What geological activity occurs when two tectonic plates move away from each other?

When tectonic plates move away from each other, the geological activity that occurs is known as divergent activity. This process is characterized by the creation of new crust as magma rises from beneath the Earth's surface and solidifies at the mid-ocean ridges or rift valleys. As the plates separate, they create space for magma to fill, leading to volcanic activity and the formation of new oceanic or continental crust.

This divergence can lead to the development of features such as mid-ocean ridges, which are underwater mountain ranges formed by this upwelling of magma. Additionally, as the plates pull apart, it can also create earthquakes due to the stress and friction at the plate boundaries.

The other options refer to different types of tectonic interactions. Transform refers to plates sliding past each other, convergent involves plates moving toward one another leading to subduction or mountain building, and subduction is the specific process where one plate is forced under another. Each of these processes results in distinct geological features and activities, but they are not associated with the movement of plates away from each other, which is why divergent is the correct identification for this question.