POSTED: 01:30 a.m. HST, Jan 07, 2011
I don't know the exact number of rock walls on Oahu and doubt that anyone does, but there must be hundreds of miles lining the valley walls.
Each and every one of the tens of thousands of rocks in those walls has a story that involves weathering, erosion, transportation and deposition.
Actually, every particle of clay and every grain of sand has a history. In the Hawaiian Islands it began millions of years ago when hot liquid lava poured out of cracks that connected through conduits deep into the molten interior of Earth.
It is ironic that a small fraction of these rocks should now find themselves in a wall that is designed to keep the earth in place and deter the very processes that formed the valleys from the lava that created the islands.
The picturesque valleys were once filled with rock that has been removed by a complex interaction of processes driven by chemical decomposition, physical breakdown, gravity and running water.
Streams get credit for creating valleys, but a stream a few feet wide is not capable of cutting a valley a mile wide through the action of running water alone.
Rainwater contains oxygen and carbon dioxide from the atmosphere, both of which work slowly but relentlessly to break down the minerals within the rocks. This softens the less resistant parts and leaves the more resistant one rounded and held loosely in place by the weathered remnants surrounding it.
Eventually an unstable boulder remains embedded in the hillside waiting for an opportunity and an impetus to break loose and move downhill.
Every rainstorm loosens bits of the weathered pieces as they become increasingly unstable. Fragments of any size from clay to boulder eventually make it to the bottom of the valley either by slow stepwise descent, where one might sit for centuries or millennia as it continues to weather, or in a rapid descent that we call a rockfall.
Eventually, rock fragments of all sizes find their way to the bottom of the valley where streams carry them away.
The rocks and boulders that lie passively in stream beds did not grow there. After their descent downslope they have slowly moved downstream by running water that rolls and bounces them along the stream bed.
The largest particles require the biggest floods to move them. Flooding is a climatic variable, and the amount of water in any stream is variable, sometimes barely moving, sometimes raging in what we quantify as 100-, 1,000- or 10,000-year floods.
Streams do not cut their valleys. They merely sweep away debris that has fallen into their reach by tumbling, sliding and rolling down the valley walls.
We attempt to deter these processes, but the fact remains that they do not care about us or require our permission. Rocks continue to roll downhill despite our objections and our efforts.
We can neither avoid nor prevent geological processes. Like it or not, we are part of nature, and we need to remember that nature bats last and geology always wins.