Hawaii’s volcanoes are different from most volcanoes around the world. In other places when a volcano erupts people flee for their lives. In Hawaii we get in our cars and drive down to the lava flow to check it out. In many ways they are gentle volcanoes; they are volcanoes with aloha. Unlike 90% of the earth’s volcanoes, which are located at the boundaries of tectonic plates, Hawaii is smack dab in the middle of a plate. It is this difference, the location and source of Hawaii’s magma, which makes our volcanoes relatively viewer-friendly.
Hawaii’s volcanoes are located in the middle of the Pacific Plate. Like all tectonic plates that form the crust of the earth, the Pacific Plate is moving—inching ever so slightly to the northwest. From deep in the earth a “hot spot” of magma rises upward and melts through the plate. This fixed plume of magma builds up volcanoes as the Pacific Plate is dragged across it. The magma from the hot spot is basic (as opposed to acidic or intermediate) lava. Basic lavas such as basalt, the most common Hawaiian lava, are relatively low in silicas and more fluid. With a well-developed plumbing system to bring the magma up and out, the lava erupts gently and flows easily. Layer after layer, the Hawaiian hot spot has laid down lava, building up gigantic islands from the ocean floor. This volcanic action has built the Hawaiian Islands extending from Kure Atoll and Midway Island all the way to the Big Island. When the volcanoes are close to the hot spot, like Mauna Loa, Kilauea, and Loihi, they are very active and growing. As they get pulled away from the hot spot, the volcano’s magmatic plumbing is disrupted and not as efficient. The volcanoes become less and less active until finally, they are completely severed from the hot spot and become extinct. With waning activity and finally extinction, these volcanoes sink, erode, and slough away as landslides. Finally sinking underneath the ocean surface again, they become seamounts.
The Big Island is a great little microcosm of Hawaii’s volcanic development. Our island is actually made up of seven volcanoes-five on the surface and two underwater. At the far northwestern end, furthest from the hot spot, are Mahukona and Kohala. The top of the submarine Mahukona is a couple thousand feet below the surface of the ocean. It once was probably a few thousand feet above sea level but has sunken and eroded over the years. Kohala is the oldest on the surface and last erupted about 60,000 years ago; its eastern side is deeply eroded. Next is Mauna Kea, it hasn’t erupted for about 3,300 years. Despite its lack of activity it is still the tallest mountain on the island at 13,790 feet above sea level. In fact, with its huge mass building up off the ocean floor, it is the tallest mountain in the world rising 32,000 feet high. Hualalai, which forms the bulk of North Kona last erupted in 1801. The Kona International Airport is built upon much of that flow. Surrounding Hualalai and covering over half of the island’s surface is Mauna Loa. Mauna Loa last erupted in 1984 and, besides being the world’s largest volcano, it is also one of the most active. Youngest on the surface is Kilauea; in near continuous eruption since January of 1983 it is arguably the most active volcano on earth. Out at sea from Kilauea is Loihi. Rising 15,000 feet off the ocean floor it has 3,000 feet more to go before it pops through the surface of the ocean and becomes the island’s next volcanic peak. Like a great conveyer belt of material, new volcanoes rise off the ocean floor as old ones sink and erode back down.
In contrast to Kilauea and Mauna Loa, volcanoes like Pinatubo and Mt. Saint Helens are located at the boundaries of tectonic plates. Where the plates meet they rub up against each other. All of this massive contact creates great friction and heat that remelts the continental crusts and creates acidic magma that is silica rich. This lava is slow flowing and explosive. Building up through lots of other geologic segments these volcanoes can literally blow their top. The blast of these eruptions, along with landslides and super-heated pyroclastic flows all combine to make these volcanoes incredibly dangerous and deadly. Mt. Saint Helens 1980 eruption in Washington killed 57 people, despite warnings and evacuations by the U.S.G.S. In contrast, Kilauea and Mauna Loa, with nearly 80 eruptions between the two of them over the last 200 years, has resulted in only two deaths directly related to volcanic eruptions. Kilauea is often touted as the world’s only active drive-in volcano. Last year it had over 2.5 million visitors to its summit caldera-a world-class volcanic tourist attraction.
Despite their gentle reputation, Hawaii’s volcanoes are a mighty force. They can be destructive and deadly. Since the current eruption began it has destroyed over 180 homes. Rare explosive eruptions called phreatic explosions have occurred when groundwater has mixed with magma creating great steam blasts. Around 1790 one such eruption produced volcanic gases that suffocated approximately 80 Hawaiians who were crossing Kilauea. The great phreatic eruption of 1924 killed a photographer who was burned by falling ash and crushed by a large boulder strewn into the air. Another photographer died in 1993 when the lava bench he was on gave way and slid into the sea amidst flowing lava. Pele surely destroys all in her path. Mauna Loa’s lower southwest rift zone will one day erupt. Its lava flows may descend with speed and great mass where now thousands of homes exist. Yet when the immensity of Hawaii’s volcanoes is considered, the incessant flow of magma imagined, these volcanoes are, by comparison, mellow. This hot spot in the sea has created islands of aloha whose beauty and spirit are unique to the world.