Geology 111G                                                                                    18 March 1999

Lecture 23.  Causes of Plate Motions

 

Sea-Floor Spreading

Magnetic Anomalies

Types of Plate Margins

Summary of Plate Tectonics

Causes of Plate Motions

 

I.  Sea-Floor Spreading:  Newly-formed ocean crust at spreading ridges preserves the record of magnetic reversals.  This was the first conclusive evidence for the dynamic nature of the earth's lithosphere, and provided a mechanism for continental drift.

A.  Magnetic anomalies:  The stripes formed as crust moves away from the spreading ridges.  The Earth's magnetic field is recorded in new basaltic crust created at the ridges, and when reversals of the field take place, the polarity of the stripes changes (see transparency 71).  These stripes were discovered by means of ship borne magnetometers originally designed to detect submarines.  The anomalies are disposed symmetrically about the spreading ridge.

1.  Half-spreading rates on either side of ridges have been calculated in a range of 1-9 mm/year.

B.  Generation of new oceanic lithosphere:  sea-floor spreading    results in fomation and movement of new oceanic plates away form spreading ridges.

1.  This process, first postulated in 1964, provided a mechanism for continental drift and revived Wegener's hypothesis.  Difference from the old view is that instead of plowing through the oceanic crust, the continents are passengers in moving lithospheric plates, essentially moving on a conveyor belt.

2.  A corollary of sea-floor spreading is that, because the Earth is not expanding in size, oceanic lithosphere must be destroyed by subduction at the same rate it is formed.

C.  Transform faults:  Strike-slip faults that connect spreading ridges, act as plate boundaries and permit the movement of plates past one another.  Important conceptual advance in mechanics of plate tectonics.

 

II.  Types of Plate Margins

A.  Divergent Margins:  plates move away from one another.  There is an evolution in the development of a divergent margin.

1.  Continental rift:  upwelling mantle heats bottom of continental lithosphere, may apply extensional stress.  Surficial expression consists of uplift, normal faulting, formation of a rift valley.

2.  New ocean basin:  formation of oceanic lithosphere marks the creation of 2 plates where formerly 1 had existed.  Surficial expression consists of a narrow seaway, newly formed ridge at the spreading center (example is Red Sea).

B.  Convergent margins:  plates move toward one another.  There are three basic types, depending upon the types of lithosphere involved.

1.  Subducting margins:  oceanic crust is thrust beneath an adjacent plate, resulting in volcanism when plate reaches 100 km.

a.  Two oceanic plates.  Results in formation of oceanic island arcs, and very deep trenches (Marianas trench where Pacific plate descends beneath Philippines plate is -36198 ft; -11040 m).

b.  Oceanic and Continental Plates.  Oceanic plate descends beneath Continental Plate.  Example is subduction of Nazca Plate beneath South America.

2.  Collisional margins.  Involves the collision of two continental plates after an ocean basin is consumed by subduction.  Partial subduction of one continental plate forms thick crust and generation of tremendous topography by isostasy (Mt. Everest is 29028 ft; 8853 m).  Example is collision of India and Eurasian plates; Iberian and Eurasian plates.

a.  Some evidence that continental crust near Matterhorn has been to a depth of greater than 50 km.

b.  Indian continental crust extends 600 km beneath Asiancontinental crust, forming thickest known continental crust on earth (80 km, = 2 thicknesses of crust) and the high Tibetan Plateau by isostasy.

C.  Transform margins.  Plates move past one another on transform faults that cut through the lithosphere.  May have features locally of either extension or shortening, because plate movement may not be exactly parallel to continental margin.  Example is San Andreas fault.

 

III.  Summary of Plate Tectonics.

A.  Earth’s lithosphere consists of rigid plates averaging 100 km thick.

1.  Margins of plates are marked by earthquake and volcanic activity caused by plate interactions.

2.  Plate interiors are relatively stable geologically and have fewer earthquakes.

B.  Relative plate motions are divergent, convergent, or transform.

1.  Three basic types of convergent margins.

C.  Lithosphere is formed at divergent margins and consumed at convergent subducting margins.

D.  Lithosphere is rigid and decoupled from the ductile asthenosphere at the Low Velocity Zone, allowing lateral movement.

E.  Plate Tectonics forms a unifying theory for the Geologic Sciences that allows important predictions about the earth.

1.  Distribution and location of resources.

2.  Locations and recurrence of geologic hazards, including earthquakes and volcanism.

 

IV.  Causes of Plate Motions.  The drivers of plates are not completely understood.  Originally it was believed that the plates moved passively on moving asthenosphere; new evidence suggests that the plates contribute to their own movement.

A.  Plate-generated movement.

1.  Slab pull.  Hypothesis that the weight of the cold slab descending into the asthenosphere pulls the whole plate along.  Normal faulting in the ocean crust provides evidence for these plates experiencing tensional stresses.

2.  Ridge push.  Hypothesis that topography of the elevated mid-ocean spreading ridges drives the plates by gravity away from the ridge.  Calculations suggest that plates could move down the gentle slope formed by the thermal profile of the lithosphere-asthenosphere boundary.  However, during initial rifting, plates begin to move without the help of an elevated ridge.

B.  Mantle-generated movement.

1.  Mantle convection cells.  Rising warm rock from the mantle spreads laterally upon encountering the lithosphere, applying shear stress and causing it to move.           

a.  Shallow convection model:  only the asthenosphere convects, down to a depth of 700 km.

b.  Deep convection model:  the whole mantle down to the core convects.

2.  Thermal plumes.  Vertical columns of of upwelling mantle 100-250 km in diameter lift the overlying lithosphere and spread laterally, which applies drag to the lithosphere.  Many thermal plumes lie at divergent plate margins.  Iceland is an example.

C.  Plates probably move as a result of a combination of the above mechanisms. Plate separation may be initiated by mantle convection or a plume, but the subsequent formation of a topographically high spreading ridge may then drive the plate in a certain direction.