Final answer:
Lava domes usually grow over a period of several years following an explosive eruption of silica-rich magma.
Step-by-step explanation:
The correct answer is 2) lava domes that usually grow over a period of several years following an explosive eruption of silica-rich magma.
Lava domes are formed when sticky and viscous lava cannot flow easily and instead piles up around the vent of a volcano. This type of lava is typically rich in silica and has high gas content, which causes explosive eruptions. As the magma is erupted and cools down, it hardens and gradually forms a dome-shaped structure.
An example of a volcano with lava domes is Mount St. Helens in Washington State, USA. After its explosive eruption in 1980, multiple lava domes have formed within the crater over the years as a result of subsequent eruptions.