United States Meteorite Impact Craters

About This Website 

This steadily expanding website presents a list of known and possible impact crater location within the United States, 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 website is written and curated as a research resource by Robert Beauford, Ph.D., with the much appreciated assistance of reviews and comments from users. (send comments to: robert@impactcraters.us).

What has been included and why

The craters listed here largely conform to those listed for the USA in the Planetary and Space Science Centre Earth Impact Database (PASSC database), maintained and hosted by the University of New Brunswick, Canada.  A link to the PASSC database can be found at the bottom of this page.

Decisions regarding which craters to include and exclude among confirmed impacts listed on this website are based on published literature, which I have tried to consistently and specifically cite.  In each case, I have looked for clearly and appropriately published examples of the most widely recognized and least ambiguous categories of evidence for impact origin, meaning (1) shatter cones, (2) grain scale evidence of shock pressures associated with impact, such planar deformation features (PDFs) in quartz or related features in zircon, (3) high pressure mineral polymorphs  that are nearly unique to impact craters, such as the dense quartz polymorphs, coesite and stishovite, or the high pressure polymorphs of zircon or rutile, reidite and TiO2 II, respectively, or (4) the unambiguous presence of meteorite fragments or of impactor components in associated glass or target rock.  Out of these various forms of evidence, shatter cones, PDFs, and traces of the impacting meteorite account for the evidence that has confirmed the vast majority of currently recognized impact craters in the USA.

At last check (edit: Nov., 2018), the overall list of impact structures and craters included here deviates from the PASSC Earth Impact Database in only 3 ways, as follows:  Alamo and Weaubleau are listed in this website as confirmed craters (see individual pages for published impact evidence upon which I based the decisions and for additional references), and Calvin, Michigan, is listed here as an unconfirmed impact crater, as I have been unable to locate any published or unpublished description of any generally recognized evidence indicating an impact origin.  Decorah has also been recently added here based on the 2018 publication of impact evidence (see the associated website page for article).  The Alamo and Weaubleau sites clearly show impact evidence, but like Beaverhead or Santa Fe, lack unanimously recognized crater boundaries.  The strength of evidence varies, and these choices of inclusion and exclusion simply represent a current 'best effort' on my part, and should be considered critically, based on the evidence presented in the relevant scientific literature.  I invite and welcome qualified comments and criticisms.

Why Studying Impact Craters Matters

According to the PASSC database, there are currently (2018) only 190 known and confirmed meteorite impact craters on the planet earth.  Only 30 well evidentiated meteorite imact craters are located in the United States of America.  These 30 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.

So, what specifically motivates impact crater science?

1. the potential for impacts to serve as analogs for other planetary surfaces
2. understanding the nature and scale of impact risk, and mitigating such risk
3. understanding the formation process of our planet and solar system - which is, fundamentally, a long series of accumulating impacts at varying scales
4. quantifying past and present energy flux in planetary environments
5. the importance of impact craters and cratering processes in forming or controlling the location of recoverable resources, both on earth and off-planet

Each of these points is explained in somewhat greater detail below.

Analogs for understanding other planetary surfaces

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 search for resources or perhaps even build colonies - are overwhelmingly characterized, petrologically, lithologically, and morphologically, by impact cratering.  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 190 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 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, though some of the icy bodies undergo analogous resurfacing processes.  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 relatiely few intact craters.

Understanding and mitigating ongoing asteroid impact risk

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 an interval between less than 50,000 and a million years, meaning that several have occured in the time since humanity began its climb from incoherent australopithecines, just a few million years ago, to become the sublime creators of daytime.  And the 'big ones' - planet killing, civilization ending impacts approaching or exceeding the scale of the KT (or K-Pg) 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 or less.  In other words, impacts capable of utterly and irrevocably ending 'life as we know it,' permanently altering the future course of humanity, or altering the destinies of nations, have occured 1000s of 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 shorter-term rewards.

[understanding the formation process of our planet and solar system - section]

[quantifying past and present energy flux in planetary environments - section]

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 and impact science, we would have nothing against which to normalize data, no conception of the deep interior of the planet, no understanding of the planets ancient or modern internal heat budget, 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 impact associated faults or where excavated large impact craters.

Recommended Initial Reading:

Anyone wishing to develop an in depth undertanding of the scientific study of meteorite impact craters would do well to begin by reading Bevan French's book, 'Traces of Catastrophe,' and Osinski and Pierazzo's (editors) recent volume 'Impact Cratering Processes and Products'.  Each of these provides an excellent overview of the subject of impact crater science, and just as important, each contains a substantial bibliography of more in-depth literature.  They provide a solid, modern introduction to the scientific discipline.

