Revised 8 / 06 (Monroe 6th ed.)

Continental Drift and Plate Tectonics - Chapter 2




Continental Drift: Introduction

Evidence and Support

Plate Tectonics: Introduction

Plate Tectonics: a local view

Mechanism of Plate Tectonics


For additional information, refer to...

GeoTours background information on Tectonics (READ THIS FOR SURE!)

AskGeoMan section on Global Tectonics

GeoMan's Grants Pass High School Geology Home Page

GPHS class notes on Continental Drift

GPHS class notes on Plate Tectonics: an overview

GPHS class notes on Divergent Plate Boundaries

GPHS class notes on Convergent Plate Boundaries



Really only 2 earth processes

Construction and destruction

Destruction is surface weathering, erosion, etc.

Construction is tectonics

Earthquakes, volcanoes, mountain building in general

Revolution in the Earth Sciences

Similar to revolutions in the other sciences

Biology - Darwin in mid-1800's

Natural Selection and Evolution

Physics - Einstein in early-1900's

Non-Newtonian mechanics

Basic change in the way geologists view and interpret internal earth processes

A "whole earth" type of model

No so during the early days of formal geological thought (1700's & 1800's)

Concerned with the continents - not the oceans

Continents seems static - no lateral movement

Some vertical movement was recognized


Mountain building

Possibly some marginal accretion, but basically stable

Geologists worked in a vacuum (when there's only 3 in all of Europe and North America...)

Little or no interdisciplinary discussion

Tough to make advances in an academic vacuum

Enter Plate Tectonics

Changed all of the above

That's what revolutions do!

Resulted largely from study of the ocean basins

Mapping after WWII

Military and communications - more on this later

Active planet - powered by immense internal forces

Produce a constantly changing surface

Very slow rate, overall (see Strickler's 2nd Law of GeoFantasy)

Theory states that the earth's crust is broken into a relatively small number of sections, or plates

Move around in response to internal, or external, forces which we don't fully understand

There's a mouthful!

Most surface features related to plate margin interactions

Differential plate motions cause the continental land masses to "drift" relative to each other

Constantly changing land/sea inter-relationships

Drift has affected all aspects of the surface of the earth

Including biological evolution - will discuss in detail later

Theory unified geologic thought

Ties together nearly all facets of Earth Science

Must be true! 99.67% of all geologists believe it

Only the most recent theory

May improve on earlier work, but still not dogma

At least not yet, and when it is it will be useless

"The safe course leads ever downward into stagnation." Muad'dib

So far it has stood the test of time

And what a test it has been

Gained prominence in the early 1970's

Caused a rush to re-interpret the earth in light of the new model

Some very exciting times


Continental Drift

Not really a new idea

Proposed many times in the past

Usually based on similarities of coastlines - Africa/South America

Little or no back-up evidence

1620 - Sir Francis Bacon

Commented on Atlantic similarities

Couldn't be by accident alone

Extremely perceptive for the time!

1658 - François Placet

Agreed with Bacon

"Separated in the Biblical flood"

mid-1800's - Antonio Snider-Pelligrini

Similar fossil plants in coal beds of Europe and North America

Reconstructed a 'supercontinent'

1908 - Taylor & Baker

Correlation of global mountain chains

All these early ideas were pretty casual

Based on landforms - not the ocean floor

Sparked little interest

Ran counter to existing dogma

Continents permanent and fixed

Any suggestion of movement was "geological fantasy"

DIGRESS TO: GeoFantasy

Alfred Wegener (Monroe; fig. 2-2, p. 35)

Instructor of Meteorology

Noted trans-Atlantic similarities

Seems everyone started here

Read article on fossil correlations in 1911 (Snider-Pelligrini?)

Sparked his obsession

1915 - published his theory with supporting evidence

"The Origin of Continents and Oceans"

Basically said that the continents had been joined into a supercontinent he called "Pangaea" (Greek: all land)

Split in the Jurassic

Initial response less than overwhelming

Developed into a full fledged battle by 1924

R.T. Chamberlain (American) - "Can we call geology a science when there exists such differences of opinion on fundamental matters as to make it possible for such a theory as this to run wild."

Baily Willis (American, 1944) - Wegener's theory is a fairy tale and should be ignored due to its deleterious effect on students

In Wegener's defense it must be noted that he took a truly multidisciplinary approach to the matter

A very easy target

Just a weatherman - not an "expert" in any of the fields he was leaning on for support

Lots of religious fervor

Pleas for "open-mindedness"

Guardians of the truth against idiotic speculation

Easy for opponents to pick holes in specifics

Regional vs. local again!

