Plate Tectonics: an overview

 

Including...

Introduction

Overview of plate tectonics

Plate boundaries

What are the plates moving around on?

Let's look closer at plates and plate boundaries

Zones of Divergence

Zones of Convergence

Zones of Lateral Movement

Other features and problems

Plate tectonics and the formation of igneous rock

Mechanism of Plate Tectonics

 

Introduction

Continue our look at the earth's crust

Learned yesterday that the crust is broken into numerous "plates"

Will start with a closer look at the theory of Plate Tectonics

Revolution in the Earth Sciences

Theory unified geologic thought

Ties together nearly all facets of Earth Science

Only the most recent theory: 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

Basic change in the way geologists view and interpret the earth

Resulted largely from study of the ocean basins

Mapping after WWII - Military and communications

Where to hide our nuclear missile submarines

And how to track the Soviets' with passive sonar

Exciting time for geology

The spreading ridges were recognized first

As the continents spread apart, new oceanic crust is formed

Like a scab over a cut

But what happens to all this new crust?

Somewhere it has to be consumed by the earth

Or it would keep getting bigger to accommodate the additional crust

So where was the excess crust going

Additional research found deep trenches in the ocean floor immediately sea-ward from linear chains of volcanoes

Trenches are the deepest portions of the oceans (Marianas: -35,810')

Incredibly active seismic areas with deep earthquakes 45° angle

Clear that they were in some manner associated with the volcanos

These volcanic chains can be off-shore (ex. Japan, Aleutians)

Or on-shore (ex. Andes, Cascades)

In any event, long chains of extremely active volcanoes

Also with lots of earthquake activity

Proposed that old oceanic crust is consumed in these deep trenches

Hugo Benioff: describe & define Benioff Zone

Plate Tectonics attempts to explain global tectonic processes and results

 

Overview of plate tectonics

Active planet - powered by immense internal forces

Produce a constantly changing surface, but at a very slow overall rate

See Strickler's 2nd Law of GeoFantasy

There are no catastrophic processes, only catastrophic events

Earthquakes and volcanos may seem catastrophic, but...

They are really the result of very slow processes

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

Poorly understood forces cause these rigid crustal plates to move relative to each other

Different plates move at different velocities

Ranging from 1 to >10 centimeters per year

This may not seem like much, but, over the course of geologic time...

Most surface features related to plate margin interactions

 

Plate boundaries

Two (or more) plates interact along huge linear zones of faulting

DIGRESS TO: faults

Represent fractures along which movement has occurred

Three kinds (normal, reverse, strike-slip): Details next week!

Tectonic activity is common here

Earthquakes, volcanoes, and mountain building in general

Thanks to friction and Newton's First Law of Motion...

Earth tries its best to keep the plates from slipping

But, as stress increases to the breaking point...

The fault will rupture

Accumulated strain will be released in a seismic event (earthquake)

The plates will shift along the boundary

Only to lock-up again and begin the process anew

The amount of energy involved here is immense

Beyond our ability to comprehend, as well as restrict or control

All we can do is study the process, try to keep out of the way, and, if we can't, hang on tight

New crust generated at spreading centers

Older crust is consumed at trenches

Volcanic arcs are surface expression of the subduction process

More on both later

 

What are the plates moving around on?

DIGRESS TO: brittle vs. ductile

The outer portion of the earth is fairly complex

Not a simple crust/mantle situation

Lithosphere: the outermost layer

Includes the crustal plates as well as high strength, brittle material beneath them

Approximately 60 miles thick

Asthenosphere: below the Lithosphere

Low strength, ductile material extending down to 150 miles

Mesosphere

The bulk of the mantle material

 

Let's look closer at plates and plate boundaries

Plate margins are where most activity happens

Three possible motions to the interaction

 

Zones of Divergence

Spreading centers - location of the formation of new crustal material

Where 2 plates are moving away from each other

Deep rifts are opened through the crust

Allow magma from the upper mantle to rise to the surface and cool

Attempt to "heal the wound"

