Friday, June 3, 2016

Man as the Great Unconformity of the Grand Canyon


Carina Nebula from Hubble Spacestaton/NASA
[T]he earth was formless and empty…and God said, …“Let there be a vault between the waters to separate water from water.” … God called the vault “sky.” … And God said, “Let the water under the sky be gathered to one place, and let dry ground appear.” … “Let the land produce vegetation: seed-bearing plants and trees on the land that bear fruit with seed in it, according to their various kinds.” And it was so. The land produced vegetation: plants bearing seed according to their kinds and trees bearing fruit with seed in it according to their kinds. … And God said, “Let the water teem with living creatures, and let birds fly above … And … “Let the land produce living creatures according to their kinds: the livestock, the creatures that move along the ground, and the wild animals, each according to its kind.” And … “Let us make mankind...rule over the fish in the sea and the birds in the sky, over the livestock and all the wild animals, and over all the creatures that move along the ground.”
                                                                                           --Genesis 1         

This biblical sequence of creation on earth is not so far off. For nine billion years after the Big Bang (13.7 billion years ago) the earth floated formless and empty in a primordial void, part of the gaseous cloud that has become the Milky Way Galaxy. Then, 4.5 billion years ago, a portion of this gaseous cloud spun out into our solar system, leaving the earth within that happy critical zone of life, 93 million miles from the sun. 

The oldest rocks on earth are just under four billion years old. Floating down the Colorado river on a raft through the Grand Canyon we peer back through nearly half of earth's history. The canyon has exposed a not quite continuous cross-section of time that we can touch and see. It is humbling. 

Lees Ferry/Siegel
Like all raft trips through the Grand Canyon, we departed from Lees Ferry, close to the Arizona/Utah border, just 10 miles south of Glen Canyon dam and Lake Powell. A cable crossing the river there marks river mile 0. Since 1921 the U.S. Geological survey has measured the daily water flow in cubic feet per second (cfs) at Lees Ferry to monitor compliance with the Colorado River Compact. 

Before construction of Glen Canyon dam in 1963, the Colorado  was flood-prone, sediment-laden, and seasonally warm. The mean daily flow rate through the Grand Canyon was 16,610 cfs, similar to now, but there were large fluctuations: the mean flood flow (spring run-off) was 86,500 cfs, with a 10-year flood peak of 124,000 cfs; peak flows occurred in excess of 300,000 cfs, scouring the river clean of deposits. The temperature of the water varied with the seasons, with water temperatures dropping to freezing in the winter time, and warming up to 85 degrees in summer. (Myers/Becker/Stevens Fateful Journey, 1999)

Today, after construction of the Glen Canyon dam, river flow is highly regulated. Flows change hourly, daily, and seasonally depending on Canyon Dam operating criteria for hydroelectric power production, the relative levels of Lake Powell (upstream of Grand Canyon) and Lake Mead (downstream of Grand Canyon). This creates daily "tides" in the river with the elevation of the water changing many feet. During the first 30 years of operation of the dam, daily water flow fluctuated up to 28,000 cfs, which resulted in 15 ft. "tides." Since 1992 the Grand Canyon Protection Act implemented more regular flows to protect the environment. Flow rates are higher in summer; the water is perennially 47 degrees where it exits the Dam and it warms up very slowly as it moves down-river, to no more than 60 degrees at Diamond Creek (river mile 226) in summer. The river is under the dominion of man. Id.



Colorado Drainage Area
We proceeded south down Marble Canyon, the section of river from Lees Ferry to the Little Colorado River. Marble Canyon was named by John Wesley Powell during his exploration down the river in 1869. Even though the rocks at river level are mostly redwall limestone--dating back 363 million years--Powell observed the walls to be polished and shiny and marble-like: "The limestone of the canyon is often polished, and makes a beautiful marble. Sometimes the rocks are of many colors - white, gray, pink, and purple, with saffron tints," said Powell. And so it is today.

Marble Canyon marks the Western Boundary of the Navajo Nation, the largest (27,425 sq. miles) semi-autonomous Native American territory in the United States. This was established by treaty in 1868. Present population on Navajo lands is approximately 174,000. Our guides told us that there is a dispute with the federal government whether tribal lands extend to the center of the river, or to the canyon rim.

