Seamounts are underwater mountains defined by oceanographers as features that ascend at least 1,000 meters above the surrounding seabed. These structures are almost always volcanic in origin, forming through the accumulation of erupted lava and igneous material. Scientists estimate there are over 100,000 seamounts taller than 1,000 meters globally, though the majority remain unmapped and unstudied.
The Role of Plate Tectonics and Magma
Seamount construction begins with the movement of Earth’s tectonic plates and the physics of magma generation beneath the oceanic crust. Plate motion dictates where and how magma can rise to form submarine volcanoes. Magma generation frequently occurs through decompression melting, particularly in the upper mantle.
In the mantle, rock is generally solid due to intense pressure. When the overlying pressure is reduced, such as at a spreading center, the rock’s melting point lowers, causing it to partially melt. This lower-density magma then begins its ascent through the lithosphere, the rigid outer layer of the Earth. The location of this magma upwelling determines the two main types of seamount formation: at plate boundaries or within the plates themselves.
Formation at Mid-Ocean Ridges
A primary setting for seamount construction is at divergent plate boundaries, known as mid-ocean ridges. Here, oceanic plates are pulling apart from each other. As the plates separate, the release of pressure on the underlying mantle triggers decompression melting.
The resulting magma rises continuously to fill the gap, cooling and solidifying to form new oceanic crust. Seamounts develop along the ridge when the magma supply is localized or the volcanic activity is focused. Formation is often linked to fracture zones or structural weaknesses adjacent to the main spreading axis, which provide conduits for the magma to erupt. The lava extruded at these plate-boundary seamounts is predominantly basaltic.
Formation via Hotspots
The second major mechanism involves intraplate volcanism, occurring far from the edges of tectonic plates. This formation is attributed to mantle plumes, or “hotspots,” rising from deep within the Earth’s mantle. These plumes are thought to originate near the core-mantle boundary and remain relatively fixed in position for millions of years.
As the rigid oceanic plate moves across this stationary hotspot, the plume melts through the overlying crust, generating a volcano directly above it. Plate movement carries the newly formed volcano away from the magma source, cutting off its supply and causing it to become extinct. The hotspot then penetrates the plate in a new location, forming a younger volcano.
This sequential process creates a linear chain of seamounts with a distinct age progression. The oldest seamounts are the farthest from the currently active volcano. The Hawaiian-Emperor Seamount Chain illustrates the plate’s movement over the fixed Hawaiian hotspot, providing a geologic record of the plate’s direction and speed over time.
The Seamount Lifecycle: From Peak to Guyot
Once the volcanic activity ceases, the structure begins a process of degradation. If the seamount grew large enough to briefly emerge above the ocean surface, wave action rapidly erodes its peak, flattening the summit. Simultaneously, the oceanic crust begins to cool and contract as it moves away from the mid-ocean ridge.
This cooling and contraction cause the seafloor to slowly subside, or sink, over millions of years. A seamount whose top has been eroded flat and subsequently sunk below sea level is called a guyot, or tablemount. Guyots are defined by their characteristic flat summits, which often contain evidence of shallow-water fossils, confirming they were once near the surface.