You could have been a mountain climber 12 million years ago
Posted: Wed Apr 30, 2008 7:46 am
You could have been a mountain climber 12 million years ago
A U.S.-German study has shown that mountains in the western United States formed much earlier than once thought by geologists--specifically about 12 million years ago, approximately 9 or 10 million years earlier than previous research had shown.
by William Atkins
Sunday, 27 April 2008
German geologist Andreas Mulch, a professor of tectonics and climate at the University of Hannover (Germany), lead the study that showed the central Sierra Nevada Mountains reached their present height about twelve million years ago.
The Mulch expedition went from the Coast Ranges, near the Pacific Ocean, into the Central Valley, and on into the Ruby Mountains, in northeast Nevada, to analyze deposits of volcanic glass. The Ruby Mountains are within the Basin and Range region of Nevada and Utah.
The discovery by Mulch and his team is important for the understanding by scientists of the geologic history of all mountain ranges throughout the world. It helps them to more accurately model global climates from early times on Earth to the present, and further on into our future.
In fact, Mulch stated, “All the global climate models that are currently being used strongly rely on knowing the topography of the Earth." [Stanford: “Study: Mountains reached current elevation earlier than thought”]
Mulch commented that scientists have shown with past studies that the Sierra Nevada Mountains and the Rocky Mountains in the western section of the United States contribute to weather and climate patterns as far away as the European continent.
He stated, "If we did not have these mountains, we would completely change the climate on the North American continent, and even change mean annual temperatures in central Europe. That's why we need to have some idea of how mountains were distributed over planet Earth in order to run past climate models reliably." [Stanford]
The Mulch team analyzed hydrogen isotopes (elements with the same atomic number) in water contained within volcanic glass (natural glass produced by the rapid cooling of molten lava ejected from volcanoes, cooled so fast that crystallization is not allowed to occur).
This information provided them with the necessary data to conclude the timing of the uplift of the Sierra Nevada Mountains to their current peak altitude above the surface of Earth.
Specifically, the ratio of hydrogen isotopes in the volcanic glass tells the researchers the changes that occured in the water vapor within air as it was contained over the Pacific Ocean and as it blew onto the North American continent over many millions of years of time in Earth’s past.
Most of the rain fall (and continues to fall) on the windward side of the western U.S. mountains (on the western side as Pacific winds come in), so water falling to the ground contains heavier isotopes of hydrogen (as compared to lighter isotopes) than it does when it falls on the eastern slope of the mountains (the region that gets less rain overall).
Generally, the windward side of these mountains receives more rainfall, making them greener with plants, trees, and other flora. On the other hand, the other side of the mountains usually receives much less rain, and often such areas are found to be desert or semi-desert climates.
The difference in rainfall amounts and the resulting landscapes between the two sides are often called the “rain shadow,” or any area of land on Earth that receives small amounts of precipitation due to the effect of a barrier (such as a mountain range), which causes the winds to drop their moisture content before reaching that land mass, which usually has desert-like conditions.
And, the higher in elevation the mountain barriers have reached, the more difference in the two regions; thus, producing a more distinct "rain shadow" effect between the dry and wet sides of the barrier.
Thus, according to the April 23, 2008 Stanford report, “By determining the ratio of heavier to lighter hydrogen isotopes preserved in volcanic glass and comparing it with today's topography and rainwater, researchers can estimate the elevation of the mountains at the time the ancient water crossed them.”
Rainfall that has fallen in the past, throughout the history of Earth, is well preserved within volcanic glass. This ability to retain a record of water occurs because when volcanoes erupt, particles of extremely hot molten rock are violently flung out into the air.
They hit the atmosphere and quickly cool—so fast do they cool that they almost instantaneous freeze into glass—what is called volcanic glass.
The volcanic glass has an amorphous (unstructured) internal appearance, as opposed to minerals, which have an ordered crystalline structure. Its unstructured state allows tiny holes (crevasses) to develop inside, which permits water to seep in and to be stored.
Once the glass becomes full of water (fully hydrated), it basically stops changing isotopically (that is, its chemical composition does not change).
Mulch states, "It takes probably a hundred to a thousand years or so for these glasses to fully hydrate.” [Stanford] However, when dealing with millions of years, such small time frames allow researchers, such as Mulch, to better estimate the uplift of mountains.
The volcanic glass studied by the Mulch team had a range of being deposited on the Earth from 600,000 years to over twelve million years. This time period occurred during massive upheavals of the tectonic plates (segments of the Earth's crust that slowly move relative to each other over time) throughout the interior of the Earth.
Mulch stated, "For the first time, we were able to document that we can track the [development of the] rain shadow on both sides of the mountain range over very long time scales." [Stanford]
The paper (“A Miocene to Pleistocene climate and elevation record of the Sierra Nevada (California )” by Mulch and his colleagues is published on the online Early Edition of the Proceedings of the National Academy of Sciences.
Mulch’s team includes A.M. Sarna-Wojcicki (United States Geologic Survey, Menlo Park, California, U.S.A.), M.E. Perkins (Geology and Geophysics, University of Utah, Salt Lake City, Utah, U.S.A.), and C.P. Chamberlain (Geological and Environmental Sciences, Stanford University, Stanford, California, U.S.A.).
Actually, Mulch performed the research while he was a postdoctoral student at Stanford University.
The abstract to the paper states, “Orographic precipitation of Pacific-sourced moisture creates a rain shadow across the central part of the Sierra Nevada (California) that contrasts with the southern part of the range, where seasonal monsoonal precipitation sourced to the south obscures this rain shadow effect. Orographic rainout systematically lowers the hydrogen isotope composition of precipitation ( Dppt) and therefore Dppt reflects a measure of the magnitude of the rain shadow.”
“Hydrogen isotope compositions of volcanic glass ( Dglass) hydrated at the earth's surface provide a unique opportunity to track the elevation and precipitation history of the Sierra Nevada and adjacent Basin and Range Province. Analysis of 67 well dated volcanic glass samples from widespread volcanic ash-fall deposits located from the Pacific coast to the Basin and Range Province demonstrates that between 0.6 and 12.1 Ma the hydrogen isotope compositions of meteoric water displayed a large (>40 ) decrease from the windward to the leeward side of the central Sierra Nevada, consistent with the existence of a rain shadow of modern magnitude over that time.”
“Evidence for a Miocene-to-recent rain shadow of constant magnitude and systematic changes in the longitudinal climate and precipitation patterns strongly suggest that the modern first-order topographic elements of the Sierra Nevada characterized the landscape over at least the last 12 million years.”