mar 110: introductory oceanography ocean basins and plate tectonics

MAR 110: Introductory Oceanography

Ocean basins and plate tectonics

Rev. 21 September 2006 Ocean basins and plate tectonics 2

Geologic hazards, part 1

• Tectonic activity plays a major role in the evolution of ocean basins.

• Tectonic activity lies behind most major geological hazards: volcanic eruptions, earthquakes, and their aftereffects.

• Three of the greatest natural disasters of the past 200 years were geological in nature.– Tambora, 1815– Krakatau, 1883– Indian Ocean earthquake and tsunami, 2004



• Tambora is a volcano on the northern shores of Sumbawa.– From 5 April to 11 April 1815, Tambora came alive with

several large eruptions – the largest in recorded history. The series of eruptions finally ended in July of that year.

– The largest eruptions, from 10 April-11 April, began as “three columns of fire rising to a great height,” and culminated in the ejection of 150 cubic kilometers of magma and ash (more than 150 times Mount St. Helens).

– 90,000 died as a result of the eruptions, associated pyroclastic flows, and famines that followed.



• Tambora (continued):– The effects of Tambora were felt far from Southeast Asia.

Dust and aerosols from the eruption blocked sunlight, triggering in the following year (1816) what became known as the “year without a summer” in northeastern North America and western Europe.

• Average global temperatures dropped about 0.3 °F, which was enough to significantly disrupt agriculture in North America and Europe.



• Krakatau (Krakatoa) is a volcano in the Sunda Strait west of Java (not east of Java as in the movie title).– From 20 May to 27 August 1883, Krakatau went through a

series of increasingly violent eruptions, culminating in a final series of cataclysmic explosions that began mid-day on 26 August and ended on 27 August.

– Two-thirds of the original island disappeared.

– The sound of the final blast was heard more than 4,600 km away, from Rodriguez Island and Sri Lanka to Australia.

– At least 36,000 died in the eruptions and related tsunami (with waves up to 40 m high).


Geological hazards, part 5

• On the morning of December 26, 2004, a massive earthquake struck along a subduction zone in the Indian Ocean off the coast of Sumatra and the Andaman Islands.– The earthquake was determined (by the U.S. Geological

Survey) to be 9.1 magnitude on the Richter scale, which makes it the third largest earthquake in history.

– The massive slip on the ocean floor triggered a tsunami that raced through the Indian Ocean, killing more than 200,000.

• One victim was reported from Kenya.


John McPhee, in his book, Basin and Range, wrote that if he had to restrict his writing on plate tectonics to one sentence, it would be, “The summit of Mt. Everest is marine limestone.”

His sentence deftly captures the importance of plate tectonics theory today. Plate tectonics explains the origin of the Earth’s major structures, ranging in size from continents and ocean basins down to mountain ranges, rift valleys and oceanic islands, and from th highest peaks of the Himalayas to the deepest trenches of the Pacific.

– From Upheaval from the Abyss


The results of plate tectonic processes abound, from the mighty folds of the ancient and well worn Appalachians, the majestic escarpments of the East African Rift Valley and the Palisades of the Hudson River, and the volcanoes that imperil cities in Iceland, Indonesia, Japan and Mexico. Even idyllic island resorts in Hawaii and the Caribbean owe their existence to the movements of great slabs of the Earth’s crust. The theory explains all volcanism and most earthquake activity on Earth. Its importance reaches far beyond the earth sciences, too.



For instance, one cannot understand the diversity of life on this planet without some knowledge of both evolution and plate tectonics. In many parts of world, billions of dollars are spent to prepare buildings, roads and humans for the inevitable volcanic and earthquake event.



Distribution of the oceans

• Ocean basins and continents are unevenly distributed, with land (29 percent of the Earth’s surface) concentrated in the Northern Hemisphere and ocean (71 percent of the Earth’s surface) dominating the Southern Hemisphere.– NH: 39.3 percent land, 60.7 percent water

– SH: 19.1 percent land, 80.9 percent water


Ocean basins

• While the ocean basins are interconnected, four are largely separated by continents.– Pacific Ocean

– Atlantic Ocean

– Indian Ocean

– Arctic Ocean

• Another ocean was recognized by the International Hydrographic Organization in 2000.– The Southern Ocean was defined as waters between 60

degrees S and Antarctica.


Polar asymmetry

• In the Northern Hemisphere, the Arctic Ocean is an ocean surrounded by continents.

• In the Southern Hemisphere, Antarctica is a continent surrounded by oceans.


Arbitrary boundaries

• Longitude 150 degrees E between Australia and Antarctica separates the Pacific and Indian oceans.

• Longitude 70 degrees W between Cape Horn and Antarctica separates the Atlantic and Pacific oceans.

