http://rst.gsfc.nasa.gov/Sect18/Sect18_2.html
navigation image map
_________________________________________________________________
In many respects, impact craters form in a similar manner to explosion
craters where explosives are buried at some depth and then ignited.
But, the energy needed to form an impact crater begins to act at the
point of contact at the surface. As it burrows into the target (rock
or water), it has an effective (not real) center of energy release. An
impact crater involves energies on the order of hundreds to millions
of kilotons (equivalent nuclear detonation energy). A series of panel
cartoons, and accompanying descriptions, shows the sequence of
formation of an impact crater. The final result is shown as a
cross-section.
_________________________________________________________________
Cratering Mechanics
No other natural event is as powerful, devastating, or potentially
catastrophic as a major impact. Consider one capable of producing a 50
km (31 mi) wide crater, excavated to a depth of 5 km (3 mi): the
energy expended is thousands of times greater than the simultaneous
detonation at one point of all the nuclear explosive devices
(euphemism for bombs) manufactured to date. We gain some idea of these
magnitudes from this logarithmic (log-log) plot of the crater
frequency as a function of energy at impact (or detonation) given in
joules.
Crater frequency (in years) or likelihood of an impact event (average
number of years until the next occurrence of an impact of a given
size) versus the energy released in joules during the impact itself.
Below is a similar diagram with some additional information. The word
"Siberia" equates to "Tunguska" in the upper diagram.
To appreciate the magnitudes of large impact events, keep in mind that
the 12.5 kiloton device exploded at Hiroshima was equivalent to about
10^14 J (Joules), the Mount Saint Helens volcanic eruption involved 6
x 10^16 J, and the largest earthquakes release up to 10^18 J (note:
the relation between energy in Joules and in kilotons[kt] of explosive
TNT is given by 1 kt = 4.186 x 10^12 J). In this context, the impact
that produced the Sudbury structure (215 km [134 mi] initial diameter)
in Canada released about 10^23 J, roughly 100,000 times greater than
earthquakes of magnitude 9.0 on the Richter scale (Sudbury, then,
could have generated an earthquake-like response on the order of
magnitude 14). Slightly larger is the Chicxulub crater in the Mexican
Yucatan, reputed to be evidence for the catastrophic impact event that
hastened the demise of the remaining dinosaurs (many families and
types had already diminished or reached extinction before this event).
In common, both earthquakes and impacts are the fastest known large
geologic phenomena, each causing ground disturbances that last only a
few minutes at most after their initiation times.
(Note: there is a log linear relation also between crater size
[diameter] and energy release, not shown on the above diagram. The
scaling formula relating energy to diameter can be approximated by D =
0.1 times the cube root of the energy E in kilotons. By way of
example: the cube root of 1000 kilotons [a megaton] is 10, so the
diameter of the crater from an event of that magnitude is 0.1 x 10 or
1 kilometer.)
18-3: The Zhamanshin crater, in Asia, is 13.5 km (8.3 miles) in
diameter. From the above graph, are there enough nuclear warheads in
the arsenals of all nations combined to make a crater of this size if
they are exploded simultaneously underground at one place? Are there
enough atomic bombs to bring about nuclear winter? Roughly, what is
the time likelihood of an impact of the size needed to have something
like a nuclear winter forced on the Earth? (And, did you see
"Armageddon" or "Deep Impact" in 1998? Does this potentiality for an
impact catastrophe worry you?) ANSWER
The source of this tremendous impact energy is the direct consequence
of a great solid mass moving at high velocity. Remember from physics
that kinetic energy (K.E.) = 1/2 mv^2, where m is the moving body's
mass, and v is its velocity. To gain a sense of the magnitude
involved, consider this calculation. Let a 30 m (98 ft) diameter iron
body (in effect, a large meteorite) weighing about 200,000 metric tons
(around 440 million pounds) strike Earth at a typical, in-space
velocity of 30 km (19 mi) per second (not hours!) (20 mps corresponds
to 72,000 mph). This impact would generate about 20 megatons
(TNT-equivalent) of energy (about 10^17 joules) that would cut out a
crater about a kilometer and a half (almost a mile) wide and 185 m
(607 ft) deep. This is the size of Meteor (Barringer) Crater, which we
will examine later. The ejection process would scatter most of the
excavated rocks to a radius of at least 10 km.
The very large kinetic energies (K.E) owing to the great masses (m)
and high velocities (v)(remember from physics: K.E. = 1/2 mv^2)
involved in impact cratering are converted to dynamic, fast moving
transient shock waves that diverge hemispherically from the line of
impact. These are compressional waves that have high amplitudes. The
pressures generated are given in either of two units: Gpa
(Gigapascals) or Kb (Kilobars) (a bar is equivalent to 0.971 of an
atmosphere of pressure, namely 14.7 pounds per square inch). A kilobar
is 1000 kb; a megabar is a million kb; 1 GPa = 10 kb. The shock waves
decay (decrease in value) as they spread out from the impact point.