Wegener would have loved the 1st Law of GeoFantasy

DIGRESS TO: Arm-wavers (global/regional thinkers) are always easy targets

J. Tuzo Wilson - early regional work on Plate Tectonics

Carl Sagan - saved by a pretty face and TV

In any event, his opponents severely took him to task!

Pointed to the lack of a mechanism (describe)

Wegener had actually proposed two possible mechanisms

Lunar/solar drag

Centrifugal pull due to rotational velocity

Both "proven" to be inadequate

Sufficient to significantly discount Wegener's work

Wegener stoutly defended his work

Disappeared in Greenland in 1930

Wegener did have supporters

Alexander DuToit - South African

Tried to establish a sound data base

Geological studies in Africa and South America

Proposed 2 supercontinents

Gondwana - southern hemisphere

Laurasia - northern hemisphere

Separated by the Tethys Sea

Arthur Holmes - Scottish

Accepted Wegener's ideas completely

Looked for a mechanism

Mantle thermal convection

Whole-mantle vs. shallow-mantle


Evidence for Continental Drift

The plate tectonic model begins with an acceptance of continental drift

Once accepted the rest follows

Fit of Coastlines (Monroe; fig. 2-3, p. 36)

One of the earliest arguments

Easy to see

Problems with early reconstructions (no surprise here!)

Trying to represent a 3-D planet on 2-D maps

Match of existing coastlines wasn't perfect

Carey (1955)

Joined at the 6560' isobath

Edge of continental shelf

Much better match

Bullard (1965)

Computer-generated best fit

3000' isobath

Stratigraphic & structural similarities (Monroe; fig. 2-4, p. 37)

If they fit so good, some geologic/geomorphic features should extend across

The torn newspaper/puzzle concept

A close look at regional & local features provided many matches

SE Brazil and SW Africa

Nearly identical stratigraphy - almost perfect match

Coal beds and regional stratigraphy of the Appalachians and Greenland/England/Scotland


Primarily related to glacial features

Evidence for extensive Permian/Pennsylvanian glaciation in the southern hemisphere

Many locations currently near the equator

Uniformitarianism suggests that this is a problem

"Direction of flow" indicators suggest that the glaciers moved onto land from what is now ocean

This is also a problem - all current glaciers move the other way

Reconstruction of Pangaea clarifies the problem (Monroe; fig. 2-5, p. 38)

Climate-sensitive sedimentary rocks

Sed rocks form at the surface and can be influenced by surface conditions

Fossils (Monroe; fig. 2-6, p. 39)

Assumed free migration of land animals if continents were joined

Within limits set by environmental needs - restricted only by habitat needs and natural boundaries

Attempt to trace common lines of descent up to the split

Then divergence of species

Ex. - Australian marsupials

Difficult to assess this data - very spotty (isn't paleontology always spotty!): Expand

Didn't do much to convert unbelievers

Recent work tends to support drift, but still essentially circumstantial

Paleomagnetism (Monroe; fig, 2-7, pg. 40)

A big deal in the post-war 40's and 50's

"Black Box" geology - geophysics

DIGRESS TO: the tendency of magnetic minerals to orient themselves relative to magnetic north upon crystallization

Stanley K. Runcorn (British)

Measured fossil magnetism in many rocks - preCambrian to recent

Calculated the average north pole region for each age

This is a complex topic

Runcorn basically found that:

Younger rock tend to align best with the current magnetic polar position

Increasing divergence with age

Also, each continent had its own North Pole

"Apparent Polar Wandering Curve" (Monroe; fig, 2-8, pg. 41)

Since the magnetic poles probably don't move about very much, then the land masses must have moved

Correlation of North America & Europe indicate they were joined until the Late Triassic

Runcorn published in 1962

Still lots of skepticism at this time

Many were swayed by Runcorn's basically incontestable data

Good example of the difference between the impact of hard empirical data and inferred evidence based on assumptions and common sense

The last (and possibly most convincing) clues came later from the seafloor

Government finally began to care about the topography of the sea floor after WWII

Defense and communications

Hide our missile subs and track the Soviet's

Mapping of seafloor indicated that there is a range of mountains running N-S down the middle of the Atlantic ocean

(Monroe; fig. 2-10, pg. 43)

Part of a larger, continuous chain which circles the planet

All volcanic in origin - basalt

As mentioned above, magnetic minerals align to magnetic north at time of crystallization (Monroe; fig, 2-9, pg. 42)

Oceanic basalt has lots of iron

Analysis of magnetic signatures of aligned magnetite grains

Identified linear magnetic anomalies

Also, evidence shows that north and south regularly switch poles!

