Apreciado visitante y lector …

…estamos en el final del proceso de reordenación de la web www.estructuras-de-impacto.impact-structures.com. Hemos incrementado el número de temas, los hemos configurado de un modo más claro y hemos añadido más textos e imágenes (con imágenes adicionales procedentes de nuestra colección de impactitas).

   Los autores de esta web (ver Contacto)

La reorganización, basada en la versión en inglés de www.impact-structures.com, era y es una tarea laboriosa. Por este motivo puede observarse que todavía hay textos pendientes de su traducción al español (en cada caso marcados en rojo). Pedimos excusas por este pequeño inconveniente y aseguramos que las restantes traducciones estaran listas en un futuro muy cercano.

Esta forma de proceder puede dar lugar a algún malfuncionamiento a lo largo del proceso (básicamente por interferencias entre versiones en inglés y español). Gracias de antemano por tu paciencia y comprensión!

Marte en la Tierra

La sonda Mars Express de la ESA ha enviado una serie de nuevas imágenes de un cráter de impacto elongado en el hemisferio sur de Marte. Probablemente, el cráter fue causado por una cadena de proyectiles y de este modo puede considerarse un modelo para ilustrar la formación de la cuenca de impacto elongada de Rubielos de la Cérida en España.  


Cuenca de impacto de Rubielos de la Cérida. Para más información clicar aquí.

Rubielos de la Cérida impact basin: New impact breccia dikes in Paleozoic silicate rocks

Breccia dikes (dike breccias) are a prominent feature in impact structures, and they have told us a good many about the processes of impact cratering. Among the yet known roughly 200 terrestrial impact structures, Azuara and Rubielos de la Cérida are providing the probably by far best insight into these fascinating geological configurations concerning abundance, exposure accessibility, and diversity with regard to geometry, dimensions and material (also see breccia/dikeshttp://www.impact-structures.com/spainrubielos-breccia-dikes). Recent extensive construction works (storage reservoir, freeway, railroad) near Lechago, Calamocha (CAL; see Fig. 1, red circle) have yielded quite a few nice geological exposures once more documenting the almost everywhere existing impact signature thus underlining that construction work in the large impact region of the Azuara and Rubielos de la Cérida structures may be a hard undertaking – as was impressively shown when the new Autovía freeway line had to cut through the strongly destroyed Paleozoic of the Iberian Chain in the northern rim zone of the Azuara impact structure between Daroca and Cariñena (click here http://www.impact-structures.com/impact-spain/the-azuara-impact-structure/the-2005-autovia-mudejar-geological-exposures/ «The 2005 Autovía Mudéjar geological exposures»).

Below we show images of some newly exposed impact breccia dikes near Lechago exhibiting their characteristic properties and, for comparison, similar dikes and dike systems from the companion Azuara impact structure (location in Fig. 1).

Focusing here on breccia dikes in Paleozoic silicate rocks bears in mind the frequent claim of some Spanish geologists, especially from the Zaragoza university, that the impact breccia dikes are all karst phenomena. Their confusion of breccia dikes with karst features, however, meets some difficulties in the case of silicate siltstones. Also fault breccias can clearly be excluded since these dikes are filled with allochthonous material as is typical for impact-induced highly energetic injection processes.

Image001Fig. 1. Location map.

Image003Fig. 2. Thick impact breccia dike cutting through Paleozoic siltstones at the new storage reservoir near Lechago (in the red circle of Fig. 1)

Image005Fig. 3. For comparison: impact breccia dike in Paleozoic siltstones; road to Santuario de la Virgen de Herrera (1 in Fig. 1). Azuara impact structure.

Image007Fig. 4. For comparison: thick impact breccia dikes cutting roughly perpendicular through the bedding of the Paleozoic siltstones. Autovía Mudéjar in course of construction; near Cariñena (2 in Fig. 1). Azuara impact structure.

Image009Fig. 5. Large system of impact breccia dikes in Paleozoic siltstones at the new storage reservoir near Lechago. Note that the dominant set of dikes is cutting sharply across the bedding (see Fig. 6). In the top: Unfolded post-impact Tertiary sediments.

Image011Fig. 6. Segment of the wall in Fig. 5.

Image013Fig. 7. For comparison: System of impact breccia dikes cutting sharply through Paleozoic silicate rocks. Autovía Mudéjar in course of construction; near Cariñena (2 in Fig. 2). Azuara impact structure.