When a shock wave meets a free surface, it is in part reflected as a
tensional wave.
The first truly modern explanation of how an impact crater is formed
was presented by Eugene M. Shoemaker in a 1963 paper. The key
illustration from this paper is reproduced here:
Sequential steps in the formation of an impact crater, as proposed by
E.M. Shoemaker.
This next diagram (adapted from H. Melosh) summarizes some of the
major aspects of the pressure distribution and shock wave-produced
phenomena associated with an incoming bolide (fancy term that applies
to the causative projectile, such as a meteorite or an
asteroid/comet):
The incoming bolide penetrates to a distance approximately twice its
diameter. As it does, it is itself destroyed by tensional waves, as it
vaporizes and breaks up into melt drops and fragments. The ground
target is impinged by the generated shock waves. At pressures around
70 GPa, the target material (usually rock) is vaporized; rocks
subjected to waves between 50 and 70 GPa are melted. The hemispherical
volume of rocks that experience shock pressures between about 5 and 50
GPa is affected by processes involved in shock metamorphism - whose
effects are considered in the next two pages. Beyond, to a level
around 1 GPa, the rocks are fractured and brokened into pieces that
accumulate as breccias (rock fragments [clasts] held in a matrix of
finer particles and melt that is quenched into glass) which are
deposited both within and beyond the resulting crater. The shock waves
thus break up the target rock, setting it into motion along
trajectories (the curved lines with arrows; only a few such lines are
shown) that carry the now disrupted pieces (some as large as houses
but grading in size downward to pea and dust sizes) upward and
outward. In this way the crater is excavated by the combination of
vaporization, melting, and fragmentation that removes nearly all rock
that underwent pressures down to a few GPa and relocates it as ejecta.
Rock beyond the final transient crater is displaced and disrupted,
with fractures that open receiving injected debris.
Craters are round or circular (in plan view) and bowl-shaped (as seen
in a cross-section) for this reason: the shock wave can be considered
as starting from a point (actually a narrow zone of some length
related to the penetration of the bolide). This waves travels radially
from the point of contact at a uniform velocity. Thus the effects,
including excavation, are equal in all horizontal directions (hence, a
circle) and approximately so in the directions that define the
hemisphere whose outer limit is the true crater.
Impact cratering is unique among natural geologic processes in that
very high pressures are attained almost instantly; these decay in
minutes or less so that the resulting crater is produced in a very
short time. The shock metamorphic phenomena so developed are also
unique. Volcanic processes can also produce craters but the processes
generally occur over much longer time spans and do not achieve
pressures above about 2-3 GPa and hence do not impose the shock
metamorphic features in impactites (general term for any rock that is
affected by shock waves from an impact) that are diagnostic of impact
events. Pressures within the crust and mantle can reach well above 5
GPa with increasing depths but the processes involved (such as the
weight of the overburden and superimposed tectonic stresses) are
applied much more slowly, so that there are no shock wave features
imposed.
Let us now follow, second by second, the formation of a large or
complex crater (one greater than about 5 km [3 mi] wide that has a
central peak and concentric slump walls). We use a series of sideview
panels created by Dr. Raymond Anderson of the Iowa Geological Survey
Bureau (and used here with his permission) to show the steps in
developing the Manson structure (see page 20-3a for an in-depth survey
of this Iowa crater). The writer, during the 1960s, when he was
working primarily on impact structures, is generally credited with
"proving" the impact origin of this very large crater, which, at one
time, many thought was the "smoking gun" that killed the dinosaurs
about 65 million years ago. But a new age date found it to be 74
million years old, which disqualified it as the culprit. This circular
structure, 32 km (20 mi) in diameter, whose centerpoint is some 130 km
(81 mi) northwest of Des Moines, Iowa, is largely intact but now
buried under 30 m (98 ft) of glacial debris.
Each of the following schematic diagrams represents a stage in the
sequence of mechanics of formation of a large (complex) crater; read
text for description; the number in the upper right circle indicates
the time in seconds or minutes after the initial moment of contact
between the incoming bolide (probably an asteroid) and the ground
surface. While these diagrams (prepared by R. Anderson) apply
specifically to the Manson structure, they apply to the cratering
process in general.
Time zero, at the initial point of contact by the incoming bolide
(asteroid?)
