Introduction to Ocean Sciences

Chapter 4: Plate Tectonics: Evolution of the Ocean

Guide to Reading and Learning

Until recently, most texts have treated plate tectonics as a theory, not as an accepted process. However, for the first time we are able to include figures in this text that show the actual rates and directions of movements of the tectonic plates measured by technologies that did not exist a decade or so ago. As a result, plate tectonics is no longer a theory. It is an accepted fact that there are tectonic plates and that these plates are in constant motion. Of course, many of the details of how and why they move, how they have migrated in the Earth’s past history, and how their current directions and rates of motion will change over time are not known.

This chapter describes what we now know of the processes of plate tectonics and adds information about new and emerging theories related to how plate tectonics works. As plate tectonic processes are described for each different type of plate margin, the topographical features of the sea floor and continents that are associated with these processes are described and the ways these features are formed are discussed.

Also described are the ways in which topographic features of the plates and plate margins are modified as they move into or away from the margin; examples are provided of where on Earth’s current surface such features are found. After reviewing the processes that occur at plate margins, the chapter describes the many hot spots that do not lie on plate boundaries and the interiors of the plates themselves. Hot spots are of particular interest because they appear to exist somewhat independently of the processes and motions of the tectonic plates themselves and their origins are not well understood. For example, until just the past several years, it was believed that hot spots remained fixed with respect to the Earth itself and only the tectonics plates moved over them. However, we now know that both the tectonic plates and the hot spots migrate, and this has yet to be explained.
The last section in the chapter reviews the characteristics of the major ocean basins and how these characteristics are related to the history of plate tectonics in each ocean basin. You will want to refer frequently to the inside front cover map of land and seafloor topography as you read this chapters, especially when you get to the last section.

Before you read this chapter, be sure to study at least the “Essential to Know” sections of Critical Concepts 1, 2, and 3, as a basic understanding of density-driven convection and isostasy is essential to grasping many of the concepts in this chapter. This is the first chapter in which familiarity with the critical concepts listed at the beginning of the chapter is absolutely essential to your understanding of many of the key concepts of the chapter.  For all later chapters, you should make sure you comprehend all the critical concepts listed for that chapter.

Chapter 4 Essential to Know 

Critical Concepts used in this chapter

CC.1, CC.2, CC.3, CC.7, CC.11, CC.14

 
4.1 Structure of the Earth

  • The continents and ocean basins are continually being reshaped by a process called plate tectonics that originates deep within the Earth.

Layered Structure of the Earth

  • The Earth consists of several layers ordered by density.
  • The core is about 7000 km in diameter and composed primarily of iron and nickel. It consists of a solid central core and an outer liquid core
  • The mantle that surrounds the core is about half as dense as the core. The upper mantle is called the asthenosphere and is thought to be plastic and, as a result, convection occurs within it.
  • The asthenosphere is surrounded by the outermost layer, the lithosphere, which is very thin, varying from about 100 to 150 km thick beneath the oceans and about 250 to 300 km under the continents.

Lithosphere, Hydrosphere, and Atmosphere

  • The lithosphere consists of two types of thin crust—oceanic and continental—fused to a thin layer of solid upper mantle material. The crust and fused upper mantle material are broken into a number of separate tectonic plates.
  • The tectonic plates “float” on the plastic mantle and rise or sink toward an isostatic equilibrium if their average density changes.
  • The oceanic crust surface is lower than the continental crust surface because oceanic crust is thinner and has higher density than continental crust.
  • Continental crust has such a low density that it always remains at the surface. However, oceanic crust can be subducted into the mantle and new oceanic crust can be formed. This is the basic process of plate tectonics
  • The submerged edges of the continents are called the continental shelf
  • At the edge of the shelf the slope changes at the shelf break and the seafloor descends in an area called the continental slope. At the base of the continental slope the seafloor either flattens out to become abyssal plain or descends further into a deep trench seaward of which lies the abyssal plain.
  • Most of the abyssal plain is flat and covered in layers of sediments but there are many rolling abyssal hills in some areas.
  • The hydrosphere consists of all the water in the oceans and all surface freshwater.
  • The atmosphere is an envelope of gases comprised of mostly nitrogen and oxygen.

