Ross Sackett's amateur telescope making
Ross Sackett's amateur telescope making
   The Wells Creek Impact Structure is the preserved geological signature of a complex meteorite crater that formed
between 100 and 300 million years ago.  Since mammals and birds evolved about midway through this timespan there
is an even chance that the impact was witnessed by warm-blooded animals.  Here is what you might have seen if you
were standing at the target point.

Impact minus two weeks

   The Wells Creek impactor was likely a small asteroid, about 1/4 mile in diameter (this is estimated from the size of
the crater, with some assumptions about the impactor’s composition and speed).  An object this size would first
become visible to moderate telescopes about 16 days before impact, though detection would be a matter of luck—a
scope pointed at just the right place at the right time.  The object appears as a faint star, brightening slowly and
moving a little each day against the background stars.  Photos taken over several days would reveal its orbit and
confirm that the object would strike the earth.

   Closing in at a typical earth-crossing asteroid’s approach velocity of 10 miles per second the impactor passes
within the moon’s orbit 6 hours 17 minutes before impact.  It would still be a faint pinpoint of light, below the threshold
of naked eye vision even on a dark clear night.

  At impact minus 2-1/2 hours the impactor is now visible to dark-adapted eyes on the night side of the planet.  From
ground zero it appears to rise very slowly in the sky, brightening noticeably over a matter of a few minutes.  As it gets
brighter, its motion against the background stars slows.  If you begin running now you might survive long enough to
witness the impact and the beginning of the cratering process.

   By impact minus two minutes the asteroid can be seen in the bright day sky, perhaps looking like a glint of sun off a
jet’s wing, but brightening rapidly.  From ground zero it hangs almost motionless in the sky, heading directly towards
you now.  By this time there is no point trying to run.

Impact minus 7 seconds

   The asteroid hits the top of earth’s atmosphere only seven seconds before it strikes the ground.  The air ahead of
the asteroid is compressed and heats to incandescence.  Friction with the air heats and melts the surface of the
asteroid, forming a trail of sparks and dense smoke.  However just an inch or two below the surface the asteroid
remains quite cool, most of the heat of atmospheric entry carried away by the ablated crust.  Although the
atmosphere slows smaller meteorites to a fraction of their initial speed and ablates off much of their bulk, an object
the size of the Wells Creek impactor arrives on the surface with nearly all of its mass and solar orbital velocity intact.

From impact to plus 0.02 seconds: Contact and compression

   The impactor penetrates the surface to a depth of only about its own diameter, in the case of Wells to 1/4 mile,
losing most of its 10 miles per second speed in only two-hundredths of a second.  The tremendous kinetic energy of
the impact creates two shockwaves, one penetrating the asteroid and the other spreading into the target rocks.  At
the contact surface between the asteroid and the rock pressures exceed those at the center of the earth.  Much of
the impactor and adjacent target rock are heated and vaporized as the compression and rarefaction waves pass
through, resulting in an explosion that releases the energy equivalent of 3,500 megatons of TNT—this is 60 times as
powerful as the largest hydrogen bomb, and equivalent to several hundred thousand Hiroshimas.  Whatever remains
of the pulverized impactor melts and mixes with molten target rock. This mixed impact melt lines a rapidly expanding
spherical explosion cavity within the target rock.

   Smaller meteorites up to the size of a car are slowed by the atmosphere to the point that they gouge a mechanical
divot in the ground, but can sometimes survive intact.  House-sized meteorites tend to explode in the atmosphere,
raining down fragments falling at terminal velocity under the influence of gravity, much slower than their cosmic orbital
velocity.  In contrast, the hypervelocity impacts of larger objects like those at Meteor Crater and Wells Creek are
explosions, leaving a proportionately much larger hole and little of the original asteroid left to collect.