At the instant of impact (0.0 sec), the target consisted of an average
of 90 m (295 ft) of Mesozoic sedimentary rocks (mainly Cretaceous in
age) (in green) underlain by 495 m (1624 ft) of Paleozoic sedimentary
rocks (light blue). These rocks lie unconformably on top of
Proterozoic sandstones and other red clastics (yellow), whose
thickness increased to nearly 3 km (1.9 mi) to the southwest. This
entire section rests on top of Precambrian crystalline (granites and
metamorphic) rocks (red) buried at depths to almost 4,600 m (15,088
ft).
Panel 2 - Sequence of steps involved in the development of the Manson
structure.
As the incoming impactor (or bolide, a general term that includes both
comets and asteroids) impressed onto this late Cretaceous surface, at
0.15 seconds, it was totally fragmented and vaporized. At it
penetrated into the rock, it imparted its energy (about 2 x 10^23 J)
in the form of supersonic shock waves that generated compressive
pressures ranging up to a megabar (1,000,000 atmospheres). We usually
find such pressures only at depths well into the Earth (100s of km).
Rocks just beyond the point of impact vaporized. An initial curtain of
ejecta, consisting of gases and melted rock, streamed upwards in a
steep cone, within which is a momentary partial vacuum caused by the
projectile passage. The energy released also generated electromagnetic
waves that extended into the atmosphere.
Panel 3 - Sequence of steps involved in the development of the Manson
structure.
At 0.6 sec the shock wave had progressed along an enlarging
hemispherical front well into the target, severely transforming rocks
at pressures ranging to about 600 kilobars (kb) (or 60 Gigapascals
[Ga], a fashionable new pressure unit) close to the line of
penetration. A fraction of the target (up to 10% of the total that the
impactor eventually displaces) melted. Some of that molten rock
carried downward along with the now-compressed and mobilized rock
underwent fragmentation. Some of it pushed out of the crater and fell
back nearby, and some literally squirted out as tiny blebs that might
have traveled hundreds of miles out of the atmosphere, and then
returned to Earth as tektites (glass "pebbles"). A fireball, similar
to that caused by atmospheric burning at surface detonations of
chemical or nuclear explosions, started to form. Within a few seconds,
the excavation phase of the crater commenced, where the shock wave
first compressed the rock and then a trailing wave (rarefaction wave)
moved through, causing tensile fragmentation. As the waves spread
outward and down, decreasing in intensity, peak pressures dropped to a
few 10s of kilobars.
Panel 4 - Sequence of steps involved in the development of the Manson
structure.
By 6.9 seconds, the initial or transient crater, arising from
vaporization, melting, and direct ejection and from centrifugal
"shoving" of the target matter outward under compression, had reached
its maximum depth. At Manson, this rapidly growing crater front cut
down through the Mesozoic, Paleozoic, and Proterozoic sedimentary
overburden, well into the Precambrian crystalline rocks. Most of this
earthen material received shocks to varying degrees and the effects of
these pressure waves were permanently imposed on the rocks. Trailing
tension (rarefaction) waves continued to decompress these rocks and
break them into fragments ranging from microscopic in size to objects
bigger than a house that eventually came to rest as deposits called
breccias.
Panel 5 - Sequence of steps involved in the development of the Manson
structure.
By 11.0 sec, as excavation continued, the peak shock pressures at the
wave front had now decayed to under 20 kb (2 Ga). As the pressure
waves advanced outward from ground zero (point of impact), they kept
breaking the rocks into fragments and blocks, launching them into
ballistic trajectories that started particles downward and then swung
them up, above the still growing crater along arcuate paths. Most of
the material left the site along low to moderate angle paths.
Generally, particles deeper and farther out from the impact center
left later and usually fell on top of particles that left earlier from
near-surface positions. Thus, the ejecta layers tended to deposit in
reverse order relative to their initial position in the stratigraphic
sequence, with ones lower in the target falling on top of upper ones
(although some mixing occurred). Beyond the edges of the crater walls,
rock units experienced faulting and folding. Along the upper walls,
sedimentary (layered) rocks pealed back so that the layers might even
overturn.
Panel 6 - Sequence of steps involved in the development of the Manson
structure.
For Manson, the crater reached its maximum excavation diameter around
25 sec, as the last voluminous ejecta emerged. Its upper walls were
especially unstable and began to fail along steep concentric fractures
and faults. At the central bottom of the crater, the rock below began
to rebound upward.
Panel 7 - Sequence of steps involved in the development of the Manson
structure.
At the transitional 26 second mark, the last ejecta were well into
flight. Slices of rock just past the walls now began an inward sliding
along faults. The crater base had started an upward movement that soon
led to a central peak. This rock material probably behaved plastically
as it almost flows upward (a good analogy is the inner blob of water
that shoots up into a momentary "crater" forming by dropping a stone
into a pond ). The collapse of the upper walls may have aided in this
effect by pushing downward toward the center.
Panel 8 - Sequence of steps involved in the development of the Manson
structure.
