Submarine Canyons and Abyssal Fans England and Scandinavia, in the Gulf of Mexico, and in

In many places, sea-floor maps show deep valleys called the Beaufort Sea on the northern coast of Alaska and submarine canyons eroded into the continental shelf western Canada. In recent years, oil companies have ex- and slope. They look like submarine stream valleys. A plored and developed these offshore reserves. Deep canyon typically starts on the outer edge of a continen- drilling has revealed that granitic continental crust lies tal shelf and continues across the slope to the rise (Fig. beneath the sedimentary rocks, confirming that the con- 11–18). At its lower end, a submarine canyon commonly tinental shelves are truly parts of the continents despite leads into an abyssal fan (sometimes called a subma- the fact that they are covered by seawater. rine fan), a large, fan-shaped pile of sediment lying on

The Continental Slope and Rise the continental rise. Most submarine canyons occur where large rivers At the outer edge of a shelf, the sea floor suddenly steep-

enter the sea. When they were first discovered, geolo- ens to about 4º to 5º as it falls away from 200 meters to

gists thought the canyons had been eroded by rivers dur- about 5 kilometers in depth. This steep region of the sea floor averages about 50 kilometers wide and is called the continental slope. It is a surface formed by sediment accumulation, much like the shelf. Its steeper angle is due

Submarine

Continental

primarily to gradual thinning of continental crust in a

canyon

shelf

Continental slope

Deep sea

transitional zone where it nears the junction with oceanic

floor

crust. Seismic profiler exploration shows that the sedimentary layering is commonly disrupted where sediment has slumped and slid down the steep incline.

A continental slope becomes less steep as it gradually merges with the deep ocean floor. This region, called the continental rise, consists of an apron of terrigenous

sediment that was transported across the continental shelf Abyssal

fan

and deposited on the deep ocean floor at the foot of the slope. The continental rise averages a few hundred kilo-

Figure 11–18 Turbidity currents cut submarine canyons into meters wide. Typically, it joins the deep sea floor at a

the continental shelf and slope and deposit sediment to form depth of about 5 kilometers.

a submarine fan.

192 CHAPTER 11 OCEAN BASINS

ing the Pleistocene Epoch, when accumulation of glacial The landward wall (the side toward the continent) of the ice on land lowered sea level by as much as 130 meters.

trench is the continental slope of an active margin. It typ- However, this explanation cannot account for the deeper

ically inclines at 4º or 5º in its upper part and steepens portions of submarine canyons cut into the lower conti-

to 15º or more near the bottom of the trench. The continental slopes at depths of a kilometer or more. Therefore,

nental rise is absent because sediment flows into the submarine canyons must have formed under water, and a

trench instead of accumulating on the ocean floor. submarine mechanism must be found to explain them. Geologists subsequently discovered that submarine canyons are cut by turbidity currents. A turbidity cur-

䊳 11.7 ISLAND ARCS

rent develops when loose, wet sediment tumbles down the slope in a submarine landslide. The movement may

In many parts of the Pacific Ocean and elsewhere, two

be triggered by an earthquake or simply by oversteepen- oceanic plates converge. One dives beneath the other, ing of the slope as sediment accumulates. When the sed-

forming a subduction zone and a trench. The deepest iment starts to move, it mixes with water. Because the

place on Earth is in the Mariana trench, north of New mixture of sediment and water is denser than water alone,

Guinea in the southwestern Pacific, where the ocean it flows as a turbulent, chaotic avalanche across the shelf

floor sinks to nearly 11 kilometers below sea level. and slope. A turbidity current can travel at speeds greater

Depths of 8 to 10 kilometers are common in other than 100 kilometers per hour and for distances up to 700

trenches.

kilometers. Huge amounts of magma are generated in the sub- Sediment-laden water traveling at such speed has

duction zone (Fig. 11–20). The magma rises and erupts tremendous erosive power. Once a turbidity current cuts

on the sea floor to form submarine volcanoes next to the

a small channel into the shelf and slope, subsequent cur- trench. The volcanoes eventually grow to become a chain rents follow the same channel, just as an intermittent sur-

of islands, called an island arc. The western Aleutian face stream uses the same channel year after year. Over

Islands are an example of an island arc. Many others oc- time, the currents erode a deep submarine canyon into

cur at the numerous convergent plate boundaries in the the shelf and slope. Turbidity currents slow down when

southwestern Pacific (Fig. 11–21). they reach the deep sea floor. The sediment accumulates

If subduction stops after an island arc forms, vol- there to form an abyssal fan. Most submarine canyons

canic activity also ends. The island arc may then ride and fans form near the mouths of large rivers because the

quietly on a tectonic plate until it arrives at another sub- rivers supply the great amount of sediment needed to

duction zone at an active continental margin. However, create turbidity currents.

the density of island arc rocks is relatively low, making Large abyssal fans form only on passive continental

them too buoyant to sink into the mantle. Instead, the is- margins. They are uncommon at active margins because

land arc collides with the continent (Fig. 11–22). When in that environment, the sediment is swallowed by the

this happens, the subducting plate commonly fractures trench. Furthermore, most of the world’s largest rivers

on the seaward side of the island arc to form a new sub- drain toward passive margins. The largest known fan is

duction zone. In this way, the island arc breaks away the Bengal fan, which covers about 4 million square

from the ocean plate and becomes part of the continent. kilometers beyond the mouth of the Ganges River in the

The accretion of island arcs to continents in this manner Indian Ocean east of India. More than half of the sedi-

played a major role in the geologic history of western ment eroded from the rapidly rising Himalayas ends up

North America and is explored more fully in Chapter 20. in this fan. Interestingly, the Bengal fan has no associ-

Note the following points:

ated submarine canyon, perhaps because the sediment

1. An island arc forms as magma rises from the mantle supply is so great that the rapid accumulation of sedi-

at an oceanic subduction zone. ment prevents erosion of a canyon.

