United States Meteorite Impact Craters

The study of 'crater morphology' is the study of the 3-dimensional structures that are left behind following hypervelocity impacts.  Many factors affect the size and shape of these stuctures, and these factors vary on different planets.  The size, velocity and angle of entry of an impactor, the composition of the impacted surface, the gravity at the point of impact, the density of the overlying atmosphere, and subsequent weathering of the impacted surface all affect crater morphology.  On earth's surface, we may assume that the role of gravity and the thickness of the atmosphere are more or less constant, meaning that crater morphology is a function of impact energy, impact angle, and the composition of the impacted surface.  When you consider the variability of earth's water, ice, soil, and rock-covered surface, this still leaves a lot of room for variability.  

Today, we distinguish 3 basic categories of impact crater: simple, complex, and multi-ringed basins.  Multi-ringed basins are very large.  There are very few (maybe only 1) known multi-ringed basins on earth, so for now, this chapter is going to concentrate on helping the reader understand simple and complex impact craters. 

History - Building a science of crater morphology

The first meteorite impact craters recognized were simple, bowl shaped depressions with raised rims.  They were distinguished on the basis of their morphology and the presence of meteorite remnants.  Today, these are called 'Simple' craters.  They look very much like what one might intuitively expect from an object hitting the ground at high speed.

As the diameter of an impact crater increases, the structure becomes less intuitive.  Instead of a bowl shaped depression, researchers find a central peak, an uplifted column of rocks.  Though these uplifted rocks may not protrude above the surface, they are often hundreds of meters higher than where they might be anticipated in the ground.  Worse yet, because these craters are generally tens to hundreds of millions of years old, they often find little to no remaining rim or excavated basin.  And when a ring-shaped depression is present around the central uplift, it is much shallower compared to the crater diameter than at a simple impact crater, typically about 1/9th of the crater diameter.  One cannot blame early researchers for not looking at a shattered and jumbled column of uplifted rocks and immediately thinking... 'meteorite impact!'  It took decades for the scientific community to build an understanding of the multi-kilometer complex craters resulting from larger impacts, and their identification remains one of the more formidible tasks in geology today.

The First Recognized Examples - Simple Craters

[page development note - see L. J. Spencer's 1933 synthesis of known impact craters [citation can be found in Boon and Albritton, 1938.  then see Boon and Albritton (1938) for a literature review and discussion of simple craters known at that date and Boon and Albritton 1936, 1937, 1938 for a review of the earliest interpretation of Ries and Steinheim as complex craters.  At least 4 authors, including Boon and Albritton, interpreted Ries and Steinheim as impact craters between 1931 and 1938.]

Complex Craters - from Cryptovolcanic to Cryptoexplosive

Complex craters were recognized as a distinct class of geological structure before it was recognized that they originated from meteorite impacts (e.g. Bucher, 1921, 1932, 1936). 

The term 'cryptovolcanic' was coined by W. Branco and E. Fraas, in 1905, to describe the Steinheim impact crater in Germany (Branco and Fraas, 1905).  The explosive structure, described as roughly circular, with a shattered and deranged uplifted central area surrounded by a depressed basin, and bounded by faults, was attributed to hidden volcanic processes.  The earliest descriptions were remarkably consistent with how one might now briefly describe a complex impact structure.  Much of the terminology used today was borrowed forward from early descriptive literature as the paradgim shifted from volcanic to impact.  The descriptive geology and mapping remain relevant.  

W. H. Bucher described several similar 'cryptovolcanic' structures in the US, between 1921 and 1936, including Serpent Mound (1921), and Wells Creek (1932).

Workers at places like Kentland, Indiana, and Steinheim, Germany, set out to understand these explosive structures that had no apparent volcanic origin.  They were termed cryptovolcanic structures.  The cryptovolcanic explanation proposed that these structures were formed by the escape of high pressure gasses from the subsurface.  While this proved wrong, the work deliniated a substantial set of the world's first complex impact craters with remarkable accuracy.  (note number out of number listed in several of these earlier works - include Boon and Albritton)  

The earliest description of an impact origin for complex craters came in 1936, with work by Boone and Albritton.  It is hard to understand how daring this interpretation might have been at the time.  Even the impact craters on the moon were still poorly understood.  They were generally understood to be volcanic in origin, and debate over whether they might be of meteorite impact origin was both new and contentious! 

It is remarkable how much Boone and Albritton got right in this early article.  Boone and Albritton characterized these structures... [explain their early three stages model - Compression and rebound, and list the structures they listed with how many proved later to be of impact origin.] and described general features of complex structures.

