Important Note (temporary):
Geoscience world has made the February 2012 issue of 'Elements' magazine available for free download on a temporary basis. The email I received said that this will continue only until early February. When I read this issue, a while back, I found it to be one of the best recent summary publications of the subject that has come into print - as important as Bevan French's book, 'Traces of Catastrophe,' or Osinski and Pierazzo recent volume 'Impact Cratering Processes and Products.' There are 7 major articles on impact craters in the publication, in addition to its normal content, and each of these has been written by leading scholars in the field. Each is a 'state of the science' summary that serves as a solid introduction to an aspect of the discipline. The bibliographies of these articles are solidly developed, and are powerful resources in themselves. Each article is available for individual download. I recommend that anyone with a sincere interest in the subject of meteorite impact craters and impactites take this opportunity to download (and even print!) each of these articles. The publication is normally quite expensive, at $20 or so an issue if I recall correctly, and I have been meaning to post a note suggesting that they have done such an outstanding job that the publication is an excellent value even at full price. I am even more delighted to be able to deliver news of its free availability! The website is here: http://elements.geoscienceworld.org/content/8/1.toc
What this site contains
This continually evolving website lists meteorite impact craters that are located within the United States and for which diagnostic evidence of hypervelocity impact origin has been published in scientific literature. The website also presents an incomplete list of unconfirmed impact location, grouped as such, as well as a few pages that are intended to provide a basic introduction to impact crater science and to the methods and techniques behind the identification of terrestrial impact craters.
The craters listed here largely conform to those listed for the USA in the Planetary and Space Science Centre Earth Impact Database, maintained and hosted by the University of New Brunswick, Canada (PASSC database).
As of Oct. 9, 2013, this website deviates from the PASSC database in 3 ways, as follows: Alamo and Weaubleau are listed as confirmed craters (see individual listings for published unambiguous diagnostic impact evidence and additional references), and Calvin, Michigan is listed as an unconfirmed impact crater, as I have been unable to locate any published or unpublished example of currently recognized criteria. The Alamo and Weaubleau sites clearly show impact evidence, but both are primarily breccia units, and lack unanimously recognized crater boundaries. Beaverhead and Santa Fe both set a prior precedent for recognition of a crater based on the presence of concentrated unambiguous indicators of impact recognized independently of a clear structural boundary. These choices of inclusion and exclusion should be considered critically, based on evidence presented in the relevant scientific literature.
According to the PASSC database, there are currently (2013) only 183 known and confirmed meteorite impact craters on the planet earth. 28 of these are located in the United States of America. These 28 locations, and the remainder of their terrestrial counterparts, offer a unique opportunity to understand both how our own planet was formed and the environments we hope to someday explore and inhabit on other planetary and asteroidal surfaces.
Why Studying Impact Craters Matters
Each of these points is detailed below.
Impact craters tell us about the surfaces of other planetary bodies in the solar system as well as about the history of our own planet.
Impact cratering is, debatably, the single most widespread and important geological process in our solar system. Every large mass in the solar system accumulated by impacts. Today, impactites may define the lithology and petrology of more exposed solid surfaces in the solar system than any other single process, possibly including volcanism. More importantly, impact crater morphology and impactite lithologies make up the materials on the surface of virtually every planetary and sub-planetary body in the solar system upon which we are likely to ever walk.
Planets with atmospheres are buffered from impacts, but present their own challenges. Venus is a boiling hell of hot, acidic gas, and Titan presents a reactive and frigid, thermally conductive environment that makes earth's moon look like a paradise beach. We will never walk the 'surface' of the gas giants, for reasons beyond enumeration. The hard, cold, airless, and accessible surfaces within this solar system - the surfaces upon which we will some day build colonies - are overwhelmingly characterized, petrologically, lithologically, and morphologically, by impact processes and products. Excepting some relatively intact volcanic surfaces on Mars, this is true for essentially every rocky or icy body, from the smallest asteroids to the earth and moon's planetary neighbors. The gas and fluid processes on these bodies and within their surfaces are taking place in the context of rocks that are fractured, metamorphosed, and emplaced largely by impacts.