Bi-laterally symmetrical bands of magnetic minerals oriented to north and south (Monroe; fig. 2-11, pg. 44)

Indicate that the sea floor is spreading apart, with new basalt magma rising to fill the gap

Generation of new oceanic crust, which then moves away from the volcanic axis...

...and goes where? More on this later

Some scientists began to put it all together, ask questions such as:

IF: Oceanic ridges/rises represent spreading centers

THEN: Age of basaltic oceanic crust should become older with distance from a ridge

Glomar Challenger - deep sea drilling ship

Actually CIA spy boat with drilling as its cover

Looking for sunk Soviet subs

Sent to test hypothesis

Drilled 8 sites in the south Atlantic

Cored thru sediments and into basaltic crust

All relatively young (<200 m.y.) (Monroe; fig. 2-12, pg. 45)

Results support drift

2 cm per year (at that location) (Monroe; fig. 2-14, pg. 46)

Several interesting observations/interpretations:

Ocean basins extremely youthful features

Oldest oceanic crust found is <200 million years old (Monroe; fig. 2-12, pg. 45)

Continental rocks up to 4.1 billion years old

Floating (and growing) through time in a 'sea' of constantly renewing oceanic crust

But what has happened to all the oceanic crust formed before 200 MYA?

There must have been some

Where did it go?

It was clear by the mid 1960's that the beginnings of a new global tectonic framework were in place

Hard to argue that the continents were fixed in place

Had drifted quite a bit

Needed a new theory which would account for drift, as well as internal process

Mountain building: commonly on continental margins

Magmatic activity: associated with mountain building

Metamorphism: associated with both

They all work together to build the earth


Video: Earth Revealed - The Birth of a Theory


Plate Tectonics

The theory which ties them all together

Exciting time for geology

Much of the theory based on continuing observations of the sea floor and continental margins

The initial determination was that, as the continents spread apart, new oceanic crust is formed at the mid-oceanic ridges

But what happens to it then?

Somewhere it has to be consumed

If not, the earth would have to keep getting bigger to accommodate the additional crust

Originally proposed, but generally discounted

Earth would have had to start out real small if spreading rates historically approached today's rates

Harry Hess in the early 1960's

Proposed that old oceanic crust is consumed at island arcs

Linear chains of volcanoes

Commonly arc-shaped

Located sea-ward of continents

ex. Japan, Aleutians

Can be on-shore

ex. Andes, Cascades

In any event, long chains of extremely active volcanoes

Also lots of earthquake activity

Commonly composed of intermediate lava and pyroclastics

DIGRESS TO: mafic vs. felsic (compositional differences) & pyroclastics

In addition, researchers found deep trenches in the ocean floor immediately sea-ward from the island arcs

Deepest portions of the oceans

Marianas Trench: -35,810'

Incredibly active seismic areas

Deep earthquakes

45° angle

Clear that they were in some manner associated with the volcanic arcs

Hugo Benioff: describe & define Benioff Zone

Anyway, putting all this together, we end up with the model of which is referred to as plate tectonics

Attempts to explain global tectonic patterns and effects

Refer to world seafloor map

Describe general aspects of the theory

New crust generated at spreading centers

Consumed at trenches

Island arcs are surface expression of the subduction process

More on that later

The outer portion of the earth's interior is fairly complex

Not a simple crust/mantle situation


High strength, brittle material

Approximately 60 miles thick

Includes crustal rocks


Low strength, ductile material

DIGRESS TO: brittle vs. ductile

Down to 150 miles

"Low velocity layer"

Global feature


The bulk of the mantle material

"Stronger" than the Asthenosphere

DIGRESS TO: "Strong/weak" adjectives & how they relate to seismology


Plate tectonics: a local view

Pacific "Ring of Fire"

Chain of active earthquakes/volcanoes around the Pacific Ocean

Relatively narrow, linear zones of activity

Related to plate margin interactions

Plate margins are where most tectonic processes occur

Individual plates are moving around relative to each other

Due to these varying velocities, there are interactions where two or more plates are in contact

Three possible motions to the interaction (Table 2-1; pg. 47)

DIGRESS TO: faults

Represent breaks in the crust along which movement has occurred

Three kinds (normal, reverse, strike-slip)

Zone of Divergence (Monroe; fig. 2-15, pg. 48) (Monroe; fig. 2-16, pg. 49)

Spreading centers - location of the formation of new crustal material

Tensional features, therefore predominantly normal faulting

Crustal wounds - scab over with new basalt (blood of the earth)

Newly differentiated mantle material

Shallow earthquakes

Down to 12 miles

Also fewer quakes than at Benioff zones

Less slippage area?