Studying the Earth’s Interior

  • The processes occurring below the lithosphere are difficult to study because we cannot visit or sample the mantle directly. Instead studies of volcanic rocks, meteorites, and the transmission of earthquake energy through the Earth must be used.
  • Seismic tomography is a technique that uses earthquake waves passing through the planet’s interior in a manner similar to human body CAT scans. This technique provides three-dimensional images of the Earth’s interior. Seismic tomography is beginning to reveal some details of the complexity of the Earth’s interior and the convection processes occurring there.

4.2 Plate Tectonics

  • Over geological time oceans and continents have formed, and disappeared, and been replaced many times.

Driving Forces of Plate Tectonics

  • .Movement of lithospheric plates is thought to be caused by convection in the mantle driven by heat that is generated partly by radioactive decay in the Earth’s core and mantle.
  • Very little is known about the details of the convection cells, but both a deep convection that extends to the core mantle boundary and a shallow convection may exist. It has been hypothesized that there may be several mesoplates in the mantle that lie below the lithospheric plates and move independently..
  • Downwelling of lithospheric plates supporting oceanic crust takes place at the deep trenches that are found at the base of the continental slope around some continents. This downwelling is called subduction, and the subducted blocks of lithosphere may, in some instances, descend all the way to the core–mantle boundary before being reheated sufficiently to allow the material they contain to rise again.
  • Areas of mantle upwelling are thought to include two areas where superplumes of warm mantle material are rising, one beneath much of Africa and another beneath the southwest and central Pacific Ocean.
  • Mantle upwelling also takes place at a number of geographically small hot spots scattered throughout the globe. Hawaii is an example. The plume of upwelling material at hot spots like Hawaii is thought to originate near the core–mantle boundary, but plumes at other hot spots may originate from shallower depths within the mantle or in superplumes.
  • The principal process that causes the tectonic plates to move across the Earth’s surface is the drag created when one edge of the plate is downwelled at a subduction zone. However, lithospheric plates may also move due to gravity as young warm new oceanic crust at the oceanic ridges slides downhill toward older crust that has cooled and sunk isostatically.
  • It was once thought that mantle upwelling occurred at the oceanic ridges that run through the center of the ocean basins, but it is now believed that the oceanic ridges are formed by mantle material from just below the crust. This mantle material rises to fill the gap left as the plates are pulled apart when subduction drags the plates apart.

Present-Day Lithospheric Plates 

  • There are, at present, seven major plates—Pacific, Eurasian, African, North American, South American, and Indo-Australian—and several smaller ones.
  • Plates migrate a few centimeters per year, but this is enough to move them across the entire planet within a few tens of millions of years.
  • The processes that cause oceanic crust to be formed and subducted occur at plate edges. Plates can be expanded or progressively destroyed, modified at their edges, and moved across the planet’s surface.
  • Most earthquakes occur at plate boundaries as the plates move relative to each other. Many of the Earth’s mountain ranges were originally created at plate boundaries.

Spreading Cycles

  • The continents have apparently been collected into one supercontinent, split apart, recollected into a supercontinent, and split apart again a number of times.
  • The present continents were transported to their current locations during the time that has elapsed since a supercontinent broke apart about 225 million years ago.

4.3 Plate Boundaries

  • Plate margins are either convergent, divergent, or transform. Adjacent plates may have oceanic crust at both edges, have continental crust at both edges, or have continental crust on one edge and oceanic crust on the other.

4.4 Convergent Plate Boundaries

  • There are three types of convergent boundaries: continental crust–oceanic crust; oceanic crust–oceanic crust; and continental crust–continental crust.
  • In the first two types of convergent plate boundary, the oceanic crust is subducted into the mantle.
  • Subduction occurs because oceanic crust, which is warm and lower density when it is first formed, progressively cools, becomes denser, and sinks isostatically to float lower on the asthenosphere.  It eventually becomes dense enough to sink entirely into the mantle.

Subduction Zones Next to Continents

  • At convergent margins where one plate has oceanic crust and the other continental crust next to the margin, the plate with the oceanic crust is subducted. These margins are characterized by a deep trench, narrow continental shelf, exotic terranes and coastal mountain ranges, and a line of volcanoes on the nonsubducting continental crust–edged plate.

