by Mark Bostick
Impactite, The Meteorite Children
I've alway's like the beginning of the Bruce Willis movie, Armegeddon.
Our view is racing around the moon to see a mostly blue planet, half lit by the sun. "This is the Earth the time the dinosaur roamed a lush and fertile planet." A large meteoroid enters the screen. "A piece of rock six miles wide changed all that." Large meteoroid becomes a large meteorite. "It hit with a force of ten thousand nucluar weapons. A trillion tons of dirt and rock hurtled into the atmosphere." The impact mushrooms out into a large fireball the leaves our atmosphere before it is done standing. "Creating a suffocating blanketing dust the sun was powerless to penetrate for a thousand years. It happened before, it will happen again. Its just a question of when." Our view now swings to the black of the dark side of the planet. If you know the word "meteoritonics", you can probably imagine a few of the events that would happen next. The resulting impact, besides killing the majority of life on the planet, created a lot of impact altered rock, known in the meteorite world as impactite.
Meteorite Hunter Pioneer, Harvey Nininger, created the rules for hunting meteorites. One of the first rules for finding meteorites was to go where meteorites have already been found. A simple rule, but one that history shows was always overlooked. Overlooked until currant meteorite hunters (with the help of currant tools and currant transportation) took up the ropes. No longer do stewn fields weather away uninterested by man. Most now have been hunted, rehunted, hunted again and in several cases....still being hunted. But it is not only the meteorites that drive the collecting hobby, but the meteorite impactite as well. Go where meteorites have fallen. yes. Go where meteorites have hit with such force that it has altered the Earth, you know it.
Meteorite Impactite, which I term loosely as earth rock altered by a meteorite impact, has interested me as much as the meteorites themselves. Although obscured by the natural erosion system of the Earth, evidence of meteorite impacts can be found around the world. Impact created minerals, partially melted breccia, impact craters and shatter cones are tell-tell signs of the meeting of our planet with a meteoroid. The types of impactite as as diverse as the meteorites themselves.
MELTS AND GLASSES
Tektites have been a source of controversy for many years now. Many articles have been written on the subject and most people now believe the impact origin, so I will ignore most of the more common (although very interesting) tektites found in Australia and South Asia. Many great writings on the origin of tektites and formation can be found on the 'net. This leaves for me some of the other glass impactites. Created in seconds under thousands of degrees of temperature and pressure, impact melts are usually a combination of several target rocks. (Target rocks are surface rocks that were directly hit with an impactor). Traces of the original impactor can sometimes be found trapped in its matrix. Melts are uniform in their composition yet variable in texture.
Darwin glass is found around the remains of a one kilometer diameter crater southeast of Mt. Darwin in Tasmania, Australia. It is typically grayish-green in color and often translucent. Originally, Darwin Glass was thought to be a tektite. Now, it is generally recognized now as a meteorite crater glass. Crater glasses have a rougher appearence over tektite glasses and are generally more porous. The photo below shows several pieces of Darwin Glass. (Clockwise from the upper-left corner) The two small ones on top are very typical of Darwin Glass. The first one is thin a very transparent when held to the light, while the second has a shape I have seen the most common. It reminds me of a tree branch. The Darwin Glass in the upper-right corner is very porous and light in color. Weathering can probably be blamed for its loss of color. The Darwin Glass below it is very dark in comparison and has a smooth skin. The largest one in the photo has a shape that reminds me of a mummy skull, the look of strips wrapped around this piece can be found on many specimens from this site. This proves meteorite collectors see things in rocks other then meteorites, and supports the crater belief over the old tektite theory.
Irghizite glass are small impact created glasses, usually under an inch long. Discovered at the 13.5 km-wide Zhamanshin crater, Kazakhstan The elements Ni, Co. and Cr occur in irghizites at levels so much higher then the local rock that impact origin is without doubt. These glasses were formed as flows and are often vesiculated. Examination of vesicles has found the presence of organosilane and organosiloxane, compounds never before found in nature. Irghizites come in all different shapes, the most common I have seen are what appear to me to be broken strips of the melted glass flow (imagine tearing the small piece of a Trizler Liqorize strip. Ones that remind me of snakes with big heads, sometimes twisted sometimes not and then a splatter like shape. The spatter like shape is often smoother on one side then the other.
Tektite with quartz
Okay, I couldn't resist not including at least one South Asia tektite. It is believe that for the mass of tektites from South Asia to have been created from a meteorite impact that it would have had to collided with a quartz bed. Shocked and unshocked quartz has been found in these tektites. This tektite is from Guang Dong, China. It weighs 160.3 grams and has a quartz inclusion.
