Shock Metamorphism - Remoe Sensing Application - Completely Remote Sensing, GPS, and GPS Tutorial
Shock Metamorphism

By far the best indicators of an impact event are preserved in the rocks that were close enough to ground zero to experience shock pressures of 20 to 500+ kb. A kilobar (kb) is the pressure produced by the weight of a thousand atmospheres, or about twice that exerted by water at the deepest ocean bottom. It's also equivalent to the weight effect of about 3 km (2 mi) of overlying rock; those pressures are usually static. Shock pressures are dynamic, with rapid, almost instantaneous rises in a state of compression as the shock wave passes. These pressures are greatly in excess of those that occur in upper crustal rocks from internal forces bringing about conventional metamorphism. The rocks undergo unique changes or alterations described as produced by a process termed shock metamorphism.

The features associated with increasing shock pressures are summarized in this table (from C. Koeberl):

With increasing pressures (and corresponding rises in temperatures resulting from "energy deposition" in the rocks associated with compression), the tendency is for individual minerals to undergo phase changes (into higher density forms) and then to melt, while a fraction of the rock experiencing the highest pressures is vaporized. Note that the post-shock densities of the still solid rocks decreases to the right.

A general plot of shock phenomena as functions of specific temperature (T) and pressure (P) appears here:

Pressure/Temperature conditions for shock metamorphism and conventional crustal metamorphism diagram (blue field).

Pressure and heat generated by the shock waves transform the crystal structures of individual minerals in spectacular ways. The common mineral quartz (which crystallizes in the hexagonal system) is perhaps the best recorder of shock-induced changes. Planar deformation features (see below) develop in quartz over a wide range of pressures, from 2 to 7 GPa (they also form in other minerals, such as the feldspars, but at somewhat different ranges). Under high pressure quartz transforms to a phase called coesite (crystallizing in the monoclinic system). At even higher pressures, another form of silica, SiO2, known as stishovite, (tetragonal system) occurs, although it may be unstable at high temperatures. The phase diagram (stability of a substance at various pressures and temperatures) for SiO2 is shown below, along with a photomicrograph of coesite.

Phase diagram for silica.

Coesite (central dark blue-gray) rimmed by multicrystalline quartz.

Rarely, diamonds (which on Earth are found in rocks once deeply located where pressures are high or in the laboratory using high pressure presses) have been found at a few impact craters (and in meteorites made up of highly shocked rock; see elsewhere on this page). Here is one from the Ries, made presumably from graphite in pre-impact target rock):

Diamond found in rock breccia at the Ries crater.

At even higher pressures, crystals may undergo atomic-structural displacements that convert them to glasses without passing through a melt stage. These diaplectic glasses usually retain their original shapes (e.g., grains), giving rise to forms known as thetomorphs (crystals that retain their original shape while having been converted to the glassy state). The photo below shows a small hand specimen of granite collected by the writer (NMS) from among the ejecta tossed out by the Sedan nuclear cratering explosion (100 kiloton device) within alluvium at the Nevada Test Site. This specimen contains only glass thetomorphs, in which the individual crystals (including the larger multi-sided phenocryst of feldspar) have remained intact without any melt-like internal flow.

A sample of granite (with a large phenocryst) that was converted entirely into a glassy state without any disruption of texture as high pressure shock waves passed through it during the Sedan nuclear cratering event.

Conversion of crystals to glass (discussed in more detail below) is a hallmark of the high degree of damage from strong shock waves. This photomicrograph shows a crystal of quartz converted to a near-isotropic state in a rock collected at the Azuara impact structure in Spain:

Nearly amorphous (isotropized) quartz.

Shock metamorphism is progressive, that is, the effects increase or change in style as shock pressures increase. This style change is evident in this series of X-ray spectrometer diffractograms, made from Cu K-alpha radiation on powder mounts of material, extracted from eight quartzite samples, which the writer collected as ejecta from the Sedan nuclear cratering explosion.

X-ray diffractograms of Sedan quartzites.

