Monday, December 05, 2011

Lava Arch and Hawaiian Geology

Lava Arch near Mele Kohola on the Big Island of Hawaii. You can tell a lot about the geologic history of a place by looking at its cliffs. Cliffs often cut through multiple strata and the depth, composition, angles, etc. of those strata tell you how an area was formed. For example, whether it was sedimentary ocean bottom thrust up through seismic activity (Coastal California) or whether it was littered with polished stones from glacial activity (parts of Colorado).

One afternoon, we went for a walk down the coast along the steep lava cliffs. We were heading North from Mele Kohola (on the Hawaii Paradise Park coastline), ostensibly looking for the black sand beach (which turned out to be several miles away). Along the way, there were all sorts of lava caves and arches to view and study. This one illustrates the different eruptions very nicely. You can see how layers of lava and ash stack on top of the other like a stack of pancakes. Some layers are harder (lava) than others (ash) and, as the waves erode them at different rates, this results in caves and arches. Other caves result from lava tubes where the lava cooled around a flow and, as the flow emptied into the sea, left an open tube. You will not see the little pancake layers if it was formed by a lava tube.

Note, lava is also composed of different minerals, often each rising or sinking into different layers (within a given flow), depending on the mineral's relative weight, viscosity, etc. Silicon floats to the top, leaving a glossy mirror-like sheen on the top of some lava. As shown here, there are also iron-rich layers that turn reddish brown as they oxidize (think rust or red cinders). If reddish brown is from iron, what is that pink rock composed of? As it turns out, the "pink rock" is actually where the lava is covered by a thin layer of pink coraline algae. While most algae is soft, coraline algae deposits onto the rock in a hard mineral-like layer. As it is a living sea organism, you only see it at and below the water line.

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