Here's why:
* Rift valleys are formed when tectonic plates pull apart, causing the Earth's crust to thin and stretch.
* The East African Rift Valley is a prime example of this process, with a vast system of valleys and volcanoes stretching for thousands of kilometers.
* In some areas within the East African Rift Valley, the continental crust is estimated to be as thin as 15-20 kilometers, compared to the average continental crust thickness of 30-40 kilometers.
Note: While the East African Rift Valley is the most prominent example, other rift valleys around the world also have areas of very thin crust.