• Longitude 20 degrees E between the Cape of Good Hope and Antarctica separates the Indian and Atlantic oceans.

• The Bering Strait separates the Arctic and Pacific oceans.


The crust, part 1

• The crust is the solid poriton of the lithosphere; it is the geosphere’s interface with the atmosphere, biosphere, and hydrosphere.

• The crust is made of rocks, which in turn are made of one or more minerals.– Minerals are naturally occurring inorganic solids

characterized by an orderly internal arrangement of atoms and with fixed physical and chemical properties.


The crust, part 2

• Rocks are classified as one of three types:– Igneous– Sedimentary– Metamorphic

• Igneous rocks form from the cooling and crystallization of molten magma.– Magma that cools slowly within the crust forms coarse-

grained rocks such as granite.– Magma that reaches the surface becomes lava; lava cools

quickly and forms fine-grained rock such as basalt or even glassy rocks such as obsidian.


The crust, part 3

• Sedimentary rocks are composed of compacted and cemented fragments of rock and mineral grains, of partially decomposed remains of organisms, or of minerals precipitated from the water.– Sediments form from rocks that undergo physical and

chemical weathering at or near the Earth’s surface.

– Sediments are transported via water, wind, or ice, and typically settle in low-lying areas. In time, the sediments are buried beneath other sediments and compacted into layers of solid rock.

– Examples include sandstone, shale, limestone, and salt.


The crust, part 4

• Metamorphic rocks are derived from other rocks that are subjected to high pressures, high temperatures, and/or chemically active fluids.– Metamorphic rocks are crystalline like igneous rocks.

– Marble is metamorphozed limestone.

– Quartzite is metamorphozed sandstone.

• Bedrock is typically igneous, with metamorphic rock locally.– Sedimentary rocks overlie igneous or metamorphic rocks.


The crust, part 5

• The crust and the ridgid upper portion of the mantle comprise the lithosphere proper.

• Crust comes in two main types:– Continental, composed mostly of granite and rich in silica

and aluminum (sial).• Continental crust is thick (20 to 90 km), and is less dense than

oceanic crust.• Continental lithosphere ranges from 100 to 150 km in thickness.

– Oceanic, composed mostly of fine-grained rock such as basalt and rich in iron and magnesium (sima).

• Oceanic crust is thin (5 to 10 km).• Oceanic lithosphere is no more than 100 km in thickness.


The crust, part 6

• The lithosphere floats on the aesthenosphere, a deformable region of the upper mantle that exhibits plastic-like behavior.– Because continental crust is less dense than oceanic crust,

it is more buoyant, thus floats higher than oceanic crust; this is known as isostasy.

• Where ocean depths are shallow (usually less than 1,000 m), the bottom is typically of continental crust; in deeper waters (more than 4,000 m), the bottom is typically of oceanic crust.


The rock cycle

• External and internal geological processes transpofrm rock from one type to another in the rock cycle.– Rocks and their component minerals are constantly

recycled.

– The rock cycle is extremely slow.


Ocean bottom profile, part 1

• The average ocean depth is 3,800 m.– Large areas are less than 200 m deep

– Other areas are as deep as 11,000 m

• Bottom features range from extremely flat to the most rugged mountains in the world.

• Geological processes range from violent volcanism to a gentle rain of sediment and organic debris.



• Continental margins: There are three distinct zones as one moves seaward from the shore.– The continental shelf is a gentle slope that extends from the

shore out to a depth of about 130 m.

– The continental slope continues on from there, dropping rapidly in depth until about 3,000 m.

– The continental rise is a narrow transition zone between the base of the continental slope and either a flat ocean basin or a deep sea trench beyond.

• Continents end at the base of the continental rise.



• Continental shelves are characterized by a gentle slope, generally about 1 degree (about 2 m/km) seaward.– They are generally wider along tectonically passive

continental margins and narrower along tectonically active continental margins.

• The dropoff along a continental slope ranges from 1 to 25 degrees seaward, the average is about 50 m/km.– Along passive margins, the continental slope may merge

with the continental rise.



• Continental rises have somewhat steeper slopes than the ocean floor beyond.– Along passive margins, sediments may spread out from the

continental rise to form vast abyssal plains.

– Along active margins, the continental rise may descend into deep-sea trenches.



• Submarine canyons may cut into the continental slopes and shelves.– Many are carved by turbidity currents, a type of submarine

mass movement akin to an avalanche.• Turbidity currents may reach speeds of up to 100 km/hr.

– Sediments carried by turbidity currents are deposited onto submarine fans, akin to alluvial fans.

– Examples of submarine canyons include Hudson Canyon and Monterrey Canyon.



• Ocean basins include topography ranging from flat plains to trenches, seamounts, and submarine mountain ranges.