The peak pressures acting on each sample are unknown. I redrew the strip chart record for each sample by arranging the sequence shown from left to right in the order of increased shock damage, based on visual criteria under the microscope. Peaks near 20° , 27° , 36° , and 39° represent quartz reflection planes (crystal indices are on the right). Those peaks near 28° , 29° , and 31° associate with feldspars. The peak at 27° (101 plane) is especially sensitive to the degree of crystal structure integrity. As the level of shock damage increases, peak height diminishes as this structure undergoes progressive disorganization, beginning in the quartz with the development of microfractures (samples A-2 and 767-1) and proceeding to the diaplectic glass stage (samples A-8 and A-6), at which the crystal structure becomes extremely disordered.

We see shock metamorphic effects best in thin sections (thin slices of rock ground to a thickness of 0.03 mm) under a petrographic microscope. In the next series of illustrations, we present these features as photomicrographs. Most sections were viewed in cross-polarized light; PP indicates plane-polarized light.

At lower levels of shock pressures, the prime effect on minerals is to shatter them through cleavage and fracturing. Quartz can experience rhombohedral cleavage in non-impactites but it is rare. This cleavage has been observed in shocked rocks, as shown in this photomicrograph of a West Hawk Lake thin section:

Rhombohedral cleavage in quartz.

One unique change results from submicroscopic breakdown and slip along crystal planes that produce planar deformation features (PDFs). We show good examples of these features in quartz and feldspar - two very common rock-forming minerals - in thin sections under a petrographic microscope. In thin sections, these may appear as just narrow, usually straight lines (the intersection of the planar feature with the thin section) or they may seem broader because they are "decorated" (darkened by tiny bubbles). Here is an example of each:

Two sets of planar deformation features in a quartz grain from the Nordlinger Ries crater.
Decorated planar features (because of their orientation [0001) they have been called basal lamellae] in quartz from the Charlevoix crater.

Shown next on the top (PP) are decorated PDFs in quartz, within a granitic rock, recovered as core from the Manson structure that was studied by the writer in 1993. Shock damage may be so intensive that it induces a brown discoloration, called "toasting", as seen (bottom image, PP) in this cluster of quartz crystals (interpreted by the writer as caused by the shattering of a single crystal in this granite clast from Manson).

Color photomicrograph of decorated PDFs from the Manson structure.
Color photomicrograph of 'toasting' in quartz crystals from the Manson structure.

The writer first encountered planar features when I studied rocks involved in the Hardhat 5 kiloton explosion in a small granodiorite pluton near Rainier Mesa at the Nevada Test Site.

Multiple sets of undecorated PDFs in quartz abound within a sandstone (top image, PP), involved in the Sedan nuclear-cratering event. When hydrofluoric (HF) acid etches a slice of shocked rock, it selectively removes disordered silicate material within PDFs, leaving a gap. In the bottom image is a quartz grain from a Sedan sandstone, as examined at high magnification under an electron microscope, that confirms this removal, suggesting PDFs consist of disordered SiO2, converted to glass that is more susceptible to etching. Note that the PDFs are indeed remarkably planar.

Color photomicrograph of undecorated PDFs in sandstone from the Sedan nuclear cratering event.
Part of a Sedan quartz crystal that contains 7 sets of close-spaced PDFs.
Electron photomicrograph of a Sedan quartz grain in which the PDFs have been etched out by acid; shows their appearance at high magnification.

Planar features resulting from the passage of a shock wave have several distinctive orientations relative to the quartz axis. One such orientation is considered diagnostic since it rarely occurs in tectonites containing quartz, i.e., rocks stressed by mountain building processes; nor has it been found in quartz found in crystalline rocks such as granite and gneiss. This is the so-called omega feature, named for the crystallographic plane whose pole (line normal to the plane) is tilted 23.5 degrees with respect to the optical c-axis for quartz. (Omega is a Greek letter, seen in the next illustration; it looks like a "w") The Miller indices for the omega plane is {1013}, with second 1 being a negative (not showable on this screen). Here is a plot of orientations (made using a Universal stage) of PDFs found in a thin section cut from the drill core recovered at the Manson structure; although a range of orientations is observed, the maximum lies in the 20-25 degree interval, which denotes the omega feature; the second most common orientation is denoted by the Greek letter for 'pi'.

