Phreatic Friday

In Hawaii

Manana Island, also known as Rabbit Island for a rabbit-raising operation that was there from the 1880s until they overran the island and were eliminated in the 1970s, lies off the southeasternmost tip of O‘ahu in the Hawaiian Islands. Manana is a tuff cone, a small vent where volcanic ash erupted (when ash solidifies, it’s called tuff, even though it isn’t necessarily tough). Although most of the volcanic material in Hawaii is dark iron-rich basalt, this little 360-foot-high island consists of light-colored rock because the eruption was phreatic, meaning the eruption interacted actively with groundwater or perhaps sea water.

The reactions between the ash and water converted the rock to palagonite, a rock formed by the alteration of the basaltic material to lighter-colored minerals like feldspar and others. The original basaltic glass in the tuff altered by hydration to calcite and zeolites which now hold the rock together, and the iron and magnesium more or less went away as part of the alteration process.

The vent at Manana is essentially the northeasternmost of a string of vents in the Koko Rift System which defines the edge of O‘ahu, and which is the youngest part of the Ko’olau Volcano. Ko’olau is the more easterly and younger of the two volcanos that make up O‘ahu.

From Eakins and others, 2003, Hawaii’s Volcanoes Revealed: USGS Geol. Invest. Series Map I-2809.

In the Hawaiian chain, the islands are generally younger as you progress to the southeast because the Pacific Plate is moving northwestward above a relatively stationary mantle hot spot so the active, youngest volcanism is at the southeastern end of the chain. Ko’olau was most active, creating O‘ahu, about 1.8 to 2.6 million years ago, but the youngest eruptions, those here at the southeastern edge of the island, are around 33,000 to 40,000 years old, with some as young as 7,000 years (Rowland & Garcia, 2004, Western Pacific Geophys. Meeting Guidebook, U. of Hawaii). O‘ahu is probably dead, volcanically, but only barely.

Emperor and Hawaiian seamount chains, from USGS.

From USGS (Modified from a drawing provided by Maurice Krafft, Centre de Volcanologie, Cernay, France.)

We often refer to the “hot spot track,” but it’s really the Pacific Plate that’s moving over a more-or-less relatively stationary spot in the mantle. Hot spots do move, a little, but compared to the motion of crustal plates, they are pretty much fixed. The 6000-km-long Emperor and Hawaiian Seamount Chains, and the Hawaiian Islands, mark the 70-million-year passage of the Pacific Plate over the hot spot. The bend in the trends represents a change in Pacific Plate movement direction about 43 million years ago.

The islet in my photo in front and to the right of Manana is called Moku Hope (or Kaohika’ipu) and it is part of the same Koko Rift as Manana, but its composition and color are different – it is the remnant of a reddish scoria volcanic flow rather than an ash vent. It differs because the lavas that flowed there did not interact violently with water that altered the minerals, as did the material at Manana.

“Palagonite” comes from such rocks at Palagonia, Sicily. The word “tuff” ultimately comes from Latin tophus, the name for the stone-like uric acid deposits in the body that result from gout, but in the 17th century it came to mean any porous rock, and finally in the early 1800s was limited to solid volcanic ash deposits (porous or not). “Phreatic” in reference to groundwater is from a Greek word for “well” or “spring.”

Manana and Kaohikaʻipu Islands are part of a State Seabird Sanctuary today.

I took the photo in 1973 when I was in Hawaii to man an exhibit at the first “kidney stone convention” I attended – the annual meeting of the American Urological Association. The presentation received the First Prize for a Clinical Investigation / Scientific Exhibit.

Photo of olivine tuff from O‘ahu. The large olivine grain near center is about 0.7 mm across. The white material is probably altered volcanic ash and glass. Collected in 1973.