Regmaglypts on clasts from the Puerto Mínguez ejecta, Azuara multiple impact event (Spain)

Fig. 1. Amazingly similar: Regmaglypts on the surface of the Tabor meteorite and on a limestone clast from the Puerto Mínguez impact ejecta.

Among the various deformation features exhibited by the carbonate clasts in the Puerto Mínguez ejecta deposit (striae, nail prints, grooves, rotated fractures, irregular fractures with complex bifurcations, mirror polish, etc), regmaglypted clast surfaces are a prominent feature (Fig. 1). For the first time described by K. Ernstson (2004), they establish clear evidences of an aerial transport of quite a few clasts of the ejecta.


Source: Cascadia Meteorite Laboratory, Portland State University

Fig. 2. A Gibeon meteorite exhibiting distinct regmaglypts.

Regmaglypts (or thumbprints) are reliefs commonly reported for the surface of some meteorites (Figs. 1, 2). The depressions originate from dynamic air pressure [continue …] and from selective erosion by material melting (ablation) off the surface of the meteorite on its passage through the atmosphere. The relief may show polygonal, spherical, rounded or elliptical shape, and a pattern like fingers over wet clay is frequent.

We suggest the ablation features observed in the ejecta have originated from a similar process that is carbonate melting and ablation off the surface on the passage of the ejecta clasts through the heated impact explosion cloud.

Of course, the depressions on the Puerto Mínguez clasts in a way remind of lapiés (karren) features on surfaces of exposed limestones. Lapiés consist of shallow, straight grooves or runnels incised into the limestone by solution. Because of the striae and polish regularly coating the regmaglypted surfaces, and because the regmaglyptic clasts are embedded as individuals in the ejecta matrix (Fig. 3), an in situ formation as lapiés can clearly be excluded.

Fig. 3. The regmaglypted individual embedded in the Puerto Mínguez impact ejecta is incompatible with an in situ dissolution («lapiés» formation).

Alternatively, the «lapiés» depressions existed already before the impact and survived rock fracturing, excavation, and emplacement of the ejecta. This can be excluded because many clasts are regmaglypted all around (Fig. 4), and the frequently observed sharp-edged ridges of the regmaglypts (Fig. 4) would not have survived the excavation and ejection process.

Fig. 4. Front and rear of a regmaglypted limestone clast from the Puerto Mínguez ejecta. The regmaglypts all around and the sharp ridges exclude a «lapiés» formation before excavation and ejection.

In a few cases, the ablation by obvious carbonate melting of the clasts has eaten deeply into the clast (Fig. 5) reminding of similar distinct ablation features of some meteorites (Fig. 6).

Fig. 5. A regmaglypted limestone clast from the Puerto Mínguez ejecta exhibits prominent ablation reaching deeply inside into the limestone clast. For comparison see the meteoritic ablation features in Fig. 6.

Fig. 6. The Derrick Peak, Antarctica, meteorite. Image courtesy of NASA.

An extended article on the Puerto Mínguez regmaglypted ejecta including many images can be read here.

Cutting into an impact crater rim: excavation and modification signature of the impact cratering process

Road constructors are the friends of the impact geologists. Without their work, most of the highlighting impact outcrops in the Spanish Azuara and Rubielos de la Cérida impact structures would not exist, and many of the rocks typifying impact would not have been discovered. In the last decade, kilometers and kilometers of new geological exposures have been prepared, and we mention the road cuts between Luco de Jiloca and Lechago, at the Puerto Mínguez, between Navarrete and Barrachina, between Fuendetodos and Azuara, between Lécera and Muniesa, between Fuendetodos and Jaulín, and many more. Not only the road cuts but also the many new quarries mostly exploited for road construction material have supplied new geologic outcrops of high geologic importance, as for example the quarries between Belchite and Puebla de Albortón, the many temporary quarries between Navarrete and Barrachina, the large quarries of Corbalán, San Blas, Villafranca del Campo, near Muel, and so on.

 Location map.           

The road cut at the crater rim and view down into the Rubielos de la Cérida impact basin.


