Tuesday, December 23, 2014

Landscape Change – How Fast?

A major question from the beginning of geology as a science has been how fast does change take place? From the Literalist read of the Bible, it would seem 6,000 years is far too short a time to permit the development of tens of thousands of meters of sediment, with remains of primitive life-forms at the bottom and advanced life-forms preserved at the top. The first rough estimates of the rate of sedimentation were made in England, by thoughtful natural scientists measuring how fast mud accumulated in a pond. These early geologists had already mapped thick stacks – thousands of meters of distinctive layers - of sediment in cliffs, road-cuts, and quarries. They had seen the same sequences long distances away, implying the same sedimentary process was happening over a very wide area. Finally, they had realized that for mud and sand to accumulate to thousands of meters of thickness, would take at minimum many millions of years. This was really the first baby step of geoscience.

Q: Hello, my name is Jurgen and I am currently enrolled in an AP Environmental Science class and have a question about river formation. I hope you can answer my question.
How long does it take for a gully or rill to be formed into a river if there is a constant stream or supply of water running through the land?  Thank you.
--Jurgen P

A: Time for a gully to become a river can vary wildly from less than a hundred to many millions of years. Generally, most terrains are in some sort of equilibrium and don't change much over time – unless disturbed by something, like a tectonic event. This is sometimes called "punctuated equilibrium." The change of a feature from one form to another (like a gully to a river) implies a permanent shift in the rainfall regime - some form of climate change – or tectonic uplift.

Change from a gully to a river could also have a lot to do with human intervention. I've walked down 10-meter-deep, steep-walled gullies that were really mini-canyons (Arroyos) in SE Arizona. These apparently didn't begin to form until man introduced cattle in the late 19th Century. Early journals from some of the first visitors describe “grass that was belly-high to a horse.” These cattle quickly wiped out the native prairie grasses by over-grazing the landscape. When Arizona earned its statehood in 1912, it had a human population of about 12,000 people, but an estimated cattle population of perhaps 10,000,000. Soils started disappearing rapidly with no roots to hold them, and small rivulets began to rip through the landscape and form small canyons in less than a century. Events like this, and the 1930's Dust Bowl, lead to the formation of the US Bureau of Land Management and the US Soil Conservation Service during the 20th Century.

Tectonic uplift can also weigh in powerfully, but tectonic shifts are generally relatively slow - slow at least in typical human time-frames. The Grand Canyon only began to form (cut down through pre-existing Precambrian to Mesozoic rocks) about 70 million years ago. The actual timing of the initial incision and the final down-cutting is still being argued today by geologists as more evidence accumulates, but it appears to have been quite rapid at the beginning.

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