chapter 3 earth structure and plate tectonics oceanography an invitation to marine science, 7th tom...

Chapter 3Earth Structure and Plate Tectonics

OceanographyOceanographyAn Invitation to Marine Science, 7thAn Invitation to Marine Science, 7th

Tom GarrisonTom Garrison

Chapter 3 Study Plan

• Pieces of Earth’s Surface Look Like They Once Fit Together• Earth’s Interior Is Layered• The Study of Earthquakes Provides Evidence for Layering• Earth’s Inner Structure Was Gradually Revealed• The New Understanding of Earth Evolved Slowly• Wegener’s Idea Is Transformed• The Breakthrough: From Seafloor Spreading to Plate

Tectonics• Plates Interact at Plate Boundaries• A Summary of Plate Interactions• The Confirmation of Plate Tectonics• Scientists Still Have Much to Learn about the Tectonic

Process

Chapter 3 Main Concepts

• Earth’s interior is layered, and the layers are arranged by density. Each deeper layer is denser than the layer above.

• Continents are not supported above sea level by resting mechanically on a rigid base. Instead, continents rise to great height because they “float” on a dense, deformable layer beneath them. In a sense, they are supported at Earth’s surface in the same way a boat is supported by water.

• The brittle surface of Earth is fractured into about a dozen tile-like “plates.” Movement of the subterranean material on which these plates float moves them relative to one another.

• Continents and oceans are formed and destroyed where these plates collide, flex, and sink.

• The centers of the relatively lightweight continents are often very old. But because plate movement often forces them back into Earth, the heavy ocean floors are invariably young – no more than 200 million years (only about 1/23rd the age of Earth).

• Compelling evidence for plate movement is recorded in symmetrical remnant magnetic fields in the ocean floors.

Earth’s Interior is Layered Inside

• Density is a key concept for understanding the structure of Earth

• Density measures the mass per unit volume of a substance.

Density = _Mass_ Volume

• Density is expressed as grams per cubic centimeter.

• Water has a density of 1 g/cm3

The Study of Earthquakes Provides Evidence for Layering

What evidence supports the idea that Earth has layers?

The behavior of seismic waves generated by earthquakes give scientists some of the best evidence about the structure of Earth. Low frequency pulses of energy generated by the forces that cause earthquakes can spread rapidly through Earth in all directions and then return to the surface.

(a) Earthquake waves passing through a homogenous planet would not be reflected or refracted (bent). The waves would follow linear paths (arrows).

(b) In a planet that becomes gradually denser and more rigid with depth, the waves would bend along evenly curved paths.

(c) P Waves (compressional waves) can penetrate the liquid outer core but are bent in transit. A P-wave shadow zone forms between 103 and 143 from an earthquake’s source.

(d) Our earth has a liquid outer core through which the side-to-side S waves cannot penetrate, creating a large “shadow zone” between 103 and 180 from an earthquake’s source. Very sensitive seismographs can sometimes detect weak P-wave signals reflecting off the solid inner core.

Earth’s Inner Structure Was Gradually Revealed

A cross section through Earth showing the internal layers.

Note: in the expanded section the relationship between lithosphere and asthenosphere, and between crust and mantle.

The thin oceanic crust is primarily basalt, a heavy dark-colored rock.

The thicker continental crust is granite, a speckled rock.

The mantle is thought to consist mainly of oxygen, iron, magnesium, and silicon.

The outer and inner core consist mainly of iron and nickel.

Earth’s Layers May Be Classified by Composition

When we examine the chemical and physical properties of Earth’s layers, we see that a cool, rigid, less dense layer (the lithosphere) floats on a hot, slowly-flowing, more dense layer (the asthenosphere).

Layer Physical Properties

Lithosphere Cool, rigid, outer layer

Asthenosphere Hot, partially melted layer which flows slowly

Mantle Denser and more slowly flowing than the asthenosphere

Outer Core Dense, viscous liquid layer, extremely hot

Inner Core Solid, very dense and extremely hot

Earth’s Layers May Also Be Classified by Physical Properties

Note that Earth is density stratified; that is, each deeper layer is denser than the layer above.

Layer Chemical Properties

Continental Crust Composed primarily of granite

Density = 2.7 g/cm3

Oceanic Crust Composed primarily of basalt

Density = 2.9 g/cm3

Mantle Composed of silicon, oxygen, iron, and magnesium

Density = 4.5 g/cm3

Core Composed primarily of iron

Density = 13 g/cm3

(Right) The concept of buoyancy is illustrated by a ship on the ocean. The ship sinks until it displaces a volume of water equal to the weight of the ship and its cargo.

