Here's why:

* Rift Valleys: These are areas where the Earth's crust is being pulled apart, creating a long, narrow depression. This process is known as rifting.

* Rift Eruptions: As the crust thins and pulls apart, magma from the Earth's mantle rises to the surface, leading to volcanic eruptions. These eruptions are often characterized by large volumes of lava flowing out of fissures, creating extensive lava fields.

Examples of Rift Valleys and Rift Eruptions:

* East African Rift Valley: One of the most prominent rift valleys, stretching for thousands of kilometers across East Africa. It's home to numerous volcanoes, including Mount Kilimanjaro and Mount Kenya.

* Mid-Atlantic Ridge: A vast underwater rift valley that runs down the center of the Atlantic Ocean, responsible for creating new ocean crust.

So, rift valleys are the defining geologic feature associated with rift eruptions. The process of rifting creates the pathways for magma to rise and erupt, leading to the formation of extensive lava flows and other volcanic features.