Why does it say that continents stay “somewhat intact” rather than fully?

We’re glad you noticed that subtle wording! Here’s what it means:

Remember that Africa and South America split apart by about 110 million years ago. Therefore, the fact that we can still see their puzzle-like fit (especially when we include their continental shelves) means that they have stayed largely intact (which is another way of saying they have retained their basic shapes) even as the entire Atlantic ocean opened up between them. However, you also know that these continents haven’t remained fully intact forever, because they were once merged together in Pangaea, which certainly did not stay intact when it broke apart. Similarly, Pangaea formed as earlier continents had merged together. That is the basic reason why we say that continents stay “somewhat intact” over time.

One related question you might wonder about is whether plates made of continental crust ever break apart or merge together, and the answer is that they do. In fact, geologists have found many places around the world where this appears to have happened in the distant past or is currently underway today. Here are just a few examples:

• Sometimes we find earthquake-causing faults in the middle of continents. Some of these probably represent ancient faults that once marked plate boundaries.
• Rising mantle rock can start to create new rifts that can eventually make a continent break apart. One example is occurring in eastern Africa today, and it explains how the Red Sea opened up between the Arabian Peninsula and the rest of Africa.
• Over time, islands (such as in island arcs) can merge together to form a new continent or merge with an existing continent. For example, much of the northwest coast of North America (including coastal regions of Alaska, British Columbia, Washington, Oregon, and California) once consisted of individual islands that were carried by plate motion into the rest of North America.
• In a somewhat similar way, Japan was once made up of a larger number of smaller islands that have merged together into the fewer islands of today. The Japanese islands are still undergoing this merging process, and might eventually either join the rest of Asia or perhaps even grow into a new continent.

Indeed, thanks to modern understanding of plate tectonics, geologists can now tell you a lot about the geological history of almost any place on Earth. Therefore, if you are curious, you might wish to look up some of the details that describe the history of your own region of the planet.