Abyssal plains are indeed lower than mid-ocean ridges, and this difference in elevation is primarily due to the geological processes at play in the ocean floor.
Mid-ocean ridges are underwater mountain ranges that form at divergent tectonic plate boundaries. As tectonic plates pull apart, magma rises from the mantle to fill the gap, which creates new oceanic crust. This process makes the mid-ocean ridges elevated compared to the surrounding ocean floor.
In contrast, abyssal plains are found at the ocean’s depths, typically at depths of around 3,000 to 6,000 meters (about 10,000 to 20,000 feet). These plains are formed by the accumulation of sediments that gradually bury older oceanic crust and features. Over millions of years, sediments from various sources, such as the remains of marine organisms and particles washed down from the continents, build up on the ocean floor. This accumulation lowers the elevation of the abyssal plains relative to the higher mid-ocean ridges.
Additionally, the oceanic crust is denser than continental crust, and as the older crust moves away from the mid-ocean ridges, it cools and sinks due to its increasing density. This further contributes to the depth of the abyssal plains. All these factors combined explain why abyssal plains are lower than mid-ocean ridges.