Here's why:

* Plate Tectonics: The Earth's outer layer is made up of large, moving plates called tectonic plates. These plates interact at their boundaries, causing most of the Earth's geological activity.

* Types of Plate Boundaries:

* Convergent Boundaries: Where plates collide. This leads to mountain building (orogeny), earthquakes, and volcanic activity.

* Divergent Boundaries: Where plates move apart. This leads to the formation of new crust, mid-ocean ridges, and volcanic activity.

* Transform Boundaries: Where plates slide past each other horizontally. This results in earthquakes.

Other areas with geological activity:

* Hotspots: Areas within the Earth's mantle where plumes of hot rock rise to the surface. These can create volcanic islands and chains, even in the middle of tectonic plates.

* Fault Zones: Areas where the Earth's crust has fractured. These can be active and cause earthquakes.

While most geologic activity happens at plate boundaries, it's important to remember that the Earth is a dynamic system and activity can occur in other locations as well.