KofelsiteIt is believed by many that about 12,000 years ago a meteorite slammed into Otztal, near Kofels in Austria, creating a crater 3-4 km across. One of the many minerals created by this impact was Kofelsite. Also known as Koefelsite. Kofelsite is very heterogeneous, ranging in color from white to dark brown. Shock-metamorphosed mineral fragments and shock melted glass are found in veins around the crater. The origin of Kofelsite has however, been one of debate. Some believe in the meteorite impact, some a landslides, and others say a landslide created by the meteorite impact. The specimen pictured above is 120.0 grams and is typical grey colored. Broken faces on this piece show the flow lines created while this mineral hardened.
Ries Melt Bomb
This glass melt down was created during the Ries impact, which will be discussed more below). A scattered glass bomb that indicates temperatures upwards of 2000°C (Horn 1972). Most have very prominent longitudinal skin wrinkles. These bombs were thrown through the air and many have aerodynamic shapes. The glass of the Melt Bombs do not weather as much as the Suevite they are found in. So over time nature carves them out of the Suevite leaving them neatly on the surface for us collectors. Melt Bombs are called "fladden" by local geologists. ("Fladden" is German for a flat, roundish shape). This "Fladden" weighs 177.6 grams, over 3 1/2 inches long and over 2 inches wide it is big for a tektite, and average size for a melt bomb.
The Aouelloul crater is located 45 km southwest of Chinguetti, Mauritania. Discovered by air in 1951, it is 250m in diameter and about 11m in depth. A large amount of fused silica glass has been found in the area, but only one small meteorite fragment has been recovered from the crater. Most of the Aouelloul Glass pieces are under 5 grams and around a centimeter in size. The Glass was formed almost completely out of limestone. The Aouelloul Glass in the photo about weighs 21.98 grams and is the second largest piece known, It makes me think of something you might get if you put Darwin Glass and Monturaqui Impactite together into one stone....Aouelloul Glass. OK, Maybe not.
Libyan Desert Glass is one of the most popular impactites amoung collectors, when held up to the light, it usually reveals internal layering of bubbles and smoky, white, brown and sometimes colorfull curtains of fine particulate matter. It is believed to be a melt sheet formed by meteoritic impact around 28 millions years ago. Found widely scattered over an area 130 km by 53 km in a remote part of the Libyan desert, (Doesn't always seem to be a hard to reach place?). Most pieces are recovered from sand free corridors between sand dunes. Wind driven desert sands sculpts, shapes and polishes fragments into strikingly beautiful and unique specimens. Prized pieces exhibit pseudo-thumbprints, fluting pitting and smoothly polished surfaces. The Libyan Glass above is 151.3 grams and is a broken fragment. A white cloud can be seen inside its yellow-green matrix.
Sudbury Meltsheet slice
About 1.87 billion years ago, an object from space 9 kilometers in diameter slammed into Canada, a little north of Sudbury, Ontario creating one of the largest impact structures on the earth. An impact so big it cracked the earth, in fact, they still have earthquakes because of it! My favorite of all the Sudbury impactites is the meltsheets. This was the closest rock to ground zero and the most drastically altered. Anything closer then this meltrock was either blown to bits, pulverized, or vaporized! Most of the slices made from these rocks clearly show the "streaks" and/or "pools" that the rock formed as it cooled. Some slices have small spots of nickel positive metal in them. This is most likely the original impactor.
The slice above weighs 638 grams. It measures 21cm x 17cm x 1cm. Parallel lines of different melted county rock can be seen all the way across this slice.
Sudbury Melt Enclosure
The photo above is from a Sudbury Melt sheet slice from my collection. Lots of metal is speckled throughout this slice, mostly around the edges. A much larger percent of metal then is typically found in this glass impactite. However, what holds my interest is the oval round enclosure almost in the middle. This appears to be a rock that was injected late in the impact, shortly after the heat wave melted the target county rock. Metal can be seen around the edges and in the fracture like lines in the enclosure.
The Dellen impact structure is a terrestrial, deeply eroded, complex impact structure, situated about 300 km North-Northwest of Stockholm, Sweden. The target rocks are homogeneous (they were heated pretty even when being formed) and consist of an intermediate, granodioritic granite composed of quartz, potassium feldspar and biotite. The specimen in the photo above weighs 251 grams and measures 10 cm by 8cm.
In 1931, stories about iron rocks around craters convinced R. A. Alderman, from the University of Australia, to investigate the remote site 8 miles southwest of a town named Henbury in the Northern Territory of Australia. What a site he found. 13 impact and explosion craters! The largest crater measures 160 meters and the smallest 6 meters, all grouped nice together in a classic strewn field ellipse. Thousands of small irons were also found around the craters. This is a piece of one of the craters. Notice the burnt black coating and the shattercone like like. These lines were probably created for sheer force during the crater process that made the holes rather then by soundwaves that create shattercones. A close look at the rock shows fine lines less then 1mm wide and across the whole stone. Specimen weighs 191.9 grams. While this May not classified as a melt. It appears to have been affected by the heat blast.
