Final answer:
Volcanoes have a gradual slope because lava flows from the volcano, cools, and solidifies, creating a buildup that shapes the slope. This is the primary process, while other factors such as erosion and tectonic activities also contribute to shaping the volcano over time.
Step-by-step explanation:
The question 'Why do volcanoes have a gradual slope?' is primarily concerned with the features of volcanic eruptions and the geologic processes that shape them. The correct answer to this question is B) Lava cooling and solidifying. This process occurs when lava flows from a volcano and hardens into rock, gradually building up the sides of the volcano to create a slope. Factors such as erosion by wind and water, tectonic plate movement, and underground magma chambers also influence a volcano's shape over time, but the initial slope is due to the solidification of lava.
Additionally, the creation of the Hawaiian island chain is an example of how volcanic activity over a hot spot can gradually build up islands through repeated eruptions. The cooled lava adds layer upon layer, contributing to the island's growth and the volcano's sloped profile. This gradual slope can be further shaped by other factors such as tectonics and erosion, which are all key concepts in understanding the various geological processes at work.