Volcanos, earthquakes and climate
Challenges and opportunities
Sustainable resource development
The 5 Cs (cattle, cotton, citrus, climate, and copper) have long been touted as characterizing Arizona. Two, climate and copper, are indigenous. Nature has given the state an arid climate with its beauty and challenges, and nature is expressed through geology, which includes one of the world's great endowments in metals, especially copper. This web page trouches on some of current research and the context for Arizona and copper, plus the links to other aspects of the state's natural history.
Copper resources in Arizona
As can be seen in the following map, Arizona has by far the largest cluster of copper deposits in the United States, and it is one of the largest groupings in the world (second only the the central Andes of northern Chile and southern Peru).
Volcanoes: Nearly all copper deposits in Arizona – which are some of the world's largest – are of a type that formed when magma (molten rock = melt) rose into the Earth's upper crust (the upper 5 or 6 miles or about 10 kilometers) and cooled, releasing water that was dissolved (incorporated) in the melt. Some 50-80 million years ago Arizona looked much like the modern Andes in Chile – a high mountain belt (probably averaging >3 km or 10,000' above sea level). These water-bearing magmatic systems concentrated copper (by a factor of about 1,000 times normal crustal concentrations) in to "small" volumes of rock (a cubic kilometer or so) but "altered" (chemically modified) much larger volumes. Where such fluids reach the surface, they can form large volumes of chemically modified rock. Yellowstone National Park is the most famous US example (though different in detail from ancient Arizona -- the magmas at Yellowstone are of a type that do not produce copper deposits). Closer analogies can be seen in many modern volcanic areas from Japan to New Zealand to Chile to the Cascades of the Pacific Northwest.
[See the parts of the animation below which illustrates the ore formation in a major copper district.]
So why are there so many copper deposits in Arizona when volcanoes are common around the world?
Answer: Preservation and exposure! As can be seen in the map above, most copper deposits are in the lighter blue areas. The various shades of blue correspond to all the places there was the ancient magmatism that could have, and probably did, form deposits. HOWEVER the light colored areas are those places where the "right" depths have been brought near the surface but also preserved from erosion – this is the result of faulting (earthquakes) and thinning of the crust.
Earthquakes and faults: The thick crust of 60 million years ago was held up by dynamic processes related to plate tectonics. When the tectonic regime changed (roughly 30 million years ago), the very think crust began to spread out under its own weight (think a blob of asphalt on a hot summer day). In the upper part of the crust, this was expressed in the form of multiple earthquakes leading to faulting and tilting of the upper crust, much like a series of dominoes or books falling over on a shelf. This led to large parts of southern and central Arizona's land surface representing ancient cross sections through the crust. This process brought the ore-forming settings near the surface. It also buried parts under shallow younger cover and represents one of the great unknowns, but potential opportunities for other deposits.
[See the parts of the animation below which illustrate faulting and tilting of a major copper district.]
So what about the other "C" -- climate?
Answer: Preservation and enrchment depend on climate.
Climate: Southwestern North America has been in an arid climate zone for much of the past 200 million years. Climate is important for many reasons, including the impact that it has on erosion (not much rain, not much erosion) and on weathering processes. Both, it turns out, are important for copper deposits in the southwest. Erosion has been relatively limited given the broadly arid conditions, thus the upper, copper deposit-bearing parts of the crust have tended to be preserved (contrast this, for instance with tropical Indonesia where copper-rich domains are being eroded within a few million years of formation). Another key factor from climate is the role that it plays in chemical enrichment or, if you will, "soil formation" on copper deposits. In arid climates, copper minerals very near the surface decompose by natural processes and the copper moves down 10s to 100s of feet into less weathered rock where it is redeposited. This adds to the concentration, further enriching the ores and, in some cases, making them easier to treat during mineral production.
The attached animation (Quicktime movie; click to play) shows the geologic history of a major copper district in western Nevada. These panels are based on over 100 man-years geologic work by many geologists coupled with information from deep drilling.
The movie shows the geologic evolution first backward then forward in time. It starts with the present day and goes back to the time (about 170 million years ago) of the volcanic and ore-forming events. It shows the sequence of faults and tilting that have turned on its side what was once an upright 7 km (4 mile) section through the upper part of the Jurassic crust (dinosuars absent!).
MODERN VIEW – GEOLOGIC CROSS SECTION (after faulting and tilting, compare with Jurassic view below)
JURASSIC VIEW – GEOLOGIC CROSS SECTION (after volcanism and ore formation but before faulting and tilting)
There are innumerable opportunities for basic research ranging from the fundamental geology (my main interests) to issues having to do with the environment (what are the natural backgrounds and baselines? how can one mitigate the effects of mining?) to the issues of mineral resource discovery and best-practices mining (how do we find and sustainably produce needed resources?) and, ultimately, to issues of public interest (how do we make the best scientifically sound decisions for the benefit of all, for future generations, and the future of our world?).
Resource issues are important to all of us. One of the goals of our group's research is to contribute the fundamental geologic understanding necessary to make the best decisions for our times and for the future. As part of this effort, we contribute to the broad-based goals of the UA's Lowell Institute for Mineral Resources ("IMR") which draws on the interests, skills and talents from across the UA campus (Public Health to Engineering to Social Sciences to Agriculture to Physical Sciences and beyond) all to add to our knowledge, share diverse viewpoints, and educate the next generation. We collaborate with many groups in other universities, in Federal and State agencies, in industry and in NGOs to cooperatively address these major challenges.
Volcanos, Earthquakes and Climate – Why Arizona is the Copper State
Dept. of Geosciences, University of Arizona, 1040 E. 4th St., Tucson AZ 85721-0077 All contents copyright ©. All rights reserved.