2. The island arc eventually becomes part of a continent.

ACTIVE CONTINENTAL MARGINS

3. A continent cannot sink into the mantle at a subduc- An active continental margin forms in a subduction zone,

tion zone because of its buoyancy. where an oceanic plate sinks beneath a continent. A long,

4. Thus, material is transferred from the mantle to a narrow, steep-sided depression called a trench forms on

continent, but little or no material is transferred the sea floor where the oceanic plate dives into the man-

from continents to the mantle. This aspect of the tle (Fig. 11–19). Because an active margin has no grad-

plate tectonics model suggests that the amount of ual transition between continental and oceanic crust, it

continental crust has increased throughout geologic commonly has a narrower shelf than a passive margin.

time. However, some geologists feel that small

Island Arcs 193

Oceanic crust

Pluton

Magma

Benioff zone earthquakes

Lithosphere Asthenosphere

Figure 11–19 At an active continental margin, an oceanic plate sinks beneath a continent, forming an oceanic trench.

Island

Trench Volcano arc Oceanic crust

Asthenosphere Lithosphere

Rising magma

Partial melting

Figure 11–20 An island arc forms at a convergent bound- Figure 11–21 Mataso is one of many volcanic islands in the ary between two oceanic plates. One of the plates sinks, gen-

Vanuatu island arc that formed along the Northern New erating magma that rises to form a chain of volcanic islands.

Hebrides Trench in the South Pacific.

194 CHAPTER 11 OCEAN BASINS

Island

Both are common in all ocean basins but are particularly

Trench Volcano arc

abundant in the southwestern Pacific Ocean. Seamounts

Oceanic

and oceanic islands sometimes occur as isolated peaks

crust

on the sea floor, but they are more commonly found in chains. Dredge samples show that seamounts, like oceanic islands and the ocean floor itself, are made of basalt.

Seamounts and oceanic islands are submarine volcanoes that formed at a hot spot above a mantle plume. They form within a tectonic plate rather than at a plate boundary. An isolated seamount or short chain of small seamounts probably formed over a plume that lasted for only a short time. In contrast, a long chain of large islands, such as the Hawaiian Island–Emperor Seamount

Asthenosphere

Chain, formed over a long-lasting plume. In this case the

Lithosphere Rising

lithospheric plate migrated over the plume as the magma

magma

Partial

continued to rise. Each volcano formed directly over the

melting

plume and then became extinct as the moving plate car- ried it away from the plume. As a result, the seamounts and oceanic islands become progressively younger toward the end of the chain that is volcanically active to- day (Fig. 11–23).

After a volcanic island forms, it begins to sink. Three factors contribute to the sinking:

1. If the mantle plume stops rising, it stops producing magma. Then the lithosphere beneath the island cools and becomes denser, and the island sinks. Alter- natively, the moving plate may carry the island away from the hot spot. This also results in cooling, con- traction, and sinking of the island.

2. The weight of the newly formed volcano causes iso- static sinking.

Figure 11–22 (a) An island arc is part of a lithospheric

3. Erosion lowers the top of the volcano. plate that is sinking into a subduction zone beneath a conti-

nent. (b) The island arc reaches the subduction zone but can- These three factors gradually transform a volcanic island not sink into the mantle because of its low density. (c) The is-

into a seamount (Fig. 11–24). If the Pacific Ocean plate land arc is jammed onto the continental margin and becomes

part of the continent. The subduction zone and trench step continues to move at its present rate, the island of Hawaii

back to the seaward side of the island arc. may sink beneath the sea within 10 to 15 million years.

A new submarine volcano, called Loihi, is currently forming off the southeast side of Hawaii. As the Pacific plate moves northwest and volcanism at Loihi

amounts of continental crust are returned to the increases, this seamount will become a new Hawaiian mantle in subduction zones. This topic is discussed

island.

further in Chapter 12. Sea waves may erode a flat top on a sinking island, forming a flat-topped seamount called a guyot (Fig. 11–25). A reef commonly grows on the flat top of a

䊳 11.8 SEAMOUNTS AND

guyot while it is still in shallow water. Animals and

OCEANIC ISLANDS

plants living in a reef require sunlight and thus can live only within a few meters of sea level. However, ancient

A seamount is a submarine mountain that rises 1 kilo- reef-covered guyots are now commonly found at depths meter or more above the surrounding sea floor. An

of more than 1 kilometer, showing that the guyots con- oceanic island is a seamount that rises above sea level.

tinued to sink after the reefs died.

Seamounts and Oceanic Islands 195

NIIHAU OAHU 3.7 2.6 MOLOKAI

The Eight Principle Islands Principal

of the Hawaiian Archipelago

See inset

Figure 11–23 The Hawaiian Island–Emperor Seamount Chain becomes older in a direction going away from the island of Hawaii. The ages, in millions of years, are for the oldest volcanic rocks of each island or seamount.

Seamounts

Oceanic

Volcanic island

crust

(Hawaii)

Direction of plate Mantle movement

Figure 11–24 The Hawaiian Islands and Emperor Seamounts sink as they move away from the mantle plume.

Volcanic island

Guyot formed by

Seamount

wave erosion

Extinct reef

Subsidence of sea floor

(c)

(d)

Figure 11–25 (a) A seamount is a volcanic mountain on the sea floor. Some rise above sea level to form volcanic islands. (b) Waves can erode a flat top on a sinking island to form a guyot. (c) A reef may grow on the guyot and (d) eventually become extinct if the guyot sinks below the sunlight zone or migrates into cooler latitudes.

Submarine Canyons and Abyssal Fans England and Scandinavia, in the Gulf of Mexico, and in