The mountainous chain of young basaltic rock at the active spreading center of an ocean is called a mid-ocean ridge.

Here's a breakdown:

* Mid-ocean ridge: These are underwater mountain ranges that form at divergent plate boundaries.

* Young basaltic rock: The rock is formed from magma that rises from the Earth's mantle and cools quickly.

* Active spreading center: This is where tectonic plates move apart, allowing magma to rise and create new crust.

The mid-ocean ridge system is the longest mountain range on Earth, stretching for over 40,000 miles. It's a vital part of Earth's geology, responsible for creating new oceanic crust and influencing ocean circulation.