A BIG hOle in the middle of nowhere...




Where ?

On September 13 1996, whilst heading for Petrified Forest National Park, I briefly visited the first-suspected, best-preserved and most famous meteor crater on Earth. Since it became known the crater received several names : Franklin's Hole, Coon Butte, Coon Mountain, Barringer (Meteor(ite)) Crater (used in scientific literature), and Meteor Crater (7/1946).
The location (35° 1' 41" north, 111° 1' 21" west, altitude 1,561.5 m or 5,123 ft) is situated in northern Arizona (Coconino County), about 65 km (40 mi) east of Flagstaff and 40 km (25 mi) west of Winslow. Junction 233 (Meteor Crater Road) leads 8 km (5 mi) southwards to the crater from Interstate 40 (better known under its old name of Highway/Route 66).
Canyon (or Cañon) Diablo is a deep narrow canyon to its west.


Prehistory.

Between Mars and Jupiter lies the asteroid belt, an area where innumerable small crumbs and big icy boulders orbit the Sun.
Many orbits of comets overlap those of asteroids. Collisions among them send fragments into their own orbits, causing some of them to cross the Earth. Earth regularly crosses paths of (former) comet tails.
Every body in the Solar System (with a stiff and observable surface) that has been visited by spacecraft is pock-marked with the scars of impacts. Earth too has been and is still being bombarded from space. More than 150 impact sites are known on Earth ; of these, Meteor Crater is the youngest and largest in the past 50,000 years.
Due to friction (ablation) with Earth's atmosphere, the smallest intruders from space will never reach the surface.
In the course of time, volcanism, earthquakes and other tectonic phenomena, redeposition (sediments) and erosion (weathering) by heat, wind and water have obliterated most impact traces. The majority of the remaining large impact sites haven't been discovered yet as they must lie on the ocean-floor and are thus invisible and inaccessible.
The object that caused Meteor Crater presumably was a piece of a shattered asteroid. For aeons it wandered through the Solar System. Gravitational interactions with other Solar System members finally made its path cross with Earth's where it ended its journey 49,700 ± 850 years ago (1991)...


Meteoroid, meteor, meteorite.

The name of a potential invader changes along its track ; whilst in space it's called a 'meteoroid', during its fiery plunge through the atmosphere it becomes a 'meteor' (what laymen call a 'shooting star'), and the extraterrestrial leftovers after a fall or an impact are termed 'meteorites'.


Formation.

An iron meteor (or group of meteors) tens of meters (more than 150 ft) across and weighing several hundred thousand tons came hurrying along (10 - 20 km/s, 6 - 12 mi/s) from the northeastern sky as a very brilliant bolide (fireball) hitting a rocky high plateau in Arizona. This cataclysmic event left a more than 200 m (750 ft) deep and 1.2 km (4,000 ft) wide gaping wound in the surface of the Earth. The high impact velocity explains why this relatively small meteor caused such a substantial crater size.
Everything happened in just a few seconds.
The kinetic energy that was suddenly explosively released has been estimated from several to more than 20 million tons of T.N.T. ! It primarily converted into ultrahigh pressure and heat.
By comparison, the uranium bomb dropped on Hiroshima/Japan (I saw a full-scale replica of Little Boy at the National Atomic Museum on Kirtland Air Force Base - Albuquerque) had an explosive force of 'only' 15 ± 3 kiloton...
The tremendous pressures created unusual minerals and in some meteorite fragments small concentrations of graphite transformed into tiny diamonds.
The hypersonic velocity caused intense shock waves in the surrounding atmosphere, rocks and meteorite.
Within a radius of several km (mi) all living things were obliterated.
Several hundred million tons of limestone and sandstone turned into rubble and were ejected.
Huge blocks of rock (e.g. 'Whale Rock' on the west boulder field) were deposited onto the rim, the rest fell back in successive layers within several km (mi) from the point of impact.

A dense hot cloud ascended quickly carrying debris, molten rock and droplets of iron-nickel which rained down again. In 1946, meteorite hunter Harvey Harlow Nininger found a large area of tiny condensed metallic spheroids northeast of the crater. I saw them on a small dish at the Meteorite Museum (Institute of Meteoritics, University of New Mexico) in Albuquerque.
The cloud gradually dissipated.
During and immediately after the impact additional fragments that were broken off from the main mass during its passage through the atmosphere possibly came down as well (some are on display in the local museum).
The fall of the biggest chunks created secondary craterlets.
Only a small fraction of the original mass got lost from friction during its descent through the atmosphere. A sizable part of the iron meteorite was strewed across a wide area and the remainder evaporated, melted and fused with rock.
The heaviest fragment is the 638 kg (1,406 lb) Holsinger Meteorite, on display at the local museum. A magnet stuck onto a large piece I touched at the Steele Visitor Center of Lowell Observatory (Flagstaff) and I noticed other fragments at the Earth Sciences department of The Field Museum of Natural History in Chicago (Illinois). Many tons found their way to other museums or disappeared into private collections. Meteorites from this fall are still for sale.

