History Written in Lava Bom...

A stroll anywhere on the island is a lesson in geology for an observant hiker. Much of the Big Island is a raw land where the bones of the earth show through. Lava flows, pu'u, craters, and the massive volcanoes dominate the landscape. On the older areas of the island erosion and soil formation softens the raw rock. Where there has been enough rainfall and time, canyons cut deep into the island, again exposing the geologic history of the land to view.

Learning to look at the land and see the process by which it formed is a wonderful process of discovery. So often the history of the land is written clearly, legible to anyone with a little knowledge who takes the time to look.

A posting by Tom over at Pacific View alerted me to an interesting feature just a short walk from the dormitories at Hale Pohaku. Just to the east of the dorm is a small rocky rise, nothing notable to attract the eye when seen from a distance, even the word hill seems overly generous. When you do take the walk over, you quickly find that this little hill is covered with lava bombs.

Getting up one morning I had a couple hours before lunch was served, a short walk was an attractive idea. I put on my boots and made my way across the rock strewn mountainside. The view from the little hill is impressive, from this vantage high on the side of Mauna Kea you look across a landscape studded with pu'u, the dozens of cinders cones that attest to the many millennia of eruptions that have occurred here. The enormous shield volcano of Mauna Loa dominates the southern horizon. In the distance you can see to the south side of the island where plumes of steam and other gasses reveal the ongoing eruption of Kilauea.

As I sat on a large lava bomb, I began to puzzle out the history of the land around me. I was atop the site of an ancient eruption, a place where liquid lava made it to the surface, and the release of gas pressure caused the liquid rock to bubble violently, launching masses of lava across the landscape.

It was clear the the hill was one side of the eruptive site, the the vent itself lay hidden somewhere at the eastern base. A second hill, slightly higher, lies a little further east, covered with the same cinder and lava bombs as the first. The eruption could not have been too violent, or very long lasting, as these hills are quite modest, dwarfed by some of the truly large pu'u that are visible all across this southern flank of Mauna Kea.

Most of the lava bombs here are classic spindle bombs and a few ribbon bombs. On the second hill there is a nice example of a classic cowpie bomb, a large blob that was still semi fluid when it landed, forming a typical cowpie shape on impact.

What is missing from this eruption is the usual pu'u shape. There is no conical shape and no clear crater. I suspect there is more to the story written here... It is common in these sort of eruptions to have several phases occur. When first reaching the surface the lava is quite gassy, the eruption is much like the opening of a soda bottle, a rush of foamy material gushing out. As the pressure is released, the gassy magma results in a stream of molten rock spraying from the vent and showering the nearby landscape with cinder. Look at a piece of cinder and note the resemblance to a solidified piece of foam.

After some of the gas has a chance to escape, the eruption becomes less violent, larger pieces are thrown from the vent, some still molten, to fall as lava bombs atop the piles of cinder around the vent. A flow of fluid magma also begins to emerge. Without the intense gas pressure, this stream moves as a liquid, flowing out of the vent. The piles of cinder offer little resistance to this heavy mass, it moves downhill, destroying the southern wall of the cinder cone formed during the early eruption and carrying it away as a lava flow begins.

This story has played out time and again in these typical cinder cone eruptions. We have watched many cinder cones go through a series of phases like this, Parícutin in Mexico is one of the most famous, but the same sequence was also witnessed at Pu'u O'o at Kilauea.

Am I sure that that is what happened here? No. Just my best guess from reading the landscape. A likely sequence of events. But that is the fun of it, like a crime scene investigator, trying to puzzle out the story from the evidence lying about. Keeping my eyes open, learning from the universe around me, something I make a habit of doing.