Plate+Tectonics


 * Earth's Magnetic Field notes (pg. 44-46)**

The geographic poles or true north and south poles, are where Earth's rotational axis intersects the surface.

When heated above a temperature known as the Curie point, these magnetic minerals lose their magnetism.

Rocks that formed thousands or millions of years ago and contain a "record" of the direction or magnetic poles at the time of their formation are said to posses fossil magnetism or pale magnetism.

Rock magnetism- magnetized minerals not only indicates the direction to the poles but also providers a means of determining the latitude of their origin.

The inclination of the paleomagnetism in rocks indicates the latitude of the rock at the time it became magnetized.

Rock magnetism provides a record of the direction and distance to the magnetic poles at the time a rock unit was magnetized.

Magnetic poles had migrated through time known as polar wandering or that the flows moved.

If the magnetic poles remain stationary their apparent movement is produced by continental drift.



One of the main objections to Wegner's hypothesis appears to have stemmed form his inability to identify a mechanism that was capable of moving the continents across the globe.

Wegener suggested two possible mechanisms for continental drift.

One of these was the gravitational force that the Moon and Sun exert on Earth to produce the tides in which the tidal force would affect Earth's outer-most layer, which would slide as detached continental fragments over the interior.

Wegener also incorrectly suggested that the larger and sturdier continents broke through the oceanic crust, much like ice breakers cut through ice.

Over the next two decades a much better picture of large expanses of the seafloor slowly and painstaking began to emerge.

From this work came the discovery of a global oceanic ridge system that winds through all of the major oceans in a manner similar to the seams on a baseball.

One of the segments of this interconnected feature extends down the middle of the Atlantic Ocean and hence was named the Mid- Atlantic Ridge.

Earthquake studies conducted in the western Pacific demonstrated that tectonic activity was occurring at great depths beneath deep-ocean trenches.

Flat-topped seamounts, named guyots, were discovered hundreds of meters below sea level.

In Hess' now classic paper, he proposed that oceanic ridges are located above zones of convective upwelling in the mantle.

As rising material from the mantle spreads laterally, seafloor is carried in a conveyor-belt fashion away from the ridge crest.

One Hess's central idea was that "convective flow in the mantle caused the Earth's entire outer shell to move."

Hess proposed that the continents were carried passively by horizontal part of the connective flow in the mantle.

Geomagnetic reversal the north magnetic pole becomes the south magnetic pole and vice versa.

When rocks exhibit the same magnetism as the present magnetic field they are said to possess normal polarity, whereas rocks exhibiting the opposite magnetism are said to have reverse polarity.

The major divisions of the magnetic time scale are called chrons and last roughly 1 million years.

Magnetometers are very sensitive instruments