Plate Tectonics: Divergent Plate Boundaries


Review of plate tectonics

Active planet - powered by immense internal forces

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

Strickler's 2nd Law of GeoFantasy: There are no catastrophic processes, only catastrophic results

Plate boundaries

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

Tectonic activity is common here

The amount of energy involved here is immense

Spreading centers vs. subduction zones


What are the plates moving around on?

The outer portion of the earth is fairly complex

Not a simple crust/mantle situation like was originally assumed

Lithosphere: the outermost layer

Includes the crustal plates

High strength, brittle material beneath them

Approximately 60 miles thick

Asthenosphere: below the Lithosphere

Low strength, ductile material extending down to 150 miles


Divergent plate boundaries

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

Basalt: dark, fine-grain, high density igneous rock

The "Blood of the Earth"

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

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

Ophiolites:A section of seafloor

A true miracle of preservation: Should be subducted

Describe basic stratigraphy

Sediments / Basalt / Sheeted Dikes / Gabbro / Ultramafics

Volcanic features of divergent boundaries

Basalt is both fluid and dry (low volatile content)

Therefore, non-explosive

Characterized by fluid eruptions and shield volcanoes


Videodisc movies & stills

Go to rock display


Divergent boundaries - second day

Crustal stress

The movement of large areas of the crust vertically & horizontally

Immense stresses at an extremely slow rate (see Strickler's 2nd Law of GeoFantasy)

Different sections of the crust are moving at different velocities

Therefore they interact at their edges - plate margins

All this up and down involves distorting the crust

Several things can happen

Break - fractures & joints

Break & slip - faults


What happens depends on

Rock type


Type and magnitude of the force (stress)


Breakage in the crust

When cold rock moves, it can break

Must be relatively cold and brittle

Called fractures, or joints

Faults: A fracture where the sides have moved relative to each other

Usually a planar surface

Hangingwall vs. footwall

Several types of faults, based on the relative sense of motion

Normal faults - hanging wall down

Common at spreading centers

Usually the result of tension and crustal lengthening

Horst & Graben (see fig. 8-13; pg. 171)

Actually Valley Building, not Mountain Building


Tectonics at Spreading centers

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 subsides to a lower elevation

Results in the Guyots described by Hess


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

Triple junctions

EXAMPLE: super ball in drawer, cracks in pavement

Only two remain active - third commonly fails

Saudi Arabia vs. Africa; Baffin Bay


Other faulting associated with divergent plate boundaries

Zone of Lateral Movement

Strike-slip faults

Nearly vertical fault with horizontal motion

Bisect and offset the spreading ridges

San Andreas fault - good example


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