Area of high heat flow

Hot rocks expand

Commonly form long, relatively narrow topographic highs

In addition, the location of a linear, near surface magma chamber will help cause doming of the overlying basaltic crust

Actual spreading takes place near the crest of the rise

As plate moves away from spreading center

Rock cools and begins to adjust to a lower elevation

Extensively studied since the 1970's

Exciting areas for study

High heat flow

Lots of volcanism

Pillow basalt

Indication of submarine volcanism

Other interesting aspects to spreading centers

Black smokers

High temperature vents

Support exotic life forms

Tube worms, etc.

Some speculation that life originated in similar environments

New and different life forms

DIGRESS TO: Sulfide deposits

Ophiolites (Monroe; fig. 2-19, p. 55)

Describe in detail

Refer to the Josephine Ophiolite GeoTour

Zone of Lateral Movement (Monroe; fig. 2-10, p. 43)

Transform faults

Strike-slip faults

Bisect and offset the spreading ridges (Monroe; fig. 2-20, p. 56)

San Andreas fault (Monroe; fig. 2-21, p. 57)

Zone of Convergence

Zone of collision between two plates

Reverse faulting and crustal shortening

Thrust fault - special case of a reverse fault (angle <20°)

There are 3 main types... (Monroe; fig. 2-18, p. 53)

Continental vs. oceanic plates

Oceanic plate is subducted due to density differences

Location of many shallow to deep focus quakes

Extend to depth beneath the overriding plate

A deep trench commonly occurs where the subducting place plunges beneath the overriding plate

Collects sediments

These, too, are subducted

Remelting at depth generates magma which forces its way to the surface

Volcanic arc chains

Can be on land (Andes, Cascades (Monroe; fig. 2-21, p. 57))

Commonly mid-range composition (Andes = andesite)

Oceanic vs. oceanic plates

Volcanic arcs usually at sea

Results in "island arcs"

Japan, Aleutians

Continent vs. continent collisions

Starts with ocean to ocean

Himalaya Mountains, Urals, Atlas

Other features and/or problems

We said that there are 7 major plates

Failed to mention the many "microplates" which occur

Mediterranean region - very complex

Open and close often in "recent" history

Triple junctions (Monroe; fig. 2-16, p. 49)

Only two remain active - third commonly fails

Back arc spreading

Between the arc and continent

Josephine Ophiolite

Spreading centers without corresponding subduction zones

Africa is surrounded (and cut) by spreading centers!

Hot Spots and Mantle Plumes

Mid-plate volcanics

Hawaiian/Emperor chain (Monroe; fig. 2-22, p. 58)

Columbia River basalt (?)

How they form is a problem


Mechanism of Plate Tectonics

What causes the plates to move?

Always been the weak point of the theory

The rallying cry of the non-believers

Unimaginable forces!

Lots of possibilities have been proposed

Lunar drag

Centrifugal pull due to rotational velocity

These first two were Wegener's original proposal

Demonstrated to be too small

Giant catfish in the center of the earth

Gravitational sliding

Away from the topographic highs of the spreading centers

Expansion of the earth

Possible mechanism to start it off

Lithosphere cracks

New material rises to fill the void

Density differences between descending lithosphere and mantle

Drags plate behind

Pulls open at the ridges

Slab-pull, ridge-push (Monroe; fig. 2-25, p. 61)

Mantle Convection (Monroe; fig. 2-24, p. 60)

Currently popular theory

Most probably a combination of several of the above

As well as others not imagined yet!


Video: Plate Dynamics


For additional information, refer to...

GeoTours background information on Tectonics

AskGeoMan section on Global Tectonics

GeoMan's High School Geology Home Page

GPHS class notes on Continental Drift

GPHS class notes on Plate Tectonics: an overview

GPHS class notes on Divergent Plate Boundaries

GPHS class notes on Convergent Plate Boundaries


GeoMan's Home Page | RCC Index | High School Geology Index


You are GeoManiac number since April 1, 1997