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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.
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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. Using mainly a microscope to examine thin sections
made from these rocks, I discovered many phenomena that were not
described in the literature (some results of similar effects in
impactites had been published but in journals not known/accessible to
me). Then, I attended a Conference on Cratering at the New York
Academy of Sciences in 1964, presenting the results of my studies of
nuclear explosion-induced shock features in granite, basalt, and
quartzites, which proved of great interest to attendees who were
studying impact craters. I likewise 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.
I realized that the impact cratering process generated huge pressures
of the same order as calculated from the nuclear explosion process. My
contribution to the field was thus to provide direct experimental
evidence that impact sites had indeed 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). In effect, I provided the
"missing link". To reiterate - since the pressures calculated (but
obviously not actually measured) 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
events such as impacts (no other mode of origin has proved plausible;
this conclusion must stand until geologists find another process that
reaches such pressures by purely terrestrial mechanism[s] - pressures
of that magnitude exist deep within the Earth but rocks from such
depths never can reach the surface).
As a bit of serendipity, I met Dr. Bevan French at that Conference. In
1967, we two happened to sit together on a field trip that was
visiting impact structures in Missouri. We began to discuss the need
for bringing the researchers into impact cratering and nuclear
explosion effects together in some type of meeting. Thus was born the
eventual Conference on Shock Metamorphism of Natural Materials, which
in turn led to grouping the papers presented into a Book by the same
name, published in 1968, which has remained the reference "bible" for
this new field.
The Shock Metamorphism book cover.
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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).
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
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,
SiO[2], 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 SiO[2] 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.
18-10: How many different sets of PDFs (i.e., different orientations)
can you discern in the upper Manson photomicrograph? ANSWER
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 SiO[2], 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.
18-11: Visually, what do the biotite kinks remind you of that you have
seen before in this Tutorial? ANSWER
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 SiO[2] 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% SiO[]2.
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.
navigation image map
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Primary Author: Nicholas M. Short, Sr.