It is the creation and destruction of oceanic crust, then, that is the mechanism for Wegener’s
drifting continents. Rather than drifting across the oceans, the continents ride on a conveyor
belt of oceanic crust that takes them around the planet’s surface.
One of the fundamental lines of evidence for continental drift is the way the coastlines of
continents on both sides of the Atlantic Ocean fit together. So let’s look at how seafloor
spreading moves continents in the Atlantic by looking more closely at figure 3 above. New
oceanic crust is forming at the mid-ocean ridge that runs through the center of the Atlantic
Ocean basins, which is called the Mid-Atlantic Ridge. Stripes of different magnetic polarity
are found on opposite sides of the Mid-Atlantic Ridge. These stripes go all the way to
the continents, which lie on opposite sides of the Atlantic. So new seafloor forming at the
Mid-Atlantic Ridge is causing the Americas and Eurasia to move in opposite directions!
Lesson Summary
- Using technologies developed to fight World War II, scientists were able to gather data
that allowed them to recognize that seafloor spreading is the mechanism for Wegener’s
drifting continents. - Bathymetric maps revealed high mountain ranges and deep trenches.
- Magneticpolaritystripesgivecluesastoseaflooragesandtheimportanceofmid-ocean
ridges in the creation of oceanic crust. - Seafloor spreading processes create new oceanic crust at mid-ocean ridges and destroy
older crust at deep sea trenches.
Review Questions
- Describe how sound waves are used to develop a map of the features of the seafloor.
- Why has no ocean crust been located that is older than about 180 million years when
the oldest continental crust is about 4 billion years old? - Describe the major features of mid-ocean ridges, deep sea trenches, and abyssal plains
and their relative ages. - Describe continents move across the ocean basins as if they are on a conveyor belt
rather than as if they are drifting, as we Wegner’s original idea. - Explain why the following scenario is impossible: Oceanic crust is not destroyed at
oceanic trenches, but new crust is still created at mid-ocean ridges. - If you were a paleontologist who studies fossils of very ancient life forms, where would
be the best place to look for very old fossils: on land or in the oceans? - Imagine that Earth’s magnetic field was fixed in place and the polarity didn’t reverse.
What effect would this have on our observations of seafloor basalts?