In short, there are currently only 183 locations on earth's surface that offer, in any meaningful sense, an analog to the primary geological context of our future off-planet exploration, resource extraction, and eventual colonization. This means more than just the shape of the surface of the land. Impact craters are 3-dimensional objects. On surfaces that preserve the impacts of the Late Heavy Bomdardment, meaning Mars, the moon, Vesta and other large to mid-sized asteroids, most of the solar systems large rocky moons, Mercury, and so on (essentially everywhere we can actually go), the upper crustal surface is composed of a megaregolith. This is a shattered zone of rock extending many kilometers below the surface (about 10-11 km on Mars or the moon).
Impact induced faulting and brecciation defines the shape of this zone. The scale of prior impacts, combined with the body's gravity, define its depth, its porosity and, along with impact heating, governs the possible distribution of fluids, mineralized zones, or ices within it. Above this is a zone of finer megabreccia composed of large blocks of shattered rock mixed with impact melt and the churned remnants of the impacted upper surface. This is overlain by a surficial regolith, the rough equivalent of our soil (though sterile), composed of the proximal and distal ejecta (shattered material flung from impacts) of more recent impact events. These are not unique layers. Each blends into the next. Planetary weathering and lava flows, even very large ones, are often merely thin veneers built upon this sequence.
Earth is not like this. Our surface is young, and is constantly recycled due to active plate tectonic processes that are nearly unique in the solar system. As a result, our granitic and granodioritic continents, our deep sediment filled basins, our alluvial valleys and erosional surfaces, and our intensely biological soils can tell us very little about what we will find both on and below the surface of other bodies in the solar system. For that, we must look at our few intact craters.
Impact craters help us to understand and control the potential threat of future impacts
The solar sytem is not a neat and clean place. There are literally billions (French, 1998) of large objects whirling around the sun. Some of these share common or similar orbits with earth or the other inner planets. Many others lie in the asteroid belt between Mars and Jupiter. A vastly larger number form the Kuiper belt and Oort cloud at the outer edges of the solar system. To say that the earth has been heavily impacted in its history is a profound understatement. The planet is, in fact, an accumulation of 6 trillion-trillion kilos of material, all of which accreted through impacts at one scale or another.
What this implies for the future can be a bit scary. Small impacts are constant. Impacts large enough to create small (<100 meter) craters seem to occur at least once a century, and possibly more frequently. Impacts capable of destroying a large city are about as common as extreme (but not the most extreme!) volcanic events. Regionally destructive impacts, capable of permanently altering the destiny of any small nation in which they occur, appear to happen at intervals of 50,000 to a million years, meaning that several have occured in the time since humanity began its climb from incoherent australopithecines to the sublime creators of daytime TV, just a few million years ago. And the 'big ones' - planet killing, civilization ending impacts of or approaching the scale of the KT boundary impactor that killed off the dinosaurs - occur about once every hundred million years, while their smaller, but still globally significant, companions traipse in at intervals measured in the tens of millions of years. In other words, impacts capable of utterly and irrevocably ending 'life as we know it,' and permanently altering the future course of humanity on this planet, have occured on earth well over 100 times since life appeared, well over 3 billion years ago. Understanding the nature and scope of this threat is an effort worth making, expecially considering that the exploration that is involved offers its own rewards.
Resource Recovery On Earth and In Space
The world's impact structures have played repeated and important roles in geophysical exploration for oil, gas, coal, rare earth elements, copper, nickel, barium, zinc, iron, silver, gold, platinum, and water. Resource producing impacts include the Sudbury structure, which is one of the planet's leading current sources of nickel and copper!
The materials from which planets and asteroids are composed start out thoroughly mixed. Ores and 'resource' mineral deposits are natural concentrations of useful atoms. Even on earth, finding these natural concentrations is hard. Because they produce prolonged localized heating and provide both conduits and energy to drive long-term hydrothermal systems, Earth's impact craters have produced some of the planet's most productive ore bodies and other resource concentrations. To exist in space, on any significant scale, humanity is going to find it necessary to find, recover and refine resources on other planets and among the solar system's smaller bodies. Impact melting and impact heat driven aqueous fluid systems are the solar system's most likely concentrators of off-planet useable resources.