Only recently, the new road cutting into the Rubielos de la Cérida impact basin rim in the ascent between Alfambra/Escorihuela and El Pobo/Cedrillas has prepared a breath-taking extraordinary continuous geologic exposure of currently about 2 km length. The exposure does not show only the unimaginably disastrous forces of the impact excavation and modification of the Permotriassic/Buntsandstein and Muschelkalk rocks leaving a gigantic megabreccia, but also reveals large-scale rock deformations hitherto obviously unknown to geologists. We want to call them stop-and-go deformations.




The impact stop-and-go deformation is characterized by a multiple rapid sequence of erosion, sedimentation, folding, faulting and flow in a limited rock unit. This extremely peculiar process is not explicable by “normal“ geological forces and is understood only by the complex impact excavation and modification movements with permanently and in a short time strongly varying stress fields probably supported by the action of water and shock-produced volatiles.

Simple model of stop-and go deformation.



More stop-and-go deformations: megabreccia near Barrachina.

Also see http://www.impact-structures.com/impact-spain/the-rubielos-de-la-cerida-impact-basin/megabreccias/


The Jaulín impactite (Azuara, Spain)

About 30 km north of the center of the Azuara impact structure (Spain) near the village of Jaulín (0°59.3′ W; 41°27.2′ N), a peculiar breccia is exposed. The breccia, not mapped geologically thus far, is intercalated between fossil-rich Jurassic limestones and brownish Miocene(?) gypsum marls. The breccia is unconformably overlying the Mesozoic rocks (Fig. 1) and may penetrate the  limestones (breccia dikes, Fig. 2) as well as corrode them (Fig. 3).

Fig.1. Unconformable contact between the Jaulín breccia and the Jurassic limestones.

Fig. 2. Penetration of the breccia into the Jurassic host rock in the form of a dike.


Fig.3. Corrosion features in the contact zone between breccia and host rock. Neither aeolian nor karstification processes are responsible of the corrosion, which probably is related with decarbonization.


On cursory inspection, the greenish rock looks like a massive bone breccia (Fig. 4). On closer examination, the «bones» prove to be limestone clasts having become hollow or more or less completely skeletal (Figs.5, 6).

Fig. 4. At first glance, the breccia reminds of a massive bone breccia.

Fig. 5. Close-up of the carbonate breccia clasts.

Fig. 6. Close-up of hollow and skeletal clasts embedded in the matrix. Note that the decomposition with possible relics of carbonate melt is confined to the interior of the cobbles reminding of the construction of a wasp’s nest.

Beginning (Fig. 7) and complete fragmentation of the clasts is observed. Beside these decomposed clasts, fragments of the limestone host rock are intermixed in the breccia (Fig. 8). They frequently show distinct whitish rims (Fig. 9) which we interpret as the result of beginning decarbonization from enhanced temperatures. Occasionally, the fragmented clasts remain coherent giving evidence of confining pressure upon emplacement (Fig. 10). For the present, it is not clear whether the hollow and skeletal clasts originate also from the local limestones, but from the discussion below we have to assume they are allochthonous.

Fig. 7. Breccia sample exhibiting a decomposed, fragmented clast and distinct flow texture of the matrix.

Fig. 8. Evidence for the erosive strength of the breccia emplacement process: fragments of the limestone host rock have been carried away and embedded in the greenish matrix. Note the beginning «conglomeratization» (increasing roundness) of the clasts.

Fig. 9. Whitish rims of limestone fragments in contact with the breccia matrix: evidence of beginning decarbonization due to enhanced temperatures.

Fig. 10. Fragmented but coherent limestone clast as indicative of confining pressure during the emplacement.

The clasts are immersed in a greenish matrix (Fig. 8) partly exhibiting flow texture as indicated by lined-up small elongated clasts (Fig. 7).  In thin section (Fig. 11),  the matrix shows to be fine-grained carbonate streaked with irregular bands of poorly rounded quartz grains in a slightly different matrix. Quite a few quartz grains exhibit planar deformation features (PDFs) as in proof of shock metamorphism (Fig. 12).

Fig. 11. Breccia matrix composed of dark carbonate material streaked with irregular bands of poorly rounded quartz grains in a slightly different matrix. Photomicrograph, plane light; the field is 10 mm wide.

Fig. 12. Several sets of decorated planar deformation features (PDFs) in a quartz grain from the breccia matrix. Photomicrograph, crossed polarizers; the field is 220 µm wide.

