The Galapagos archipelago sits on the northern edge of the Nazca tectonic plate. Additionally, there is a “hot spot” where molten mantle is forced to the surface to produce volcanic islands. As the tectonic plate slips southeast a series of islands are formed in its path.
In terms of the Galapagos Islands, the oldest islands are to the east (i.e., San Cristobal, Genovesa, Española), while the youngest islands lies to the west (i.e., Fernandina & Isabela). This helps explain why there is more volcanic activity on these two western islands.