By 35 seconds, the central peak had attained its maximum height
(overshoots) and began to founder in collapse.
Panel 9 - Sequence of steps involved in the development of the Manson
structure.
By about a minute after the impact started, the central peak was well
into subsidence towards its final position and the crater walls had
commenced to slide and tumble inward to form nested or terraced rings
(see the Tycho image for an overhead view of these conditions). By
this time, some of the material that left at high angles directly
above the crater began to descend. The heavier, larger particles
settled first, because they passed through the atmosphere more
quickly, due to their momentum.
Panel 10 - Sequence of steps involved in the development of the Manson
structure.
Over the next 30 minutes or so, this fallout piled up in a continuous
blanket, inside and outside the crater. Other materials expelled at
lower angles formed a wider apron of ejecta that these later fallout
deposits covered. Small particles and dust from the event carried
hundreds of miles. Manson material has been found in a thin layer at
sites in South Dakota, up to 500 km (311 mi) away and the finest sizes
traveled in the stratosphere probably well beyond North America
(likely global in extent).
18-4: To recapitulate, specify the time or time interval at which each
of these stages in the Manson crater formation was important: a)
Maximum melting of rock; b) Maximum depth of transient crater; c)
Moment when shock wave had decayed to about 20 kilobars (roughly the
lower limit at which signficant shock features are produced in the
rocks; d) Maximum excavation of fragmented rocks; e) Inward failure of
crater walls; f) Start of central peak rise; g) Collapse of central
peak; h) Deposition of fallout. ANSWER
At the time of impact, 74 million years ago, the Manson area was
almost certainly under water, because the region lay within a shallow
sea. This impact should have produced a tsunami-like disturbance
(steep fronted waves that travel at velocities >800 kph [about 500
mph]). Large-body impacts into the open oceans probably spawn huge
waves, whose initial heights may exceed 325 m (1,066 ft). If this was
so at Manson, the various ejecta deposits would not have formed in the
usual sense, because they would have entered disturbed waters and
would be irregularly deposited or stirred up by waves moving back into
the crater area. The seas retreated a few million years later, leaving
the land exposed to erosion. About 10% of the crater's upper
structures and deposits eroded. Now glacial deposits of the
Pleistocene Age cover the crater, which has no surface expression at
all. We show a cross-section (side view) of the Manson crater, as it
remains today (glacial cover is a thin gray line):
Cross-section through the 32 kilometer wide Manson structure in Iowa.
The dashed yellow line marks the boundary of the final transient
crater, modified upwards centrally by the rise of its surface along
the central peak. The curved concentric maroon or black lines are
fault planes, bounding slides of bedrock that dropped downward to help
create terraces. Their outer limits define the maximum (apparent)
crater diameter.
18-5: Drillers at the surface above Manson cannot see what they are
"aiming" for because of the glacial cover. But, suppose geophysical
surveys have outlined the main elements of the crater, so the drill
team knows where the center and the rim are located underneath. What
would they encounter, as evident in the recovered drill core, if they
drilled a) at the center; b) half way out (in the "moat"); and c) into
the rim? ANSWER
Much of what has been shown in the above panel cartoons that follow
the sequence of events during impact cratering can be reproduced at
laboratory scales. This next set of sequential panels are photographs
taken by a high speed camera of the development of the ejecta curtain
from an impact of a small (centimeter-sized) projectile fired into
loose sand from a gas-gun that accelerates the projectile to high
velocities.
Time sequence photos of the ejecta curtain from impacted loose sand;
experiment done by Donald Gault and associates at Ames Research
Center.
In the above experiment, vertical black-painted sand was inserted as
columns (using thin celluloid tubes to contain this sand) in the
target material. After the impact the target sand was sealed by a
liquid glue and then sectioned, one of which is shown here:
Cross-section through the sand target in the above gas-gun impact,
showing deformation at the crater base.
The black sand markers just below the crater base show an abrupt
bending towards the rims on either side. This confirms that the shock
waves induce motions in the sand that roughly parallel the growing
surface of the forming crater. Thus, transport of ejected sand is
outward at angles; below the final crater base the deformation broadly
follows this motion.
For anyone seeking a comprehensive and technical treatment of
cratering mechanics, we recommend the book "Impact Cratering: A
Geologic Process," by H.J. Melosh, 1989, Oxford Monographs on Geology
and Geophysics No. 11, Oxford University Press.
Having now grasped some idea of what happens when impact craters are
produced, it could prove interesting to you to run your own
calculations in determining Impact Cratering Effects. This Web site
brings up a page that allows you to enter various parameters to
determine what is predicted to happen when an incoming asteroid or
comet strikes the Earth.
navigation image map
_________________________________________________________________
Primary Author: Nicholas M. Short, Sr.