Volcanoes

  • Volcanoes form 10 to 100 km away from the continent edge on the crust of the nonsubducting plate as the subducted oceanic crust and sediments descend at an angle, are heated, and melt under the nonsubducting plate.
  • These volcanoes often erupt explosively because the subducted material contains water and other constituents that become gases as they rise and pressure is reduced.

Exotic Terranes        

  • Exotic terranes are formed when small pieces of continental crust or extinct volcanoes on the subducting plate reach the subduction zone and are too large and low-density to be subducted. Instead they are scraped off and welded to edge of the continent with the nonsubducting plate.

Subduction Zones at Magmatic Arcs

  • At convergent margins where both plates have oceanic crust next to the margin, the older (thus, coldest and higher-density) oceanic crust of one plate is subducted below the oceanic crust of the adjacent plate. These margins have a deep trench and narrow continental shelf, sometimes a chain of low sedimentary arc islands, a chain of magmatic arc volcanic islands on the nonsubducting plate, and sometimes a back arc basin behind these islands.

Magmatic Arc Volcanoes

  • Volcanoes form on the nonsubducting plate, often creating an arc of volcanic islands on this younger plate. These volcanoes erupt explosively.
  • At both types of subduction zone, the volcanoes that form are set back on the nonsubducting plate by a distance that reflects the age of the oceanic crust on the subducting plate. If old, cold, and dense crust is being subducted, its angle of descent is steeper and the volcanoes on the nonsubducting plate form nearer to the edge of this plate.

Back-Arc Basins

  • If the rate of subduction of a plate is high enough it can exceed the rate at which the two plates move toward each other. In this case, the edge of the nonsubducting plate can be stretched and thinned, creating a back arc basin that may be stretched so much in some cases that magma rises to form new oceanic crust in the basin.

Collisions of Continents

  • At convergent margins where both plates have continental crust next to the margin, neither plate edge can be subducted because both plate edges are continental crust and their density is low. These convergent margins are characterized by high mountain chains at the margin and a high plateau on the overriding plate that is formed when one plate rides under the other.

4.5 Divergent Plate Boundaries    

  • New oceanic crust is formed at divergent margins called oceanic ridges, which are characterized by a range of undersea mountains that occasionally rise above sea level.

Oceanic Ridges

  • Oceanic ridge mountains are aligned in a chain that follows the margin between plates, and there is usually a central rift valley in which the sea floor is deeper than the adjacent mountains.
  • When two plates are pulled apart at an oceanic ridge, magma rises up between the separating plates and solidifies to form new oceanic crust.
  • Because the newly formed oceanic crust is warm, it floats high in the asthenosphere, creating the elevated oceanic ridge mountain chain.
  • After new oceanic crust is formed it cools slowly, its density is increased, and it sinks isostatically as it moves away from the plate boundary. It is also progressively covered by layers of sediments, reducing the topographic roughness.

Oceanic Ridge Types  

  • There are three types of oceanic ridge distinguished by differences in their spreading rate.
  • Oceanic ridges with moderate spreading rates, as in the Atlantic Ocean, have a well-defined central rift valley aligned down the middle of the ridge chain. This valley is typically 25–50 km wide and 1–2 km below the adjacent peaks.
  • At faster-spreading oceanic ridges, such as the East Pacific Rise, the mountains are lower and less rugged, and the central valley is absent or much less well-defined.
  • At the slowest-spreading oceanic ridges, most magma is cooled and solidified before it reaches the seafloor. Where the plates pull apart, the new oceanic crust is formed by material that is already solid before it rises to form the new seafloor. This type of ridge has very few volcanoes compared to the other types.

Oceanic Ridge Volcanoes

  • There are many volcanoes along most oceanic ridges. However, they erupt quietly, without violent explosions. Humans are often unaware of the eruptions.
  • Oceanic ridge volcanoes are nonexplosive for several reasons. First, the magma does not contain sediments and water, unlike magma in subduction zone volcanoes. Second, the water pressure is high and elevates the boiling point of water so that it does not boil even when molten magma is erupted and cooled by direct contact with the water. Third, water movements carry heat away from the eruption site and bring very cold water continuously into the area where magma is in contact with the water.

