Terms in this set (20)

1) The shapes of many continents are such that they look like they are separated pieces of a jigsaw puzzle. For example, look in the top right map of the previous page at the shape of the east coast of North and South America relative to the shape of the West coast of Africa and Europe.
2) Many fossil comparisons along the edges of continents that look like they fit together suggest species similarities that would make sense only if the two continents were joined at some point in the past. Likewise, there are geological formations that appear to end at the boundary of one continent and continue on the boundary of another continent.
3) A large amount of seismic, volcanic and geothermal activity is found along the conjectured plate boundaries.
4) Ridges such as the Mid-Atlantic Ridge appear to be regions where plates are separating and the seafloor is spreading. The ridges are produced by lava welling up from between the plates as they pull apart. Likewise, mountain ranges appear to have formed where one plate is subducted. For example, the Himalayan Mountains are still growing, pushed up by the motion of the Indian Subplate as it passes under the Eurasian plate.
5) Magnetic anomaly stripes are observed in seafloor spreading zones such as the Mid-Atlantic Ridge. These stripes correspond to reversals of the direction of magnetization for seafloor rocks. The stripe pattern results because the Earth periodically reverses the polarity of its magnetic field. As the seafloor spreads, the direction of the current magnetic field is fixed into the rocks as they solidify from upwelling magma. Therefore, the stripes are a direct indication of seafloor spreading. For many geologists, this explanation of the magnetic stripe anomalies was the clinching argument leading to an acceptance of the plate tectonic hypothesis.