Evidence for plate tectonics, part 1

• Francis Bacon only noticed the general similarity in shape between Africa and South America – he wrote nothing about a supposed fit between the two.

“The very configuration of the world itself in its greater parts presents conformable instances which are not to be neglected. Take for example Africa and the region of Peru with the continent stretching to the Straits of Magellan, in each of which tracts there are similar isthmuses and similar promontories, which hardly can be by accident.”

Francis Bacon, Novum Organum



• Abraham Ortelius did write in 1596 that the coastlines of the Americas appeared to have been joined in the past with Europe and Africa.

“But the vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of [Europe, Africa and the Americas], where they face each other – I mean the projecting parts of Europe and Africa, of course, along with the recesses of America.”

Abraham Ortelius, Thesaurus Geographicus


. . . the great ocean basins are permanent features of the earth’s surface and they have existed, where they are now, with moderate changes of outline, since the waters first gathered.

– Bailey Willis, Principles of paleogeography



• Eduard Suess proposed a southern supercontinent, Gondwanaland, early in the twentieth century.

• Frank Bursley Taylor did propose a theory of continental drift in 1908, but it was limited to an explanation of the origin of equatorial mountain belts in the drift of continents away from the poles – hardly a comprehensive theory of continental drift as proposed by Alfred Lothar Wegener, thus I don’t regard Taylor as anything near Wegener’s equal.



• Alfred Wegener noticed the similarity in shapes of the coastlines of the Americas, Africa, and Europe.

“Please, look at a map of the world! Does not the east coast of South America fit exactly with the west coast of Africa as if they had formerly been joined? The correspondence is still better if one compares not the present coasts but the lines of descent to the deep sea.”

Alfred Wegener in a 1910 letter to his fiancée, Else Köppen

• Wegener presented his ideas publicly in 1912.


Wegener’s argument was simple: About 300 million years ago, all large land masses were united to form one supercontinent, Pangaea. Beginning at about 150 million years ago, Pangaea began breaking up and the fragments drifted apart, and, in some cases, collided again, eventually becoming the continents we recognize today. (Wegener also postulated that the process of division and collision had been active before Pangaea came into existence, but found it difficult to reconstruct pre-Pangaea geography.)



Wegener produced maps of how he thought the continents had been assembled or disassembled at various stages in time. He presented the geological, geographical, climatological and paleontological evidence in support of his theory, and he explained why competing theories did not work .




• British geologist Arthur Holmes, though not a fan of Wegener, proposed in 1930 that convection currents in the mantle could drive continental drift.

• Marie Tharp discovered a rift valley in the middle of the Mid-Atlantic Ridge in 1952.

• Tharp and Bruze Heezen used several types of data – including location of earthquake epicenters – to trace a seismic belt running through mid-ocean ridges throughout the world.


As Tharp plotted the more northern profiles, she noticed a large valley at the center of the Mid-Atlantic Ridge. Although the valley wasn’t as prominent in the three southern profiles, it was still there, typically 1000 fathoms deep and nine to 30 miles wide – as deep as the Grand Canyon, but much wider. In the six weeks it took her to prepare the profiles, Tharp became convinced that she was looking at a rift valley and told Heezen so.



Heezen did not want to hear the news. A rift valley indicated that the Earth’s crust was spreading apart, and that might mean the continents on either side of the Atlantic were getting farther and farther away from each other. If the continents were drifting apart, one would have to conclude that there was something to Alfred Wegener’s crackpot theory. Speaking out in favor of continental drift would be an act of professional suicide.



Heezen looked at Tharp’s profiles. No matter how hard he tried to make the rift valley disappear, it would not. He groaned, and said, “It can’t be. It looks too much like continental drift.



The Atlantic belt of earthquake epicenters follows the crest of the Mid-Atlantic Ridge and its prolongations into the Arctic and Indian Oceans with a precision which becomes more apparent with the improvement of our knowledge of the topography and of epicenter locations. These are all shallow shocks. Their apparent departure from the narrow crest of the ridge seldom exceeds the probable error of location.

– Maurice Ewing, Bruze Heezen, and Marie TharpThe Mid-Atlantic Ridge Seismic Belt


The crest is 30 to 60 miles wide, very rough, and on a typical section shows several peaks at depths of about 800 to 1100 fathoms. There is usually also a conspicuous median depression reaching depths of about 2300 fathoms. This is interpreted as an active oceanic rift zone which continues through the African rift valleys.

– Maurice Ewing, Bruze Heezen, and Marie TharpThe Mid-Atlantic Ridge Seismic Belt



• On March 26, 1957, Bruce Heezen gave a talk on the mid-ocean ridge seismic belt at Princeton University. Harry Hammond Hess, a leading geologist of the time stood up after the talk and said “Young man, you have shaken the foundation of geology.”



