The Island of Hawaiʻi, commonly known as the Big Island, is a landmass built entirely by volcanic activity from the seafloor. The most straightforward answer to how many volcanoes make up this island is five, representing the major shield volcanoes visible above sea level. This count becomes more complex when considering the older, submerged structures that form the island’s deep foundation. These volcanoes have coalesced over hundreds of thousands of years, creating the largest island in the Hawaiian archipelago.
Identifying the Five Main Volcanoes
The Big Island is a composite structure formed by the overlapping shields of five separate volcanoes. The oldest is Kohala, located in the north, which forms the northern tip of the island. Kohala is considered extinct, having last erupted approximately 120,000 years ago.
Dominating the northern half is Mauna Kea, a massive shield volcano considered dormant, with its last eruption occurring about 4,500 years ago. Mauna Kea is renowned as the tallest mountain in the world when measured from its base on the ocean floor to its summit, standing over 33,500 feet high.
On the western side is Hualālai, the third-most active volcano on the island. Hualālai last erupted in 1801, classifying it as an active volcano with a high threat potential due to its proximity to populated areas. This volcano rises over 8,200 feet above sea level.
Mauna Loa is the most massive volcano on the planet by volume, covering more than half of the Big Island’s landmass. It is an active volcano with a very high threat potential, having erupted 33 times since 1843, including a recent eruption in 2022.
The final volcano is Kīlauea, located on the southeastern flank of Mauna Loa. Kīlauea is considered one of the world’s most active volcanoes, having been in an almost continuous state of eruption for decades. It is the youngest of the visible structures and is classified with a very high threat potential.
The Sixth Submerged Structure
The island’s formation includes a sixth structure, Māhukona, an older, submerged shield volcano located off the northwest coast near Kohala. This structure is entirely underwater and is now considered extinct.
Māhukona forms the foundational platform upon which the younger, visible volcanoes were built. Its main shield-building stage ended approximately 470,000 years ago. Evidence suggests it was once above sea level before it began to sink due to subsidence.
The summit of Māhukona eventually subsided below the ocean surface between 435,000 and 365,000 years ago. Scientists identified features like a drowned coral reef and a major slope break at depths of about 3,770 feet below sea level. Geologists include Māhukona in the total count because it played a definitive role in the island’s initial formation.
Monitoring Current Volcanic Activity
The dynamic geological nature of the Big Island requires constant surveillance, primarily conducted by the Hawaiian Volcano Observatory (HVO). HVO maintains an extensive network of instruments to track the two most active volcanoes, Kīlauea and Mauna Loa. This monitoring ensures the timely detection of changes that could precede an eruption.
One primary monitoring method is tracking ground deformation using Global Positioning System (GPS) stations and tiltmeters. These instruments measure tiny changes in the ground’s elevation and movement, indicating the swelling or deflating of the volcano as magma moves beneath the surface.
The network also uses seismometers to record seismic activity. An increase in the number and size of earthquakes often signals the movement of magma toward the surface. Monitoring also includes measuring the volume and chemical composition of volcanic gases, such as sulfur dioxide, which escape from vents and fissures. This comprehensive approach allows scientists to understand the internal plumbing of the shield volcanoes and provide warnings to the public.