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Before much was known about impact craters, and their mechanics of formation, most geologists considered terrestrial craters since proved to be impact to have a volcanic or some other purely terrestrial origin. But, the pressures involved in any known endogenic process that occurs in the upper crust were quite low. Pressures produced in underground nuclear explosions (or generated in the laboratory by firing high speed missiles at rocks) as determined by calculations and direct measurements proved to be very much higher than those associated with volcanism, mountain building, etc. The rocks affected by these explosions or firings had unique effects in their minerals. Rocks found in impact structures had the same effects and many peculiarities not observed in any other kind of crater. It was concluded that impacts and explosions alter the rocks they act on by what is now known as shock metamorphism. That is the topic of this page. Its applicability to the origin of the black glass tektites is considered.





Note: The writer (NMS) is one of the first geoscientists to work in and develop the specialty field of shock metamorphism. This happened serendipitously. While working for the Atomic Energy Commission at the Lawrence Livermore Laboratory (California), in 1960 I became inquisitive about the effects of nuclear explosions on the rocks surrounding them. I discovered many phenomena that were not described in the literature. Then, I attended a Conference on Cratering at the New York Academy of Sciences in 1964 and heard/saw several papers describing effects on rocks found at supposed impact craters. These were the same features as I had noted in the nuclear explosion rocks. These explosions generated huge pressures of the same order as calculated from impact. My contribution to the field was thus to demonstrate that impact sites had experienced great pressures (up to one half to one megabar) beyond levels known from any other terrestrial near-surface process (volcanic explosions produce overpressures of less than 50 kilobars). Since the pressures calculated for shock metamorphism imposed on the impact rocks corresponded to that observed in nuclear explosion rocks on which the causative pressures (hundreds of kilobars) had been directly measured by instruments emplaced around the nuclear explosions, the argument is strong that structures containing these shock effects can only have been produced by high pressure-generating impacts (this conclusion must stand until geologists find another way to reach such pressures by purely terrestrial processes).


Shock Metamorphism

By far we find the best indicators of an impact event 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, whereas shock pressures are dynamic, with rapid, almost instantaneous rises 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 shock metamorphism. These changes can be grouped into stages of increasing shock metamorphism, as displayed in this diagram:

Changes in rocks subjected to a range of shock pressures, presented as grades or levels.

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, with a fraction of the rock experiencing the highest pressures to vaporize. 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).

18-6: What is the lower limit of shock pressures at which some physical change of state (including phase transformation of one mineral to another) defining a stage of shock metamorphism occurs; the upper limit(s)? ANSWER

We show the P-T diagram (as facies [phase] fields) for conventional metamorphism in the lower left. Pressure and heat generated by the shock waves transform the crystal structures of individual minerals in spectacular ways. The common mineral quartz, under high pressure transforms to a phase called coesite. At even higher pressures, another form of silica, SiO2, known as stishovite, occurs, although it may be unstable at high temperatures. Planar deformation features (see below) develop over a wide range of pressures.

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. 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 six-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.

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 other criteria. 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 11 illustrations, we present these features as photomicrographs. When the sample is viewed in plane-polarized (PP) light, we include the symbol PP in parentheses; otherwise, if there is no symbol, we are viewing the section in cross-polarized light.

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. Shown on the top (PP) are decorated (darkened by tiny bubbles) 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.

18-7: How many different sets of PDFs (i.e., different orientations) can you discern in the top photomicrograph above? ANSWER

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.

Electron photomicrograph of a quartz grain in which the PDFs have been etched out by acid; shows their appearance at high magnification.

In the next pair of photomicrographs, the top image is a single set of PDFs, arranged en echelon (slanted) in alternate twins, 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" (we also refer to these as deformation lamellae):

Photomicrograph of feldspar twins in which one set has PDFs and the other has begun to convert to isotropic glass.

Lenticular structure in feldspar, a form of kinking caused by shock deformation.

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 . As pressures enter the 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).

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

Color photomicrograph of melted feldspar in a Manson granite.

18-8: Visually, what do the biotite kinks remind you of that you have seen before in this Tutorial? ANSWER

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 369°.

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. On the top is a microscopic view (PP) of the breccia (called suevite, locally) from the Ries crater in Bavaria that contains shock-melted rock (brown flow bands) and occluded fragments of quartz with PDFs. In the bottom photo is a melt from the Manicouagan (Quebec) crater, whose composition is close to that of feldspar, in which crystals of feldspar have grown in place rapidly as the melt quenched.

Quenched shock melt and a grain of quartz with PDFs; from the Ries crater of Bavaria.

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. They are not present as such in breccia rocks, associated with volcanic explosions, where pressures rarely exceed 10 kb. Their presence 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 available to conduct shock experiments on materials. One was a 16-inch cannon barrel and loading chamber obtained from a decommissioned battleship. One several occasions I fired flat-nosed shells at rock targets (in the enclosed housing), recovered the samples, and studied the shock effects (mainly fracturing). 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.

At this time, the writer invented a new way to shock rocks. Using some of the principles by which atomic devices are detonated, he designed the implosion tube method, 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 cutting on a lathe, and then examined as thin sections under a petrographic microscope. This next picture is a photomicrograph of planar features developed at an estimated 200 kilobars as a sample of unshocked sandstone from the Nevada Test Site is imploded upon.

Planar features in shock-imploded sandstone.

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. 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. Most of the phenomena found in rocks from impact structures were reproduced by this technique (including the only known example of calcite glass).

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 ranged from about 60 to 60% SiO2. No minerals occur within tektites but melted quartz glass 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 found the answer to the water quandry. During his first year at Lawrence Livermore Laboratory, he 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, 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.

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Primary Author: Nicholas M. Short, Sr. email: [email protected]