Earthquakes

Geology 111G Lecture 22 18 April 2005

Plate Tectonics: Continental Drift and Earth Magnetism

Continental Drift

Geologic Evidence

Earth Magnetism

Earth's Magnetic Field

Paleomagnetism

Geophysical Evidence for Continental Drift

Sea-Floor Spreading

Magnetic Anomalies

Types of Plate Margins

Summary of Plate Tectonics

Causes of Plate Motions

I. Continental Drift. Hypothesis that the continents have not remained fixed throughout geologic time, but have moved relative to one another and relative to the earth's magnetic poles. Alfred Wegener first proposed this concept between 1911-1915. He envisioned a former supercontinent, Pangea or Pangaea, composed of all the existing continents.

A. Wegener's arguments for the existence of Pangea.

1. Restorable shape of continents. West edge of Africa and east coast of South America match at the 200-m water depth.

2. Mismatch of late Paleozoic climatic belts and latitudinal zones.

a. Modern climatic belts are roughly parallel to lines of latitude, and we infer that this has been true in the past.

b. Permian glacial deposits are scattered across the Gondwanan continents and now lie at different latitudes. Reassembly and movement of Gondwana to South Pole solves this problem.

B. Hypothesis was discredited during Wegener's lifetime for lack of a mechanism (1925, Sir Harold Jeffreys). Wegener envisioned that the continents moved through the oceanic crust like icebreakers, but Jeffreys demonstrated that the rocks were not strong enough to do this and that there was no known mechanism capable of moving the continents this way.

1. Continental drift awaited the advent of plate tectonics for a viable mechanism.

II. Earth Magnetism. Provides an independent physical clue that continents have in fact moved relative to the earth's magnetic field (EMF).

A. Earth's magnetic field. Earth's rotation causes it to be a large magneto, like a bar magnet. The field forms a sheath around the globe described by lines of force.

1. These emerge from south magnetic pole and disappear at north magnetic pole.

2. They are resolved into two components relative to the physical earth.

a. Inclination: Angle between lines of force and horizontal plane, or surface of earth.

b. Declination: Angle between lines of force and true north (direction to pole of rotation).

B. Paleomagnetism. Study of ancient states of the EMF. Some rocks lock in the EMF when they are formed by deposition or cooling following eruption. This weak magnetism is termed natural remanent magnetization, and can be measured using a magnetometer, which is shielded from the modern EMF. NRM does not necessarily correspond to modern EMF, as we will see below.

C. Mismatches of rock magnetization and present EMF provide evidence for movement of the continents.

1. Deduced from changes in inclination and declination of rocks of different ages.

2. Assumes that pole of rotation has not moved through geologic time.

D. Reversals of the EMF. An early finding of paleomagnetists was that the EMF has undergone periodic reversals of its orientation in the past, such that past states of the EMF have been equal and opposite tot he modern field. Because these reversals are global in extent, they represent a series of events that can be calibrated to the Geologic time scale.

1. Normal polarity: EMF or remanent magnetization same as normal field.

2. Reverse polarity: EMF or remanent magnetization opposite to normal field

a. Polarity events. Time intervals during which the polarity of the field is oriented consistently.

b. Polarity epochs. Groups of events dominated by a particular polarity.

III. Geophysical evidence for Continental Drift. Mismatches in the paleomagnetic inclination measured in rocks of different ages demonstrate that either the continents have moved relative to Earths axis of rotation, or that the EMF has not always been oriented parallel to that axis which appears unlikely.

IV. 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.

V. 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.

VI. 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 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.