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, a reversal of 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 606 km2 (234 sq mi) in area and 14,000 cubic kilometres (3,400 cu mi) in volume, and thus constitutes just under 6% of the island of Hawaii.
The Volcano is cut by multiple deep gorges, 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 landslide destroyed the northeast flank of the volcano, reducing its height by over 1,000 m (3,281 ft) and traveling 130 km (81 mi) across the sea floor. This huge landslide may be partially responsible for the volcano's foot-like shape. In 2004, marine fossils on the flank of the volcano were found, 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 the phenomenon of geographic isolation, resulting in an ecosystem radically different from that of other places. Invasive species introduced by man present a problem to Kohala's ecosystem, as they push native species out of their habitat. There are several initiatives to preserve Kohala's ecosystem. Crops, especially sweet potato (Ipomoea batatas), have been harvested on the leeward side of the volcano for centuries as well. 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.
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 then 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.
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 20 km (12 mi) 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.
The windward side of the Kohala mountains 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 Waipiʻo 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 in to the back of what have thus become the largest valleys (Waipiʻo and Honokane Nui).
The natural habitats in the Kohala district range across a wide rainfall gradient in a very short distance—from less than 5 in (127 mm) a year on the coast near Kawaihae, to more than 150 in (3,810 mm) a year near the summit of Kohala Mountain, a distance of just 11 mi (18 km). At the coast are remnants of dry forests, and near the summit lies a cloud forest, a type of rainforest that obtains much of its moisture from "cloud drip" in addition to precipitation.These large cloud forests dominate its slopes. This biome is rare, and contains a disproportionate percentage of the World's rare and endemic species. The soil at Kohala is nitrogen-rich, facilitating root growth.
Elevation: 5,480 ft (1,670 m)
Type: Shield volcano, Hotspot volcano
Age of rock: Oldest dated rock:460,000 BP
Estimated: 1 million
Volcanic arc/belt: Hawaiian-Emperor seamount chain
Last eruption: About 120,000 years ago
Easiest route: Kohala Mountain Road