What this page contains: This is one of a group of several pages dedicated to communicating the nature of diagnostic evidence for impact crater identification and the specific tools and techniques used in this science. If you are new to impact crater science, you might want to start by reading [Crater Identification] and [What Makes a Confirmed Crater?] before returning to this and other specific topic pages. Please note that this website is perpetually under construction in an ongoing effort to make it more understandable and more useful.
Smashing Rocks to Bits - An Introduction
The shock produced by hypervelocity impacts produces changes in affected rocks and minerals ranging from broad regional morphological changes such as excavation and faulting to grain scale impact metamorphism such as melting, phase changes, and unique patterns of fracturing.
Each stage of the impact process leaves signatures in the affected rocks. The shock wave propagated during contact and compression produces grain scale impact metamorphism while instantly redistributing and melting or vaporizing massive quantities of rock. Excavation redistributes rocks on a massive scale, creating unsustainable slopes, lofting dust, gas, and large fragments into the air, and creating compression and friction heating between grains. During this phase, masses of disrupted rock move as fluids (acoustic fluidization), zones of unrelated rocks are mixed, and accoustically fluidized solids or melted portions are forcefully injected into surrounded rocks. Both of these stages occure very quickly. (for timing, Melosh and Ivanov, 1999; Kenkmann, 2002) The third stage of crater formation, collapse, is a much slower process. Falling rocks and dust are superpositioned over other layers, particles are sorted, unstable slopes shift and fall, and the centers of complex structures rebound, sometimes hundreds of meters from their compressed position. During this stage, the great hole in the ground that has been carved by the impact flattens out with rebound and collapse, and is filled in by falling fragments of cooling rock. The collapse stage blends seamlessly into the later stages of weathering, during which cooling of heated rocks, redistribution of water, cementation of disrupted sediment, and the slow flattening of the landscape commences.
Nomenclature - Naming Smashed Rocks
The various rocks affected by the impact are termed impactites. Formally defined (Stoffler and Grieve, 2007), impactites are "rocks affected by one or more hypervelocity impact(s) resulting from collision(s) of planetary bodies." Impactites are understood and described in terms of their position before and after the impact event as well as by their texture, how and to what extent they have been altered by shock, and by the type or types of rock that make them up. On earth, most impactites are from a single, known impact event. On other planetary bodies within the solar system that lack a substantial atmophere, such as Mars or the Moon, virtually the entire surface of the ground is covered in impactites, and the specific impact that originated many of the affected rocks is difficult to determine. This is the case for two reasons: First, because the lack of an atmosphere makes a much larger number of hypervelocity impacts possible, and secondly because the surfaces of these bodies experience extremely slow weathering, thus preserving the impactite character of a surface regolith that would quickly decay to soil or otherwise weather away on earth.
Impacts on earth can typically be understood in terms of a single impact origin. The nomenclature for these rock groups, developed by the International Union of Geological Sciences, and published in a paper by Stoffler and Grieve in 2007, is based on subdivision into 3 large groups. These are Shocked rocks, meaning those that are shock metamorphosed, but not melted or brecciated (shattered), Impact Melt rocks, meaning those which are composed significantly or completely of material that has been completely melted by the energy of impact, and Impact Breccias, meaning rocks that are composed of clasts (broken fragments) that are cemented together by a matrix composed of smaller rock particles or by rock particles and melt glass. Each of these broad groups is subdivided into smaller groups in order to create a complete naming system capable of describing the rock types found in the vicinity of impact craters. The individual mineral grains and rock textures of impactites in each of these groups are also used to further characterized impacts according to stages of increasing shock metamorphism. These are detailed in Stoffler and Grieve as well.
Unfortunately, while the overall classifacatory nomenclature can be applied to most craters, the more subtle stages of shock metamorphism can only be applied in a very incomplete manner for impactites formed in sedimentary rocks. This is because the carbonate rocks, limestone and dolostone, do not exhibit the necessary diagnostic microscopic or macroscopic textures and features necessary for this type of classification to be undertaken. The same challenge is posed by shales, which comprise one of earth's more common rock groups. Sandstone, however, is brilliant for preserving detailed impact information.
Variation in impact metamorphism is dependent upon both impact energy, which generally decreases radially outward and downward from the impact point, and upon rock or mineral type. Different minerals are metamorphosed in different ways and different host rocks respond differently to shock pressures. In general, shock metamorphism effects occur between about 5 and 100 GPA (Stöffler and Grieve, 2007). Explained in oversimplified terms, this is because below 5 GPA, minerals tend to retain the ability to deform elasticly without undergoing metamorphism (Hugoniot Elastic Limit). Above 100 GPA, almost everything turns to vapor. Impact energies at the center of a large impact can reach several hundred GPA (Stöffler and Grieve, 2007).
