Earth Structure and Plate Tectonics• How do we explain the existence of land and

oceans, and the features that are found on (land) and within (oceans) them?

• To understand the origin and structure of the oceans, we must first understand the structure (and origin) of the Earth itself

The Earth is layered

• The Proto(early) Earth was entirely molten, such that gravity segregated the elements and compounds according to their densities

• Because each deeper layer is denser than the layer above, the Earth is said to be density stratified (density = mass/volume)

• The highest density material exists at its center (core); the least dense material at its surface (crust)

The Earth is layered• Similarly, any gases formed in the interior

were vented to the surface (less dense)• Over time, the accumulated gases produced

an envelope of air (the atmosphere) or condensed to form surface water

• If we could slice the Earth into halves, we would see an arrangement of concentric interior shells, much like the layering of an onion

The Earth is layered

• The Earth’s structure consists of a series of concentric layers, or spheres, which each differ in density, chemistry (or composition), and physical properties

• The compositional layers of the Earth consist of– The crust; thin, outermost layer– The mantle; thick, middle layer– The core; densest, inner layer (iron and nickel)

The Earth is layered

But wait! There’s more…

• The uppermost layer, the crust, is lightweight and brittle, but crust beneath the ocean differs in thickness, composition, and even age from the crust of the continents

• In fact, it is because of these differences that the continents and oceans exist in the first place!

• Let’s explain….

The Earth is layered

• The Earth is further classified into layers based on physical (not chemical) differences:– Lithosphere– Asthenosphere– Lower mantle– Core

Increasing depth

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Lithosphere

• The lithosphere is Earth’s cool, rigid layer. It consists of all of the crust, and also of the uppermost cool and rigid portion of the mantle• The lithosphere consists of oceanic and

continental crust as well as the upper mantle• “litho” = rock

Lithosphere

Asthenosphere

• The asthenosphere is the hot, partially melted, slowly flowing layer of upper mantle below the lithosphere

• Unlike the lithosphere which is cool, rigid, and brittle, the asthenosphere is plastic, and capable of flow

• It is also thicker; extending to a depth of ~400 miles beneath the Earth’s surface

Asthenosphere

The Lower Mantle

• The lower mantle is the portion of mantle that extends to the core

• The lower mantle is similar in composition to the asthenosphere, but it is less plastic (more rigid) than the mantle above it

• This is because, despite its intense heat, the lower mantle is subject to increased pressure at an increased depth

Lower Mantle

Core• The core has two parts

– Outer core: dense, viscous liquid– Inner core: solid, extremely dense

• Extremely hot! Recent evidence indicates that the inner core may be nearly 10,000 F! ̊�– Just as they did during the formation of Earth,

radioactive elements within the core and mantle continue to decay releasing heat to the rest of the planet

Geologic differences between oceans and continents

• The Earth’s crust consists of oceanic and continental crust– Oceanic crust is thin and is made up of

heavy basalt– Continental crust is thick and is made up

mostly of granite, which is lighter than basalt

– Very important!

Geologic differences between oceans and continents

• Remember, continental crust is thicker and less dense than oceanic crust

• As a result, land floats on the asthenosphere and rises higher than the thin, heavier oceanic crusts, which sinks much deeper into the asthenosphere

• To better understand this, let’s consider the principles of buoyancy….

Continents rise above the ocean because of isostatic equilibrium

• Buoyancy is the ability of an object to float in a fluid by displacing a volume of that fluid equal in weight to the floating object’s own weight

• For example, a steel ship can float because it displaces a volume of water equal in weight to its own weight– An empty cargo ship displaces less water than a

ship loaded with cargo

Continents rise above the ocean because of isostatic equilibrium

• Buoyancy supports a cargo ship in water in the same way that it supports lithospheric plates floating in the asthenosphere

• Less dense objects float higher than more dense objects (which sink deeper)

• Crustal plates float at an ‘elevation’ which depends on their thickness and density– A mountain rises above the ocean in isostatic

equilibrium; it has displaced as much asthenosphere as is equal to its mass

• Because the continental crust is not exceptionally dense, it can project above sea level; however, the dense oceanic crust is almost always submerged

How about this heat?!!?• Even after 4.6 billion years, heat continues to

flow from within the Earth• This heat is very important as it powers the

construction (and destruction) of mountains and volcanoes, causes earthquakes, moves continents, and shapes ocean basins!

