Geography
How Volcanic Hotspots Create Chains of Seamounts and Guyots
Quick fact
The Hawaiian-Emperor seamount chain is over 5,800 kilometers long and includes at least 80 volcanoes, with the oldest dated to about 80 million years old—far older than the current islands.
Why this is interesting
Have you ever wondered how the Hawaiian Islands—and the long trail of underwater mountains stretching to the northwest—formed? What could possibly create a chain of volcanoes in the middle of an ocean plate?
Read the full explanation
Understanding How Volcanic Hotspots Create Chains of Seamounts and Guyots
Imagine a blowtorch fixed in one place under a moving sheet of metal. As the metal moves, the torch melts a line of holes. Earth’s crust (the plates) does the same thing: there are narrow, fixed columns of unusually hot material rising from deep in the mantle—these are called mantle plumes. The surface expression of a plume is a hotspot, where magma melts through the crust and builds a volcano. But the plate is not stationary; it slowly drifts (a few centimeters per year). So the volcano is carried away from the hotspot, and the hotspot erupts in a new spot, building another volcano. Over millions of years, this creates a chain of volcanoes—each one older the farther it is from the hotspot. When these volcanoes are underwater, they are called seamounts. If the top of a seamount is flat, it is called a guyot. The flat top forms because when the volcano was above sea level as an island, waves eroded its top into a flat surface; then it sank (subsided) beneath the ocean, leaving a flat-topped underwater mountain.
A deeper explanation
The process works because hotspots are thought to be anchored in the deep mantle, possibly at a boundary layer like the core–mantle boundary. While the exact mechanism is still debated, the key is that the plume remains relatively stationary while the lithospheric plate moves over it. This is a direct consequence of plate tectonics: divergent plate boundaries, subduction zones, and mantle convection all contribute to the horizontal motion of plates. The Hawaiian Islands are a perfect example: the Big Island is currently over the hotspot, and it’s the only actively erupting island. As you move northwest, the islands get progressively older and more eroded, and eventually they become seamounts and guyots. The Emperor Seamounts, which continue the chain northwestward, show a bend that records a major change in Pacific Plate motion about 47 million years ago. This demonstrates that hotspot trails can be used to reconstruct the past movements of plates. It also explains why many islands and seamounts are linear chains rather than scattered randomly. Understanding this mechanism helps geologists predict which volcanoes might be active and how the seafloor around them has evolved. The difference between a seamount and a guyot is also a clue to the history of the volcano: guyots have had time to be eroded at the surface and then subsided, indicating they formed when sea level was higher or the crust was shallower.