The island of Oahu, known for its distinct mountainous landscape, was formed entirely by volcanic activity. The question of how many volcanoes are on Oahu has a complex answer because the term “volcano” can refer to different geological structures. While the island’s massive landmass was built upon two primary, foundational shield volcanoes, dozens of smaller, younger volcanic features also exist. Understanding the count requires distinguishing between the original shield volcanoes and the later eruption points.
The Foundation: Oahu’s Two Primary Shield Volcanoes
The core of Oahu consists of two enormous shield volcanoes that erupted millions of years ago, forming the main body of the island. A shield volcano is characterized by its broad, gently sloping profile, built up over time by highly fluid lava flows. These are the foundational volcanoes that created the landmass.
The older of the two is the Waiʻanae volcano, which began its activity approximately four million years ago. The Waiʻanae Range, which runs along the western side of the island, represents the deeply eroded remnants of this original structure. Lava flows from the main shield-building stage of Waiʻanae are dated to a period between 3.8 and 2.95 million years ago.
The second major structure is the Koʻolau volcano, whose main shield formed more recently, between 2.7 and 1.8 million years ago. The Koʻolau Range, which runs parallel to the eastern coast, is what remains of this volcano. The striking, sheer cliffs on the windward side are the result of massive erosion and a catastrophic landslide that dramatically reshaped the volcano’s eastern flank.
The immense volume of lava extruded by these two volcanoes created the vast majority of the island’s surface and subsurface structure. Their enormous size and age place them in the main shield-building stage of the Hawaiian volcanic life cycle. However, they do not account for the numerous, more recognizable volcanic cones found near the city of Honolulu.
The Secondary Features: The Honolulu Volcanic Series
The common confusion regarding Oahu’s volcano count comes from numerous smaller, more recent features known collectively as the Honolulu Volcanic Series (HVS). These features are not new volcanoes, but rather post-erosional vents that erupted long after the main Koʻolau shield volcano went dormant. This type of volcanism represents the final, distinct rejuvenated stage in the Hawaiian volcanic life cycle.
Geologists have identified between 30 and 40 separate vents and cones that make up the HVS, most of which are concentrated in the southeastern portion of the island. These secondary eruptions were often explosive because the rising magma interacted with groundwater or shallow ocean water, creating steam-driven blasts. This explosive activity formed specific structures like tuff cones, which are composed of consolidated ash and fragmented rock.
The most famous example of this activity is Diamond Head, a classic tuff cone towering over Waikiki. Other prominent landmarks belonging to the HVS include Punchbowl Crater in the center of Honolulu, and the Koko Head complex. The Koko Head complex includes Koko Crater and Hanauma Bay, a marine embayment formed within a tuff ring.
The HVS is also responsible for offshore features, such as Manana Island, often called Rabbit Island. These smaller vents created the coastal plain upon which much of the modern city of Honolulu is built, adding a layer of younger volcanic rock on top of the older Koʻolau lavas. Because of the sheer number of these distinct eruption points, the count of “volcanoes” on Oahu is often cited as being much higher than the two primary shield volcanoes.
Extinct or Dormant: The Current Status of Oahu’s Volcanic Activity
The two primary shield volcanoes, Waiʻanae and Koʻolau, are considered extinct, meaning they are no longer capable of erupting. They have moved far from the hot spot that created them and have been significantly eroded over millions of years. Their volcanic activity is considered permanently finished within the current geological framework.
The Honolulu Volcanic Series, however, is classified differently because it represents the rejuvenated stage of volcanism, a low-volume, sporadic return of magma activity. The most recent well-dated eruption from the HVS occurred approximately 30,000 to 40,000 years ago. This relatively recent activity means the HVS is classified as dormant, rather than extinct.
A volcano is considered dormant if it has the potential to erupt again, even if it has not done so in tens of thousands of years. While the possibility of future eruptions in the HVS exists, it is considered very low given the long intervals between eruptions and the island’s distance from the mantle hot spot. Any future activity would likely be similar to the HVS, producing isolated, smaller cinder or tuff cones.