The Evidence• Despite the fact that the continents and ocean

basins are composed of rigid rock, their shapes and sizes change slowly over time

• How is this possible? When viewed over millions of years, they no longer appear rigid, but instead flow ever so slowly in reaction to the high temperatures and pressures that exist in the Earth’s interior– Our maps merely represent a snapshot of continents

and oceans in motion over time

The Evidence: Parallelism of the shorelines

• A simple look at a world map reveals continents that look as though they could fit together; their coastlines appear to match up with each other, as if they were pieces of a giant jigsaw puzzle puzzle

• From the time accurate charts became available in the 1700’s, observers noticed the remarkable coincidence of shape of the Atlantic coasts of South America and Africa

• The fit of all continents around the Atlantic at a water depth of ~137 meters as calculated by Sir Edward Bullard in the 1960’s

• Why such a water depth?

The Evidence: Continental drift

• In 1912, Alfred Wegener proposed the theory of continental drift, the idea that all Earth’s land was in fact once joined into a single supercontinent, called Pangaea

• Discovery of coal (the fossilized remains of tropical plants) in Antarctica, and similarities in fossils found across separate continents supported his theory

Pangaea

The Evidence: The Ring of Fire

• In the 1930’s, a “ring of fire”, a circle of violent geological activity surrounding much of the Pacific Ocean, was identified

• Scientists were beginning to reveal a worldwide pattern of earthquakes and volcanoes that did not occur randomly, but rather, were concentrated in zones that extended in lines along Earth’s surface (thank you seismologists!)

Evidence: The Ring of Fire

Evidence: Radiometric dating• The discovery that unstable, naturally-occurring

radioactive elements lose particles from their nuclei (and subsequently change into new elements) enabled scientists to age rocks, by measuring the ratio of radioactive atoms to stable atoms in a sample (radioactive decay occurs at a predictable rate)

• Surprisingly, the age of the ocean floor was found to be no more than 200 million years, much less than the estimated age of Earth

Evidence: Radiometric dating

• While the oceans were found to be relatively young in age, the continents were aged at 4 billion years, MUCH older than the oceans

• Why the difference? Why are oceans younger than the continents? How could this be?

• The key to this mystery is explained by the process of seafloor spreading; the creation of new ocean floor and subsequent pushing (spreading) of the continents

Seafloor Spreading

• New ocean floor (basaltic crust) develops at mid-ocean ridges and then spreads outward

• The continents on either end of the ocean would be spread apart (continental drift) as a result

• Continental drift, then, must be caused by the same forces responsible for creating new ocean crust

Seafloor Spreading

• Seafloor spreading, and continental drift, is powered by convection currents; slow-flowing circuits of material within the asthenosphere

Plate Tectonics

• The discovery that the upper part of the mantle (asthenosphere) was in fact plastic (capable of flow) advanced the theory of plate tectonics

• Plate tectonics describes the large-scale movements of the Earth’s lithosphere over the asthenosphere, driven by heat dispersed from the mantle (and can explain the theory of continental drift!)

Plate Tectonics

• A key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates which ride on the fluid-like asthenosphere

• To support this theory, newly-formed oceanic crust would be hot, and would cool and shrink in volume as it moved from the spreading center, and indeed it was!