SHATTERCONES, A SOUND FOSSIL
Shattercones, are an impact rock with a very distinctively patterned structure. Often conically-shaped, these rocks have close-spaced "grooves" or "lines" that radiate outward from the apex. The apex, or point, of the stone points towards the impact area. Think of these as a sound fossil. An Earth rock that has recorded the sound wave of the a large object from outer space. A result of the high pressure and high velocity shock wave produced from an impacting meteorite. The mechanics that produce shattercones are poorly understood, and only the largest mining explosions and nuclear tests generate enough heat and pressure to even slightly mimic shatter cone formation.
Here I am holding a Beaver Head Shattercone from an Impact Site that it is in both Montana and Idaho, USA. There is some disagreement among various geologists as to which formation these belong in. Most put the host rocks in the Gunsight Formation which is the youngest member of the Lemhi Group. The Gunsight is very late Proterozoic to early Cambrian. That puts a maximum age on the impact of 800-1,000 million years. We don't have a very good minimum age yet. Some have pegged the minimum at 500 million years. More shattercones have been discovered in similarly aged rocks (perhaps the same formation?) at two very distant locations in Idaho. These new shatter cones cover a large area and have not been reported in literature. They are likely part of the Beaverhead impact which is projected to have a diameter of 75-100 km and which landed in east central Idaho. Some of the rocks containing the Beaverhead shatter cones were thrust into Montana later. The shattercone above is 5508 grams or 12lbs 3.4oz. It measures about 11 inches high, 6 inches wide, and 5 inches in depth. Beaver Head is considered by many the best place in the world to collect shattercones. This nice 3 dimensional cone shows why.
THE BRECCIA's (OR FOR MY FRENCH FRIENDS, THE BRECCHIA'S
Suevite (pronounced SWAY-vite) or suevite breccia is a polymict impact breccia with clastic matrix and mineral clasts in various stages of shock metamorphism. Or in layman terms, its a rocks filled with other rocks inside it. Usually including impact melt particles which are in a glassy or in a crystallized state. This brecciation makes Suevite one of the more interesting impactites.
Ries Otting Suevite
About 14.7 million years old an the impact of a large meteorite or comet nucleus with the Earth’s surface at Nordlinger Ries, Germany, Czechoslovakia in the Moldau Valley crated a crater and this suevite breccia. This is the impact that also created the gem green moldavite tektites. In the Otting Quarry Suevite you can see looks of glass bombs (black), crystalline fragments (speckled) and fragments of Malmain limestone (light grey). Most of the fragments embedded in suevite were changed by the influence of heat. This suevite resembles the regolith that covers the lunar surface. The origin of the Ries site was a debate for hundreds of years, until July 1960, when Shoemaker came to the Ries site in search for coesite, an impact created mineral. In a few days the samples he collected tested positive and the mystery solved. The slice above weighs 363.3 grams. Ries have a very consistent look.
Ries impact created 14 known types of Suevite. The Otting the most common. One of the more rare and colorful types is (or was) found at the Polsingen Quarry. This location has been closed from collecting for some time now. The red color comes from the inclusion of hematite (red iron ore). This Suevite contains no glass and is highly vesicular, although very solid. It is believed to be the crystalline melt from the center of the crater. The Slice above weighs 277 grams. While I have labeled it as a Suevite here, since it is commonly called such, the Polsingen variety is rather an impact melt rock and not a suevite.
The Boxhole Crater is almost a perfectly circle,175 meters across. Recognized inJune 1937 by Boxhole Railroad Station in North Territory, Australia. The rim rises 10-17 meters above the crater floor, depending on which side you are referring to. The impact happened about 30,000 years ago and left thousands of small meteorite irons. This is a slice of Breccia from the crater. Notice how one side seems almost unaffected while the other side is very brecciated. This slice weighs 89.5 grams.
The impact at Sudbury created a nuclear bomb like mushroom cloud. Onaping is a lithified fallback breccia composed of melt glass and country target rock fragments from this mushroom cloud.An ejecta blanket of debris hundreds of yards (meters) deep. "Bucky Ball Fullerine" with terrestrial carbon in the form of C-60 and C-70 is found in this type of Sudbury impact rock. Greenish-Blueish dust covered Tektites can also be found within its matrix.
Sudbury Metal Rich
Here is another and more common meteorite impact created specimen from the Sudbury Astrobleme Impact in Canada. A very nice piece of what I could be referred to as a metal rich breccia This is came from the ring of the impact crater and is part of a network of metallic and silicate minerals that has been mined for many years. Basically speaking all Sudbury metal-rich breccias and sulfides such as this one contain some amount of iridium, cobalt and selenium. In addition, most of these specimens will have nickel, iron, sperrylite and pentlandite. This slice weighs 653 grams and has an unpolished face.