Impact plus 0.02 to 13 seconds: Excavation

   The shock front compresses the rock ahead of the expanding cavity.  As the pressure waves pass through the
target rock is pulverized and set in motion, much of it to be ejected from the crater in a wall of debris—the ejecta
sheet—thrown out from the growing crater.  This material will fall to earth around the crater creating an ejecta blanket,
thickest on the rim of the crater and tapering off with distance.  Most of this material will fall close to the rim of the
crater, but some released just after impact may be ejected at such velocity that it travels great distances (in very large
impacts some of the ejecta may be traveling at earth escape velocity into solar orbit). But only perhaps half of the
material within the crater is destined to be ejected—a significant proportion is compressed into the floor and walls of
the growing crater.  

   The intense shock waves passing through the fractured target rock alter its properties in a surprising way: it begins
to behave more like a fluid than a solid.  One theory is that acoustical energy—the intense rumble of the passing
shockwave—fluidizes the rock so that it responds like the surface of a pond disturbed by a falling pebble.  The
fluidized fractured strata below the impact compress and bend downwards away from the expanding crater.  As we will
see, this fluid-like behavior continues even after the initial crater reaches its maximum extent.  As these underlying
layers are shocked and compressed they fracture in a characteristic way, creating shattercones in the rock directly
beneath the center of the growing crater.  Shattercones are diagnostic of impact craters and nuclear explosions; as
far as we know they occur nowhere else.  The discovery of shattercones led to the realization that a number of
presumed volcanic craters on earth were really the traces of meteorite impacts.

   The crater reaches its maximum depth about 13 seconds after impact, when the expansion of the crater, ejection of
debris, and the depression of underlying strata cease.  At this point the crater is a parabolic basin a little more than
four miles in diameter and about 1-1/4 miles deep.  This transient cavity is lined with a mix of melted and fractured
debris that will eventually cool to form a breccia, a kind of petrologic peanut brittle.  As the name transient cavity
implies this deep bowl-shaped crater is temporary, and immediately its shape begins to change.

Impact plus 13 seconds to several minutes: Modification

   In an impact this large the target rock remains fluid even after the transient cavity reaches its maximum extent.  
Within seconds the compressed and still-fluid rock beneath the crater springs back up, like the skin of a trampoline or
the splashback from a plopping pebble.  This creates a rising hump of uplifted strata in the center of the crater that
can briefly rebound above the level of the original ground surface.  Simultaneously, the walls of the crater begin to
collapse down and spread outwards.  At about this point the energy that originally fluidized the rubble has attenuated
sufficiently that the target rock once again behaves more like heavy rock, and the raised central peak “freezes” in its
uplifted position.  One guess is that the rumbling set up by the passing shockwave has quieted down to the point that
the rock acts like rock again. The core of the central uplift at Wells is composed of strata that were originally 2500
feet below the bottom of the transient cavity. Though today the peak has eroded away, you can still see the upturned
strata on the edge of this central uplift.  In impacts larger than Wells the rock remains fluid longer, and the central
hump drops again to form an expanding ripple, creating a central ring rather than a peak.  Truly huge basin impacts
like Chicxulub and those under the basalt “seas” of the moon remain fluid even longer, and are surrounded by
multiple frozen ripples.

   With creaks groans and shudders the local landscape adjusts to the fresh injury.  The rim, floor, and central peak
fracture as the surface tries to return to something like its original shape.  These fractures extend deep into the strata
below the crater, and can be seen today as ring and radial faults in the rocks now exposed on the surface at Wells.  
As the steep slopes of the rim slump under the pull of gravity the basin becomes wider, and the floor shallows.  Over
the next few minutes the final “relaxed” crater comes to be about 8 miles wide and 2000 feet deep, with a central uplift
many hundreds of feet above the floor of the basin, surrounded by rubble from the severely fractured strata that once
overlain the central peak.  Surrounding this is a ring of cooling breccia—impact melt and fragmented target rock—
containing some traces of the original impactor.  Out towards the edges of the basin rubble slumped from the crater
walls cover the impact debris.  The strata at the rim of the final crater are uplifted—a legacy of the original explosive
impact—and mantled with ejecta from the crater.  At the rim the blanket of ejecta is several hundred feet thick,
tapering off about 3 times the radius of the crater—that is 12 miles from ground zero.  For miles beyond this loose
bombs of debris (some the size of a house) have peppered the landscape with their own irregular secondary craters.  
Rays of small secondaries may extend tens of miles from the crater.

