Science · Grades 3–6

Ocean Sea Basins

Reading comprehension lesson about ocean sea basins — the vast underwater landscapes that cover most of our planet.

Reading Passage

Ocean Sea Basins

The ocean floor is not flat. It has mountains, valleys, plains, and trenches as dramatic as any landscape on land — and in many cases far larger. Earth’s ocean is divided into five major basins: the Pacific, Atlantic, Indian, Southern, and Arctic. The Pacific Ocean is by far the largest, covering more area than all of Earth’s landmasses combined. Each basin has a distinct shape carved over millions of years by the movement of tectonic plates, volcanic activity, and the accumulation of sediment.

One of the most dramatic features of the ocean floor is the mid-ocean ridge system, a continuous underwater mountain chain that winds around the entire planet for about 40,000 miles. At the center of the ridge lies a rift valley where tectonic plates are slowly pulling apart. Molten rock, or magma, rises from the mantle, fills the gap, and hardens into new oceanic crust in a process called seafloor spreading. The Mid-Atlantic Ridge runs down the center of the Atlantic Ocean and is spreading at a rate of about one inch per year, slowly widening the Atlantic.

At the edges of ocean basins, where oceanic plates collide with continental plates, the denser oceanic crust is forced downward into the mantle in a process called subduction. This creates deep ocean trenches — the deepest places on Earth. The Mariana Trench in the western Pacific, formed by the subduction of the Pacific Plate beneath the Mariana Plate, reaches a depth of nearly 36,000 feet — deep enough to swallow Mount Everest with more than a mile to spare. The pressure at that depth is more than 1,000 times the pressure at the ocean surface.

The deep ocean floor is also shaped by sediment. Fine particles of clay, the remains of microscopic marine organisms, and material washed from the continents accumulate on the ocean floor over millions of years, building up thick layers of sediment. Much of the deep ocean floor is covered by abyssal plains — nearly flat, featureless regions formed by sediment filling in and smoothing the rugged volcanic terrain. Scientists study ocean sediment cores to reconstruct past climates, because the remains of ancient marine organisms preserve a record of past ocean temperatures and chemistry.

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