Plate Tectonics/Seafloor spreading

• An important concept to understand is that warm, newly formed oceanic crust is less dense, and becomes more dense (shrinks in volume) as it ages and cools

• As a result, newly formed oceanic crust would rise higher than the ocean floor (crust) surrounding it, and older crust farther from the spreading center would be cooler and more dense, resulting in deeper ocean floor

• Remember, denser material will sink deeper into the asthenosphere than lighter material. The deeper oceanic crust sinks into the asthenosphere, the deeper the ocean basin!

e.g., lighter continental crust

e.g., heavier oceanic crust

Plate Tectonics/Seafloor spreading• Sure enough, when sediments were aged

outward from the spreading center, they increased in age towards the continents

• In fact, ocean spreading occurs at a slow, but comprehensible rate. The Atlantic Ocean is growing (extending outward from its center) at a rate of 2.5cm per year, while the Pacific Ocean is spreading at a rate of 8-12cm per year (that’s nearly 5 inches!); your fingernails grow ~3.6cm per year

Plate Tectonics/Seafloor spreading

• If the oceans are continually growing, the creation of new oceanic crust must be balanced by the destruction of crust elsewhere! Otherwise, the Earth would be continually expanding (which evidence confirms it is not)

• In fact, scientists have identified that crust plunges down into the mantle along edges of the Pacific Ocean; a process known as subduction

Subduction• Subduction occurs appropriately enough at

subduction zones; regions of crust destruction

Subduction

Oceans Thirteen

• According to the theory of plate tectonics, the Earth’s outer layer consists of ~13 separate lithospheric plates floating on the asthenosphere

• When heated from below, the plastic asthenosphere expands and rises, lifting and cracking the rigid crust above it, creating new plates.

Plate Tectonics

• Dense oceanic crust is constantly created (at spreading centers) and subducted (at subduction zones), while the less dense continental crust rides high in the lithospheric plates, rafting along on the slowly moving asthenosphere below

• Explains why oceanic crust is so young, relative to the much older continental crust

Plate Tectonics: The Major Players

Plates interact at plate boundaries

• Not surprisingly, volcanic activity and earthquakes are frequently concentrated in spreading centers and subduction zones.

• What’s more, some of the Earth’s major mountain ranges are found along subduction zones (current or historic)

• Plates interact with each other at boundaries; they diverge (come apart), converge (come together), or slip past each other

Ocean basins form at divergent boundaries

• Divergent plate boundaries are the spreading centers; regions of crustal formation

• A divergent plate boundary is the line along which 2 plates are spreading apart from each other

• New oceanic (basaltic) crust forms from plumes of the heated mantle below

• Divergent plate boundaries are identified as mid-oceanic ridges

The Mid-Atlantic Ridge

Ocean basins form at divergent boundaries

• Mid-oceanic ridges, such as the Mid-Atlantic ridge, are clearly found in the middle of the ocean basins, but this was not always the case

• Divergent plate boundaries (and subsequent ocean formation) originate beneath continental crust, and result in a growing ocean basin– Begins as a rift valley; quite literally a valley

created by a geological rift or spreading

• The Red Sea is just a baby as far as seas (oceans) go; a new ocean is also forming along Africa’s rift valley

Tectonic forces are widening the Red Sea (on the left) and pushing the Sinai Peninsula farther away

from Africa at a rate of ~1cm per year

Ocean basins form at divergent boundaries

• As the crust is uplifted and thinned by the rising magma plume, the plates move apart, and oceanic crust is laid down

• Remember, oceanic (basaltic) crust is much denser than continental crust, and so the rift valley ‘drops’ below sea level, filling with water

• The break-up of Pangaea was the result of several divergent plate boundaries forming

The Growth of the Atlantic Ocean• A mature ocean takes

millions of years to form; the Atlantic is ~210 million years old

• The Pacific Ocean is much older; what remains of the ‘super ocean’ once surrounding Pangaea

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Plates come together at convergent boundaries

• What happens when two plates come together, or converge?

