INTRODUCTION

Tremendous forces are at work where tectonic plates meet. Let’s take a closer look at what happens at the three types of plate boundaries starting with Divergent Boundaries.

INSTRUCTION

Divergent Boundaries

Most divergent plate boundaries are located along the crests of oceanic ridges. These plate boundaries can be thought of as constructive plate margins because this is where new oceanic lithosphere is generated. Look again at the divergent boundary in Figure 7A on page 255. As the plates move away from the ridge axis, fractures are created. These fractures are filled with molten rock that wells up from the hot mantle below. Gradually, this magma cools to produce new slivers of seafloor. Spreading and upwelling of magma continuously adds oceanic lithosphere between the diverging plates. (Section 9.3 in your textbook)

Oceanic Ridges and Seafloor Spreading

Along well-developed divergent plate boundaries, the seafloor is elevated, forming the oceanic ridge. The system of ocean ridges is the longest physical feature on Earth’s surface, stretching more than 70,000 kilometers in length. This system winds through all major ocean basins like the seam on a baseball. The term ridge may be misleading. These features are not narrow like a typical ridge. They are 1000 to 4000 kilometers wide. Deep faulted structures called rift valleys are found along the axes of some segments. As you can see in Figure 9, rift valleys and spreading centers can develop on land, too.

Seafloor spreading is the process by which plate tectonics produces new oceanic lithosphere. Typical rates of spreading average around 5 centimeters per year. These rates are slow on a human time scale. However, they are rapid enough so that all of Earth’s ocean basins could have been generated within the last 200 million years. In fact, none of the ocean floor that has been dated is older than 180 million years.

Figure 9 The East African rift valleys may represent the initial stages of the breakup of a continent along a spreading center. A Rising magma forces the crust upward, causing numerous cracks in the rigid lithosphere. B As the crust is pulled apart, large slabs of rock sink, causing a rift zone. C Further spreading causes a narrow sea. D Eventually, an ocean basin and ridge system is created. Relating Cause And Effect What causes the continental crust to stretch and break?

Continental Rifts

When spreading centers develop within a continent, the landmass may split into two or more smaller segments. Examples of active continental rifts include the East African rift valley and the Rhine Valley in Northwest Europe.

Figure 10 East African Rift Valley This valley may be where the African continent is splitting apart. Interpreting Diagrams What stage in the drawings on page 259 does this photograph show?

The most widely accepted model for continental breakup suggests that forces that are stretching the lithosphere must be acting on the plate. These stretching forces by themselves are not large enough to actually tear the lithosphere apart. Rather, the rupture of the lithosphere is thought to begin in those areas where plumes of hot rock rise from the mantle. This hot-spot activity weakens the lithosphere and creates domes in the crust directly above the hot rising plume. Uplifting stretches the crust and makes it thinner, as shown in Figure 9A. Along with the stretching, faulting and volcanism form a rift valley, as in Figure 9B.

The East African rift valley, shown in Figure 10, may represent the beginning stage in the breakup of a continent. Large mountains, such as Kilimanjaro and Mount Kenya, show the kind of volcanic activity that accompanies continental rifting. If the stretching forces continue, the rift valley will lengthen and deepen, until the continent splits in two. At this point, the rift becomes a narrow sea with an outlet to the ocean, similar to the Red Sea. The Red Sea formed when the Arabian Peninsula rifted from Africa about 20 million years ago. In this way, the Red Sea provides scientists with a view of how the Atlantic Ocean may have looked in its infancy.

PRACTICE

1.    Take notes on the above information.

2.    Click here (http://videos.howstuffworks.com/science-channel/29268-100-greatest-discoveries-sea-floor-spreading-video.htm) to watch a video about sea floor spreading.

ASSESSMENT

1.    1. Turn in your notes.

2.22. Take the Lesson 10-02 and Lesson 10-04 Quiz.

2.