Earth Science: Geology, the Environment, and the Universe

Chapter 17: Plate Tectonics

Problem of the Week

Splitting At the Seams

Imagine that you are flying over the surface of Earth. You fly over the planet, taking note of the mountains, canyons, vast plains, and even the varied topography of the seafloor. You marvel that these wonders appear to have been around for a long time and still seem so stable. Your communications and photos tell of a planet covered by water and old, stable landforms.

However, as you have been discovering, Earth is a dynamic planet. This quick flyover did not take time to investigate Earth the way that humans have over the last five hundred years or so. As you know, landforms change, sometimes slowly, sometimes over night. Earth’s crust is a moving mass of various large slabs called plates. These plates, whose movement is fueled by the heat within Earth, bump and grind against each other in some places—convergent zones—and move apart in other places—divergent zones. It is the interaction of this convergence and divergence of plates that you will be thinking about more closely in this problem.

Think About This:

In your text, page 455, (**Note, if map can be inserted on the Web page that would be good!) there is a map of the world that shows plate boundaries, the direction that the plates are moving at the boundaries, and the relative rates of movement at the boundaries. Notice that there are two places on Earth designated as zones of extension within continents. At these places, divergent boundaries extend onto continents. Take a close look at the one that is shown extending deep into the African continent. This zone is called the Great Rift Valley and is predicted to be the next great ocean as the continent splits allowing the Indian Ocean to flow into the area.

Problem:

Geologists usually associate a divergent zone, where new crust is forming, with a complimentary subduction zone, where crust is being pulled down into the mantle. You can see this on the map. Look closely at the Great Rift Valley. There is a divergent zone several thousand miles to the west and another several hundred miles to the east, and there is no subduction zone between them. This means that crust is being created between these two zones.

What kinds of geologic activity might take place on the African Plate due to having divergent boundaries on all but the northernmost boundaries?