Kohala () is the oldest of five volcanoes that make up the island of Hawaii. Kohala is an estimated one million years old—so old that it experienced, and recorded, the reversal of Earth's magnetic field 780,000 years ago. It is believed to have breached sea level more than 500,000 years ago and to have last erupted 120,000 years ago. Kohala is in area and in volume, and thus constitutes just under 6% of the island of Hawaii.
Kohala is an extinct shield volcano cut by multiple deep gorges, which are the product of thousands of years of erosion. Unlike the typical symmetry of other Hawaiian volcanoes, Kohala is shaped like a foot. Toward the end of its shield-building stage 250,000 to 300,000 years ago, a volcanic landslide destroyed the northeast flank of the volcano, reducing its height by over and traveling across the sea floor. This huge landslide may be partially responsible for the volcano's foot-like shape. Marine fossils have been found on the flank of the volcano, far too high to have been deposited by standard ocean waves. Analysis indicated that the fossils had been deposited by a massive tsunami approximately 120,000 years ago.
Because it is so far from the nearest major landmass, the ecosystem of Kohala has experienced geographic isolation, resulting in a unique ecosystem. There are several initiatives to preserve this ecosystem against invasive species. Crops, especially sweet potato (Ipomoea batatas), have been cultivated on the Leeward side of the volcano for centuries. The northern part of the island is named after the mountain, with two districts named North and South Kohala. King Kamehameha I, the first King of the Kingdom of Hawaii, was born in North Kohala, near Hawi.
Geology History
Studies of the volcano's lava accumulation rate have put the volcano's age at somewhere near one million years. pp. 41–43 It is believed to have breached sea level more than 500,000 years ago. Beginning 300,000 years ago, the rate of eruption slowly decreased over a long period of time to a point where erosion wore the volcano down faster than Kohala rebuilt itself through volcanic activity. Decreasing eruptions combined with increasing erosion caused Kohala to slowly weather away and subside into the ocean. The oldest lava flows that are still exposed at the surface are just under 780,000 years old. It is difficult to determine the original size and shape of the volcano, as the southeast flank has been buried by volcanic lava flows from nearby Mauna Kea and Mauna Loa. However, based on a sharp increase of incline on the submarine slope of the volcano at a depth of , scientists estimate that in its prime the portion of Kohala that was above the water was more than wide. Geologists estimate that the volcano was once about twice its current size.
The volcano is so old that it experienced, and recorded, a reversal of magnetic polarity (a change in the orientation of Earth's magnetic field so that the positions of the North and South poles interchange) that happened 780,000 years ago. Fifty flow units in the top of exposed strata in the Pololu section are of normal polarity, indicating that they were deposited within the last 780,000 years. Radiometric dating ranged mostly from 450,000 to 320,000 years ago, although several pieces strayed lower; this indicated a period of eruptive history at the time.
Kohala was devastated by a massive landslide between 250,000 and 300,000 years before present. Debris from the slide was found on the ocean floor up to away from the volcano. Twenty kilometers wide at the shoreline, the landslide cut back to the summit of the volcano, and is partially, if not largely, responsible for the volcano losing in height since then. The famous sea cliffs of the windward Kohala shoreline stand as evidence of the massive geologic disaster, and mark the topmost part of the debris from this ancient landslide. There are also several other unique features found on the volcano, all marks made by the decimating collapse.
Kohala is thought to have last erupted 120,000 years ago. However, several samples of a more recent date have been obtained. Two samples collected about apart, obtained from Waipio Valley on the volcano's east flank in 1977, were dated reliably to 60,000 years. Another double set of samples, collected from the western part in 1996, was measured to be 80,000 years in age. The samples have been treated with skepticism by the scientific community, and their ages have been directly challenged. The rocks collected from the west site have not been tested by subsequent experiments. The lava samples collected from the eastern site were allegedly found under a Mauna Loa lava flow, which had been previously dated to 187,000 years. The Kohala lava flows could be as young as 120,000 years although they may be even younger.
In 2004, unidentified marine fossils were found at the base of the volcano inshore. A research team led by University of Hawaii's Gary McMurtry and Dave Tappin of the British Geological Survey found that the fossils had been deposited by a massive tsunami, above the current sea level. The team dated the fossils and nearby volcanic rock at about 120,000 years old. Based on the rate of Kohala's subsidence over the past 475,000 years, it was estimated that the fossils were deposited at a height of about , well out of reach of storms and far above the normal sea level at the time. The timing corresponds with the last great landslide of nearby Mauna Loa. Researchers hypothesize that the underwater landslide from the nearby volcano triggered a gigantic tsunami, which swept up coral and other small marine organisms, depositing them on the western face of Kohala. Mauna Loa, an active volcano, has since repaired its flank. Marine geologist Tappin believes that a "future collapse on this volcano, with the potential for mega-tsunami generation, is almost certain."
