath the Australian Plate.

Continental fragments that share their geologic origin and history with the land mass, but whose connection with it has been altered (but not severed) by rifting or other tectonic activity, are also natural components of the margin.

Submarine ridges
According to article 76 paragraph 6, submarine ridges are part of the continental shelf, but because they are not submarine elevations they cannot be natural components of the continental margin. The distinction between submarine ridges and submarine elevations in terms of article 76 is not clearly established in the CLCS Guidelines (1999). A logical basis for this distinction may lie in a thorough assessment of the morphologic, geologic and tectonic relationship of the sea-floor high to the land mass (Brekke and Symonds 2004). 

Demonstration of natural prolongation is not by itself sufficient to demonstrate that a feature is a natural component of the continental margin. Submarine elevations that are natural components of the continental margin share morphological continuity, crustal characteristics, geologic origin, and tectonic evolution with the adjoining land mass. A submarine ridge shares one or more, but not all of these characteristics with the land mass and is therefore a natural prolongation, but it is not a natural component of the margin.

For example, a submarine ridge must be morphologically connected to the land mass for it to be a natural prolongation, but is not a natural component of the land mass because it has a different geologic origin and history. The geology of a submarine ridge can vary along its length, and may share its geologic origin and history with the associated land mass along some, or none, of its length.

The degree of similarity of geologic origin and history required to demonstrate that a submarine feature is “a natural component of the continental margin” may depend upon the nature of crust of the land mass. Continental crust is formed as a result of multiple tectonic and refractive processes and can be variable in composition. Assessment of the geologic and morphologic continuity between the land mass and a sea-floor high, and therefore classification of the high as a submarine ridge or as a submarine elevation that is natural component of the continental margin, would have to consider the inherent variability in the composition of continental crust.

An example of a submarine ridge might be a transform ridge extending between the continental margin where it initiated, during the early phases of continental break up and the onset of sea floor spreading, and the deep ocean floor. With increasing distance from the continental margin, the affinity of the rocks along the transform ridge could change from continental to oceanic. The location of the transition zone could be difficult to identify, but in practice it would probably be sufficient to demonstrate that its geologic nature changes along its length for the feature to be classified as a submarine ridge. 

The extent of the continental shelf is limited to 350 nautical miles along a submarine ridge. If a location could be established where the ridge was entirely oceanic and no longer a geologic prolongation of the land mass, then seaward of that position the ridge would be a “ridge of the deep ocean floor” and no longer eligible to generate foot of slope positions.

The Macquarie Ridge Complex consists of a series of ridges that extend south from New Zealand for 1,600 kilometres. The majority of the rocks in the Macquarie Ridge Complex are oceanic in origin, although they are not part of a modern sea-floor spreading system. The ridge complex is thought to have originated as a series of short spreading ridge segments connected by oceanic fracture zones that progressively rotated, lengthened, coalesced, and became uplifted as a result of changes in relative motion between the Australian and Pacific plates. However, at the northern end of the ridge system the rocks are continental, related to the land mass of New Zealand. The nature and exact location of the boundary between rocks of continental and oceanic affinity along the ridge complex are unknown. Because the nature of the geology changes along the length of the ridge, and the ridge is morphologically connected to New Zealand, it may be an example of a submarine ridge (i.e., a natural prolongation but not a natural component of the margin).



Figure 2. The Macquarie Ridge Complex is a major submarine feature extending south from the New Zealand land mass. It includes rocks of both continental and oceanic origin.
Ridges and fossil convergent margins
Continents are dominantly built by the suturing of crustal fragments which have been brought together at a convergent plate boundary. When convergence ceases, these crustal fragments may span the paleo-boundary between the two fused plates. Figure 3 illustrates some examples of how sutured fragments might form submarine elevations, submarine ridges or ridges of the deep ocean floor. 

In 3 (a) the sutured volcano is part of the land mass. There is continuity of morphology, geology and tectonic history with the land mass and therefore the volcano’s submerged extent is a submarine elevation. 

In 3 (b) the suturing of the volcano occurs seaward of the exposed landmass. The volcano does not occur beneath the landmass and the transition in geology and tectonic history along the submarine extent of the feature  implies that it is a submarine ridge.

  In 3 (e) the sutured fragment is completely incorporated within the crustal structure of the onshore land mass. Even though it is not exposed onshore there is continuity of morphology, geology and tectonic history with the land mass. The feature is therefore a submarine elevation which is a natural component of the margin.



Figure 3. Submarine elevations and submarine ridges across a fossil convergent margin.

In 3 (c) and (d) oceanic crust is part of the land mass and therefore its submerged extension is part of the continental margin. The outer edge of the continental margin will be defined at the point of maximum change in gradient at the base of the slope. Continental prolongation ends at that point. Example 3 (c) could be an example of a submarine elevation comprised of oceanic rocks, perhaps depending on its shape. Example 3 (d) may be an example of a submarine ridge, a natural prolongation of the land mass at one end but a “ridge of the deep ocean floor” at its seaward limit.

Other submarine features
Neither article 76 nor the CLCS Guidelines (1999) state that submarine elevations, submarine ridges, and oceanic ridges constitute the complete set of possible elevated submarine features. There are many types of elevated submarine features that lie beyond the shelf, slope and rise and are therefore not part of the continental margin. It does not necessarily follow that these features are oceanic ridges or part of the deep ocean floor.

Examples of elevated submarine features that can lie beyond the rise and might not be considered oceanic ridges or part of the deep ocean floor include hot-spot volcanic seamount chains, oceanic plateaus, submerged continental rift blocks, volcanic arc segments and isolated intra-plate volcanoes. The distinction is partly one of size – these features are all limited in extent and are not “oceanic” in scale.

If these features lie within the shelf, slope and rise surrounding a land mass then they are part of the continental shelf of that land mass, and they are either a submarine ridge or a submarine elevation that is a natural component of the margin. Foot of slope positions could be defined around the edges of these features. In contrast, if part of an oceanic ridge lies within the rise surrounding a land mass, then some, but not all of its length may be a submarine ridge or submarine elevation. 

Summary
The New Zealand Continental Shelf Project differentiates among sea-floor highs using the following criteria.


• If the feature is continental in origin and there is a continuous morphological, geological and tectonic connection with the land mass, then it is a submarine elevation that is a natural component of the margin.
•  Regardless of its origin, if the feature has been accreted or sutured to the continental margin and is morphologically and geologically continuous with it, then unless it is an in situ ridge of the deep ocean floor or is in situ deep ocean seafloor, it is a submarine elevation that is a natural component of the margin.
• If an island arc is morphologically and geologically and geologically connected to the land mass then it is a submarine elevation that is a natural component of the margin. 
• If the feature is of oceanic origin (i.e., either a spreading ridge or a transform ridge) then unless it is that part of the ridge that has been accreted or sutured to the margin by tectonic activity, or is otherwise integrated into the land mass, then it is part of the deep ocean floor and is an oceanic ridge.
• Regardless of its origin, if the feature is morphologically continuous with the continental margin, and is not an oceanic ridge, then it is either a submarine ridge or a submarine elevation that is a natural component of the margin, depending on the degree of geological and tectonic continuity between the land mass and the ridge.