Sunday, June 19, 2016

The Hawai'i That Was: The Beginning of All Things, (Ba)salt of the Earth

There are lots of places that are associated with a particular kind of rock. There's the granite of the Sierra Nevada, or the sandstone of Zion National Park. Geologists think Franciscan graywacke sandstone when someone mentions the California Coast Ranges. But nearly every mainland location is really made of a variety of different rocks. That's not the case with the Hawaiian Islands. There is but one rock. It comes in many guises, but it is compositionally the same thing: basalt.
A pahoehoe flow from Kilauea from 2004. The flow was only a few days old and looks silver because of a thin layer of volcanic glass that degrades and falls away within weeks or months.
That's the starting point of our journey through The Hawai'i That Was. Hawai'i began as basalt, and until the eroded rocks are covered by coral reefs, that's all there will be, the basalt or the weathered components of the basalt. Every island in the chain began as a series of sterile tracts of the black volcanic rock.
That's not to say that basalt in Hawai'i is everywhere the same. It originates in the same place, as a "partial melt" magma deep in the Earth's mantle at a (probable) hot spot. Magma results from the melting of rocks, but rocks are made of different kinds of crystals, and different crystals melt at different temperatures. So a partially melted magma will be made of the minerals that melt at slightly lower temperatures. In the case of Hawai'i, the original rock, peridotite (or related rock like dunite), is composed primarily of olivine and a few other minerals, but the partial melt produces a rock composed of pyroxene, calcium-rich plagioclase and lesser amounts of olivine. Two kinds of lava, not easily distinguished in the field, are found in Hawaii: a sodium-depleted tholeiitic basalt (early-stage eruptions), and a sodium-rich alkali basalt (late-stage eruptions). At times, the magma will bring bits of peridotite to the surface as clots in the lava like the one in the picture below. These clots are called xenoliths ("alien rocks").
A mantle xenolith in basalt. The green mineral is olivine (it weathers to red iron oxide quickly in the moist climate of Hawai'i)
Olivine is a semi-precious gemstone, and is occasionally visible as phenocrysts in the basalt. Given the name, it's not hard to guess that the stone is green in color. Hawai'i hosts one of the few green sand beaches to be found anywhere on the planet (which we visited; the story will come in a follow-up blog).
Olivine phenocrysts in vesicular (holey) basalt at Pu'ohonua o Honaunau National Historical Park
Eruptions of basalt can vary in temperature, gas content, water content and other factors. Depending on the circumstances of the eruption, basalt can take the form of a pahoehoe flow (see the second picture above) where the surface is smooth or ropy looking. It can also take a rough and blocky aspect like the one in the picture below called an a'a flow (believe it or not students misspell this word sometimes). Lava flowing into the sea can explode into sand-sized particles (forming black-sand beaches), or pillow-shaped lobes called (not surprisingly) pillow lavas.

Explosive eruptions of basalt are fairly rare on Hawai'i, but they do happen. The rapidly cooling lava may not even form crystals, forming a glass instead. The glass can take the form of a gold-colored basaltic pumice (below), a spongy material with the consistency of styrofoam.
Basaltic pumice at the Lyman Museum in Hilo
One of the oddest materials to result from a basaltic eruption happens when molten lava flies through the air trailing long thin strands of glass called Pele's Hair. The strands are so delicate it's amazing they can be found at all, much less in bunches like the sample below from the Lyman Museum in Hilo. I've only found single strands out in the wilds.
Pele's Hair at the Lyman Museum in Hilo
Larger chunks of molten lava can twist during flight into so-called lava bombs. In the picture below, the sample is lying in a bed of cinders, the smaller bits of explosive eruptions. Take some serious advice here: if you collect lava bombs in Hawai'i and you are going through airport security, and they ask what you have in your luggage, DO NOT use the word "bomb". The results are not happy or convenient, based on a true story (thankfully not mine; I have plenty of lava bombs from California).

Basalt is the beginning of all that is in the Hawaiian Islands. The islands began as thousands upon thousands of lava flows, the soils on which plants and animals survive are derived from the weathering of basalt, and the platform on which coral reefs later grow at the end of the island's existence is basalt. Basalt forms the base on which all travels took place and basalt was the building stone of choice (the only choice).
The Mamalahoa Trail, that stretches from Kona to Puako. It was built in the 1800s.

The islands started as sterile basalt, but as the saying goes, "life finds a way". Certain native ferns and trees are able to colonize the rock even in the absence of anything resembling soil. In the picture below, native 'Ohi'a trees are growing in basalt that erupted in 1959. Barring any more eruptions (a risky proposition in this particular spot on Kilauea), this will be a rainforest in a few centuries.
Native 'Ohi'a trees growing in a recent basalt flow in the interior of the Kilauea Iki crater, which erupted in 1959.
It was June 1st. Although Mrs. Geotripper and I had been on the islands for nearly a week doing some reconnaissance, the students were now arriving at the airport, and we were gathering our class together. Our exploration of the Hawai'i That Was had reached the starting gate.