In the course of time erosion has had relatively little impact on the crater, so there is still quite a lot of the original material left. To protect the scientific integrity of the site from an increasing number of visitors one is only allowed to walk one fourth of the rim along its northwestern part.
During the next centuries the crater filled up with sand and rocks. Today its depth still reaches about 175 m (570 ft). Its shape is not round but octagonal (circumference about 4 km (2.4 mi)). The peculiar shape probably resulted from the character of the pre-existing rock formations. The crater rim rises only a few tens of meters above the surrounding flat arid plain (the highest spot is Barringer Point on the west side rising about 39 m (127 ft)) and is barely visible from the access road.


Geology.

Part of the subsoil became exposed showing the different layers that were deposited in the course of millions of years.
Four distinctly coloured strata can be discerned : the lowest and oldest consists of greyish-white Coconino sandstone about 260 million years old (from the Permian period) making up most of the crater walls, next comes the Toroweap formation containing yellowish sandstone mixed with dolomite (limestone, CaMg(CO3)2), above this lies cream-coloured Kaibab sandstone deposited about 250 million years ago containing fossils of sea creatures (e.g. ammonites), and at the top of this geologic heap is 240 million years old dark reddish-brown Moenkopi sandstone.
Nearby a silica mine (SiO2) was in operation in the 1940's (the mineral was used for glass-making). The white scars of this activity are clearly visible on any aerial photo.


First explorers.

Native Americans (Navajos ?) were the first visitors.
An army scout was the first to report (1871) about the crater which was named after him : Franklin's Hole.
Local settlers thought it was just another extinct volcano (Sunset Crater Volcano (National Monument) is a well-known extinct volcano north of Flagstaff) and called it 'Coon Butte'.
In 1886 an Hispanic shepherd named Mathias Armijo discovered strangely formed metallic pieces west of the crater near Canyon Diablo and thought they were silver.
Analysis of samples revealed mostly iron, some nickel, a little cobalt and trace elements (including iridium).
Many tons of the stuff have been recovered since.
Meteorites from this fall are still named after the first find.
This discovery eventually led to the presumption that a meteor had caused the hole in the ground.
The first scientist known to have visited the site (1891) was chemist and mineralogist Arthur E. Foote. He collected many fragments and discovered tiny diamonds within some of them.
In October of that same year geologist Grove Karl Gilbert (U.S. Geological Survey) briefly visited the crater. He was the first to put forward the impact hypothesis but revoked the idea later.
Samuel F. Holsinger (U.S. Forestry Service) told Daniel Moreau Barringer about the crater in 1902 and discovered the largest single piece of the meteorite.


Daniel Moreau Barringer (1860-1929).

This mining engineer and lawyer from Philadelphia (Pennsylvania) was convinced about the impact theory and suspected that a huge body of iron and nickel lay buried somewhere within the crater. Extraction of this ore could become a very lucrative business.
He bought the land and together with partner Benjamin Chew Tilghman (a mathematician and physicist) they formed the Standard Iron Company, without seeing the crater first. Until his death he invested money and equipment into the project and from 1903 till 1929 (with breaks) undertook numerous but vainly attempts to detect a hypothetical large metallic mass beneath the crater (the scattered remains of all this activity are still visible).
Several drillings were performed at different locations but nothing of value was found, only small meteoritic fragments were uncovered (most of them near the northeastern side).
The high cost and ambiguous results of Barringer's efforts led investors in his company to seek re-examination of the premise of a large buried mass. Astronomer and ballistics expert Forest Ray Moulton made calculations about the mass of iron that could be expected. His conclusion was that the high energy of the impact would vaporize and fragment almost the entire mass (the idea was already suggested by geologist George Perkins Merrill in 1908)...
The land is still owned by the Barringer family and is nowadays exploited for tourism.
In 1921 his second son, D. M. 'Reau' Barringer Jr., discovered the next known meteor crater near Odessa (Texas, 31° 45' north, 102° 29' west, diameter 0.168 km (0.1044 mi), circa 50,000 year).

Barringer's name has also been attributed (1970) to a 68 km (42 mi) wide impact crater on the far side of the Moon (28° 0' south, 149° 42' west), near the northern edge of the Apollo crater basin (537 km or 334 mi in diameter).

This picture was taken 8/1967 by Lunar Orbiter 5 (8/1/1967-1/31/1968),
5,068.6 km (3,149.5 mi) above the lunar surface.

