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

Pressure/Temperature conditions for shock metamorphism and conventional crustal metamorphism diagram.

18-9: 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 field (as facies) 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 among the ejecta from 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.

Color photograph example of a specimen of granite from the Sedan nuclear cratering explosion.

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, collected as ejecta from the Sedan nuclear cratering explosion.

X-ray diffractograms of Sedan quartzites.

The peak pressures acting on each sample are unknown. We 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 we view the sample 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. 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.

18-10: 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 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 microscope image of quartz grain from a Sedan sandstone.

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 right image, feldspar within a granite rock, at the Carswell Lake (Canada) impact structure, appears strongly "kinked" (we also refer to these as deformation lamellae):

Electron microscope image of quartz grain from a Sedan sandstone. Color photomicrograph of feldspar within a granite at the Carswell Lake (Canada) 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 left image below, for a sample of granite, subjected to a nuclear explosion (PP). As pressures enter the 400 kilobar range, feldspar in a Manson granite began to melt, as shown in the right photo, by dark and gray flow bands, but the rock remains intact (the quartz is still crystalline).

Color photomicrograph of granite subjected to a nuclear explosion. Color photomicrograph of melted feldspar in a Manson granite.

18-11: 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.

Color photomicrograph of grains of quartz in sandstone from the Sedan nuclear crater.

At pressures within the 400-500 kilobar range, rocks melt as though severely heated (above about a half megabar, rocks start to vaporize). 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 left 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 right 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.

Color photomicrograph of the breccia from the Ries crater in Bavaria. Color photomicrograph of melted fledspar from the Manicouagan crater in Quebec.

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.

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Primary Author: Nicholas M. Short, Sr. email:

Collaborators: Code 935 NASA GSFC, GST, USAF Academy
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