Testing Plate Tectonics Chapter 9.4 Testing Plate Tectonics

Paleomagnetism This is one type of evidence for plate tectonics. It is the study of changes in Earth’s magnetic field. Earth is like a bar magnet. That makes some of the rocks magnetic. These magnetic rocks are frozen in direction. That means that as the rock moves it’s magnetic poles stay the same.

Normal polarity – when rocks show the same magnetism as the present magnetic field. Reverse polarity – when rocks show opposite magnetism as the present magnetic field. The discovery of strips of rocks of alternating polarity, which lie as mirror images across the ocean ridges is evidence of sea floor spreading.

Earthquake Patterns There is a close link between deep-focus earthquakes and ocean trenches. The absence of deep-focus earthquakes along oceanic ridge systems is consistent with a new theory. There is a relationship between earthquake activity and where plates meet. Intermediate focus and deep focus earthquakes will happen at a subducting plate.

Ocean Drilling An ocean drilling expedition found that the age of the sediment increased with the increased distance from an oceanic ridge. This means that sediment found at the ridge is newer than away from the ridge.

Hot Spots A hot spot is an area of melting hot rock at the surface. This creates volcanic mountains and the mountains move as the oceanic crust moves over the hot spot. An example is the Midway Islands to the Hawaiian Islands – all created by the same hot spot.