Breccia formation. – The contacts between breccia and underlying autochthonous rocks as well as the peculiar characteristics of the clasts absolutely exclude a karstification process. A «normal» sedimentation and any diagenetic processes are not consistent with the observations either. From the stratigraphic position at the base of the unfolded Upper Tertiary and from the evidence of enhanced temperatures and of shock metamorphism we conclude the breccia to be an impactite related with the formation of the Azuara structure in the giant multiple impact event that, beside the Azuara structure, formed also the large Rubielos de la Cérida impact basin and crater chain (see http://www.impact-structures.com/spain/). We explain the breccia to be impact ejecta excavated from a region in the Azuara structure where shock intensities were enough to decarbonize and melt limestone cobbles and boulders and to produce PDFs in quartz grains contributing to the breccia matrix.

Hollow and skeletal limestone boulders, cobbles and pebbles are not uncommon in the impact region, and they have been found in e.g. the basal suevite breccia (see suevite)  and the impactite from Almonacid de la Cuba (see peculiarities). The process of the decomposition obviously confined to the interior of the clasts is not completely understood thus far, but we suggest two possibilities that must not exclude one another. The clasts belonged to Lower Tertiary conglomerates in the target, and they experienced

  • a shock concentration in the cobbles’ interior by shock-wave reverberation and focusing effects


  • a shock heating of the cobbles and a rapid external cooling upon ejection, only allowing the interior to be decarbonized and melted.

On excavation and ejection, the shocked limestone clasts were mixed with the greenish matrix material possibly originating also from the Lower Tertiary. The emplacement was a process of ballistic erosion and sedimentation (Oberbeck 1975) under high pressure (penetration into the Jurassic host rock that was partially fragmented and incorporated into the breccia) and still enhanced temperatures (partial decarbonization of the host rock).

With regard to its stratigraphical position, the Jaulín impactite must be seen as a special variety of the Azuara/Rubielos de la Cérida basal breccia, and, due to the observed shock effects and the relics of probable carbonate melt,  as a special type of a suevite breccia (see IUGS classification, the_suevite_page).

New varieties of the basal suevite breccia from the multiple-impact area in northern Spain

This peculiar polymict breccia uniformly exposed  at the base of the unfolded Upper Tertiary over a distance of at least 120 km is a strong clue to the Mid-Tertiary multiple impact in northern Spain. A lot has already been said and written about this breccia (Ernstson & Fiebag, 1992; Ernstson & Claudin 2002, Ernstson et al. 2003; Claudin & Ernstson 2003; http://www.impact-structures.com/impact-rocks-impactites/the-suevite-page/suevites-from-the-azuara-and-rubielos-de-la-cerida-spain-impact-structures/, http://www.impact-structures.com/impact-spain/the-rubielos-de-la-cerida-impact-basin/basal-suevite-breccia-in-the-rubielos-de-la-cerida-impact-basin/), and that is why we confine ourselves to show some new varieties exposed about 2 km northeast of Olalla in the rim zone between the Azuara impact structure and the Rubielos de la Cérida impact basin. We point to the dominating Paleozoic and Triassic components, to the flow texture, to halos around many clasts, to the remarkable fitting of fragmented clasts, to breccias-within-breccias, and we think that some of the breccias are simply beautiful.

Here, we want to mention that regional geologists from the Zaragoza university and the Madrid Center of Astrobiology (Ángel Luis Cortés, Marcos Aurell, Enrique Diaz-Martínez, and others), which basically deny an impact in that region,  consider this typical breccia a lacustrine sediment or  even a conglomerate (Aurell et al. 1993; Cortés, Nov. 6, 2002, Heraldo de Aragón).


Impact-induced carbonate-psilomelane vein in the Azuara structure of northeastern Spain


Outcrop of Muschelkalk dolomite crosscut by a dark vein of impact melt rock in the vicinity of Monforte de Moyuela.

Black vein under the microscope: light matrix of carbonate minerals (Cc), black particles and gas vesicles (gv). Long side of the figure is about 1 mm.






The full article can be read here:



key words: Azuara impact structure, Rubielos de la Cérida impact structure, carbonate melt, breccia dike, manganese, quench crystallization.

