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Why does the ocean surface bulge above large seamounts?

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The ocean surface bulges above large seamounts due to the force of gravitational attraction.1

Because a seamount represents a significant concentration of mass on the seafloor, it exerts a localized gravitational pull that is stronger than that of the surrounding, less dense deep-sea floor. This increased gravitational attraction pulls the surrounding seawater toward the seamount, causing the ocean surface to rise or "bulge" directly above the feature.1

This phenomenon is a critical aspect of modern undersea exploration. By using radar altimeters on satellites, scientists can measure these subtle variations in the height of the sea surface with high precision. Because the shape of the ocean surface is a sensitive indicator of the topography beneath it, these measurements allow researchers to map and identify previously uncharted seamounts, ridges, and other seafloor features that would otherwise be difficult to detect. Conversely, areas with less mass, such as deep-sea trenches, cause the ocean surface to downwarp.1