by: Charles O’Dale

The petrographic and geochemical study of actual rocks from the potential impact structure will bring final confirmation of the presence of an impact structure. In case of a structure that is not exposed on the surface, drill-core samples are essential. Good materials for the recognition of an impact origin are various types of breccia and melt rocks. These rocks often carry unambiguous evidence for the impact origin of a structure in the form of shocked mineral and lithic clasts or a contamination from the extraterrestrial projectile.


Magnesioferrite is a magnesium iron oxide mineral, a member of the magnetite series of spinels (Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl₂O₄ in the cubic crystal system). Spinel-bearing spherules condensed from the Chicxulub
impact-vapor plume Denton S. Ebel, Lawrence Grossman



Unraveling the simultaneous shock magnetization and demagnetization of rocks

J. Gattacceca, M.Boustie, E.Lima, B.P.Weiss , T.de Resseguier, J.P.Cuq-Lelandais


In the natural case of a hypervelocity impact on a planetary or asteroidal surface, two competing phenomena occur: partial or complete shock demagnetization of pre-existing remanence and acquisition of shock remanent magnetization (SRM). In this paper, to better understand the effects of shock on the magnetic history of rocks, we simulate this natural case through laser shock experiments in controlled magnetic field. As previously shown, SRM is strictly proportional to the ambient field at the time of impact and parallel to the ambient field. Moreover, there is no directional or intensity heterogeneity of the SRM down to the scale of ∼0.2 mm3. We also show that the intensity of SRM is independent of the initial remanence state of the rock. Shock demagnetization and magnetization appear to be distinct phenomena that do not necessarily affect identical populations of grains. As such, shock demagnetization is not a limiting case of shock magnetization in zero field.

As a consequence, when it can be recognized in a rock, SRM must be considered as a reliable record of the direction and intensity of the ambient magnetic field at the time of impact. The natural process of hypervelocity impact where a rock carrying a remanent magnetization is shocked in the presence of an ambient field can be studied as the simple superimposition of shock demagnetization and shock magnetization. For this there are now a variety of techniques that allow experimental study of both phenomena separately or simultaneously as in this study.

These results have potential implications for the paleomagnetic study of meteorites, and lunar rocks, and for the understanding of the magnetic signature (as studied through paleomagnetism and/or magnetic anomalies) of terrestrial, lunar and Martian impact craters

Geophysical method to measure variations of the Earth’s magnetic field related with rocks of different magnetic properties. Magnetic anomalies in and around impact structures may result from displacement of magnetized rocks in the impact cratering process, decomposition of existent rock magnetization (by shock, for example), and formation of new magnetic phases in rocks (e.g., by chemical alterations, by acquiring a thermal remnant magnetization). Magnetic geo-signatures are instrumental in identifying impact structures ie: Carswell.
Several mechanisms related to impacts may radically change the magnetic properties of target rocks. Peak pressures in autochthonous rocks may reach ~30 GPa at impact, which is sufficient to produce shock demagnetization and remagnetization effects.


Impact melt rocks are basically volcanic rocks, such as basalt lava, and they attest to the extreme conditions generated by the impact event. Pressures and temperatures in the target rocks surrounding the point where the asteroid or comet hits are so high that large volumes or rock can be instantaneously melted. Pieces of this melt can cool rapidly to form glass and be incorporated in suevites. However, sometimes, so much melt is produced that it forms a pool in the central parts of an impact crater to form crater-fill deposits. This pool of melt then cools slowly over time and solidifies to form a new rock, which we term impact melt rock. An impact melt rock contains only a few fragments of target rock, maybe up to ~25% fragments in extreme cases. If the melt contains a lot of fragments of target rock, then we term this an “impact melt breccia”. Impact melt rocks can be found in crater-fill deposits  and in ejecta deposits.

Shock pressures in excess of roughly 60 GPa (600 kbar) are required for total rock melts. Impact melts are extremely uniform in their composition but highly variable in texture. They are composed predominantly of the target rocks but may contain a small but measurable amount of the impactor.

The Manicouagan impact crater – 10 m block of mafic gneiss embedded within the impact melt cliff, north shore of Memory Bay.

Melt rocks in impact structures may also result from frictional melting in strong dynamic metamorphism





Rock that was formed by the recrystallization of a pre-existing rock in response to a change of mainly temperature and pressure (metamorphism). Metamorphic rocks are, e.g., marble (metamorphic limestone), gneiss, schist.




Incoming meteoroids enter the earth’s atmosphere at 11 km/sec to 72 km/sec. Ram pressure between the air and the object create a very high temperature plasma at the front of the meteor. This plasma becomes visible at between about 120 km and 75 km above the earth. Energy goes into melting and vaporizing stone and metal. Energy is shed as material ablates.  In a couple of seconds most meteors are have been consumed. The left-over debris is called meteoric dust or just meteor dust.