When the distal ejecta blankets from these impacts are better understood and identified in stratigraphic context at distance, they will probably provide, through well constrained conodont analysis, the most accurate means yet available for dating these 3 structures.
No trace of the impactor has been reported for any of the 3 Ozark Plateau impact sites. Numerical models have shown that the dynamics of a large impact should result in the complete destruction of the impactor. In such an impact, the meteorite should be reduced to gas, plasma, and very small particles. An exception to this (Maier, 2006) was discovered at the 70 kilometer Morokweng impact crater in South Africa, where a 25 cm chondrite fragment was recovered within the impact melt sheet. Other impacts over 4km, in keeping with models, have not produced any remants of impactors above microscopic sizes (Martel, 2006), though relatively successful attempts to characterize the impactors associated with certain craters have been undertaken using trace element abundances and very small fragments.
Impactites in Carbonate Targets
(bring in the carbonate impactite reference)
Most rock types will recrystallize to form rock similar to igneous varieties or will remain frozen as glass when they cool after impact melting. Recognition and interpretation of impactites in predominantly carbonate (limestone and dolostone) environments is made somewhat more challenging than in other rock groups due to the fact that melted carbonates do not form glasses and recrystallize as carbonates that are challenging to distinguish from their pre-melt parent rocks. This means that establishing a visual distinction between sheet melts and unimpacted rocks and between suevites, or melt bearing breccias, and lithic breccias, which contain little or no melt material, in carbonate impact structures can be a little tricky.
Shocked rocks: may be represented by rocks that have merely been moved by an impact, or that have been metamorphosed to any extent by shock pressure from an impact, but have not been shattered (non-brecciated) and have not been completely melted. Shocked rocks are subdivided into 'stages' of shock metamorphism (from 0 to 5) based on their degree of shock alteration. (Stöffler and Grieve, 2007) The indicators for grouping shocked rocks by stage are different for different rock types. The classification structure applicable to some sandstones found at the Missouri sites can be found at Stöffler and Grieve, 2007, in table 2.11.6. No classificatory scheme currently exists for grouping carbonates or shales into shock stages.
Impact Melts: are rocks that were completely melted by impact events and that have typically resolidified to form glass or rock groups analogous to those produced by igneous activity (volcanism). In the case of carbonates such as are found at the Missouri impact structures, the primarily carbonate impact melts have recrystallized to form carbonates fairly similar to their parent rocks. According to the classification scheme developed by Stöffler and Grieve, 2007, impact melts are sub-grouped according to their clast content into clast rich, clast poor, and clast free impact melt rocks. In other words, they are classified according to the percentage of the rock volume that is made up of fragments of unmelted rock. Unmelted clasts in impact melts may be present due to incomplete melting or due to entrainment (capture from surrounding rocks) during movement of the melt. The melted portions of impact melts (as opposed to the clasts) can be further subdivided according to their degree of crystallization. This is largely a function of cooling speed, since rocks that cool very quickly freeze as glassy, or non-crystalline rocks, and rocks that cool very slowly crystallize completely, leaving no glassy remnants between crystals. Rocks that have cooled at an intermediate speed provide the third subcategory, which is defined by crystals suspended in glass or glass between crystal grains. The term glassy is used to describe the first group, hypocrystalline describes rocks that contain some glass and some crystalls, and holocrystalline describes completely recrystallized melt rocks that contain no glass. All of this is lined out in Stöffler and Grieve, 2007, but the rocks from Missouri sites primarily employ only the holocrystalline portion of this nomenclature.
Impact Breccias, like the other two major impactite groups, are also subdivided according to variations within the group. Impacts shatter rocks. The resulting rock type, composed of broken rock fragments, is called breccia. Breccias are rocks composed of fragments of broken rock bound by a matrix that may either be composed of smaller fragments of broken rock, by solidified melted rock, or by a mix of these two. The 3 primary subcategories (Stöffler and Grieve, 2007) are monomict lithic breccias, which are composed only of lithic clasts (rock fragments), all of which are of the same type of rock, polymict lithic breccias, which are composed entirely of rock particles, but with particles originating from two or more different rock groups, and suevites, which are distinguished by the presence of melted material in the matrix between larger rock clasts. The melt can make up anywhere from a fraction of the matrix to virtually the entire matrix.
The nature and composition of impactite rock groups are determined largely by their position in relation to the impact location before and after the impact, and thus by the energy (shock or heat) to which they are exposed during and after the impact process. Rocks blown into the air, throroughly mixed, and falling together with melted material, for instance, will form polymict suevite breccias. Rocks that remain essentially in place and absorb energy from a shock wave that passes through them may produce melts or any stage of shock. Thus, by examining an impactite, you can understand something of its history, and by looking at the position of a rock within an impact site, you can develop some preliminary impression of what you might find. This concept should not be overstated, however, since impact craters are some of the most jumbled and chaotic geologic environments on the planet. There is, however, a sense to the jumbling, and impactites of a similar type tend to be zoned according to the distribution of energy and directions of motion during and after crater formation.