Rift Zones

  • At divergent margins within a continent, a tectonic plate splits apart to form two new plates. This type of margin is characterized by a rift valley along which are found volcanoes and lakes. These margins quickly (at least in geological time) become oceanic divergent margins if the rifting continues.

New Oceans

  • If rifting continues, the continent is separated permanently and a new ocean is formed with its own oceanic ridge formed by rising magma.
  • The new continent edges formed at a rift zone progressively cool and sink as they move away from each other to form the sides of a new ocean. These edges eventually become passive margins that are characterized by a flat coastal plain cut by river valleys, and a wide continental shelf.

4.6 Transform Faults and Fracture Zones 

  • Transform faults occur where two plates slide past each other.
  • Transform faults are needed at intervals along convergent or divergent margins to accommodate the movement of the plates on a spherical surface.
  • These faults are characterized by rugged topography on either side of the fault. This topography remains as a fracture zone when the plate edges become fused again at the end of the transform fault.
  • There are frequent earthquakes along a transform fault but there are few or no earthquakes in the fracture zone as the topography of the transform fault has now been transported entirely onto one or another of the two plates where it is now no longer in the area where the two plates meet and slide past each other.

4.7 Hot Spots 

  • Hot spots are locations of mantle upwelling. Volcanoes formed at hot spots can be very tall.
  • As the plate moves, these volcanoes move away from the hot spot, cool, and sink.
  • Chains of hot spot volcanoes show the combined relative directions and rates of motion of the tectonic plates and the hot spots beneath them.
  • As islands move away from hot spots, they cool, increase in density, and sink isostatically. Their tops may be eroded when the volcano has sunk so that the area of the island above sea level becomes small. Eventually the island sinks further and becomes a submerged seamount with a flattened top.
  • Most hot spots are small in area at the Earth’s surface but some are now thought to be upwelling plumes (columns) that extend all the way from the core–mantle boundary to the Earth’s surface.
  • There are thought to be areas of upwelling in the mantle called superswells that are much larger than hot spot plumes. At present there appear to be two such superswells, one located beneath the African continent and one located beneath the floor of the southwest Pacific Ocean.

A Lesson about Science

  • The trail of islands and seamounts that extends from Hawaii to Asia was created as the relative motions of the plate and hot spot in the mantle changed.
  • Studies have shown that the age of each island along the hot spot trail increases as you move from Hawaii to Asia.
  • The islands and seamounts that are less than about 50 million years old are aligned in an approximately straight line, but older islands form an approximate straight line that is angled in a more northerly direction.
  • Until recently, the change of direction in the Hawaii hot spot trail was accepted by most of the scientific community to indicate that the direction of motion of the Pacific Plate changed 50 million year ago. It was also accepted that hot spots did not move.
  • Recent studies have shown that each of the islands older than 50 million years was formed at a more northerly latitude. This means that prior to 50 million years ago, the hot spot must have moved to the south and the Pacific Plate did not apparently change direction.
  • The discovery that hot spots move is a good example of how accepted “facts” about earth science are not facts but, instead, are only the most recent consensus view and can change.

4.8 Plate Interiors

  • Processes including isostatic changes and sedimentation alter oceanic crust topography during the tens to hundreds of millions of years between its formation and its destruction at subduction zones. Continent crust topography is created primarily at plate boundaries and is modified by isostatic changes and erosional processes.

Oceanic Crust

  • As it moves away from the oceanic ridge where it was created, oceanic crust is progressively buried in layers of sediments that reduce its topographic roughness.
  • Oceanic crust also cools with age and distance from the oceanic ridge and sinks as its density is increased as a result of this cooling and the weight of accumulating sediments.
  • As volcanoes sink isostatically in tropical regions they may develop a fringing coral reef that grows upward as the island sinks. This reef becomes a barrier reef separated from the sinking island by a lagoon.  Eventually, when the island has completely sunk below sea level, it can become an atoll
  • If the volcano sinks faster than the coral can grow upward (or if the volcano is in higher latitudes), it sinks until it is small enough that its top is eroded by wave action. It then sinks further which causes the sunken volcano to become a flat-topped sea mountain called a guyot.

Continental Edges

  • Fewer than half of today’s coasts lie on the edge of tectonic plates. Coastlines that lie within tectonic plates are called passive margins.

