Which geological feature is an underwater mountain chain where new ocean floor is formed?

The underwater mountain chain where new ocean floor is formed is known as a mid-ocean ridge. This feature is created by the process of seafloor spreading, which occurs when tectonic plates pull apart at divergent boundaries. As the plates separate, magma from the mantle rises to fill the gap, cooling and solidifying to create new oceanic crust. This process not only forms the ridges but also contributes to the overall growth of the ocean floor.

Mid-ocean ridges are characterized by a central rift valley and can extend thousands of kilometers across the ocean floor, making them one of the longest mountain ranges on Earth. Their formation is a key component of the plate tectonics theory, illustrating the dynamic nature of our planet's surface.

In contrast, trenches are deep fissures in the ocean floor caused by the subduction of one tectonic plate beneath another, while island arcs are chains of volcanic islands formed above subduction zones. Seamounts are underwater mountains that rise from the ocean floor but are not formed by the same processes as mid-ocean ridges and can be extinct volcanoes. All these features have significant geological importance but do not involve the creation of new ocean floor in the same way mid-ocean ridges do.