Perseid meteor shower. Most ionization trails become visible at around 95 kilometres up.







If a meteoroid’s size, composition, speed and entry angle allow it to survive  the “meteor” phase of entry, it will slow to about 4 km/sec and enter “dark flight” at 20 km to 15 km above earth. Light emission from incandescence and ion recombination ceases. The meteor will arch into a more vertical trajectory, slow to terminal velocity of  about 0.1 km/sec and fall as a meteorite.

If the meteoroid is of sufficient size to keep it’s hyper-velocity  >12 km/sec through the atmosphere becoming an IMPACTOR, it will impact the ground and explode.  The kinetic energy of an object of mass m traveling at a speed v is = mv2/2, provided v is much less than the speed of light.

“Shrapnel” Whitecourt Meteorite, an iron meteorite..

The Ages of Meteorites

Meteorites, most of which are fragments of asteroids, are very interesting objects to study because they provide important evidence about the age, composition, and history of the early solar system. There are many types of meteorites. Some are from primitive asteroids whose material is little modified since they formed from the early solar nebula. Others are from larger asteroids that got hot enough to melt and send lava flows to the surface. A few are even from the Moon and Mars. The most primitive type of meteorites are called chondrites, because they contain little spheres of olivine crystals known as chondrules. Because of their importance, meteorites have been extensively dated radiometrically; the vast majority appear to be 4.4–4.6 Ga (billion years) old. Some meteorites, because of their mineralogy, can be dated by more than one radiometric dating technique, which provides scientists with a powerful check of the validity of the results. The results from three meteorites are shown in Table 1. Many more, plus a discussion of the different types of meteorites and their origins, can be found in Dalrymple (1991).

There are 3 important things to know about the ages in Table 1. The first is that each meteorite was dated by more than one laboratory — Allende by 2 laboratories, Guarena by 2 laboratories, and St Severin by four laboratories. This pretty much eliminates any significant laboratory biases or any major analytical mistakes. The second thing is that some of the results have been repeated using the same technique, which is another check against analytical errors. The third is that all three meteorites were dated by more than one method — two methods each for Allende and Guarena, and four methods for St Severin. This is extremely powerful verification of the validity of both the theory and practice of radiometric dating. In the case of St Severin, for example, we have 4 different natural clocks (actually 5, for the Pb-Pb method involves 2 different radioactive uranium isotopes), each running at a different rate and each using elements that respond to chemical and physical conditions in much different ways. And yet, they all give the same result to within a few percent.

Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education

ACHONDRITE: A class of stony meteorites that crystallized from magmas. The term means without chondrules.

CAI:  Calcium-aluminum-rich inclusions (CAIs) are found in chondritic meteorites. CAIs are primitive objects that formed in the solar nebula before the planets formed. CAIs are light-colored objects rich in refractory elements (that condense at a high temperature). Besides calcium and aluminum, this includes magnesium, titanium, and rare earth elements. CAIs range in size from about a millimeter to a centimeter. Meteoriticists have identified several distinct varieties of CAIs, but all share a high temperature origin. Some might be condensates from the solar nebula. Other CAIs might be evaporation residues. FCAI – rare type of CAI. F stands for fractionation, UN stands for unidentified nuclear isotope properties. FUN CAIs are characterized by 26Al/27Al ratios much lower than the canonical value of ~5×10-5(at the time of our Solar System’s formation); they also can have large isotopic anomalies in many elements.

CHONDRITE: An abundant class of stony meteorites with chemical compositions similar to that of the Sun and characterized by the presence of chondrules (see definition below). Chondrites come from asteroids that did not melt when formed and are designated as H, L, LL, E, or C depending on chemical compositions. The H, L, and LL types are called ordinary chondrites. The L chondrites are composed of silicate minerals (mostly olivine and pyroxene, but feldspar as well), metallic nickel-iron, and iron sulfide (called troilite). Most L chondrites are severely shocked-damaged, probably by a large impact on the asteroid in which they formed. The E type are called enstatite chondrites, a rare type that formed under very reducing conditions and are composed primarily of a magnesium silicate called enstatite. They are subdivided into the low-iron (EL) chemical group and the high-iron (HL) group. The C –carbonaceous chondrites– contain water-bearing minerals and carbon compounds including a variety of organic molecules such as amino acids. Carbonaceous chondrites are the most primitive meteorites–primitive in a chemical way. For example, the CI group of carbonaceous chondrites are closest in composition to the photosphere (visible surface) of the Sun.