Orientations of planar features in quartz for the sample indicated.

Pi orientation (30-35 degrees) planar features resemble omega features; here are two sets of Pi features from a quartz crystal in a Clearwater Lakes core sample:

Pi PDFs in quartz.

The PDFs do not form in thetomorphs since those result from pressures above the range of PDF formation. But this PDF-bearing crystal from the Azuara structure is an exception. The photomicrograph in cross-polarized light shows part of the quartz grain to have become dark (glassy state) A conjecture: the rock containing it was heated after the breccia was emplaced, to the extent that the grain shown is becoming isotropized.

A quartz thetomorph with preserved PDFs.

Feldspars, being also tectosilicates, develop planar features as well. Here are PDFs in feldspar within a granodiorite fragment that was shocked during the Sedan cratering event;

PDFs in a plagioclase crystal in a granodiorite block in alluvium affected by the Sedan nuclear event.

In the next pair of photomicrographs, the top image is a single set of PDFs, arranged en echelon (slanted) in alternate twins (gray bands), within a soda-feldspar crystal in granitic rock, taken from Manson. In the bottom image, feldspar within a granite rock, at the Carswell Lake (Canada) impact structure, appears strongly "kinked" (these are also refered to as deformation bands):

Photomicrograph of feldspar twins in which one set has PDFs and the other has begun to convert to isotropic glass; from Manson impact structure.
Lenticular structure in feldspar, a form of kinking caused by shock deformation; from Carswell Lake impact structure.

The micaceous mineral biotite, which consists of very thin cleavages, stacked like pages in a book, also kink easily, as shown in the top image (PP) below, for a sample of granite, subjected to a nuclear explosion (the Hardhat event); the lower image shows more detail in another crystal.

Biotite in Hardhat granite.
Kinked biotite in a rock subjected to shock from a nuclear explosion.

Other mineral species experience one or more modes of shock-induced deformation. Below are photomicrographs, the first showing strain bands in an olivine crystal, the second showing lamellae formed in entatite (a pyroxene mineral); the third pictures shock-induced twinning in calcite:

Strain bands in olivine; implosion tube specimen.
Deformation lamellae in enstatite; implosion tube specimen.
Calcite twins in a crystal present in a West Hawk Lake core sample

As pressures enter the 40 GPa (400 kilobar) range, feldspar in a Manson granite began to melt, as shown in the bottom photo, by dark and gray flow bands, but the rock remains intact (the quartz is still crystalline).

Color photomicrograph of melted feldspar in a Manson granite.

Because of the writer's close association with a major study of Manson in the 1990s, I have decided to add a page, namely page 18-3a, that the reader can access optionally to learn more about this structure from a petrographic viewpoint.

At more extreme pressures, mineral grains may convert into glass without any change in their original shapes, i.e., the texture is preserved, while the composition changes from crystalline to glassy. We show these thetomorphs in a microscopic view (PP) for quartz grains in a sandstone rock, collected from around the Sedan nuclear crater at the Nevada Test Site. The SiO2 appears to have undergone incipient vaporization, as indicated by the occasional round vesicles.

Thetomorphs of quartz (grains convert to glass while retaining their shape) in a quartzite rock fragment shocked during the Sedan event.

Here is a pair of photomicrographs. The left shows Sedan quartzite in which all grains are now glass; the right image shows the same area of a thin section in which the petrographic microscope's polarizers have been engaged (Crossed Nicols), showing here that the quartz grains are all isotropic as expected from glass (glass under the microscope shows no birefringence and hence will be dark at all positions when the microscope stage is rotated 360°).

Sedan quartzite thetomorphs; grains in left photomicrograph retain their shape but are now entirely glass as viewed in plane polarized light; this confirmed under crossed-Nicols in the microscope in which all grains are dark, as characterizes isotropic glass.