Impacts have been a fundamental geological process throughout the planet's history. As such, they teach us a significant amount about the interior and history of our planet.
Modern geophysical exploration does not stop at the surface of the planet earth. Without the corollary field of meteoritics, we would have nothing against which to normalize data, no conception of the deep interior of the planet, and no real conception of geochemical differentiation at a planetary scale.
The largest-scale and most broadly applied refining and concentration process in the solar system is (or was) the process of planetary differentiation. Every large object in the solar system, including very large asteroids, moons, and planets, has undergone a process of melting and sorting at a large scale that is termed planetary differentiation. Oversimplified and stated in brief, differentiation is the process during which large objects in the early solar system melted and seperated into dense, iron rich cores, heavy silicate mantles, and more or less light silicate crusts. This happened because the early solar system was rich in short lived radioactive isotopes of aluminum and iron. These are essentially all gone now. The decay of these radionuclides produced heat. Large bodies do not shed heat as effectively as small bodies, so they heated up to the temperatures necessary to melt. When they melted, the iron, along with various atoms that associate with iron, largely sank to the center. Heavy iron and magnesium rich silicates floated on top of this iron, and light feldspars, aluminum, calcium, and sodium rich silicates, floated at the planetary surface. .
This is why the earth has a dense iron core and is composed of progressively lighter materials as one works outward. In materials from space, we see the results of differentiation in the form of iron-nickel meteorites, the cores of shattered, differentiated planetesimals from the early solar system. Obviously, planetary differentiation concentrates some materials, such as iron, to a useful extent, but it fails to concentrate many other elements to a level we would think of as recoverable ore. For that, we need impacts, water, prolonged regional volcanism, or plate tectonics. (I'll again apologize for the oversimplification, but encourage the reader to search the subject further if interested. There are lifetimes worth of fascinating work to be done in understanding the mechanisms, physical means, and subtle results of planetary scale differentiation.)
Sorting within the solar nebula and accretionary disk, the earliest stages in the formation of our solar system, is in some ways similar to planetary differentiation, and I'll explain it in greater detail at some point. For now - It is, more or less, the process by which heavy materials wound up near the center of the solar system and light ones wound up far from the sun, around and beyond the outer planets. Though a great deal of mixing has occured since then, we still see dense, metal rich meteorites such as enstatite chondrites differing greatly from the carbon-rich or icy concentrations found in material that accumulated farther from the sun.
These two large scale and pervasive solar system processes, differentiation and sorting within the nebula, are great at concentrating some things, such as ice and iron, but very inefficient at sorting at a more subtle level. It requires tremendous energy and through-put of ore to recover poorly concentrated materials from raw materials.
On Earth, there are two geological processes that are lacking in space, and that produce the majority of our recoverable mineral resources. These are tectonic activity, with its associated volcanism and repeated recycling and refinement of crustal rock, and the action of water, which concentrates metals and other ions by several means, including leaching and precipitation, or dissolution and recrystallization, weathering, or errosional sorting. Without these largely water-related processes, we would not have the majority of earth's utilizeable metal resources, and virtually none of its lighter element resources available in recoverable abundances.
The preceeding is a lot of background to understand a few simple facts about the role of impact craters in the otherwise innert, fossil surfaces of nearly every large inner solar system body other than earth. Large impacts provide energy for sorting resources. Large impact craters (1) form slow-cooling sheet melts within crustal rocks, (2) excavate and uplift deep rocks that contain potentially useful resources not readily available on planetary outer surfaces, and most importantly, (3) leave tremendously long lived hydrothermal systems opperating along their perimeters and around their central uplifts. Recoverable resources, ranging from sulfides and carbonates to salt and metals, in the inner solar system, are likely to be found at the bounding faults and central uplifts of large impact craters.
Page development notes to self: put in google earth links for all craters http://www.gelessons.com/