Newly differentiated mantle material

Basaltic flows, pillows, and breccias

Tensional features - stretching of the crust

Therefore predominantly normal faulting

Shallow earthquakes: Down to 12 miles

Area of high heat flow: Hot rocks expand

Commonly form long, relatively narrow topographic highs

Example: Mid-Atlantic Rise (or Ridge)

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 with lots of volcanism

Several interesting aspects to spreading centers

Hydrothermal vents (black and white smokers)

High temperature vents which support exotic life forms

Tube worms, etc.

Some speculation that life originated in similar environments

Also lead to formation of massive sulfide ore deposits

Gold, copper, zinc

Zones of Convergence

Subduction zones where crustal rocks are consumed

Good example: Pacific "Ring of Fire"

Chains of active earthquakes/volcanoes around the Pacific Ocean

Relatively narrow, linear zones of activity

Zone of collision between two plates - results in crustal shortening

Reverse faulting

Three possible situations

1) 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 plate plunges beneath the overriding plate

Collects sediments which are also subducted

Remelting generates magma which forces its way to the surface

Volcanic arc chains on-shore

EXAMPLE: Andes, Cascades

2) Oceanic vs. oceanic collision

Same basic idea as above

Also results in subduction

Volcanic chain usually off-shore

EXAMPLE: Japanese Islands, Aleutian Islands

3) Continent vs. continent collisions

Massive slabs of granitic crust collide

No subduction possible so some major mountains are pushed up

EXAMPLE: Himalaya Mountains; Ural Mountains

Zones of Lateral Movement

Strike-slip faults

Bisect and offset the spreading ridges

San Andreas fault - good example

 

Other features and problems

There are 7 major plates

Need to mention the many "microplates" which occur

Mediterranean region - very complex

Open and close often in "recent" history

Triple junctions

EXAMPLE: super ball in drawer

Only two remain active - third commonly fails

Saudi Arabia vs. Africa

Baffin Bay

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

Columbia River basalts

How they form is a problem

 

Plate tectonics and the formation of igneous rock

Remember I said that the crust was essentially 2 types of igneous rock

Basalt and Granite

Granite forms deep within the crust

Associated with regions where two plates are moving towards each other

For a variety of reasons, the majority of the earth's most violent earthquakes and volcanoes occur in these areas

Example: eastern edge of the Asian continent vs. the Pacific Ocean

Collision is between a continental plate and an oceanic plate

Heavier oceanic basalt is forced beneath the lighter continental materials along a deep trench called a "subduction zone"

Subjected to increasing heat and pressure

Can cause it to undergo metamorphic changes

In extreme conditions, completely remelt to form new magma

If this magma can find a path of weakness

Generally not a problem where two plates are grinding into each other

Erupt from the surface as lava

Form volcanic mountains (called an "island arc")

Some cools at depth forming large granitic "plutons"

These are sutured to the "continental margin" (added to craton)

Basaltic crust is created at spreading centers

Moves like a conveyor belt to a subduction zone where it is destroyed

Therefore, all oceanic crust is geologically young (<200 million years old)

Continental crust, however, is essentially just along for the ride

This lighter "scum" is basically the end product of the earth's differentiation

Granitic crust has been accumulating for 4 billion years

Edges get beaten up, blocks split and join, but it basically isn't destroyed

 

Mechanism of Plate Tectonics

Always been the weak point of the theory: The rallying cry of the non-believers

Lots of possibilities have been proposed

Lunar drag

Centrifugal pull due to rotational velocity

Giant catfish (Namazu) in the center of the earth

Gravitational sliding - away from the topographic highs of the spreading centers

Density differences between descending lithosphere and mantle

Drags plate behind and pulls open at the ridges

Mantle Convection: Currently popular theory

Most probably a combination of several of the above

As well as others not imagined yet!