In 1936, W. H. Bucher, representing the cryptovolcanic camp, and Boon and Albritton, presenting an impact interpretation, called out a group of (how many) similar 'cryptovolcanic' structures that were (1) circular, (2) had a central uplift that was (3) intensely fragmented and deranged, and that were (4) marked by irregular and localized faulting, and (5) had no evidence of volcanic intrusion.  (Bucher, 1936; Boon and Albritton, 1936) [reread these and clarify this and their other early works]

In 1946, R. S. Dietz supported Boon and Albritton's interpretation of the cryptovolcanic strucutres as impact craters, calling upon the moon for evidence.  Dietz summarized the then-current debate about lunar craters, by pointing out that there is a structural distinction between smaller, bowl shaped lunar craters, similar to Barringer crater (Meteor Crater, Arizona), and medium sized craters that were distinguished by a central uplift.  He did not spell out the differences between these two sets and the larger craters, but does divide them into 3 morphological groups, distinguished by size, presaging our modern understanding of crater morphology: simple (small), complex (medium to large), and multi-ringed basins (very large).  Dietz also clarified the general description of a complex crater.  He placed them within a minimum and maximum size range, referred to the rebound peak as the 'central uplift' (the term still used today), described approximate radial symmetry and a rim, and described a characteristic ring shaped depression (now known as the 'annular basin') surrounding the central uplift.  He replaced the term crypto-volcanic with crypto-explosive.  Dietz supported Boon and Albritton's (1936) compression and rebound explanation for the central uplift.

Boon and Albritton's three-stage model of complex crater formation was reiterated upon and expanded by Dence, 1968, in a critical volume.

In 1946, Dietz observed that shatter cones were formed by a downward, rather than upward directed force, and in 1947, he explicitely challenged the cryptovolcanic model, championing Boone and Albritton's early explanation of a meteorite impact origin.  "Boon and Albritton (1) have developed evidence to show that structures of the Kentland type are the product of ameteorite impact. According to these writers, high-velocity impact, many times faster than the velocity of a shock wave in any type of rock, compresses the rocks elastically, rather than deforming them plastically, after which they are "backfired" into a damped-wave disturbance." Dietz (1947)

The work of Dietz (1959 and 1960) crystallized the concept of complex impact craters, with identification of a group of 7 related structures identifiable by a central uplift that contained shatter cones.  The case for an impact origin was strengthened in 1960 and 1961.  Chao et al. (1960) found coesite, a high pressure mineral polymorph of quartz, at Meteor Crater, Arizona (Barringer crater).  And in 1961, Cohen et al. found the same material, indicative of tremendous shock pressure, at the Kentland and Serpent Mound crypto-explosive structures.

The impact and cryptovolcanic paradigms met head to head in 1963, with articles by Dietz and Bucher simultaneously appearing in the American Journal of Science (Bucher, 1963; Dietz, 1963).  The evidence for impact origin was stronger, now backed up by a literature built around new techniques and technologies.  Structural geophysical studies, grain-scale petrographic changes, shatter cones, and high pressure mineral polymorphs presented a combined case that redefined cryptovolcanic structures as impact craters, folding the earlier descriptive work into a new paradigm that distinguished two kinds of crater - simple versus complex. 

Debate did not abruptly end, but the cryptovolcanic camp was subsequently marginalized.  Though resistance persisted in some camps well into the 1980s, a pivotal 1968 volume, Shock Metamorphism of Natural Materials, largely ignored the cryptovolcanism-vs-impact debate, now working from a foundation of a clear set of impact structures understood in terms of mutually recognized unambiguous evidence of shock associated with hypervelocity impact.  Impact crater science rode a surge of support, with investigations into shock metamorphism accompanying successes in the space program and nuclear test programs.  Impact crater science was strongly supported in the United States during the late 1960s and early 1970s as scientists worked to understand the lunar surface and returned lunar smaples.  An excellent 1990 article ('Towards a modern understanding of complex craters') by Bevan French summarized the end of this century-long tranistion in prespective and the emergence of our contemporary understanding of impact crater science, [read it again, and quate]. . 

This image shows examples of well preserved simple and complex craters on Mars.

Simple Crater Morphologies versus Complex Crater Morphologies

The modification stage of the impact process will produce the crater morphology that we later see based on a combination of impact environment and impact energy.  Smaller impacts produce simple craters and larger impacts produce complex craters.  Terrestrial (Earth) craters with a 'simple' morphology tend to be 4 km or less in diameter and have a bowl shaped depression with a raised rim.  Craters with a complex morphology, depending on impacted rock type, tend to be a minimum of about 4 kilometers in diameter, and may be any size above this. Complex morphologies of larger impacts vary, but generally express either an uplifted centers from rebounding of impacted material and a surrounding raised crater rim at some distance, or a raised ring surrounding the relatively flat center surrounded by the larger raised ring of the outer crater wall.  Both complex crater morphologies look as if a wave were frozen as it propagated outward from a stone splashing in water.  The outer edge of a complex crater is marked, like a simple crater, by an inwardly sloping crater wall and a raised rim. 

Craters between 5 and 15 km in diameter characteristically express complex crater morphologies of the first type mentioned above, a central peak and surrounding bowl that rises again to a raised outer crater rim. All of the primary deformation and faulting in craters of this scale occurs within 15 to 60 seconds.  (Kenkmann, 2002)

Larger craters have a lower depth to diameter ratio. 

The majority of the world's known impact craters exhibit complex crater morphologies, though they differ in their particulars.

Simple Craters (typically less than 1/2 km up to 2.5 to 4 km Bowl shaped depressions with a raised rim.