Bevan French's book is available online for free and is inexpensive in print.  It can be found as a downloadable PDF at: http://www.lpi.usra.edu/publications/books/CB-954/CB-954.pdf  Impact Cratering Processes and Products is worth the investment.  It can be found at http://onlinelibrary.wiley.com/book/10.1002/9781118447307 or through Amazon, at http://www.amazon.com/Impact-Cratering-Processes-Products-Osinski/dp/140519829X 

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[page development notes below this line:

abbreviate the above, and move it to chapter 1 of impact crater identification; make this a dscription of the state of the science and move summary graphics of the US crater population to this page

US craters have provided several historical firsts: first description of shatter cones (at Kentland), first recognized simple craters, first recognition of complex impact structures and recognition for the mechanism of their formation, Boone and Albritton (along with Ries, in Germany), first recognition of coesite in nature at a simple crater, and at complex craters.

US crater research benefited from a surge of research on older impact scars on earth associated with an effort ot both prepare for and understand lunar surface exploration.

Describing the timing of the 'discovery' of impact craters is a difficult and imprecise process, as most impact craters have been recognized as geological disturbances long before their impact origin was recognized, and the recognition of some sites took decades of analysis and research, sometimes even awaited discoveries in other areas of science!  Kentland, for instance was first described in (1883?), first proposed as an impact crater in 1936('37?), played a role in the recognition of the significance of coesite, shatter cones, petrographic grain-scale indicators of impact origin, and of complex craters as a class of geological structures, between the 1930s and 1960s, and was still doubted by some researchers in the 1980s!  Nevertheless, we a can build a timeline of sorts.  Catalogs of Boon and Albritton 1937, Dietz at various years.

mention north American tektite strewn field and KT and other impactite horizons

The impactcraters.us website is continually changing and growing for several reasons.  This resource is intended as a research tool and up to date compendium of information on impact craters located within the United States.  The aim of this project is to provide a current catalog of known impact craters for the United States region that is as accurate as possible, based on diagnostic evidence of hypervelocity impact published in scientific literature.  It is hoped that this may support global projects in the same vein, such as the  (PASSC database and Meteoritical Bulletin database).  The intended scope of the project is large - to provide and accurate catalog with basic metrics, a literature review, and a complete annotated bibliography for each site, with a visitor's guide and field-trip introduction to each of the ones that is exposed (not buried).  This means that the project includes ovber 150 years of research on nearly 30 sites, with extensive travel and the necessity of reading and reviewing many thousands of pages of text.  Though the effort remains incomplete and imperfect, it is hoped that it will have utility to researchers and educators, and that it may be engaging for students and the general public.  Because of both the scope of the project and the fact that it is intended to provide ongoing and expanding utility, rather than a momentary snapshot of the field, that it will remain a 'work in progress' for an indefinite time. 

This website resource is divided into TWO parts: A Guidebook to the Meteorite Impact Craters and Structures of the United States, an ongoing review and guide to the geological literature surrounding those reasonably well supported impact craters that are best supported by evidence within the U.S., and a second book Introduction to Impact Craters and Their Identification, intended to provide an introduction to the history and current research behind the recognition of impact craters as geological structures.

The guidebook chapters for each impact craters may be accessed using the links on the left-hand side of the website pages.

Tangential to the project is a list of possible impact structures that have been described in the literature or reported by site visitors.  These can be found [here - possibles] and [here - user submissions]. 

 end page development notes]

This site will remain, for the foreseeable future, perpetually under construction.  impactcraters.us is a project that I work on for the simple enjoyment of it and to facilitate others work.  I hope that the site is both useful and engaging for the reader.  Except where referenced from peer-reviewed work in the formal literature, the content of the site has not been subjected to peer review and may contain substantial inaccuracies.  If you observe any, point them out!  The site should be expected to change (and hopefully improve) from week to week.

Links to some good general reading and resources:

PASSC EARTH IMPACT DATABASE

DAVID RAJMON'S IMPACT DATABASE

TRACES OF CATASTROPHE

IMPACT FIELD STUDIES GROUP

NASA NEAR EARTH OBJECT PROGRAM

MELOSH AND BEYER - CRATER COMPUTER

MARCUS, MELOSH, COLLINS - DESTRUCTION COMPUTER

KRING - GEOLOGY OF BARRINGER CRATER