• Plates may come together in one of the following ways– Oceanic to continental plate– Oceanic to oceanic plate– Continental to continental plate

Ocean – Continent Convergence

• When an oceanic plate collides with a continental plate, the heavier oceanic plate will subduct underneath the lighter continental plate

• Due to intense heat and pressure, the subducted plate will be melted, and frequently result in volcano formation; the result of magma enriched in dissolved gases from the melting plate

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Ocean – Continent Convergence

• The familiar volcanoes of the west coast (Mount St. Helen’s and the Cascade volcanoes) are the result of the subducting Pacific oceanic plate beneath the North American continental plate

• The active volcanoes of Central America and South America’s Andes Mountains are also a product of this convergence, as are the many associated earthquakes in the region

Ocean – Ocean Convergence

• When 2 oceanic plates converge, the heavier plate will slip steeply below the lighter one into the asthenosphere, forming a deep trench

• As with the subducting oceanic crust beneath the continental crust, released gases from the subducted plate frequently result in volcanic activity, only this time, the volcanoes emerge from the seafloor, rather than the continent

• The Marianas Trench – the world’s deepest ocean trench was formed by 2 oceanic plates converging

• The Marianas Trench is ~11,033 meters (36,200 feet)

Pacific plate

Mariana plate

• The Japanese islands were formed in this way!• Earthquakes occur only on the side of the

subducting plate

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Continent – Continent Convergence• When a continental plate collides (converges)

with another continental plate, neither plate is subducted

• This is because both plates have equal density, and so they are instead compressed, folded and uplifted into each other, forming mountain ranges

• The Himalayas are a prime example; the India and Eurasian plates are still colliding, and so Mt. Everest is still growing!

Islands are formed when an oceanic plate moves over a stationary mantle plume• Volcanic island chains, such as Hawaii, are not

the result of ocean-ocean convergence, but rather the result of an oceanic plate moving over a stationary plume of mantle, called a hot spot

• Usually a hot spot is far from a plate boundary, but results from a rising mantle plume– As the plate moves over the plume, volcanic islands

are created

• The Hawaiian island chain was formed this way!

Functions as a giant assembly line; volcanoes become inactive as they move away from the hot spot

Seafloor spreading pushing plate westward

Transform boundaries

• In addition to divergent and convergent plate boundaries, plates can also slide past each other; in this case, crust is neither created nor destroyed

• Transform faults are such fractures in the lithosphere that are offset horizontally from the vertical spreading center– Remember, the Earth is a sphere and so spreading

cannot occur evenly as it would over a plane

• As the seafloor spreads, transform faults form as cracks horizontal to the spreading axis.

The mid-ocean ridge is jagged (not straight) and marked by numerous transform faults along its length, the result of plate divergence on a spherical Earth

Mid-Atlantic Ridge

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Transform boundaries

• Transform faults are so named because they transform the relative motion of the plate movement

• The potential for earthquakes at transform boundaries is great as the plate edges slide past each other

• California’s San Andreas fault is one of the most famous transform faults and the site of numerous, powerful earthquakes

• Transform faults (indicated here in red) occur when the edges of 2 plates are moving in opposite directions (illustrated best in bottom diagram)

• The San Andreas fault is the eastern boundary of the Pacific Plate. When the Pacific and North Atlantic plates slide passed each other, earthquakes can result due to stress build-up (e.g., kinks in plate)

• A continental transform boundary; can also be oceanic (underwater)

Summary• Oceans cannot grow indefinitely• As one ocean basin spreads on one side of the

Earth, another ocean basin will be affected on the other side of the planet. All ocean basins are connected (one world ocean)

• The edges of the continents are subject to rising and falling sea levels, constantly changing the boundaries between land and sea

Continental Margins

• When the amount of seawater exceeds the capacity of the ocean basins, they are overfilled and seawater spills out of them, flooding the edges of the continents

• Thus, the shorelines of the continents are always in flux (we’ll come back to this)

Summary/Wilson Cycle• During the course of hundreds of millions of

years, ocean basins ‘evolve’ through distinct stages, collectively referred to as the Wilson Cycle– Embryonic: uplift, formation of rift valley– Juvenile: divergence; young sea (ex. Red Sea)– Mature: seafloor spreading; (ex. Atlantic)– Declining: convergence/subduction (ex. Pacific)– Terminal: convergence/uplift (ex. Mediterranean)– Suturing: convergence/uplift (ex. Himalayas)