High School Earth Science: Plate Tectonics - Continental Drift — Flashcards

Evidence for Continental Drift: Magnetic Polarity    
New evidence for magnetic drift was discovered in the 1950s from studies of Earth’s magnetic history. Earth has a magnetic field like that of a bar magnet, with magnetic north and south poles. The magnetic poles are located near the geographic poles. In the 1950s, scientists studied the magnetic properties of rocks that formed at different times and in different places. They used magnetometers, which are devices that measure magnetic field direction and intensity.

Why study rocks? Rocks containing the mineral magnetite have crystals that align with Earth’s magnetic field. Magnetite crystals are like tiny magnets that point to the north magnetic pole as they crystallize from magma. Once the crystals form, their magnetic alignment, or polarity, is locked in place. Therefore, the alignment of crystals in rocks records the direction of Earth’s magnetic field at the time the rocks formed. Scientists studying the magnetic polarity of rocks compared rocks that formed at different times and places.

They made several important observations:    
- Young rocks are aligned with Earth’s current magnetic north pole regardless of the continent on which they formed.    
- Old rocks of about the same age that formed on the same continent have the same magnetic field alignment, but it is different from Earth’s current magnetic alignment.    
- Old rocks of different ages, regardless of where they formed, are not aligned with each other or with Earth’s current magnetic field.    

How could these observations be explained? One possible explanation is that the locations of Earth’s magnetic poles have changed their positions through time. However, to fit all the data, this explanation would require that Earth have had two magnetic north poles in the past. Another possible explanation—and one that is better supported by the data—is that the continents, and not the magnetic poles, have changed their positions through time. The magnetic polarity evidence supports the continental drift hypothesis. When the evidence was first discovered, it stimulated research to understand how continents can move over Earth’s surface.