Walvis Ridge MV1203 Expedition

Seamounts: What are they and how do they form?

Volcanoes are some of the most majestic and well-known expressions of the forces at work in the earth under our feet. Most people have seen photographs of glowing lava flows spilling down the sides of Hawaii’s Kilauea and of explosive eruptions from volcanoes such as Mt. Etna in Italy and Mt. Pinatubo in the Philippines. What many people don’t realize is that the majority of volcanism on earth actually occurs underwater. Seamounts are simply underwater volcanoes, built up over time by successive lava flows extruded onto the ocean floor. Guyots are seamounts that have very flat tops. They are seamounts that grew above sea level, were eroded away, and subsided below the water again as they ceased erupting and grew older.

Due to their lack of accessibility, relatively little is known about how seamounts evolve compared to volcanoes on land. Geologists have used techniques such as drilling, geophysical surveys, ocean floor mapping, and diving in submersibles to study the external shape and internal structure of seamounts. In some places, large sections of seamounts have collapsed, allowing us to look inside the seamount and see how different lava flows have been extruded over time.

Most studies so far have shown that the inner core of a seamount is dominated by pillow lavas and is thus relatively dense. As the seamount grows over time, its slopes become steeper and some of the material already present may break into pieces as it alters and other lava flows are emplaced on and around it. As the seamount grows taller and nears the surface of the ocean, explosive eruptions may also generate fragmented volcaniclastic deposits. These deposits are not very stable and may slide down the slopes of the seamount, making it wider and smoothing out its slopes.

Knowledge of what seamounts look like inside is important for both scientific and practical reasons. Geologists are interested in how liquid circulates through oceanic crust and how heat and chemicals from deeper in the earth are released into the oceans. Since much of the ocean floor is covered in sediment, seamounts that stick up above this layer can provide a relatively porous vent for hot fluids. The collapse of seamounts and islands can also have significant implications for humans. For example, old volcanic islands are often quite unstable. If such an island were to collapse and send an enormous landslide into the ocean, large waves or tsunami could propagate long distances across entire ocean basins and flood inhabited coastal areas.

Susan Schnur