• Hess was inspired to develop a theory of sea-floor spreading, based on part on Holmes’ ideas, that said that the crust, driven by convection currents in the mantle, was created and split apart in rift zones. A comparable amount of crust was destroyed by subduction into trenches, thus ensuring that the Earth did not expand.– Robert Sinclair Dietz proposed similar ideas at about the

same time, but later said that Hess should receive priority.



• Frederick John Vine and Drummond Hoyle Matthews proposed a test of the sea-floor spreading hypothesis, using patterns of magnetic reversals recorded in rocks along the ocean floor.– If sea-floor spreading occurred, the magnetic patterns

would look roughly the same on either side of a mid-ocean ridge.

– Their proposal was published in the journal Nature.



• Another researcher, Lawrence Whitaker Morley, proposed a similar test, but his very short paper on the topic was rejected at the time, first for an alleged lack of space in Nature, and then by a Journal of Geophysical Research editor who wrote:“. . . such speculation makes interesting talk at cocktail

parties, but it is not the sort of thing that ought to be published under serious scientific aegis.”

– This was after the publication of Vine and Matthews!

• Ocean research, largely conducted by Lamont-Doherty, eventually proved sea-floor spreading.


Plate boundaries

• John Tuzo Wilson, became a convert to plate tectonics and made many contributions to the development of the theory, including the determination that tectonic plates are separated by three types of boundaries.– Divergent plate boundaries

– Convergent plate boundaries

– Transform faults


Divergent boundaries, part 1

• Rifts, whether in the depths of the ocean or an land, mark divergent plate boundaries, where convection currents moving in opposite directions pull plates apart. Magma wells up through the split to become lava and cools to form new, essentially oceanic, crust.– Mid-Atlantic Ridge and Iceland

– Red Sea and East African Rift Valley

– Connecticut lower Hudson river valleys are remnants of former rifts


Divergent boundaries, part 2

• Lava that emerges along a deep-sea rift forms tube-like and pillow-like structures.

• Shallow earthquakes often accompany volcanic eruptions.

• New oceanic crust is warm and is less dense, thus rides higher on the mantle – leading to the ridges on either side of the rift. As the rock cools and condenses, it sinks.– Ocean depths increase with distance from the rift.


Convergent boundaries, part 1

• Convergent plate boundaries form where tectonic plates collide head on. The type of feature that is formed depends on the type of plates involved.– Where an oceanic plate, which is more dense, collides with

a continental plate, which is less dense, the oceanic plate is forced below the continental plate, forming a subduction zone, which features offshore trench. Subduction zones are often accompanied by a mountain range and/or volcanic arc on the continental side of the trench.

• Cascadia Subduction Zone and Cascade Range



• Features of convergent boundaries (continued):– Where two oceanic plates collide, trenches typically form

on the seaward size while island arcs form in shallow seas on the continental side.

• Sunda Trench and Sunda Arc in Indonesia

• Islands of Japan

• Aleutian Islands



• Features of convergent boundaries (continued):– Where two continental plates collide, great mountain belts

form.• Appalachians in North America

• Alps in Europe

• Atlas in Africa

• Himalayas in Asia


Transform faults

• Transform faults occur where two tectonic plates slide horizontally past one another.

• Once called fracture zones where noticed on the ocean floor.– San Andreas fault is arguably the most famous transform

fault.


Hot spots

• Hot spots are sites of long-term upwelling of hot magma.

• They may form chains of mountains or islands where tectonic plates drift over the site of the hot spot.– Hawaiian Islands and Emperor Seamount Chain

– Yellowstone


Atolls

• Atolls are ring-shaped islands surrounding a seawater lagoon.

• They form from subsidence of seamounts coupled with growth of fringing reefs of coral.– Coral growth keeps up with subsidence.

– Charles Darwin proposed the accepted theory of atoll formation after observations of atolls on the HMS Beagle expedition.


Hydrothermal vents

• Hydrothermal vents form in divergent margins along fractures in the sea floor.

• Waters as high as 400 °C discharge through the vents.– These waters are often rich in minerals, giving the water a

black color; chimneys formed from such vents are called black smokers.

• Hydrothermal vents are home to unique communities of organisms that do not depend on light from the sun for existence; rather than photosynthesis, the communities are based on chemosynthesis.


Wilson cycles

• Wilson cycles, named for John Tuzo Wilson who first described them, are cycles of repeated closing and opening of ocean basins.

• They have six stages:– Embryonic: formation of a rift valley

– Juvenile: rift valley widens and connects to an ocean

– Mature: ocean basin lined with passive margins

– Declining: subduction widespread along rim of ocean basin

– Terminal: narrowing of the sea as continents close together

– Suturing: collision of continents complete.

mar 110: introductory oceanography ocean basins and plate tectonics

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