Hawaiʻi is the classic example of a geological hotspot, a phenomenon that explains volcanic activity occurring far from tectonic plate edges. A hotspot represents a fixed point of intense heat originating deep within the Earth’s mantle. The resulting volcanism is known as intraplate volcanism. This fixed heat source is responsible for building the entire Hawaiian island chain over millions of years.
The Underlying Geological Mechanism
The Hawaiian Islands are attributed to a mantle plume, a relatively stationary column of superheated rock rising from the Earth’s lower mantle. This plume acts as a consistent heat source, delivering thermal energy to the overriding lithosphere, the rigid outer layer of the Earth.
The key to island formation lies in the movement of the Pacific Plate, upon which the Hawaiian archipelago sits. This large tectonic plate constantly moves in a west-northwesterly direction, sliding over the stationary mantle plume. As the plate moves, the plume’s heat partially melts the overlying solid rock near the surface, a process known as decompression melting.
This molten material, or magma, is less dense than the surrounding rock, allowing it to erupt onto the seafloor, continuously forming a volcano. The steady action of the plume, coupled with the plate’s movement, creates a conveyor belt effect. This pulls newly formed volcanoes away from their heat source as new ones begin to form.
The Evidence of the Island Chain
The most compelling evidence supporting the hotspot theory is the age progression of the Hawaiian-Emperor Seamount Chain, a linear trail stretching over 6,200 kilometers. As the Pacific Plate carries volcanoes away from the stationary heat source, they are cut off from their magma supply and become dormant. This results in a distinct pattern where islands become progressively older and more eroded the further they are from the Big Island of Hawaiʻi.
Scientists use radiometric dating techniques to confirm this progressive aging. The oldest exposed rocks on the Big Island are less than 700,000 years old. Moving northwest, Kauaʻi, one of the oldest main islands, has volcanic rocks dating back about five million years, eventually subsiding beneath the ocean surface.
These older, submerged mountains are known as seamounts, forming a continuous trail that records the history of the Pacific Plate’s movement. As a volcano moves off the plume, its immense weight causes the underlying crust to flex and sink. This process transforms high volcanic islands into low-lying atolls and finally into flat-topped seamounts, with the oldest dating back over 65 million years.
Where Volcanic Activity Occurs Now
The Big Island of Hawaiʻi currently sits directly above the mantle plume and is home to the only active volcanoes in the archipelago. Active volcanoes like Kīlauea and Mauna Loa are consistently fed by the plume’s magma. The current position of the Big Island marks the most recent point of sustained volcanic activity.
The hotspot is already creating the next Hawaiian landmass, a submarine volcano named Kamaʻehuakanaloa, formerly Lōʻihi Seamount. This active, growing volcano is located about 32 kilometers southeast of the Big Island’s coast. Its summit is currently about 1,000 meters below the ocean surface, and it is experiencing frequent earthquake swarms, indicating ongoing magmatic activity.
Geologists estimate that Kamaʻehuakanaloa may breach the ocean surface to become the next Hawaiian island within the next 100,000 years. The existing islands will continue their slow journey to the northwest, moving at a rate of approximately 10 centimeters per year, eventually becoming seamounts. As long as the mantle plume remains active, the process of new island formation and plate movement will continue, lengthening the seamount chain into the future.