Creation of Oahu by Volcanism

Volcanoes are magnificent formations from an opening in a planet’s crust. They form when magma, volcanic ash, and gases escape from a magma chamber below the surface of the planet. Tectonic plates cover the surface of the planet and are constantly moving. Volcanoes are generally found where these plates converge or diverge. Not only are volcanoes found in these areas but also where the Earth’s crust is stretching or thinning. There are volcanoes that form away from plate boundaries in areas that are called “hot-spots”. An example of “hot-spot” volcano formation is the Hawaiian Islands.

Figure 1: (Kreger, 2010) The image above shows that volcanoes are created by hot magma that rises from the Earth’s core and expels through the cracks of the upper mantle and ocean crust. The movement of the Earth’s plates forms these cracks on the surface of the ocean crust.

Every Island is made of one or more volcanoes that have erupted many times and emerged above sea level. A seamount is a mountain that rises above the Ocean floor that is of volcanic origin and does not rise above sea level. Most seamounts are formed from extinct volcanoes that have been eroded and grounded back down by waves. Loihi is an example of an active seamount that is currently being studied because of its assumed Island formation. Because Loihi is located before the Island of Hawaii, it is believed that it will become an Island soon enough. Loihi is slowly moving across the hot spot and growing from its continuous eruptions. By observing the growth of Loihi and the patterns of eruptive activity of older seamounts, geologists can study the growth of the Hawaiian Islands more extensively. (Normark, 1982)

Figure 2: (Rowland) The image above shows a picture of Loihi, a volcanic seamount, that is currently sitting on the hot spot and growing. It is not yet classified as an Island because it is still submerged underwater.

The Hawaiian Islands sit on a “hot-spot” that does not move. The Pacific plate gradually moves northwestwardly and carries the Islands away from the hot spot. The islands follow a particular rate of growth and after extensive observation geologists say that the hot spot is actually a column stretching from the mantle of the Earth to the surface of the ocean floor. The different types of lava found on the Hawaiian volcanoes support this theory. The Hawaiian Volcanoes show two forms of lava, Basaltic, and Alkali. When the Volcanoes are young they expel basaltic lava, which creates shield like volcanoes. When volcanoes become older, alkali lava is erupted. As the volcano moves away from the hot spot their shape becomes cone like and rough because of the alkali eruptions.

Figure 3: (Rowland) This figure above shows the movement of the volcanoes along the hot spot. As the plate movement carries the volcanoes over a fixed hot spot, different lavas are expelled.
Many Volcanoes will erupt and become Islands by growing above sea level, but in time begin to subside. Most Volcanoes that are more than 1 million years old will have subsided approximately 3-4 km. When Volcanoes and the ocean floor subside, coral reefs will build. A fringing reef is formed when corals begin to settle and grow around an Island. When the reef expands and the Island inside the reef circumference subsides, this forms an atoll. An atoll is an island that has completely subsided below sea level, becoming an open lagoon, and is surrounded by a ring of growing coral. This process of atoll formation can take as long as 30 million years. Figure 4: (USGS, 2001) The figures above illustrate the formation of atolls from volcanoes that have emerged above seawater and subsided over time. Over time the reef will grow around the volcano and as the volcano subsides the reef will remain and what would be left is an open lagoon.
The Hawaiian Islands are the youngest volcanic islands in the Pacific Region. They are a part of the Hawaiian Ridge-Emperor Seamounts chain that extends to 6,000 km long. The Emperor seamount chain, which includes seamounts from Daikakuji Seamount to Meiji Seamount, extends 2,327 km and shows a North movement pattern. The Hawaiian Ridge extends 3,750 km from Loihi to Daikakuji seamount to the northwest (Clague, 1989). This sudden bend in the chain showed that the motion of the Pacific Plate changed abruptly about 43 million years ago. Since the change the pattern of volcano movement has been continuing northwest at an average of 3-3.5 inches per year. (USGS, 2001)

Figure 5: (USGS, 2001) The figure above shows the Hawaiian Ridge that stretches about 2,700 km moving in a northwesterly direction. Past the Northwestern Islands are the Emperor Seamounts that makes a sharp bend to the North direction.
The two largest and oldest volcanoes on Oahu are Koolau and Waianae. The oldest lava was dated to be ~ 3.9 million years old from the Waianae Range. Waianae last erupted ~2.5 million years ago and makes up the western coast of Oahu. The Waianae mountain range extends 35 km long and 14 km wide and is composed of three types of lava, mainly of basaltic rock. (Wikipedia, 2013) Waianae’s original caldera was located at the head of Lualualei Valley but has been buried by erosion. Waianae has been heavily eroded. The slopes of the western side of the mountain range are steep and consist of heavy rainfall. It is where deep valleys such as Nanakuli, Lualualei, Waianae, and Makaha are found. The northern part of the range ends in steep cliffs, whereas the southern part ends gradually as it reaches the plain. Standing at the highest elevation of the Waianae mountain range is Mount Ka’ala. Found at the head of Makaha valley, Mount Ka’ala stands at 1.2 km and is the highest point on Oahu. (Wikipedia, 2013)
The Koolau Range is estimated to be 2.5 million years old. The Koolau range forms the Eastern coast of Oahu and its mountain parallels for 60 km in length and 21 km wide. The mountain’s original caldera is located near the town of Kaneohe and after erosion, appears as a hill. The Koolau range traps precipitation and results in many streams and waterfalls. Located on the eastern side of the Koolau are cliffs. These cliffs, or Pali’s, range from 150-750 meters in height and are approximately 3 km away from the sea. The highest point of the Koolau Mountain Range, Konahuanui, is found at the head of Nuuanu valley and stands at both 960 and 940 meters in height. The western slopes of the Koolau connect with the eastern slopes of Waianae to form the Central part of Oahu. (Editors of Encyclopedia Britannica)

