Underwater mountain ranges, also known as mid-ocean ridges, are formed by a process known as seafloor spreading. This process is driven by the movement of tectonic plates, which are large pieces of the Earth's crust that are in constant motion.

Here is a simplified explanation of how seafloor spreading and the formation of underwater mountain ranges occur:

1. Convection Currents: Within the Earth's mantle, the layer beneath the crust, there are convection currents. These currents are caused by the heat generated from the Earth's core. The heat causes the rocks in the mantle to rise, cool, and sink back down, creating a circular motion.

2. Divergent Plate Boundaries: At certain locations on the Earth's surface, tectonic plates move away from each other. These boundaries are called divergent plate boundaries. As the plates move apart, a gap or rift is created between them.

3. Magma Rises: The movement of the tectonic plates creates a pathway for magma (molten rock) from the Earth's mantle to rise up towards the surface. This magma fills the gap between the diverging plates.

4. Crust Formation: As the magma reaches the seafloor, it cools and solidifies, forming new oceanic crust. This new crust is added to the edges of the diverging plates.

5. Mid-Ocean Ridges: The process of seafloor spreading and the continuous addition of new crust cause the seafloor to bulge upward, forming underwater mountain ranges known as mid-ocean ridges. These ridges can extend for thousands of kilometers and can be found in all ocean basins.

6. Spreading Rates: The rate at which the plates move apart and the magma rises can vary at different divergent plate boundaries. This variation in spreading rates affects the width and height of the underwater mountain ranges that are formed.

7. Volcanic Activity: Along the mid-ocean ridges, volcanic activity is common due to the presence of magma near the surface. This volcanic activity can create new volcanic islands or seamounts, further adding to the complexity of the underwater topography.

Over time, as the seafloor spreading process continues, the mid-ocean ridges move away from the divergent plate boundaries, and they become older and cooler. The seafloor on either side of the ridge also moves away from the ridge as new crust is added. This process of seafloor spreading and seafloor aging is a fundamental aspect of plate tectonics and contributes to the dynamic nature of the Earth's surface.