Complex Craters

Central Peak Craters (Complex Craters with a Central Uplift) and Peak-Ring Craters (Complex Craters with a Raised Central Ring)

Multi-Ringed Basins

Transitional Forms

Simple craters do not simple replace excavation pits above a certain energy level, and complex craters with central peaks do not abruptly replace simple craters in the case of larger impacts, and then proceed to be replaced by mulit-ringed structures in turn.  These commonly recognized types are points in a continuum that includes, and is influenced by, a wide range of transitional features and structures, as well as a host of impact specific variables, such as fluid cover, gravity of the target body, and even type of target rock.

Weathering and Erosion of Craters

Differences from Planet to Planet

Differences in gravity, from planet to planet, result in differences in crater morphology and in the size at which transitions between crater morphologies take place.  In general, everything is bigger in lower gravity.  This means simple bowls can be found at diameters well above the range within which they occur on earth.  The transition to complex craters, similarly, happens at larger diameters. 

Lower gravity also typically, but not always, goes hand-in-hand with a thinner atmosphere.  The thicker the atmosphere, the larger an object must be in order to reach the ground at hypervelocity.  On the moon, an object the size of a grain of dust can reach the ground at many km/second.  This means that smaller hypervelocity impact craters can be found.  Also, average speed of impact varies form body to body, depending upon location within the solar system.  In general, bodies farther from the sun are struck more slowly, but this is modifed by the fact that a planet adds velocity to an in-bound impactor in accordance with its gravity.

What craters on earth really look like

While having an understanding of idealized impact crater morphology, as described on this page, is invaluable in locating and interpreting the evidence necessary in order to establish whether a geological structure is an impact crater, it is equally important to recognize that very few terrestrial impact craters actually 'look-like' impact craters.  Most, if they are exposed above the eath's surface at all, look like vaguely circular areas of slightly subdued topogoraphy, lakes, oddly deformed or positioned mountains, or like nothing at all.  Looking closely at the pictures of specific craters in this website is likely to help the researcher get a sense of this.  Out of all of the United States' meteorite impact craters, only one, Barringer Crater, strongly resembles the unweathered, idealized structures that we see on the moon or Mars.  Nevertheless, topographraphical clues can and do assist researchers. 

References and additional resources:

Branco W., Fraas E. 1905. Das kryptovulkanische Becken von Steinheim. In Abhandlungen der königl. preuß. Akademie der Wissenschaften. Berlin 1905.

no link found

Bucher W. H. 1921 Cryptovolcanic structure in Ohio of the type of the Steinheim Basin (abstract), Bulletin of the Geological Society of America, Volume 32, pages 74-75.

https://books.google.com/books?id=nitYAAAAYAAJ

[Compared the Serpent Mound to Branco and Fraas 1905 description of Steinheim, characterizing it as a cryptovolcanic structure.]

Bucher, 1925, 1933, 1963, 1965

[championed the cryptovolcanic paradigm.  Though he was mistakenm he did a great deal of work in cohering and describing the group of structures that later came to be understood as complex impact craters.]

Bucher 1932 

[Wells creek as cryptovolcanic]

Boon J. D., Albritton C. C. 1936. Meteorite Craters and Their Possible Relationship to “Cryptovolcanic Structures”. Field and Laboratory, Volume 5, No. 1, pp. 1-9.

https://sites.smu.edu/shulermuseum/publication_pdfs/field_lab/BoonAlbritton1936.pdf

Dietz R. S. 1946. Geological structures possibly related to lunar craters. Popular Astronomy, Volume 54, pages 465-467

http://adsabs.harvard.edu/full/1946PA.....54..465D

[Dietz described the basic morphological characteristics of complex impact craters, comparring them to similar structures observed on the moon, and replaced the term cryptovolcanic structure with 'crypto-explosion structure', advancing the impact paradigm.]

Dietz R. S. 1947. Meteorite impact suggested by the orientation of shatter-cones at the Kentland, Indiana disturbance. Science 105: 42-43.  DOI: 10.1126/science.105.2715.42

http://www.sciencemag.org/content/105/2715/42.extract

[observed that shatter cones were formed by a top-down force, rather than by explosion from below]

Dence 1968

[three stages, expanding upon Boone and Albritton]

French, B. M., 1998, Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures. Houston, Texas: Lunar and Planetary Institute. pp. 120. LPI Contribution No. 954. http://www.lpi.usra.edu/publications/books/CB-954/CB-954.intro.html.

Innes 1961 The use of gravity methods to study the underground structure and impact energy of meteorite craters.

Melosh, H. J. and Ivanov, B. A., 1999, Impact Crater Collapse, Annu. Rev. Earth Planet Sci. 27 pp. 385-415

Pike, R. J., 1980, Control of crater morphology by gravity and target type: Mars, Earth, Moon, Proc. Lunar Planet. Sci. Conf. 11th pp. 2159-2189

Page development notes to self:  draw graphics of the various morphologies, label parts, then expand to include transitions.  Fix the lack of citations.

Link to this as suggested reading: 

https://www.lpi.usra.edu/publications/books/planetary_science/chapter3.pdf

and to the chapter in Bevan's book.