CHONDRULE: Roughly spherical objects found in a type of meteorite called chondrites. Most chondrules are 0.5 to 2 millimeters in size and are composed of olivine and pyroxene, with smaller amounts of glass and iron-nickel metal. Two main chondrule types have been identified. Type I contain only small amounts of oxidized iron (FeO); olivine crystals in them contain only about 2 mole percent of the iron-rich-olivine fayalite (Fe2SiO4) end member. Type II chondrules contain much more FeO; olivine crystals in them typically contain 10-30 mole percent fayalite. The shapes of the mineral grains in them indicate that chondrules were once molten droplets floating freely in space.

HED: The three linked stony meteorite groups known as the HEDs are howardites, eucrites, and diogenites. They come from asteroid Vesta. (Data collected by NASA’s Dawn Mission, in orbit around Vesta from 2011-2012, strengthed the association between Vesta and HED meteorites.)

IRON: Meteorite: Iron meteorites are made, almost completely, of iron and nickel metal. They are chemically distinguished and grouped according to the abundances of the trace elements such as gallium and germanium, as well as nickel. Initially, irons were classified into four groups and were given Roman numerals I, II, III, and IV. Today 13 groups are recognized and designated further by letters A through G according to concentrations of siderophile (“iron-loving”) trace elements. Iron meteorites that do not fit into the groups are called ungrouped. The two iron-nickel alloys in iron meteorites are called kamacite (low-nickel content, usually up to 7.5 wt% nickel) and taenite (high-nickel content, ~20 to 50 wt% nickel). These alloys are rare in terrestrial rocks.

PALLASITE: A stony-iron meteorite that is a mixture of isolated silicate crystals (usually olivine) surrounded by metal.

REFRACTORY INCLUSION: Inclusions, enriched in the rare earths and the other elements mentioned in the definition of refractory. These inclusions are often referred to as Ca-, Al-rich inclusions, or “CAIs.” Most refractory inclusions contain the minerals spinel and melilite and/or hibonite.

REGMAGLYPT: Shallow depression, resembling a thumb print in clay, that is commonly seen on meteorites. Regmaglypts are formed by ablation from the surface by vortices of hot gas as a meteor falls through a planetary atmosphere.

SPINEL: MgAl2O4, magnesium aluminum oxide mineral, with Fe+2 able to substitute for Mg and with Cr or Fe+3 able to substitute for Al.

UREILITE: An ultramafic achondrite meteorite that contains interstitial carbon in the form of graphite or diamond.

Widmanstätten Pattern: A pattern found in iron meteorites due to a change in crystal structure of iron-nickel metal grains during cooling. This structural change produces a pattern of crystallographically oriented kamacite (low-nickel content, usually up to 7.5 wt% nickel) plates in taenite (high-nickel content, ~20 to 50 wt% nickel).







A meteoroid is a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom.








Synchronous impact of two (paired impact) or more impactors. A Late Triassic multiple impact has been proposed to have produced a chain of five large impact structures on the European and the American continents.

Compilation of selected terrestrial meteorite impacts during the Triassic and the postulated Late Triassic multiple impact theory, modified after Spray et al.(1998). Lucas et al.(2012)suggested an age of ∼220 Ma for the Carnian/Norian boundary, which has an age of ∼227Ma in the current International Stratigraphic Chart (Cohen et al., 2013). Impact age data from Koeberl et al.(1996), Ramezani et al.(2005), Schmieder and Buchner (2008), Schmieder et al.(2010).

Multiple impacts are observed also on the Moon, Mars, Venus and on Jupiter’s satellites Ganymede and Callisto.





The largest craters contain multiple concentric topographic rings, and are called multi-ringed basins, for example the lunar Orientale.

The Sudbury Impact Structure may be a multi-ring impact crater.





A sheet of rock that has moved sideways over neighboring strata as a result of an overthrust or folding.



Cloud of comets hypothesized to be the source of the long-period comets. Periodical disturbance of the Oort cloud has been proposed to be related with a disputed periodical increase of cometary impacts on the Earth (Shiva theory).

Visual representation of the Oort Cloud, which is littered with ice and rocks left over from the formation of the Solar System. (Image: NASA) The Oort Cloud is an extended shell of icy objects that exist in the outermost reaches of the solar system. It is named after astronomer Jan Oort, who first theorised its existence. The Oort Cloud is roughly spherical, and is thought to be the origin of most of the long-period comets that have been observed.



OVERTURNED STRATA (“overturned flap”)

Inverse stratigraphy at an impact crater rim related with the excavation process.

The high energy impact explosion, creating the Barringer Impact Crater, ejected large amounts of material out of the crater, in some cases preserving stratigraphic relationships. Notice that the normal undisturbed sequence has the Coconino (oldest) at the bottom, followed by the Toroweap, Kaibab and Moenkopi (youngest) as you move upwards. In the overturned rocks near the crater, this sequence is repeated above the Moenkopi, but in a reverse (overturned) order. The ejected and overturned strata extends 1 to 2 km from the crater.