OCEAN BASINS and CONTINENTAL MARGINS

 

 

INTRODUCTION

Prior to 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.

Another 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 Basin.

Recall 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.

 

CONTINENTAL MARGINS

There are two types of continental margins:

 

Passive margins, which are common around the Atlantic Ocean, consist of:

Active margins, which are common around the Pacific Ocean, consist of:

The Continental Shelf

The 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.

The Shelf 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

The continental 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.

Associated 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.

 

Continental Rise

The CRs 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 1000 km.

OCEAN BASINS

Ocean basins typically include the following features:

OCEAN/SUBMARINE TRENCH

Ocean 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.

 

ISLAND ARCS

The 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/HILLS

The 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 Ocean. Why?

SEAMOUNTS AND GUYOTS

Seamounts 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.

Guyots 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 tropical areas.

HYDROTHERMAL VENTS

These 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.

These blacksmokers, 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)

These 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).

The 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:

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.