At pressures within the 400-500 kilobar range, rocks melt as though severely heated (above about a half megabar, rocks start to vaporize). In this photomicrograph, a quartz thetomorphic grain has started to vesiculate (silica vaporizes).

Vesiculating quartz thetomorph.

In impactites, the melting quickly quenches into glass and may become singular masses mixed in the breccias, or as discrete layers near the bottom of the final crater. The hand specimens below are examples of breccia from several impact structures, starting with two from the Ries crater in Bavaria (called suevite, locally at the Ries crater but the term is now generic and applies to breccias at other impact structures).

Suevite breccia from the Ries crater.
Suevite from the Ries.
Suevite from the Chassenon structure.
Suevite from the Popigay impact structure.

In the top below. is a microscopic view (PP) of the breccia from the Ries crater that contains shock-melted rock (brown flow bands) and occluded fragments of quartz with PDFs. At the bottom is a photomicrograph that shows diaplectic glass of two compositions, derived from quartz and feldspar (converted to thetomorphs whose boundaries have been distorted by flow) in a Ries rock that was intensely shocked.

Quenched shock melt and a grain of quartz with PDFs; from the Ries crater of Bavaria.
Quartz and feldspar converted to glass.

At some impact structures, fractures opened beyond the true crater walls as the event proceeded were filled with injected fragments and melt. Two examples from the Rubielos de la Cerida impact structure in Spain illustrate typical fracture-filling veins;

Several filled fractures (dark brown) at the Rubielos de la Cerida structure.
Breccia filled vein at the Rubielos de la Cerida structure.

An impact-injected vein whose filling includes shock melt is called pseudotachylite. Here is an example from the Vredefort structure, in which the veins shown are at hand specimen size:

Pseudotachylite vein in the country rock just outside of the Vredefort structure.

The melts within large impact craters are commonly mixed with fragments of shocked rock. They can, however, be fairly uniform in texture and could be mistaken for volcanic flow rock, except they usually contain some fragments that display shock effects. Here are two examples

Impact melt from the Tenoumer structure; it has recrystallized; note quartz inclusion which retains faint PDFs..
Recrystallized quartz and feldspar from a highly shocked Clearwater Lake sample.

The next three show hand specimens of impact melt:

Impact melt.
Impact melt.
Impact melt from the Chicxulub structure.

The next photomicrograph shows a melt from the Manicouagan (Quebec) crater, whose composition is close to that of feldspar, in which new crystals of feldspar have grown in place rapidly as the melt quenched.

Shocked feldspar that has recrystallized; Manicouagan crater in Canada.

Thetomorphs and the types of PDFs shown above occur in nature only within rocks involved in structures that have at least some of the characteristics of impact craters. They also readily form in rocks surrounding nuclear explosions, where instruments directly measure pressures in hundreds of kilobars. And, we can make them experimentally in the laboratory using controlled explosions to create these pressure ranges, such as in the implosion tube method invented by the writer (see below). They are not present as such in breccia rocks associated with volcanic explosions, where pressures rarely exceed 10 kb. As the writer has already stated, their presence in quartz and/or feldspar is decisive proof of an impact event as the cause of a deformed structure.

The writer, while at Lawrence Livermore Laboratory, took advantage of the facilities at Site 300 which the physicists used to conduct shock experiments on materials. One was a 16-inch cannon barrel and loading chamber obtained from a decommissioned battleship. On several occasions I supervised the firing of flat-nosed shells at rock targets (in the enclosed housing), recovered the samples, and studied the shock effects. Here is that cannon.

16-inch cannon facility for sending projectiles against targets enclosed in rigid containers and held in sand within the housing on the left.

The rocks hit by these shells only showed one distinct shock effect: Quartz and feldspar crystals were invariably fractured, even shattered.

Fractured tectosilicates in a cannonized granodiorite.

In 1964, the writer invented a new way to shock rocks. Using some of the principles by which nuclear devices are detonated, I designed the implosion tube method, whose setup is shown here prior to detonation.

Field set up of the implosion tube experimental method for shocking materials.

