Explosive eruptions, like this one from Halemaumau on May 23, 1924, produce downwind ash that can become rich soil if abundant enough. (Photo courtesy of Hawaiian Volcano Observatory)

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

“Pahala Ash,” named for the town of  Pahala, refers to soil deposits along the southeast portion of the island of Hawaii.

Pahala Ash was described by several early investigators and, eventually, the term came to be used for ashes from Kohala to Ka Lae at South Point.

These ash deposits were the underpinnings of the sugar cane industry. The soils formed from these ashes sustained cane growth on relatively young flows, especially on the southeastern portion of the Big Island.

Ash deposits are the result of explosive volcanic eruptions. Recently, geologists determined that the ash deposits of Kohala, Waimea, Hamakua and North Hilo were derived from Kohala and Mauna Kea volcanoes.

These ashes are chemically distinct from ashes produced by Kilauea and  Mauna Loa.

More recent work further constrains Pahala Ash to ash deposits along the northeast rift zone of Mauna Loa, northeast Kilauea, and those found south and southwest of Kilauea’s summit, including the eastern flank of Mauna Loa. The bulk of this ash is presumed to be from Kilauea, although Mauna Loa cannot be excluded as a source.

The Pahala ash is a mixture of altered glass, rare vitric (glassy) shards, Pele’s hair, pumice, and olivine crystals. It is derived from ashfall deposits, weathered and reworked ash, and sediments.

The ash is comprised mostly of sand and silt-sized fractions. Ancient soil horizons are present in some localities.

The appearance of the ash is greatly influenced by climate. In dry areas, it is friable, in places compact, but it is mostly sandy, loose, and dusty. In higher-rainfall areas, the ash appears clay-like. The ash deposits from Ka`alu`alu to South Point appear to be loess, reworked and redeposited by wind.

Significant widespread accumulations of ash were named and used as marker beds. Some of the named ashes older than the Pahala Ash are the Moo, with an estimated age of 30-35,000 years, Pohakaa, whose age is 40,000-50,000 years, and the Kahele and the Halape Ashes, which are 50,000 to 100,000 years old.

Around Hilo, the ash layers are called Homelani Ash; this unit was originally lumped with Pahala Ash. The age of this unit ranges from 10,000 to 200,000 years old. Each one of these ashes marks an explosive episode in the history of the volcano much more significant than what is currently happening at the summit of Kilauea.

Pahala Ash has a more restricted distribution from northeast Kilauea, across Kilauea’s south flank, extending over Mauna Loa to at least South Point and to the Kahuku Pali, included ashes from 200 to 40,000 years ago.

From the results of our work, we are able to show that there have been at least eight ash events in more recent times after the Pahala ash-producing events (less than 10,000 years).

Most of these ash horizons have not been studied well enough to be used as marker beds, nor is the accumulation of ash substantial over a wide geographic area.

But the older “Pahala Ash” is recognizable over a large geographic region and therefore can be used as a stratigraphic marker.

An earlier investigator proposed to restrict the age range of the Pahala Ash to 10,000-25,000 years ago. Our recent work would restrict the Pahala Ash even further, based on mapping and radiocarbon ages, to 16,000-31,000 years old, thereby constraining the period of time where substantial ash accumulation occurred and is preserved, in places, on the flanks of Mauna Loa.

Unfortunately, this means that most type sections can only be found in drainages, arroyo, sea cliffs, and on top of the Ninole Hills above Punaluu.

Most people on the island take for granted the origin of soils underfoot. Most believe that lava flows were weathered and broken down to soil. In reality, most of the soils on the southern half of the Big Island are ashes produced by multiple explosive episodes in the volcano’s past. Fortuitously, these deposits have presented an agricultural opportunity in an otherwise rocky landscape.

So, as you cruise the Big Island and see pastureland and agricultural plots, you have to ask yourself, soil or ash…?