I. Introduction

A. Culmination of Continental Drift & Seafloor Spreading

1. Plates actually defined by J. Tuzo Wilson

a. he noted that deformation (earthquakes & volcanism) at the Earth's surface occurred in relatively narrow bands surrounding larger areas with much less tectonic activity (the plates)

b. he also defined a new kind of fault, the transform fault, in his description of the relative motions between plates

2. Jason Morgan & Dan McKenzie quantified the geometric aspects of plate motion

B. Essential Idea

1. The lithosphere of the Earth is divided into several internally-rigid segments (plates) moving relative to one another

2. Deformation only occurs at plate boundaries

3. Distribution of seismicity shows this well

II. Geometry of Plate Motion

A. Types of Plate Boundaries

1. Divergent, or constructive (= seafloor spreading)

a. plates are moving away from each other

b. associated with MOR & continental rift zones

2. Convergent, or destructive (= subduction)

a. plates are moving toward each other

b. associated with deep-sea trenches & continental collision zones

3. Lateral, or conservative (=transform or strike-slip)

a. plates are sliding past each other

b. associated with transform faults & fracture zones

B. Transform faults

1. Proposed by J. Tuzo Wilson as fundamentally different from classical transcurrent faults

2. Critical element in plate geometry

a. Define direction of relative motion

b. Lie on small circles about the pole of rotation

c. Used to define poles of rotation

C. Poles of Rotation

1. The point on the surface of the Earth around which two plates pivot & which does not move relative to either of the two plates

2. The axis of rotation is perpendicular to the Earth's surface & passes through the pole of rotation & the center of the Earth

3. The rigid motion between two plates can be described by the latitude & longitude of the pole of rotation & the angle through which the plates rotate (angular velocity, v)

a. the velocity of motion at any point along a plate boundary is a function of the angular velocity & the angular distance away from the pole of rotation

III. The Plates

A. Major plates

1. North American

2. South American

3. Eurasian - nearly all continental lithosphere

4. African - nearly surrounded by MOR

5. Antarctic - nearly surrounded by MOR

6. Indo-Australian

7. Pacific - nearly all oceanic lithosphere

B. Important minor plates

1. Philippine, Nasca, Cocos, & Juan de Fuca (or Gorda) - nearly all oceanic lithosphere: surrounding the Pacific

2. Caribbean & Scotia: associated with the North & South American plates

3. Arabian, Aegean, & Anatolian: along the African-Eurasian boundary

IV. Seismology & Plate Tectonics

A. Introduction

1. Showed that all the new seafloor generated by seafloor spreading went back into the mantle along inclined seismic zones associated with deep-sea trenches, island arcs & andesitic volcanism = Benioff (after Hugo Benioff, the seismologist who first described them) or subduction zones

2. Confirmed existence of transform faults (below "C. Focal Mechanisms")

B. Distribution of Earthquakes

1. Long, narrow belts

a. coincide with MOR crest (& continental rift zones), deep-sea trenches (& continental collisional zones) & transform faults

2. Divide Earth's lithosphere into seven large (Major) & numerous small(Minor) plates that move relative to each, but deform only at their edges

3. Shallow (<70 km deep) Earthquakes

a. Most prevalent - occur at all plate boundaries

4. Intermediate (70-300 km deep) & Deep (>300 km deep, to 700 km) Earthquakes

a. Occur only at convergent plate boundaries

C. Focal Mechanisms (Fault Plane, or First Motion, Solutions)

1. Analysis of seismograms from many seismic stations to determine the first movement received at a given seismic station from a given earthquake

2. Two kinds of motion

a. Dilatational - movement toward the earthquake source

b. Compressional - movement is away from the earthquake source

3. Type of motion plotted on stereonet

4. Can also get direction & state of stress

V. Heat Flow & Age-Depth Relationship of the Seafloor

A. Heat Flow

1. Amount of Heat Conducted Out of Earth's Interior

2. Distribution of Heat Flow

a. High at MOR crest, but decreases away from MOR crest

b. Also high near island arcs - small spreading centers

3. Unusual Aspects

a. Average Oceanic Heat Flow Equals Average Continental Heat Flow

i. continental heat flow should be higher - higher content of radioactive elements

ii. heat under ocean derived from mantle convection

b. Heat Flow at MOR Crest is Smaller Than Theory Predicts

i. hydrothermal circulation removes some heat by CONVECTION

B. Age-Depth Relationship of the Seafloor

1. New seafloor is hot - thus it it is expanded & stands high to form the crest of the MOR

2. Once the seafloor forms, it cools, contracts & gets deeper

3. The relationship between age & depth is relatively straight forward

a. for seafloor younger than ~50 million years: depth = 2500 + 350Ãage

b. for seafloor older than ~50 million years: depth = 2500 +6500(1-1/exp(age/68.5))

4. The fixed age-depth relationship for seafloor combined with varying spreading rates results in varying widths for the MOR

a. varying average spreading rates through time affects sea level

VI. Volcanism & Plate Tectonics

A. Distribution of Volcanoes

1. Long, narrow belts

a. coincide with MOR crest (& continental rift zones) & deep-sea trenches (& continental collisional zones)

i. volcanism does NOT occur at transform faults

B. Volcanism at Convergent Plate Boundaries

1. Consists of the Calcalkaline suite of rocks

a. dominated by Andesitic volcanics, &

b. Granitic plutonic rocks

2. Characterized by explosive volcanoes

a. due to high silica content & resulting viscous nature of magma

3. Probably results from partial melting of oceanic crust & sediment

C. Volcanism at Divergent Plate Boundaries

1. Consists of Tholeiitic basalt & gabbro

2. Derived by partial melting of upper mantle (low-velocity zone)

D. Hot Spot Volcanism

1. Volcanism within a plate interior, or at a divergent plate boundary

a. hots spots are believed to result from a stationary "hot spot" in the aesthenosphere

b. generates additional basaltic magma (alkalic suite, however)

2. Hot spots at divergent plate boundary - MOR above sealevel (Iceland)

a. leaves volcanic plateau in plate interiors

3. Hot spots within plate (Hawaii)

a. generates island chains in plate interiors

b. reheats lithosphere - resets subsidence curve

4. Used to determine "absolute" plate motions

VII. Driving Forces

1. Mantle convection is ultimately the driving force for the plates

a. convection is the manifestation of thermal gradients in the mantle

2. Ridge push

a. plates are pushed apart at the MOR due to the elevation of the MOR crest above the surrounding plate

b. generates 1-4 cm/yr of plate movement (Atlantic & Indian Ocean plates)

3. Slab pull

a. plates are pulled along by cold, subducting slabs

b. generates fast (5-10 cm/yr) rates of plate movement (Pacific plates)

i. can actually pull MOR along with it

4. Trench suction

a. old lithosphere actually pivots into aesthenosphere

b. trench (& continent to which it is attached) is pulled along

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