The small tube, shown on the right, is made of brass or steel. It is hollowed in the center. Into this are fitted either cored rock or mineral samples, or loose sand made up of such samples. The tube is welded shut. It is then placed with positioners along the central axis of a large, 4-inch diameter, aluminum tube (left) that is filled with a liquid explosive. Upon detonation, the cylinder is blown apart but shock waves also move inward, squeezing (imploding) the sample container which is recovered. Samples inside experience shock over a range of peak pressures up to 500 kilobars. The samples are released from the tube by sawing or trim-cutting on a lathe, and then made into thin sections for examination under a petrographic microscope. Those containing quartz grains or crystals usually display the same kinds of planar deformation features found in impactites. This next picture is a photomicrograph of planar deformation features developed at an estimated 20 GPa (200 kilobars) in a sample of unshocked sandstone placed in an implosion tube.

Planar features in shock-imploded sandstone.

PDFs also formed in feldspar crystals in a granodiorite shocked in an implosion tube:

Deformation lamellae in feldspar shocked in an implosion tube; the mottling is an artifact caused by the scanning procedure.

An interesting effect developed in one sandstone mounted in the implosion tube. Quartz progressively experienced increasing shock damage in grains from the outer edge of the sample (fracturing), through to the interior (PDFs and then isotropization at the center). This photomicrograph shows part of the gradient variation:

Progressive shock damage (increasing to the left) in quartz grains within a sandstone placed in an implosion tube.

Of particular interest is the progressive shock metamorphism of a basalt flow unit that was the site of a small nuclear cratering explosion (Danny Boy event) at the Nevada Test Site. Samples from that event showed mainly fracturing, whose numbers per unit area of thin sections made from samples recovered at progressively closer distances to the crater wall increased as the wall was reached. This is a typical view of fractured plagioclase in the basalt.

Danny Boy basalt with more fractures than unshocked basalt from the nuclear cratering explosion site.

Of more interest is the behavior of basalt at high shock pressures. Samples of the basalt were included in several implosion tube experiments. Below are 4 photomicrographs. In the top two are uncrossed (left) and crossed (right) Nicols views of a thin section of basalt that was shocked to an estimated 300 kilobars. One can see that all the light toned plagioclase laths are converted to glass thetomorphs (Feldspar glass produced by shock pressures is found in meteorites and is known as Maskelynite). As pressures increase (perhaps as high as 400 kilobars), the plagioclase begins to melt (bottom left) and the rock starts to vesiculate. Full melting of the basalt in the implosion tube (also shown in plane polarized light) appears in the bottom right. It was this work that probably led to the writer's (NMS) selection as a lunar samples investigator because the Apollo 11 site was predicted to be basalt that had experience repeated impacts.

Microscope views of strongly shocked Danny Boy basalt; left - in plane polarized light, in which the light-colored plagioclase retains its shape; right - under crossed-Nichols, the basalt plagioclase becomes isotropic (glass)
Progressive partial to full melting of Danny Boy basalt, shocked to peak pressures ~400-500 kilobars in an implosion tube experiment; both views in bright field light.

Over the course of a year (1963-64), the writer used this new technique to shock more than 30 rock types, some 25 different minerals, and several mixes of sand (the olivine and enstatite examples on this page were from the implosion tube experiment). Most of the phenomena found in rocks from impact structures were reproduced by this technique (including, at that time, the only known example of calcite glass).

Instant rock, formed by shock lithification, was discussed in connection with the material around the Arabian Wabar crater (page 18-2). Shock lithification has affected much of the lunar surface rubble (regolith and eject deposits). A sample containing loose quartz grains was converted by implosion to a "sandstone" much like Wabar rocks. Another sample containing loose grains of olivine and plagioclase was included in an implosion tube. The implosion converted it to a shock-lithified analog to a basaltic rock, as shown here:

Shock-lithified 'sandstone' made in the implosion tube; the superposed pattern is an artifact.
Shock-lithified grains of feldspar and olivine.

The writer's work on shock processes made a serendipitous contribution to a controversy raging in the 1960s regarding "mysterious" pebbles found widely scattered in Asia, Europe, Africa, and North America. These are called tektites, small pieces of black, obsidianlike glass in elongate irregular to "leather button" shapes. Below are some typical examples:

Tektites.