Icebergs sink into water so that the same proportion of their volume (about 90%) is submerged. The more massive the iceberg, the greater this volume is.

Isostatic Equilibrium Supports Continents above Sea Level

Think for a moment about the lithosphere. Why doesn’t it sink into the asthenosphere? How are features such as mountains supported?

Earth’s continental crust and lithosphere are supported on the on the denser asthenosphere. Instead of buoyancy, the term isostatic equilibrium describes the way the lithosphere is supported on the asthenosphere.

Isostatic Equilibrium Supports Continents above Sea Level

Erosion and isostatic readjustment can cause continental crust to become thinner in mountainous regions.

(a) A mountain range soon after its formation.

(b) As mountains are eroded over time, isostatic uplift causes their roots to rise. The same thing happens when a shi is unloaded or an iceberg melts.

(c) Further erosion exposes rocks that were once embedded deep within the peaks. Deposition of sediments away from the mountains often causes nearby crust to sink.

• The age of Earth has been subject to debate. Scientists now use an age of 4.6 billion years.

• The principle of uniformitarianism was introduced in 1788. This principle sates that the forces which shaped Earth are identical to forces working today.

• Catastrophism is the thought that Earth is very young, and events described in the Bible are responsible for the appearance of Earth’s features.

The Age of Earth Was Controversial and Not Easily Determined

The Fit Between Continental Edges Suggested That They Might Have Drifted

The fit of all the continents around the Atlantic at a water depth of about 137 meters (450 feet), as calculated by Sir Edward Bullard at the University of Cambridge in the early 1960s. This widely reproduced graphic was an effective stimulus to the tectonic revolution.

• Alfred Wegener’s theory of continental drift was out of favor with the scientific community until new technology provided evidence to support his ideas.

• Seismographs revealed a pattern of volcanoes and earthquakes.

• Radiometric dating of rocks revealed a surprisingly young oceanic crust.

• Echo sounders revealed the shape of the Mid-Atlantic Ridge

The Fit Between Continental Edges Suggested That They Might Have Drifted

• The ideas of continental drift and seafloor spreading were tied together in the theory of plate tectonics. Main points of the theory include:– Earth’s outer layer is divided into lithospheric

plates– Earth’s plates float on the asthenosphere– Plate movement is powered by convection

currents in the asthenosphere seafloor spreading, and the downward pull of a descending plate’s leading edge.

The Theory of Plate Tectonics

Where does the heat within Earth’s layers come from?Heat from within Earth keeps the asthenosphere flowing. This allows the lithosphere to keep moving. Most of the heat that drives the plates is generated by radioactive decay, given off when nuclei of unstable elements break apart.

(Left) The tectonic system is powered by heat. Some parts of the mantle are warmer than others, and convection currents form when warm mantle material rises and cool material falls. Above the mantle floats the cool, rigid, lithosphere, which is fragmented into plates. Plate movement is powered by gravity: The plates slide down the ridges at the places of their formation; their dense, cool leading edges are pulled back into the mantle.

The Theory of Plate Tectonics

Most Tectonic Activity Occurs at Plate Boundaries

Seismic events worldwide, January 1977 through December 1986. The locations of about 10,000 earthquakes are colored red, green, and blue to represent event depths of 0-70 kilometers, 70-300 kilometers, and below 300 kilometers.


The major lithospheric plates, showing their directions of relative movement and the location of the principal hot spots. Most of the million or so earthquakes and volcanic events each year occur along plate boundaries.

(Above) Plate boundaries in action. As Plate A moves to the left (west), a gap forms behind it (1) and an overlap with Plate B forms in front of (2).

Sliding occurs along the top and bottom sides (3). The margins of Plate A experience the three types of interactions: At (1), extension characteristic of divergent boundaries; at (2), compression characteristic of convergent boundaries; and at (3), the shear characteristic of transform plate boundaries.


The lithospheric plates interact with the neighboring plates in several ways. (1) Divergent, (2) Convergent, (3) Transform.


(Right) Extension of divergent boundaries causes splitting and rifting.

The lithospheric plates interact with the neighboring plates in several ways.

