Geol 02C Historical Geology
J Bret Bennington
Paleogeography and Paleomagnetism
Paleomagnetism
One of the key discoveries of the 1950’s that forced people to take continental drift seriously was the presence of remnant magnetism in rock.
Remnant magnetism: We have already discussed how tiny iron mineral grains can become oriented to the Earth’s magnetic field as sedimentary grains settle in quiet conditions or as igneous and metamorphic rock cool. The orientation that these magnetic grains acquire has three characteristics:
Polarity - The overall North-South orientation of the remnant magnetic field in the rocks. This can be either normal (as it is today) or reversed (with north and south magnetic poles changing places.) Magnetic reversals preserved in the stratigraphic record were discovered in the 1950’s and we still do not understand what causes the periodic reversals of the Earth’s magnetic polarity.
Declination - Like a compass needle, a remnant magnetic field also points in a straight line toward the magnetic pole.
Dip - Because of the orientation of the force lines of the Earth’s magnetic field, a compass needle will also align itself vertically if allowed to tilt. At the equator the force lines are parallel to the surface of the Earth and the dip of the remnant magnetic field is zero. At the poles the force lines are vertical and the dip is 90°. In between the dip varies predictably with latitude.
To summarize, the remnant magnetic field in a rock gives an indication
of the direction to the magnetic pole and the latitudinal distance
from the pole when the rock was magnetized. Unfortunately, nothing
about remnant magnetism gives any indication of longitude (i.e. the position around the circumference of the Earth).
Wandering Poles
Shortly after it was discovered that a remnant magnetic field could be measured in rocks and that the Earth’s pole had frequently reversed its polarity in the past, paleomagneticians (paleomagicians?!) began to measure the remnant magnetic fields of a variety of young and old rocks. They quickly found that young rocks had a declination and dip that agreed with the present position of the Earth’s magnetic field, but that older rocks seemed to show that the magnetic pole had wandered in the past, occupying positions far from its present location.
However, an observation soon developed that cast doubt on the theory of polar wandering. When the polar wander paths obtained from rocks in North America and from rocks in Britain were compared, they were found to be different - as if to suggest that the magnetic pole had been in two different places at the same time in the past. Clearly, this was impossible. The solution to this puzzle was obtained when North America and Europe were adjusted on a map to connect to one another along the mid-Atlantic ridge. Assuming that the two continents were once joined brought the polar wander paths together until about the Jurassic - just when Wegener had suggested that they had begun to drift apart!
Nowadays, were refer to polar wander paths as apparent polar wander paths because we understand that it was the continents doing the wandering and not the Earth’s magnetic poles.
Paleogeography
Once the reality of continental drift was confirmed by plate tectonics, geologists realized that they could use paleomagnetic data to reconstruct the positions of the different continents at different times in the past. This practice is called paleogeography and is succeeding in producing a remarkable view of the evolving geography of the Earth. Using declination data geologists can follow the rotational orientation and from dip data they can determine the latitudinal position of land masses back through time.
Other sources of paleogeographic information
Other kinds of information besides paleomagnetism are useful for reconstructing Earth’s ancient geography.
Mountain ranges - these mark the suture zones of ancient continental collisions. For example, the Ural Mts in Russia mark the collision boundary between ancient european and asian land masses.
Faunal similarity - continents that are close together tend to share more species and genera of organisms than continents that are widely separated. The fact that all of the southern hemisphere continents shared many species of land animals and plants until the middle Mesozoic was used by Wegener to argue for the presence of a single southern land mass up to that time.
Climactic indicators and climate belts - extensive coal deposits tend to form in tropical environments at low latitudes. Evaporites are common to the dry subtropical belts. Glacial deposits are usually restricted to temperate and higher latitudes.