Structure Kohala had two active rift zones, a characteristic common to Hawaiian volcanoes. The double ridge, shaped axially, was active in both the shield and postshield stages. The southeast rift zone passes under the nearby Mauna Kea and reappears further southeast in the Hilo Ridge, as indicated by the relationship between collected lava samples. In addition, the two features align to one another more closely than does the ridge to Mauna Kea, of which it was once thought to constitute a part. The bulk of the ridge is built of volcanic rock with reverse magnetic polarity, evidence that the volcano is at least 780,000 years old. Older lava dated from the toe of the ridge was estimated to be roughly between 1.1 and 1.2 million years old.
Geologists have classified the volcano's lava flows into two units. The Hawi Volcanic layers were deposited in the postshield stage of the volcano's life, and the younger Pololu Volcanics were deposited in the volcano's shield-building stage. The rock in the younger Hawi section, which overlies the older Pololu flows, is mostly 260,000 to 140,000 years old, and composed mainly of hawaiite and trachyte. The separation between the two layers is not clear; the lowest layers may actually be in the Pololu section, based on their depositional patterns and low phosphorus content. The time intervals separating the two periods of volcanic evolution were extremely brief, something first noted in 1988.
The forest-covered summit of the volcano consists of many cones which generated lava from its two rift zones between 240,000 and 120,000 years ago. Kohala completed its shield-building stage 245,000 years ago and has been in decline ever since. Potassium-argon dating indicates that the volcano last erupted about 120,000 years ago in the late Pleistocene. Kohala is currently in transition between the postshield and erosional Hawaiian volcanic stages in the life cycle of Hawaiian volcanoes.
The United States Geological Survey has assessed the extinct Kohala as a low-risk area. The volcano is in zone 9 (bottom risk), while the border of the volcano with Mauna Kea is zone 8 (second lowest), as Mauna Kea has not produced lava flows for 4,500 years.
Kohala, like other shield volcanoes, has a shallow surface slope due to the low viscosity of the lava flows that formed it; however, the northwest shoreline boasts some of the highest seacliffs on Earth. Events during and after its eruptions give the volcano several unique geomorphic features, some possibly resulting from the ancient collapse and landslide. The volcano is shaped like a foot; the northeast coast is prominently indented across of shoreline.
There is a small string of faults on and near the main summit caldera, arranged parallel to the northern coast. The faults were originally regarded to be a direct result of the collapse; however scientists note the fault lines do not go all the way to the coast but are arranged tightly clustered around the caldera. This indicates that the fault lines may have been caused indirectly, possibly due to the sudden release of stress during the event.
Hydrology
Mauna Kea causes the trade winds to divert around it, increasing wind to super-enhanced flow over the Kohala ridge and orographic lift brings most of the rainfall to the northeastern slope of Kohala. The windward side of Kohala mountain is dissected by multiple, deeply eroded stream valleys in a southwest–northeast alignment, cutting into the flanks of the volcano. North of Kohala's summit the volcano's northwest–southeast trending rift zone separates rainfall into two streams, going southeast, into Waipio Valley, or northwest, into Honokane Nui Valley. When the volcano was still active, vertical sheets of magma, arranged in what is known as dikes, forced their way out of the magma reservoir and intruded into the rift zone, which was weakened by the collapse. As the dikes forced their way up, they formed fractures and faults parallel to the rift zone. The exertion caused by the dikes produced a series of faults along its length, forming horsts and grabens (fault blocks). Northeast of the Kohala summit, where the most rainfall occurs, the faulted structure prevents summit rainwater from naturally flowing northeast down the mountain slope. Instead, the rainwater flows down laterally and empties into the back of what have thus become the largest valleys (Waipio and Honokane Nui).
The leeward northwestern slope of Kohala has few stream valleys cut into it, the result of the rain shadow effect, as the dominant trade winds bring most of the rainfall to the northeastern slope of the volcano. The seven windward valleys of Kohala are, from southeast to northwest, Waipio, Waimanu, Honopue, Honokea, Honokane Iki, Honokane Nui, and Pololu. The size of the valleys reflects the amount of drainage they have been able to capture; Waipio, the largest, at some point drained Waimanu Valley as well, and the former headwaters of Waimanu are now one of its branches. Waimanu valley, cut off from its headwaters to the summit and left with Wai'ilikahi as its main tributary, still had managed to form a substantial floodplain, although less than half the size of Waipio. Honokane Nui, the third largest valley, has its drainage almost exclusively from the north side of the summit, and as a result is a deep, long, narrow bottomed valley with little habitable floodplain.