   All this happened in a timespan of only seconds and minutes, from first impact to the final more-or-less stable
crater.  But now the pace slows from the frenetic catastrophist sprint of impact and excavation to the slow
uniformitarian crawl of erosion, sedimentation, and faulting.  Millions of years pass with little change except the slow
grind of water and wind on the rock, softening the contours of the rim and peak, filling the basin with sediment, and
ultimately erasing all trace of the crater itself.

   Wells Creek was a complex crater, typical of impacts on earth between 2-1/2 and 14 miles in diameter.  Smaller
impacts like the famous 3/4 mile-wide Barringer Meteor Crater in Arizona form simple bowl-shaped craters, lacking the
central peak and collapsed rim of larger complex craters like Wells.  Keep in mind that 8 mile diameter Wells Creek
was originally ten times the diameter of Meteor Crater.  Craters larger than 14 miles have one or more rings, and are
called ringed or multi-ring basins.  Chicxulub has as many as four rings, extending out to a diameter of 190 miles.  
Even larger impacts probably formed in the first billion years of earth history, but all traces have been obliterated by
geological processes.  However, the airless surface of the moon does preserve much of its early history.  The dark
markings that form the face of the Man in the Moon are lava flows that cover multi-ringed basins larger than any
preserved on earth today.  Imbrium basin, the Man in the Moon’s right eye, has 6 “frozen” rings and is 2000 miles
across.  Rocks returned by Apollo astronauts date the Imbrium impact to 3.85 billion years ago, older than the earliest
unaltered (that is, non-metamorphic) rock now found at the surface of the earth.  Imbrium was the last large basin
formed on the nearside of the moon, so we believe that by 3-3/4 billion years ago the heavy bombardment of the
inner solar system was over.  Impacts have steadily become smaller and increasingly infrequent since.

Impact plus 200 million years, more or less

   Today all traces of the original Wells crater are gone.  Technically what we can see at Wells Creek today is an
impact structure, not an impact crater: a preserved geological signature, a kind of fossil, of the original impact.  The
ejecta blanket, rim, impact melt, condensed bits of the original asteroid, and central uplifted peak have all been
eroded and transported out of the region, and today reside deep in the sediments underlying the Gulf of Mexico.  The
present landscape surrounding the structure is more than a thousand feet lower than the level of the ground at the
moment of impact.  The rock now exposed at the surface was under—at the core at least half a mile under—the
original crater.  

   At the core of the impact structure is a shallow depression about three miles across and 200 feet deep.  While this
topographic basin mimics the shape of a complex impact crater (complete with a modest central hill) the appearance
is deceptive—the entire depression actually marks the central uplift of the original crater.  The upturned strata and
severely shocked rock of the original peak have eroded faster than the surrounding landscape, creating a shallow
basin where there was once a tall hill.  At the core of this topographic basin are uplifted Cambrian limestones
hundreds of millions of years older than the impact.  These strata that were once deep below ground zero were
fractured into megabreccia by the passing shockwave, leaving abundant shattercones now exposed on the surface.  

   Faults radiate from the center of the shattered strata under the former central peak, ringed by a nearly circular
fault marking the edge of the uplifted block at the center of the original crater (also ringing the topographic
depression at the center of the present structure).  Three more ring faults and several radial faults extend out to
about 4 miles from the center of the structure, marking what is believed to be the rim of the original 8 mile diameter
final crater.
Copyright 2009 Ross Sackett
What Happened at
Wells Creek, TN?