Figure 6: (Dave Black Online, 2007) The figure above illustrates the Waianae Range on the right that makes up the Western coast of Oahu, the Koolau range that makes the Eastern coast of Oahu, and the Schofield plateau located in the middle where the two ranges meet.
Current studies show that during the shield stage of the Koolau, Oahu used to be twice as big as it is today. A massive earthquake broke off the Eastern side of the Island and has travelled a great distance in sea over a million years ago. Of at least 17 landslides off of Oahu’s shore, the most prominent landslide is the Nuuanu landslide. Debris has been found in the ocean amounts to nearly one third of the island’s landmass. The Nuuanu Pali is now a steep cliff and played an important role when King Kamehameha I united the Hawaiian Islands. Soldiers were thrown off the cliff and those that were lifted back by the powerful winds were spared. (Editors of Encyclopedia Britannica)
The same kind of erosion happened on the Wailau Mountain on Molokai. Early bathymetry mapping of the Hawaiian Ridge showed high topography in the Ocean abyss. These types of erosions play an important role in shaping the Island. The Nuuanu and Wailau landslide are ranked the largest landslides on Earth. During the bathymetric survey of the Hawaiian Ridge it showed massive submerged cliffs right off the shore of both the Koolau and Wailau Mountain. These massive landslides are linked with geologic damages such as earthquakes, submergence of shorelines, and large tsunamis. (Walker, 1990)

Figure 7: (Moore, J.G. and Clague, D.A., 2001) The above is a topographical map of the landslides off the coasts of Oahu and Molokai.
During the dormant phases of the Koolau, different volcanoes arose during a second eruption along the range. This second eruption is named the Honolulu Volcanic Series, the erosions of these volcanoes also helped form the Island. The Honolulu volcanic series is a set of monogenetic volcanic events that created many landmarks of Eastern Oahu such as Diamond Head, Koko Head, Punchbowl Crater, Tantalus, Hanauma Bay, and Aliapa’akai. This series of eruptions are not like the Koolau as they only erupted once. This occurs when the magma supply to the volcano is low and the vents are not close or large enough to develop plumbing systems to continually feed magma.
Diamond Head, also known as Le’ahi, is estimated to be about 150,000 years old and has been extinct for the same amount of time. It is a cinder cone part of the Koolau Volcano and was most likely explosive due to its close proximity to sea level at the time. During the time of eruption of Diamond Head an eruption that took place at the same time built the Black Point lava shield. Diamond Head is a part of the monogenetic series and geologists believe that it will no erupt again. Presently Diamond Head is a United States State Monument and is a popular destination for tourism. Part of the volcano is used by U.S. government and is not open to the public.
Hanauma Bay is actually a crater created 32,000 years ago. A series of volcanic vents lead to explosive eruptions that shattered the sea floor. Eventually forming the Bay. The eruptions were believed to have happened underwater and blew up hot gases, rock and coral, which blanketed the area. The violent eruption of Hanauma Bay has left fragments of white coral limestone and dark basalt rock in the walls. When visiting the areas around the bay you can see these fragments along the beach. The crater walls of Hanauma Bay eroded possibly from waves, opening the bay. As the sea level rose after the explosion, the bay is now a flooded crater. As coral reef grew in the area it has become a nature preserve and is now a popular site for snorkeling. Koko Head is also part of the Koolau Volcano. It has three significant depressions the largest is Hanauma Bay. Koko head is a massive cinder cone and the best preserved. Radiocarbon dating of volcanic rocks near the area suggests that the last volcanic event occurred only 7,000 years ago. It is the youngest crater and cone of the Honolulu Volcanic Series. At the summit of Koko head includes two craters. Railway tracks were built to service and supply the bunker at the summit built in World War II. Radar antennae are located in the summit and are not open to the public. Under the Kalanianaole highway you will be able to notice the multiple layers of tuff, volcanic ash, that collected resulted in the mass of Koko head. After a series of erosions the unique rock formations and arches on the ridges of the mountain was formed. This area along the Koko rift had the most recent volcanic activity of Oahu and it might not be over yet.
Punchbowl is another volcanic tuff cone formed some 75,000 – 100,000 years ago. Formed from the eruptions of hot lava through cracks in the old coral reefs that were at the foot of the Koolau mountain range. It is now the location of the National Memorial Cemetery of the Pacific. Punchbowl was used for several ancient Hawaiian Ceremonies and also used as military purposes during World War II. (Wikipedia, 2013) (Editors of Encyclopedia Britannica)
Tantalus was formed during the demise of the Koolau volcano. It is a cinder cone that also has a summit crater that is frequently visited by hikers, bicyclist, and tourists. Tantalus is now a small residential community surrounded by the Makiki area. The southern slope of Tantalus was bombed by Japan in 1942.
Oahu was once an enormous Island originated by two volcanoes: Koolau and Waianae. Through enormous amounts of eruptions and land formations, Oahu was created. Throughout the years, erosion has taken its toll on Oahu and shaped the Island. Volcanoes have been studied for years and fortunately, Loihi gives geologists a chance for a better insight about Island formation. Starting as a seamount, Loihi will one day emerge above sea level and become an Island. Careful observation of Loihi will show the similarities of the formation of Oahu and the other Islands on the Hawaiian Ridge-Emperor Seamounts chain.

Works Cited

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Moore, J.G. and Clague, D.A., 2001, Mapping the Nuuanu and Wailau landslides in Hawaii, In Evolution of Hawaiian Volcanoes: (Ed. E. Takahashi et al.), AGU Monograph, in press.