The some 30 km (18.7 mi) wide asteroid number 3693 (discovered 9/15/1982 by Edward Bowell, Anderson Mesa Station/Lowell Observatory, Arizona) also carries his name.
This interactive asteroid ephemerides calculator determines its position in the sky (map), given some time frame, for certain locations (observatories).


Minerals of the Canyon Diablo meteorites.

Bronze-coloured troilite nodule bordered by black graphite,
and surrounded by silvery schreibersite.

The mineralogy of Canyon Diablo meteorites depends on whether they have been shocked by impact.
The most common iron-nickel alloy is kamacite. A small fraction of the material is made up of the other iron-nickel constituents, the strongly magnetic taenite and plessite (not a mineral but a mixture of fine-grained kamacite and taenite).
Schreibersite ([Fe,Ni,Co]3P) is a very hard mineral.
Troilite (FeS) occurs as nodules or as elongated lenses. It may be mixed with graphite (C), chromite (FeCr2O4), daubreelite (FeCr2S4) or other metal sulphides.
Graphite occurs as large bodies within iron or in separate masses.
Cohenite ([Fe,Ni,Co]3C) is common. It's extremely hard but unstable at normal temperatures and pressures and will break down into kamacite and graphite (over a long period of time).
Strongly magnetic haxonite ([Fe,Ni,Co]23C6) and silicates are also found.
Bits that were subjected to greater shock show partial melting, recrystallization, fine closely parallel lines (Neumann lines, slipbands, glide-twin lamellae), and other deformation. Perhaps the most well-known shock effect is the transformation of graphite into diamond and lonsdaleite (a hexagonal polymorph of cubic diamond).
Traces of germanium, gallium and iridium are also present.


'Rosetta Stone of astrogeology'.

Meteor Crater has been extensively investigated.
In 1957 (1960 ?) Eugene Merle Shoemaker, Edward C. T. Chao, B. M. Madsen and David Milton discovered the new mineral coesite at the site, followed later by a second one, stishovite.
Coesite (density 2.911 g/cm3, or, specific gravity 2.911) and stishovite (4.289 (g/cm3)) are dense crystalline polymorphs of quartz (silica, SiO2) that can only be formed under extremely high pressures. Both minerals were previously artificially created in labs (coesite in 1953 by Loring Coes, stishovite in 1961) and during nuclear explosions.
In 1963, E. M. Shoemaker demonstrated that nuclear craters were structurally similar to Meteor Crater in nearly all respects ; the sequence of layers of the ejecta blanket are deposited in reverse order to the underlying rock layers.
These finds provided irrefutable evidence for the impact hypothesis.
Both minerals were later also found at other possible impact sites (astroblemes, 'star wounds') and are since an important clue for unveiling their origin.
These and other discoveries have led some to call Meteor Crater the 'Rosetta Stone of astrogeology' (the real Rosetta Stone is a black basalt monolith discovered in 1799 at Rosetta near Rashid and the mouth of the river Nile containing several ancient writings that provided the basis for unlocking the mysteries of hieroglyphics).


Playground for Moon travellers and screen-actors.

Because of its similarity to the Moon N.A.S.A. selected the spot (1963 - 1971) as a training-ground for the Apollo astronauts. At the site the Moon travellers received a crash course in geology and meteoritics and trained, wearing their space suits, for their historic journeys to the Moon.
Signs along the road, the empty shell of an Apollo test capsule, and the obligatory Astronaut Hall of Fame, remind visitors to this.

Meteor Crater was also a source of inspiration (or the scene) for several American science-fiction movies : Damnation Alley (1977), Meteor (1979), Starman (1984), Asteroid (miniseries for television, 1997), Deep Impact (1998), Armageddon (1998).

The publicity surrounding the astronauts and movie stars resulted in an ever increasing number of visitors coming to Meteor Crater.


Tourism.

Visitor facilities at Meteor Crater are privately managed by Meteor Crater Enterprises Inc. (1955).
The site is open year round (admission charged) :
May 15 through September 15 from 6 a.m. to 6 p.m., and 8 a.m. to 5 p.m. the rest of the year.

Three specially adapted vantage points along the rim offer an overview of the crater. Little hollow tubes ('telescopes') mounted on the guard-rail of an overhanging observation deck are directed towards several salient spots.
There are guided walks (lasting about one hour) on part of the rim.
The Museum of Astrogeology covers the creation of the crater and its significance for science. The exhibition also comprises the Astronaut Hall of Fame. There are regularly scheduled lectures and short movies.
The site has officially been designated a Natural Landmark (1968) thus preserving it for future generations.

Meteor Crater draws more than 300,000 visitors a year.
Curiosity, wonder, and a desire to learn more about the natural forces which created it are probably the motives to come and gaze into this awe-inspiring 'BIG hOle in the middle of nowhere'...


References.


Image credits.


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Last updated Tuesday April 4 2000.
Herwig Ronsmans.