Accretionary lapilli from the Azuara and Rubielos de la Cérida impact structures (Spain)

Accretionary lapilli is a term originally solely related with volcanism. Accretionary lapilli are pellets that form by the accretion of fine ash around condensing water droplets or solid particles, particularly in steam-rich eruptive columns. Commonly, they exhibit a concentric internal structure, and, once formed, they can be transported and deposited by pyroclastic fall, surge, or flow processes (Allaby & Allaby, 1999; A Dictionary of Earth Sciences). Armoured lapilli is the term that is especially used in the case the ash has accumulated around a small rock fragment. The armoured-lapilli variety is frequently found in deposits of basaltic base surges.

Image 1: Accretionary lapillus (diameter 0.5 mm) from the basal
suevite breccia in the Azuara impact structure (Mayer 1990).
Photomicrograph, xx nicols.

Since similar processes are related with the turbulent explosion plume raising above the expanding excavation cavity in an impact cratering event, it is not surprising that accretionary lapilli have been found also in impact deposits. Graup (1981) describes accretionary lapilli to occur in the suevite fall-back breccia of the Ries impact structure. They are also reported for ejecta deposits of the K/T Chicxulub impact structure in Mexico (http://www.lpi.usra.edu/meetings/metsoc2000/pdf/5124.pdf,
http://www.lpi.usra.edu/meetings/largeimpacts2003/pdf/4113.pdf) and Belize (http://www.icdp-online.de/news/workshops/abstracts/EGS03/EAE03-J-06925.pdf).

Concentrations of lapilli formed lapillistone that occurs as discontinuous, reworked clasts within the megabreccia related with the Late Devonian Alamo impact (http://gsa.confex.com/gsa/2002AM/finalprogram/abstract_42158.htm).

For the Azuara impact structure, accretionary lapilli was first reported by Mayer (1990). Image 1 shows a typical lapillus from the matrix of the basal suevite breccia near Muniesa. The interior is composed of very badly sorted material (calcite, quartz, ore). The rim zone is formed by concentric layers of finer material (mostly micritic calcite). Similar accretionary lapilli are observed also in the matrix of Azuara breccia dikes.

Accretionary lapilli also contribute to the matrix material of the basal suevite breccia and breccia dikes in the Rubielos de la Cérida elongated impact basin (as part of the Azuara/Rubielos de la Cérida impact crater chain, see Highlights ).

 2 3

 4 5

Images 2 – 5: Accretionary lapilli from the basal suevite breccia in the Rubielos de la Cérida impact basin. Photomicrographs, xx nicols; the fields are 3.5, 5, 6.5 and 3 mm wide.

Images 2 – 5 show accretionary lapilli from the basal suevite breccia near Corbatón in the Rubielos de la Cérida impact basin. The lapilli are basically carbonate with some accessory silicate material (e.g., quartz fragments, Image 3) and regularly exhibit the typical «onion skin» internal structure. The angular core of the lapillus in Image 5 probably is a fragment of a former lapillus thus reflecting some kind of reworking in the lapilli formation and deposition process.

Near the village of Olalla in the northern rim zone of the Rubielos de la Cérida impact basin, a prominent breccia dike is exposed (Images 6, 7). In the early seventies, in the course of his geologic mapping activities, W. Monninger called this dike «Teufelsmauer» (Devils Wall) because of its peculiar structure and composition. At that time, impact processes and impact rocks were not well known among geologists.

 6 7

Images 6, 7: The Devils Wall breccia dike near Olalla.

 8 9

Images 8, 9: The matrix of the Devils Wall breccia dike composed of accretionary lapilli. The whitish crusts of some breccia fragments are suggested to result from high-temperature decarbonization of limestone. Polished sections, the fields are 13 and 15 mm wide.

In polished sections (Images 8, 9), the matrix of the carbonate dike breccia for the most part proves to be completely composed of accretionary lapilli (lapillistone). Limestone fragments from the disintegrated host rock of the dike abundantly make up the core of larger lapilli. In doing so, the fragments my be broken but still remain coherent within the lapillus, as can be seen in Image 10. This shows that the accretion process continued upon injection of the vapor plume material into the host rock.

Image 10: Fragments of the breccia dike host rock make up the core of larger accretionary lapilli.

Regional geologists from Spain (M. Aurell, E. Díaz-Martínez, A. L. Cortés, and others) vehemently refusing the impact origin of Azuara and Rubielos de la Cérida, suggest the basal suevite breccia and breccia dikes to be diagenetic, pedogenetic or karstification features. For these geologists, the accretionary lapilli is calcrete (caliche).

