Final answer:
The subject of this question is Geography, and the answer is 'rift valley'.
Step-by-step explanation:
The subject of this question is Geography.
At the mid-ocean ridges, the slabs of oceanic crust are separating and creating cracks. This process is known as rifting and results in the formation of rift valleys. In these rift valleys, volcanic activity may occur, leading to the creation of new crust.
Therefore, the correct answer is rift valley.
Learn more about Processes at mid-ocean ridges