The composition of most tektites ranges from about 60 to 80% SiO2. No minerals occur within tektites but melted quartz glass (Lechetelierite) smeared out by flow is rarely present. Tektites are totally devoid of water. The button shape in particular is consistent with an aerodynamic reshaping as though the glass was initially molten, rapidly cooled, and surficially remelted upon passing through the atmosphere.

Then, and more so now, the majority opinion considers tektites to be formed by the high pressure range developed during impacts on terrestrial rocks. At those pressures the rock target would be molten and the blobs of melt tossed far from the crater into, then out, and then back into the atmosphere, falling land or sea (there, the equivalent of the tektites are microtektites, in tiny sphere shapes) scattered over wide areas. Researchers now believe they have found the source impact craters for some of the tektite strewnfields: Bosumtwi crater in Africa; Australian crater(s) for the Indochinites; the Ries crater for the Moldavites (Czech Republic). The American tektites (Georgia) have not been tied to any known crater (the Chesapeake crater is now a possibility).

There has been a small group of researchers with a "minority" opinion as to tektite origin. Led by the late Dr. John O'Keefe of Goddard Space Flight Center, these individuals considered tektites to come from silicic volcanoes on the Moon. However, no volcanic structures of the silicic type have been found there and the composition of lunar rocks does not fit that of tektites. However, a strong argument made lunar tektite advocates is the total absence of water (almost all moonrocks are anhydrous). Since most terrestrial rock contain varying amount of water, some should be present if the tektites derive from rocks on Earth.

The writer (NMS) believes he has put his finger on the answer to the water quandry. During my first year at Lawrence Livermore Laboratory, I studied the glasses found in the base of the cavity produced by the world's first underground nuclear explosion, codenamed "Rainier", which took place in 1957 in volcanic tuff (containing up to 17% water) at the Nevada Test Site. Three types of glass are present: 1) pumicelike, whitish and vesicular, with moderate water content; 2) obsidianlike, black, dense glass, with less water, and 3) a pinkish dense glass totally devoid of detectable water, found injected into cracks opened up by the explosion. The first two types are shown in this view of the crater base:

Glasses similar to pumice (light-toned blocks) and obsidian (black material) at the base of the Rainier cavity.

The third type is most like tektites (except for color, an orangy-pink caused by finely dispersed copper from the wiring in the nuclear device). The most potent technique for water detection failed to find any in this glass which was derived from the tuff nearest the nuclear device and hence subject to the highest pressure. My conclusion was that at very high temperatures (at which some vaporization may occur) the shock-induced melting also completely removes any initial water by some separation process yet unspecified. This observation would seem to extrapolate to tektites, as I have proposed. Despite this evidence, Dr. O'Keefe and his supporter, Dr. Harold Urey, never gave up their lunar origin hypothesis for tektites.

While tektites are almost certainly products of a terrestrial shock event, many extraterrestrial meteorites show some of the shock features discussed on this page. Some of these meteorites are breccias; others are single rock types. Since it is generally believed that at least some meteorites come from the Moon and Mars, those so postulated were probably pushed off their parent body by impact events. Here are two meteorites that are brecciated or otherwise show some signs of shock metamorphism.

he Chergach meteorite, possibly from the Moon.
The Dhohar meteorite.

As you will see in Section 19, which considers the planets in the Solar System, rocks from the Moon collected during the Apollo missions include breccias that were lithified (some by shock?) or are mare basalts that were shocked during impacts. Here are two examples from the Apollo 16 Cayley site:

Apollo 16 breccia
Apollo 16 breccia with melt.

Shock effects in lunar samples, which the writer studied as an Apollo investigator, are shown on page 19-6.

We have now presented convincing evidence that impact craters exist and can be properly identified by their characteristic shock effects. We will close this Section with examples of impact craters that have been imaged by remote sensing methods; several were discovered using remote sensing imagery.

Source: http://rst.gsfc.nasa.gov