Shock Metamorphism¶
Contents
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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.
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 and heard/saw several papers describing effects on rocks found at supposed impact craters. These were the same 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 beyond levels known from any other terrestrial near-surface process since the 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. QED.
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:
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:
` <>`__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 <Sect18_answers.html#18-9>`__
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.
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).
` <>`__18-10: How many different sets of PDFs (i.e., different orientations) can you discern in the top photomicrograph above? `ANSWER <Sect18_answers.html#18-10>`__
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.
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).
` <>`__18-11: Visually, what do the biotite kinks remind you of that you have seen before in this Tutorial? `ANSWER <Sect18_answers.html#18-11>`__
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.
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.
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.
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).