Impact spallation in nature and experiment

Spallation is a well-known process in fracture mechanics as well as in impact cratering and has been investigated theoretically and experimentally by many researchers. Unfortunately, it is less well known that spallation can also be observed in nature as an actually existing geologic phenomenon in and around impact structures. The present WEEKLY IMAGE shows already known spallation features in conglomerates around the Azuara/Rubielos de la Cérida impact structures (Spain), now recognized to form a 120 km long impact crater chain (see

–  KLICK here; and Ernstson, K., Schüssler, U., Claudin, F. & Ernstson, T. (2003): An Impact Crater Chain in Northern Spain. – Meteorite, 9/3, 35-39 – KLICK here), and prominent spallation fractures only recently observed in ejecta (Pelarda Fm. ejecta) from this crater chain.Spallation takes place when a compressive shock pulse impinges on a free surface or boundary of material with reduced impedance (= the product of density and sound velocity) where it is reflected as a rarefaction pulse. The reflected tensile stresses lead to detachment of a spall or series of spalls.Prominent spallation effects have been reported for shocked Buntsandstein conglomerates exposed around the Azuara/Rubielos de la Cérida impact structures. Details about these geologic spallation features have been described in Ernstson, K., Rampino, M.R., and Hiltl, M. (2001): Cratered cobbles in Triassic Buntsandstein conglomerates in northeastern Spain: An indicator of shock deformation in the vicinity of large impacts. Geology, 29, 11-14., and can be found here.

Image A. Subparallel open spallation
fractures in a shocked quartzite cobble
from Buntsandstein conglomerates.

Image B. Spallation crater in a shocked
quartzite cobble from Buntsandstein

Image C. Shock experiment on artificial conglomerate.

Images D

Images E
Images D, E. Concave spallation fracture surfaces in quartzite boulders from the Pelarda Fm. ejecta.
The Images A and B show typical shock-produced spallation features in these Buntsandstein quartzite cobbles: subparallel open spallation fractures (Image A) and a concave fracture surface forming a crater after the detachment of a lens-shaped spall (Image B). This concave spall fracture near a spherically shaped reflection surface is predicted by theory (and hardly explained by any other geologic process) and can be produced experimentally as shown in Image C.
The shock experiments were performed at the Fraunhofer Institute for High-Speed Dynamics (Ernst-Mach-Institut) in Freiburg, Germany. A single-stage powder gun was used to accelerate steel projectiles. As targets, we used two quartz spheres (rock crystal) in contact, embedded in a synthetic epoxy matrix. The shots were performed with impact velocities in the range of 25 to 115 m/s, corresponding to initial impact pressures between 0.55 and 2.5 GPa (5.5 and 25 kbar). The recovered samples were cut in half (see Image C, shot 3), thin sections were made, and the results of our observations were presented in Ernstson, Rampino, and Hiltl (see above). Here, the recovered sample of shot 3 at lowest impact velocity is shown (Image C) displaying a clear spallation fracture in the right-hand sphere otherwise untouched.
In the Ernstson/Rampino/Hiltl paper published by Geology (see above), the importance of such shock-deformed autochthonous conglomerates for an easy recognition of regional impact signature has been pointed out.
Here, we report on recent observations of prominent spallation fractures in quartzite boulders from the Azuara/Rubielos de la Cérida impact ejecta (Pelarda Fm.). The Image D shows a typically deformed boulder that displays a concave spall fracture surface being a mirror image of the convex surface (sketched as white broken line in Image D) of the detached (and now missing) large spall. A similar concave spallation fracture can be seen in Image E.
The quartzite boulders (mostly Cambrian Bámbola quartzite and Ordovician Armorican quartzite) contributed to the upper part (dominating molasse sediments) of the purely sedimentary target and, upon impact, experienced moderate to strong shock before excavation and ejection. The shock is documented by abundant multiple sets of PDFs in quartzite boulders (see, e.g., the sound PDF analysis made by Dr. Ann Therriault, in: Ernstson, K., Claudin, F., Schüssler, U. & Hradil, K. (2002): The mid-Tertiary Azuara and Rubielos de la Cérida paired impact structures (Spain). – Treb. Mus. Geol. Barcelona, 11, 5-65 – KLICK here, and on shockeffects ). We assume that the prominent spall fractures in the large quartzite boulders have originated also from the initial shock event, although a formation by collision of quartzite boulders during excavation and ejection must also be taken into consideration.