The name, Meteor Crater, is a misnomer. Meteors are extraterrestrial debris
that enters Earth’s atmosphere but either skips back into space as though a
flat stone on the surface of a pond, or completely burns up during entry.
Meteorites are meteors that actually strike the Earth’s surface, be it water
or land.
Though a small and simple crater, Meteor Crater is the world’s
best-preserved impact structure and the first recognized as such a structure.
The region’s arid climate and the crater’s young age (49,000 ± 3,000
years) are responsible for its excellent pres
ervation.
Studies show that a meteorite’s size can be approximated at a 20:1 ratio
of the crater’s diameter though this depends on the variables of the object’s
mass, speed, trajectory, etc. Considering these factors, estimates indicate an
object about 40 m (130 ft), traveling at 20 km/sec (12 mi/sec) -- the RMS
encounter speed for Earth-crossing meteoroids. Total energy release at impact
was equal to that produced by a 15-megaton nuclear device exploded at a depth
of 20 m (67 ft) below the surface to produce the crater which is 1.2 km (.75
mi) diameter, and 180 m (594 ft) deep with its upturned rim rising 30 - 60 m
(100 - 200 ft) above the surrounding plateau. Today, the structure is slightly
rectangular due to a mutually perpendicular scissor-type vertical joint set
residing in the target rock.
At time of impact, a shockwave expanded hemispherically into the target
rock exceeding its dynamic yield strength by several orders of magnitude –
usually equal to thousands of atmospheres of pressure. Following this intense
compression, a rarefaction (decompression) wave moved through the rock in the
same manner but completely unloaded the compression thereby ejecting target
rock into the atmosphere as a consequence of Newton’s third law of motion
– that of equal and opposite forces. In effect, the compressed rock blasted
itself out of the ground as it was decompressed to form a rapidly expanding
cavity that would become the crater itself.
In the process of decompression, target rock is upthrust into an expanding
cone of ejecta. Remnants of the ejecta blanket still exist as inverted
stratigraphy in various regions surrounding the crater though today, it is
almost completely eroded away. Originally, deposited as a continuous bed out
to a distance of 3 km (2 mi), the ejecta would have become discontinuous
beyond that. Smaller, secondary impact structures, surround the crater having
been caused by fallback of large target fragments in excess of 30 m (100 ft)
across, and pieces of the meteorite are found throughout.
The crater formation process continues until the force of the expanding
shockwave equals that of the target tock’s mechanical strength. The crater
stops growing at this point, but the shockwave and rarefaction continue to
expand into the rock causing further alteration.
A melt lens of suevite breccia (pronounced sway-vite) composed of
fractured bedrock and meteoritic material extends to a depth of 180 m (600 ft)
below the crater’s floor which itself is comprised of a fining upward unit
of fallout 11 m (35 ft) thick. This unit is overlain by talus from the crater’s
walls and alluvium that was washed in with water that occasionally formed a
small lake in the basin. Note that suevite differs from igneous rock in that
it is composed of melted target rock and the melted remnants of the impacting
body.
Click to see an enlarged topographic map of the crater.