Divergent plate boundaries – Boundaries between plates moving apart, further classified as:

– Divergent oceanic crust – for example, the Mid-Atlantic Ridge– Divergent continental crust - for example, the Rift Valley of East Africa.


Convergent Plate Boundaries - Regions where plates are pushing together.

(Below) Compression at convergent boundaries produces buckling and shortening.


(above) Translation at transform boundaries causes shear.

Transform plate boundaries - locations where crustal plates move past one another, for example, the San Andreas fault.


Ocean Basins Are Formed at Divergent Plate Boundaries

Divergent plate boundaries – Boundaries between plates moving apart, further classified as:

Divergent oceanic crust – for example, the Mid-Atlantic Ridge

Divergent continental crust – for example, the Rift Valley of East Africa.

(Figure to the Right)

(a) As the lithosphere began to crack, a rift formed beneath the continent, and molten basalt from the asthenosphere began to rise.

(b) As the rift continued to open, the two new continents were separated by a growing ocean basin. Volcanoes and earthquakes occur along the active rift area, which is the mid-ocean ridge. The East African Rift Valley currently resembles this stage.

(c) A new ocean basin (shown in green) of the Atlantic. resembles this stage.


Seafloor spreading was an idea proposed in 1960 to explain the features of the ocean floor. It explained the development of the seafloor at the Mid-Atlantic Ridge. Convection currents in the mantle were proposed as the force that caused the ocean to grow and the continents to move.

(Right) The Mid-Atlantic Ridge showing its conformance to the coastlines of the adjacent continents. The first inset shows a detail of the ridge generated from side-scan sonar data – the central rift is clearly visible. Red and orange colors indicate the crest of the ridge; dark blue, the deeper seabed on either side. The second insert shows the location of the ridge and associated valley in the Atlantic.

The breakup of Pangaea shown in five stages beginning about 225 million years ago. (Note: Ma = mega-annum, indicating millions of years ago). Inferred motion of lithospheric plates is indicated by arrows. Spreading centers (mid-ocean ridges) are shown in red.


Convergent Plate Boundaries

Convergent Plate Boundaries - Regions where plates are pushing together can be further classified as:

• Oceanic crust toward continental crust - for example, the west coast of South America.

• Oceanic crust toward oceanic crust - occurring in the northern Pacific.

• Continental crust toward continental crust – one example is the Himalayas.

(Above Right) A cross section through the west coast of South America, showing the convergence of a continental plate and an oceanic plate. The subducting oceanic plate becomes more dense as it descends, its downward slide propelled by gravity. At a depth of about 80 kilometers (50 miles), heat drives water and other volatile components from the subducted sediments into the overlying mantle, lowering its melting point. Masses of the melted material, rich in water and carbon dioxide, rise to power Andean volcanoes.

(Above) The formation of an island arc along a trench as two oceanic plates converge. The volcanic islands form as masses of magma reach the seafloor. The Japanese islands were formed in this way.


The distribution of shallow, intermediate, and deep earthquakes for part of the Pacific Ring of Fire in the vicinity of the Japan trench.

Note that earthquakes occur only on one side of the trench, the side on which the plate subducts.

The great Indonesian tsunami of 2005 was caused by these forces; the site of the catastrophic 1995 Kobe subduction earthquake is marked.


(Above) A cross section through southern China, showing the convergence of two continental plates. Neither plate is dense enough to subduct; instead, their compression and folding uplift the plate edges to form the Himalayas. Notice the massive supporting “root” beneath the emergent mountain needed for isostatic equilibrium.


Crust Fractures and Slides at Transform Plate Boundaries

(a) Transform faults (orange) form because the axis of seafloor spreading on the surface of a sphere cannot follow a smoothly curving line. The motion of two diverging lithospheric plates (arrows) rotates about an imaginary axis extending through Earth.

(b) A long transform plate boundary, which includes California’s San Andreas Fault. Note the offset plate boundaries caused by divergence on a sphere.

A History of Plate Movement Has Been Captured in Residual Magnetic Fields

Paleomagnetism: strips of alternating magnetic polarity at spreading regions.

The patterns of paleomagnetism support plate tectonic theory. The molten rocks at the spreading center take on the polarity of the planet while they are cooling. When Earth’s polarity reverses, the polarity of newly formed rock changes.

(a) When scientists conducted a magnetic survey of a spreading center, the Mid-Atlantic Ridge, they found bands of weaker and stronger magnetic fields frozen in the rocks.