Passive Margins

  • Passive margins develop as a rift widens to create a new ocean, with new oceanic crust separating the continental crust on the two diverging plates.
  • Initially the passive margin crust is warm and, therefore, low density, and floats high in the asthenosphere. As a result, the land slopes away from the coast and most rivers do not drain into or contribute suspended sediments to the new ocean. This causes the new ocean to have high biological productivity.
  • As the passive margin moves away from the oceanic ridge, the crust cools and sinks isostatically until eventually the slope of the land is reversed and rivers now empty their suspended loads to the new ocean, which lowers productivity.
  • Organic matter deposited when the passive margin is new and ocean productivity is high is buried by sediments after the land slope reverses. These deposits may then be heated and converted to oil and gas deposits.

The Fate of Passive Margins

  • If the oceanic crust next to a passive margin cools sufficiently to become high enough in density to sink into the asthenosphere, subduction may start and the passive margin will become a subduction zone.
  • The west coast of Africa may soon (in geological time) become a subduction zone, as the oceanic crust is aging and the adjacent continental crust is elevated due to a heat buildup in the mantle below the African continent. This elevation is evidenced by the shallow and narrow continental shelf in this location compared to, for example, the shelf off the North Atlantic coast of the Americas.

4.9 Sea-Level Change and Climate

  • Changes in climate and sea level affect coastlines and seafloor topography. Both are relatively stable today but are in a constant state of change.

Climate Cycles

  • Over the past 1,000 years the Earth’s average temperature has varied by about 0.5oC, and over the past 10,000 years by about 2–3oC.
  • Two to three million years ago, the Earth’s temperature was as much as 10oC below today’s temperature. This cold period was known as an ice age.
  • Climate varies on time scales ranging from a few years to tens or hundreds of millions of years.

Eustatic Sea-Level Change

  • Sea level changes eustatically when the Earth’s climate changes. When the Earth is warmer, sea level rises and when the Earth cools, sea level falls. This change is primarily due to thermal expansion of ocean water, but also due to the creation and melting of ice (glaciers) on the continents.
  • During the warmest period since the breakup of Pangaea, sea level was much higher and covered about 40% of the current landmass.
  • At the peak of the most recent ice age, 20,000 years ago, sea level was about 100 m below its current level.
  • Sea level has risen and fallen many times since the breakup of Pangaea and the coastline has migrated back and forth accordingly.

Sea-Level Change and Continental Margin Topography

  • Wind, wave, and river erosion of coastlines and inland areas tends to flatten topography and deposit sediments in low-lying areas, further reducing topography.
  • Sea-level oscillations have subjected the area between the continental shelf edge and an elevation of several tens of meters above current sea level to alternate cycles of sediment deposition and wind, wave, and river erosion. Progressive flattening of topography by these processes has been more effective at passive margins because their topography has not been subjected to the continuous tectonic changes that affect other margins.
  • The continental shelf has many valleys that were cut by rivers and then drowned by rising sea level in the past 15,000 years.

Isostatic Sea-Level Change

  • Sea level rises or falls on various coastlines as the edge of the continent rises or falls isostatically.
  • Many coastlines in mid and high latitudes are still rising isostatically in response to glacier melt after the last ice age. On these coastlines, this isostatic rise partially offsets the current eustatic sea level rise.
  • Melted glaciers have left many steep-sided valleys that are now filled with sea water.  These valleys are known as fjords

Sea Level and the Greenhouse Effect

  • Sea level oscillations are normal, but human civilization developed in a period of unusually stable sea levels.
  • Sea level is currently rising at about 2–3 mm∙yr–1 but may rise much faster if the enhanced greenhouse effect causes the planet to warm by several degrees this century. A sea level rise of only a meter or two would inundate vast areas of land, including many major coastal cities.

4.10 Present-Day Oceans

  • All of the present-day oceans are connected. However, the Arctic Ocean is divided from the Pacific Ocean by a shallow sill and only connected with the Atlantic Ocean by a relatively narrow, deep passage.

