Moon Mountain Vista

"Big Waves -- Right in Our Own Backyard?"

Apr 27, 2005

I will bet that many of you living in the Valley of the Sun don?’t know that rocks you see every day were formed in environments much like that of present-day Sumatra and the Indochina Peninsula. Now, I would also wager that many of the residents of the Valley don?’t really care a whole lot. You may never look at the Phoenix Mountains the same again.
The Phoenix Mountains are the range that nearly cuts the city of Phoenix in half. They run from Camelback Mountain on the east end, to Moon Hill on the west. Most of the rocks exposed in them were formed very long ago, during what geologists call Precambrian time. These old rocks make up Mummy Mountain, Squaw Peak, Stony Mountain, and North Mountain. (Some of the rocks, though, like those lying on Camelback Mountain, and those of Moon Hill and Shaw Butte, are much, much younger.)
Mapping done by geologists in the last few decades, and research finished in just the last ten years has shown that they detail a sequence of Earth?’s history in which our very area was much like today?’s Southeast Asia. If you were to walk from the Piestewa Freeway in Dreamy Draw (SR-51) west over to Shaw Butte, for example, you would be going right down through a rock section (now standing on end, basically), that shows the whole evolution of that environment.
At that time, around 1700 million years ago, if you had been in a spaceship looking down, you would have seen here a long string of islands, some big, some small. Paralleling their coasts would have been rugged mountains and conical hills?—an occasional smoking volcano among them?—making for scenic backdrops for the bays and beaches of the day.
No swaying palm trees to be seen, however, or flocks of sea birds, or much of anything else. None of those existed yet?—only rocks and water, and life-forms in that water, too small to see without microscopes. The serene blue skies would have belied a rather hostile place to be, on the ground.
That type of landscape was then, and is now, the result of what we call "tectonic plate" activity. Where two giant pieces of the Earth?’s crust converge, being pushed together by planetary forces from below, one ends up overriding the other. The ensuing geography can form an "island arc", and a range of active volcanoes pops up there, due to intense melting beneath Earth?’s surface.
The accompanying picture above shows a cut-away view of the geology off Sumatra, but you could just as well take away those present-day names, and visualize the make-up of the Phoenix Mountains over a billion years ago.
Last December, off Sumatra, where two such plates have been grinding against each other for years, the build-up of stress finally became too much. It released in one big jolt, unleashing the shock waves and ocean waves that killed so many people.
That same scenario undoubtedly has repeated itself and replayed itself countless times in our planet?’s history. Some of those episodes happened right here, over and over again, in cataclysmic fashion. They have created the rocks we see in our "back-yard", but only those rocks have remained to tell the story.