(b) The molten rocks forming at the spreading center take on the polarity of the planet when they are cooling and then move slowly in both directions from the center. When Earth’s magnetic field reverses, the polarity of new-formed rocks changes, creating symmetrical bands of opposite polarity

The age of the ocean floors. The colors seen here represent an expression of seafloor spreading over the last 200 million years as revealed by paleomagnetic patterns. Note, especially the relative symmetry of the Atlantic basin in contrast with the asymmetrical Pacific, where the spreading center is located close to the eastern margin and intersects the coast of California.


Apparent Polar wandering: plate movement causes the apparent position of the magnetic poles to have shifted.

(a) The magnetic properties of rocks in North America, South America, Europe, and Africa suggest that the north magnetic pole apparently migrated to its present position from much farther south, possibly from a point in the Pacific Ocean.

(b) The problem can be solved if the pole remained fixed but the continents have moved. For example, if Europe and North America were previously joined, the paleomagnetic fields in the rocks would indicate a single pole until the continents drift apart.


Plate Movement above Mantle Plumes and Hot Spots

(above) Formation of a volcanic island chain as an oceanic plate moves over a stationary mantle plume and hot spot. In this example, showing the formation of the Hawai’ian Islands, Loihi is such a newly forming island.

Plate Tectonics Explains Sediment Age, Oceanic Ridges, and Terranes

• Age and distribution of ocean sediments– The sediment in the ocean is thinner and

younger than the age of the ocean indicates it should be

• The Oceanic ridges: Oceanic ridges are clear indicators of past events.

• Terranes: Oceanic plateaus that form by uplifting and mountain building as they strike a continent.

Oceanic plateaus usually composed of relatively low-density rock are not subducted into the trench with the oceanic plate. Instead, they are “scraped off,” causing uplifting and mountain building as they strike a continent (a-d).

Though rare, assemblages of subducting oceanic lithosphere can also be scraped off (obducted) onto the edges of continents.

Plate Tectonics Explains Sediment Age, Oceanic Ridges, and Terranes

Scientists Still Have Much to Learn about the Tectonic Process

• Why should long lines of asthenosphere be any warmer than adjacent areas?

• Is the increasing density of the leading edge of a subducting plate more important than the plate’s sliding off the swollen mid-ocean ridge in making the plate move?

• Why do mantle plumes form? What causes a superplume? How long do they last?

• How far do most plates descend? Recent evidence suggests that much of the material spans the entire mantle, reaching the edge of the outer core.

• Has seafloor spreading always been a feature of Earth’s surface? Has a previously thin crust become thicker with time, permitting plates to function in the ways described here?

• There is evidence of tectonic movement prior to the breakup of Pangaea. Will the process continue indefinitely, or are there cycles within cycles?

Scientists Still Have Much to Learn about the Tectonic Process

The Wilson cycle, named in honor of John Tuzo Wilson’s synthesis of plate tectonics. Over great spans of time, ocean floors form and are destroyed. Mountains erode, sediments subduct, and continents rebuild. Seawater moves from basin to basin.

In this chapter you learned that Earth is composed of concentric spherical layers, with the least dense layer on the outside and the most dense at the core. The layers may be classified by chemical composition into crust, mantle, and core; or by physical properties into lithosphere, asthenosphere, mantle, and core. Geologists have confirmed the existence and basic properties of the layers by analysis of seismic waves, which are generated by the forces that cause large earthquakes.

The theory of plate tectonics explains the nonrandom distribution of earthquake locations, the curious jigsaw-puzzle fit of the continents, and the patterns of magnetism in surface rocks. Plate tectonics theory suggests that Earth’s surface is not a static arrangement of continents and ocean but a dynamic mosaic of jostling lithospheric plates. The plates have converged, diverged, and slipped past one another since Earth’s crust first solidified and cooled, driven by slow, heat-generated currents flowing in the asthenosphere. Continental and oceanic crusts are generated by tectonic forces, and most major continental and seafloor features are shaped by plate movement. Plate tectonics explains why our ancient planet has surprisingly young seafloors, the oldest of which is only as old as the dinosaurs, that is, about 1/23 the age of Earth.

In the next chapter you will learn about seabed features, which the slow process of plate movement has made and remade on our planet. The continents are old; the ocean floors, young. The seabed bears the marks of travels and forces only now being understood. Hidden from our view until recent times, the contours and materials of the seabed have their own stories to tell.

Chapter 3 in Perspective

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