Sheet melts, proximal and distal ejectas, fallback breccias, etc... groups may blend into each other without clear boundaries.
Impactites formed in cabonate impact environments has, so far, been an underdeveloped area of research.
Evans et al., in a brief 2006 GSA abstract, reported 6 distinct types of breccia formed in the carbonate target rocks of Missouri's craters. These included fracture breccia (<1m clasts) and megabreccia (>1m clasts), injection breccia, dilation breccia, crystalline basement breccia, and ejecta/resurge breccia. These types reflect, somewhat, on both composition and mechanism of emplacement, and by extension, location within the structure. The authors further reflected that presence (or lack) of these types in the extant structures is more a function of subsequent erosion than original emplacement.
The area of all three impacts is underlain by Ordovician sediments containing ooids that are easily mistaken for impact spherules. Simonson, 2003, has investigated the criterian for distinguishing impact spherules in these types of potentially misleading environments.
References and Further Reading:
BRUCE M. SIMONSON, Petrographic Criteria for Recognizing Certain Types of Impact Spherules in Well-Preserved, Precambrian Successions, Astrobiology, Volume 3, Number 1, 2003. http://cmbi.bjmu.edu.cn/news/report/2003/astrobiology/11.pdf
Koeberl, C. Identification of meteoritic components in impactites. 1998,
Koeberl, C. The Geochemistry and Cosmochemistry of Impacts. 2007
Maier, W. D., M. A. G. Andreoli, I. McDonald, M. D. Higgins, A. J. Boyce, A. Shukolyukov, G. W. Lugmair, L. D. Ashwal, Pl. Gräser, E. M. Ripley, and R. J. Hart (2006) Discovery of a 25-cm Asteroid Clast in the Giant Morokweng Impact Crater, South Africa. Nature, v. 441, p. 203-206.
(An exception to the rule that large craters do not produce meteorites. A summary of the subject is at Martel, 2006, Fossil Meteorite Unearthed From Crater: http://www.psrd.hawaii.edu/June06/Morokweng.html)
Melosh and Ivanov, 1999
OSINSKI, G. R., GRIEVE, R. A. F., COLLINS, G. S., MARION, C. and SYLVESTER, P. (2008), The effect of target lithology on the products of impact melting. Meteoritics & Planetary Science, 43:19391954. doi:10.1111/j.1945-5100.2008.tb00654.x
(Addresses impacts in igneous vs sedimentary lithologies and questions regarding sedimentary impactite nomenclature.)
Osinski G. R., Spray J. G., and Grieve R. A. F. 2008. Impact melting in sedimentary target rocks: A synthesis. In The Sedimentary Record of Meteorite Impacts, Geological Society of America Special Paper 437. Editors: Evans K. Horton W., King D., Morrow J., and Warme J.Geological Society of America: Boulder. pp. 118.
Reimold et al., 2008, DEBATE ABOUT IMPACTITE NOMENCLATURE RECENT PROBLEMS,
Impactite nomenclature - Debate and clarification followed the publication of Stoffler and Grieve, 2007, is discussed here: http://www.lpi.usra.edu/meetings/lmi2008/pdf/3033.pdf)
SIMONSON B. M., Petrographic Criteria for Recognizing Certain Types of Impact Spherules in Well-Preserved Precambrian Successions, ASTROBIOLOGY, Volume 3, Number 1, 2003.
(Distinguishing impact spherules from ooids in carbonate impact environments.)
Stoffler D. and Grieve R. A. F. 2007. Impactites, Chapter 2.11. In Metamorphic rocks: a classification and glossary of terms, Recommendations of the International Union of Geological Sciences, edited by Fettes D. and Desmons J. Cambridge: Cambridge Univ. Press. pp. 8292 + Glossary.
(Impactite nomenclature - the publication is here: http://www3.nd.edu/~cneal/Lunar-L/IUGSImpactitesPaper-2007.pdf)
Stoffler and Langenhorst, 1994: (PF's)
Page development notes to self:
The Speed of Sound in Impacted Rocks (5-8 km/s, French, 1998), The passage of shock waves versus normal deformation, Increase in shock pressure correlates to an increase in temperature, Strain Rates, Transient Pressure (may exceed 500 GPa)/ Transient Strain, Yield Strengths, Melting Energy. Bars, Kbars, and gigapascals (in intuitive meaningful terms), PFs and PDFs, Feather Features, Mosaicism, Partial melting or deformation, softened grain boundaries, and complete melting, High pressure polymorphs, Diaplectic Glass: 35-45 GPa (French, 1998 citing several). Mostly from Quartz (diaplectic quartz glass) and Feldspar (maskelynite), Kink Bands.