Final answer:
Extinct volcanoes with conical tops and steep sides in the ocean basins are called seamounts, which are significant for their influence on ocean currents and biodiversity.
Step-by-step explanation:
The extinct volcanoes with conical tops and steep sides found in the ocean basins are called seamounts. These underwater mountains are formed by volcanic activity and can become islands if they rise above the ocean surface. However, when they remain submerged and are no longer active volcanoes, they are known as seamounts. These features are significant as they can affect ocean currents and are often areas of high biodiversity, being home to various marine life. Seamounts can also accumulate thick blankets of sediment over time, influencing the seafloor structure and ecology.