Here's why:

* Geosphere: This sphere encompasses the solid, non-living components of the Earth, including the crust, mantle, and core. Volcanic activity originates from the mantle and involves the movement and eruption of molten rock (magma).

* Hydrosphere: This sphere includes all the water on Earth, such as oceans, lakes, rivers, and ice. While volcanic activity can influence the hydrosphere (e.g., creating new islands or releasing gases that affect water chemistry), it's not primarily a part of it.

* Atmosphere: This sphere is the layer of gases surrounding the Earth. Volcanic eruptions release gases and ash into the atmosphere, affecting weather patterns and climate. However, the origin of volcanic activity is within the geosphere.

* Biosphere: This sphere encompasses all living organisms on Earth. Volcanic activity can impact the biosphere by creating new environments or destroying existing ones, but it's not an inherent part of the biosphere itself.