Gentle rapid in Marble Canyon/Siegel

Campsite, Day 1
 Our loosely connected group of travelers (32 of us) represents a 30 year slice of time, from our mid-forty's to mid-seventies. None of us were alive when Hitler marched into Poland. Yet we can readily see our way back to John Wesley Powell's first exploratory trip down the Grand Canyon in 1869. Through a life of learning we can feel our connection to the Pilgrims who arrived on these shores in the early 17th century. We can imagine ourselves with Garcia Lopez de Cardenas, a Spanish conquistador who with his soldiers was the first European to stand on the south rim of the Grand Canyon in September 1540. As we scan the hillsides of the canyon looking for big horned sheep, behold the soaring falcon, admire the collard lizards, marvel at the canyon bats feeding at sunset, and peer through the waters for humpback chub, we can imagine the first humans foraging in these canyons 13,000 years ago during the last ice age. It's all a blip in time--our time.

One of the Larger Rapids/Siegel
The earliest recognizably hominid species, they say, emerged approximately five million years ago. Man turned Sapien, if not modern, a mere 200,000 years ago. We did not disperse from Africa until 50,000 years ago. We can't see our way back any farther than that. Our connection to 300,000 years ago is purely academic, abstract, like our connection to the Big Bang, or to God.

The Great Unconformity


The canyon is carved out of the relatively flat Colorado Plateau. The very youngest rocks represented here (at the surface of the plateau) are 250 million years old. Those are the Kaibab Formation rocks found near the canyon rim. As the canyon cut down into the Colorado Plateau it exposed successive layers of sedimentary rocks laid down in the late Paleozoic era (260-380 million years ago). Moving down from the rim we find the Toroweap Formation, Coconino Sandstone, and Hermit Shale. Below that is found the Supai Group consisting of Esplanade shale, the Wescogame Formation, the Manakacha formation, and the Watahomigi Formation. Then comes the Surprise Canyon Formation, Redwall Limestone, and the Temple Butte Formation.

The Temple Butte formation (~380 million years old) overlies the much older Tonto Group of rocks (Muav Limestone, Bright Angel Shale, and Tapeats Shale). This Tonto Group consists of ocean floor layers--long ago uplifted to their present elevation--that date to the early Cambrian Age (545-510 million years ago). This ~130 million year gap in the record from the Temple Butte Formation to the Tonto Group is called an "unconformity" by geologists.

The Tonto Group lies directly upon considerably older Precambrian rock--The Grand Canyon Supergroup--which represents another unconformity (gap in the record of time) of more than 200 million years.

Yet a third unconformity of ~600 million years takes us down to the metamorphic rocks found at the very bottom of the canyon, the Granite Gorge Metamorphic suite. This includes the shiny Vishnu Shist which is very prominent at river level as we enter deep into the Grand Canyon.

So as we float down the river, there before us we can touch and feel 1.8 billion years of time. More than its physical beauty and majestic grandeur, it is this quality of age, this sense of time manifest, that is most impressive about the Grand Canyon. And as we crane our necks to peer at the massive canyon walls from under our sun hats it occurs to us that we are the greatest unconformity of all in this place: more than 250 million years separates us from the Kaibab Formation--the youngest rock found at the top of the canyon.

Illustration of Grand Canyon Rock Formations 

A Measure of Time

In this age of $16 trillion economies, trillion dollar budgets, billion dollar hospitals and football stadiums, billionaires running for president, and $1 million dollar mean housing prices in our city, we are used to large numbers. So how do we think of 1.8 billion years of time?

Here's one way to think of it. In examining the Vishnu Shist found at the bottom of the Grand Canyon geologists found garnets. These garnets can only be formed under immense pressure, pressure caused by mountains the size of Mt. Everest. Mountains the size of Mt. Everest were once here. 

Mt. Everest is still growing, but it is also eroding. Once it stops growing it may erode at a rate of .1 millimeter per year. Not a lot we think. We can relate to .1 millimeter a year. It's nothing to a mountain 29,029 feet tall; or so we think in our human timeframe. But at that rate of erosion, Mt. Everest will be gone--a flat plane--in 88 million years. In other words, our 1.6 billion years of time travel from the canyon rim to the bottom of the river represents enough time for Mt. Everest to be built up and be fully eroded (160 million year cycles?) 10 times. 

And it occurs to us, the great unconformity from the top of the canyon wall (250,000 years ago) to us floating down the river today is enough time for Mt. Everest to be completely eroded three times (at a rate of .1 millimeter/year).

View from The Granaries/Nikles

Will we still be around when Mt. Everest is eroded to its nubbins? What will we be like? We know one thing, the Grand Canyon which formed sometime in the last 5.5 million years will be long gone. Our global warming will have run its course and a new era of glaciation may be under way. Movies notwithstanding, if our descendants are still around, it will be right here on this small plot of earth. 

And here is our intrepid Crew: 

Hatch River Expeditions Trip: May 23-29, 2016/Buckholz






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