Pacific Ocean

  • The Pacific Ocean is the largest ocean and, on average, the deepest.
  • It is almost completely surrounded by the trenches of subduction zones and a ring of coastal mountains that restrict river input and the amount of sediment that reaches the deep ocean floor. As a result, the Pacific Ocean abyssal floor is not as flat as the Atlantic and Indian Ocean floors.
  • The Pacific has many islands—some created at hot spots, some fragments of continents, and some created at magmatic arcs.
  • The Pacific Ocean has the oldest oceanic crust, but not by a substantial amount, as all crust that existed before about 170 million years ago has been subducted.

Atlantic Ocean

  • The Atlantic Ocean is a relatively young ocean and is currently still expanding with passive margins on both sides.
  • The Atlantic has fewer islands than the Pacific and receives more suspended sediment so its abyssal topography is smoother.
  • Greenland, the largest island in the Atlantic, is actually a part of North America that is separated from the rest of the continent by only by a shallow flooded continental shelf.

Indian Ocean

  • The Indian Ocean is the youngest ocean.
  • The northern part of the Indian Ocean is dominated by the India–Eurasia collision and the world’s three largest rivers erode the newly formed mountains and carry large amounts of suspended sediments into the ocean.
  • To the east of India the Indian Ocean is bounded by the very active subduction zone of the Indonesian magmatic arc.
  • To the west of India, the Red Sea is a rift zone that becomes an oceanic ridge at its south end.
  • Passive margins are present along much of the West African coast, most of India, and Australia.

Marginal Seas

  • Arms of the ocean that are partially isolated from exchange with the major oceans are called marginal seas.
  • There are four types of marginal sea.
  • One type consists of shallow submerged areas of continental crust—for example, the Baltic Sea.
  • A second type is formed in back arc basins behind subduction zones—for example, the Java Sea.
  • The third type comprises narrow remnants of a closing ocean—for example, the Mediterranean Sea.
  • A fourth type is made up of narrow arms that form as a new ocean begins to open—for example, the Red Sea

Critical Concept Reminders:

CC.1Density and Layering in Fluids (p. 70)

  • The Earth and all other planets are arranged in layers of different materials sorted by their density. To read CC1 go to page 2CC.

CC.2 Isostasy, Eustasy, and Sea Level (pp. 71, 85, 91, 96, 97, 99)

  • Earth’s crust floats on the plastic asthenosphere. Sections of crust rise and fall isostatically as temperature changes alter their density and as their mass loading changes due to melting or to the formation of ice stemming from climate changes. This causes sea level to change on the coast of that particular section of crust. Sea level can also change eustatically when the volume of water in the oceans increases or decreases due to changes in water temperature or changes in the amount of water in glaciers and ice caps on the continents. Eustatic sea level change takes place synchronously worldwide and much more quickly than isostatic sea level changes. To read CC2 go to page 5CC.

CC.3 Convection and Convection Cells (pp. 73, 77, 87, 90)

  • Fluids that are heated from below, such as Earth’s mantle, or ocean water, or the atmosphere, rise because their density is reduced. They continue to rise to higher levels until they are cooled sufficiently, at which time they become dense enough to sink back down. This convection process establishes convection cells in which the heated material rises in areas of upwelling, spreads out, cools, and then sinks at areas of downwelling. To read CC3 go to page 10CC.

CC.7 Radioactivity and Age Dating (pp. 70, 91)

  • Some elements have naturally occurring radioactive (parent) isotopes that decay at precisely known rates to become a different (daughter) isotope, which is often an isotope of another element. This decay process releases heat within the Earth’s interior. Measurement of the concentration ratio of the parent and daughter isotopes in a rock or other material can be used to calculate its age, but only if none of the parent or daughter isotopes have been gained or lost from the sample over time. To read CC7 go to page 18CC.

CC.11 Chaos (p. 73)

  • The nonlinear nature of many environmental interactions makes complex environmental systems behave in sometimes unpredictable ways. It also makes it possible for these changes to occur in rapid and unpredictable jumps from one set of conditions to a completely different set of conditions. To read CC11 go to page 28CC.

CC.14 Photosynthesis, Light, and Nutrients (p. 95)

  • Chemosynthesis and photosynthesis are the processes by which simple chemical compounds are made into the organic compounds of living organisms. The oxygen in Earth’s atmosphere is present entirely as a result of photosynthesis. To read CC14 go to page 46CC.

 

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