File:Kohala Mountains, State Route 19, near Kawaihae, Hawaii.jpg

English: Kohala mountain, from State Route 19, near Kawaihae, island of Hawai'i.

I continued on my drive from Kona International Airport to Pu'ukohola Heiau National Historical Park along State Route 19. As I neared Kawaihae, Hawaii, Kohala, the shield volcano comprising the north tip of the Big Island, loomed in the distance.

Kohala is the oldest of five volcanoes that make up the island of Hawaii. It is believed to have last erupted 120,000 years ago. The volcano is cut by multiple deep gorges, the product of thousands of years of erosion.

A dike complex near the volcano's main caldera separates runoff into two major drainage basins, the Waipiʻo Valley and Waimanu Valley, and it maintains the volcano's shallow water table. Kohala supports a complex hydrological cycle that has been exploited to provide a water supply to island residents.

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 volano 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.

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 different flow units in the top 140 m (459 ft) of exposed strata in the Pololu section are of normal polarity, indicating that they were deposited within the last 0.78 million 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 130 km (81 mi) 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 1,000 m (3,281 ft) 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.

The volcano's lava flows are sorted into two layers. The Hawi Volcanic layers were deposited in the shield stage of the volcano's life, and the younger Pololu Volcanics were deposited in the volcano's post-shield stage. The rock in the younger Hawi section, which overlies the older Pololu flows, is mostly 260 to 140 thousand 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 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. 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 extends prominently across 20 km (12 mi) of shoreline, differing from the ordinarily smooth, rounded shape of Hawaiian volcanoes.

Kohala is dissected by multiple, deeply eroded stream valleys in a west-east alignment, cutting into the flanks of the volcano. The northwestern slope of Kohala has few stream valleys cut into it, the result of the rain shadow effect—the dominant trade winds bring most of the rainfall to the northeastern slope of the volcano.

The valleys are more than 800 m (2,625 ft) in depth, among the oldest and largest of which are the Waipiʻo and Waimanu valleys. The volcano stayed active well into the formation of these mountainside valleys, as illustrated by later Pololu lava flows, which separated into two directions and often flowed into Pololu Valley. Recent seafloor mapping seems to show that the valley extends a short way into the seafloor, and it is believed the valley formed from the tumbled-out rock from the landslide.

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.

The happy combination of small trees, bushes, ferns, vines, and other forms of ground cover keep the soil porous and allow the water to percolate more easily into underground channels. The foliage of the trees breaks the force of rain and prevents the impact of soil by raindrops. A considerable portion of the precipitation is let down to the ground slowly by this three-story cover of trees, bushes, and floor plants and in this manner the rain, falling on a well-forested area, is held back and instead of rushing down to the sea rapidly in the form of destructive floods, is fed gradually to the springs and to the underground artesian basins where it is held for use over a much longer interval.

The mountain supports approximately 155 native species of vertebrates, crustaceans, mollusks, and plants. A diverse complexion of fungi, liverwort, and mosses further add to the variety. In fact, up to a quarter of the plants in the forest are mosses and ferns. These work to capture the water from clouds, in turn providing microhabitats for invertebrates and amphibians, and their predators. Estimates on the water capacity of the forest range from 792 US gal (2,998 l) to 3,962 US gal (14,998 l) per hectare.

The mountain is also home to several bogs, which exist as breaks in the cloud forests. It is believed that bogs form in low lying areas where clay in the soil prevents proper water drainage, resulting in an accumulation of water that impedes the root systems of woody plants. Kohala's bogs are characterized by sedges, mosses of the genus Sphagnum, and the endangered ʻŌhai (Sesbania tomentosa). Other habitats include rain forest and mesophytic (wet) forests.

The same isolation that produced Kohala's unique ecosystem also makes it very vulnerable to invasive species. Alien plants and feral animals are among the greatest threats to the local ecology. Plants like the kahili ginger (Hedychium gardnerianum) and the strawberry guava (Psidium littorale) displace native species. Prior to human settlement, many major organisms such as conifers and rodents never made it onto the island, so the ecosystem never developed defenses against them, leaving Hawaii vulnerable to damage by hoofed animals, rodents, and predation.

Kohala's native Hawaiian rain forest has a thick layer of ferns and mosses carpeting the floor, which act as sponges, absorbing water from rain and not letting much of it through to the soil; when feral animals like pigs trample the covering, the forest loses its ability to hold in water effectively, and the result is a severe loss of topsoil, much of which ends up being dumped by streams into the ocean.

From 1400 to 1800, the principal crop grown at Kohala was sweet potato (Ipomoea batatas), although there is also evidence of yams (Dioscorea sp.), taro (Colocasia esculenta), bananas (Musa hybrids), sugarcane (Saacharum officinarum), and gourds of the family Cucurbitaceae. The optimal rainfall level for the sweet potato lies between 30 to 50 inches (76–130 cm) per year. A combination of factors makes the rainfall at Kohala variable both from location to location and from year to year. In addition, Kohala is buffeted by strong winds, which are directly correlated to soil erosion; ancient farmers utilized a series of earthen embarkments and stone walls to protect their crops. This technique has been shown to reduce wind by at least 20–30 percent.

In addition to walls, there are a series of stone paths that divided the farmed area into plots of variable size. These structures are unique because although many people used such systems at the time, Kohala has some of the few to survive. The leeward slopes of Kohala were used for sugar plantations in the late 19th century.Several plantations on the mountain were consolidated into the Kohala Sugar Company by 1937.

Kohala supports a very complex hydrological cycle. In the early part of the 20th century, this was exploited by building surface irrigational channels designed to capture water at the higher elevations and distribute it to the then-extensive sugarcane industry. In 1905, after 18 months and the loss of 17 lives, the Kohala Ditch, a vast network of flumes and ditches, measuring 22 mi (35 km) in length, was completed. Its has since come into use by ranches, farms, and homes.

The Hawaii County Department of Water Supply relies on streams from Kohala to supply water to the population of the island. With increasing demand, the original surface channels have been supplemented by deep wells designed to channel groundwater for domestic use.

The land around Kohala is administered as two districts, North Kohala and South Kohala, of the County of Hawaiʻi. The beaches, parks, golf courses, and resorts in South Kohala are called "the Kohala Coast."

King Kamehameha I, the first King of the unified Hawaiian Islands, was born near Upolu Point, the northern tip of Kohala. The site is within Kohala Historical Sites State Monument. The original Kamehameha Statue stands in front of the community center in Kapaʻau, and replicas of the statue are found at Aliʻiōlani Hale in Honolulu, and in the United States Capitol at the Hall of Columns in Washington, D.C.