Lecture #26: Rock Magnetism and Sea-floor Spreading

The Earth's magnetic field - invisible lines of magnetic force surrounding the earth deflect magnetized objects, such as compass needles, that are free to move. The Earth's magnetic field has north and south poles. Geophysical observations suggest that the Earth's magnetic field is generated within the liquid of the Earth's metallic outer core.

Magnetic reversals - Evidence suggests that the earth's magnetic field has periodically reversed its polarity in the past. During the present time lines of magnetic force leave the earth's magnetic south pole and go to the magnetic north pole. During a magnetic reversal the lines of force leave the north pole and go to the south pole.

Magnetic inclination - the angle with the horizontal assumed by a freely swinging magnet. Hence, the magnetic inclination varies regularly with the latitude from zero at the magnetic equator to 90° at the magnetic pole.

Paleomagnetism - Many rocks record the strength and direction of the earth's magnetic field at the time the rocks formed. Small magnetite crystals in a cooling lava flow act like tiny compass needles, preserving a record of the earth's magnetic field when the lava solidifies. Rocks stained red by iron compounds also record former magnetic field directions. These ancient rocks contain a magnetic signature that when measured shows the direction of the earth magnetic field at the time of the formation of the rocks provided they have not been heated.

Magnetic latitude - the paleomagnetic inclination is therefore a record of the place between the pole and the equator where the rock was formed.

Apparent Polar Wander - Geophysicists studying paleomagnetic pole positions during the 1950s found evidence suggesting that poles wandered all over the globe. This is called apparent polar wander. Another discovery was that the path of apparent polar wandering measured in North America differed from that measured in Europe. Rocks of the same age in Europe and North America suggested that the north pole was in two positions at the same time. Because the Earth's magnetic and rotational poles are close, geophysicists concluded that the magnetic pole has always been near the north pole that the the continents moved on the surface of the earth. Europe and North America show different poles because they were in different positions than they are today. Hence, data from paleomagnetism finally ended the debate over continental drift.

All of the early evidence for continental drift came from the continents. But if the continental crust moved, then the ocean crust must also move also. Evidence for the movement of the seafloor came from a hypothesis by Harry Hess of Princeton University.

Seafloor spreading - The seafloor is characterized by mid-ocean ridges. Hess hypothesized that the topography of the seafloor could be explained if the seafloor moves sideways, away from the oceanic ridges. Hess postulated that magma rose from the interior of the Earth and formed new oceanic crust along the mid-ocean ridges. This hypothesis was tested using paleomagnetism. When lava is extruded at any mid-ocean ridge, the rock it forms becomes magnetized and acquires the magnetic polarity that exists at the time the lava cools. As the crust moves away from mid-ocean ridges, it contains a continuous record of the Earth's changing magnetic polarity.

Mid-ocean Ridges - The existence of the ridge and its high heat flow are caused by the rise of hot mantle rock. New, young sea floor is continually being formed by basalt eruptions at the ridge crest. The magnetic reversals near the mid-ocean ridges allows the prediction of the age of the sea floor and the rate of the sea-floor motion.