Until recently, impacts by extraterrestrial bodies were regarded as,
perhaps, an interesting but certainly not an important phenomenon in the
spectrum of geological process affecting the Earth.
Our concept of the importance of impact processes, however, has been
changed radically through planetary exploration, which has shown that
virtually all planetary surfaces are cratered from the impact of
interplanetary bodies. It is now clear from planetary bodies that have
retained portions of their earliest surfaces that impact was a dominant
geologic process throughout the early solar system.
For example, the oldest lunar surfaces are literally saturated with impact
craters, produced by an intense bombardment, at least a 100 times higher
than present impact flux, which lasted from 4.6 to approximately 3.9
billion years ago. The Earth, as part of the solar system, experienced the
same bombardment as the other planetary bodies.
On the Earth, a variety of possible effects have been ascribed to impacts.
Currently, the best working hypothesis for the origin of the Earth's moon
is the impact of a Mars-sized object with the proto-Earth. This resulted
in the insertion into Earth orbit of vaporized material from the impactor
and the Earth, which condensed and accreted to form the moon.
Heat generated by early impacts may have led to outgassing of Earth's
initial crust, thus, contributing to the primordial atmosphere and
hydrosphere. Additionally, the impacting bodies themselves, may have
contributed to the Earth's budget of volatiles. This early bombardment
would also have frustrated the development and evolution of early life,
with the largest impacts having the capacity to effectively sterilize the
surface of the globe. In more recent geologic time, there is evidence that
at least one mass extinction event, notably that of the dinosaurs and many
other species 65 million years ago, is linked to global effects caused by
a major impact event. Impacts also have some economic significance.
For example, the vast copper-nickel deposits at Sudbury, Canada are likely
a related result of a large-scale impact 1850 million years ago and
several impact structures in sedimentary rocks have provided suitable
reservoirs for economic oil and gas deposits.
Most of the terrestrial impact craters that ever formed, however, have
been obliterated by other terrestrial geological processes. Some examples,
however remain. To-date, approximately 150 impact craters have been
identified on Earth. Almost all known craters have been recognized since
1950 and several new structures are found each year.
Types of craters
The morphology of impact craters changes with crater diameter. This
size-morphology relation is well illustrated with fresh-appearing craters
from the moon. Only the smallest impact craters have a bowl-shaped form.
As crater diameter increases, slumping of the inner walls and rebounding
of the depressed crater floor create progressively larger rim terracing
and central peaks.
Peak Ring Crater
At larger diameters, the single central peak is replaced by one or more
peak rings, resulting in what are generally termed impact basins.
Multi Ring Basin
This same progression in crater morphology is observed throughout the
solar system, including on the Earth; although, terrestrial craters are
less well-preserved and, hence, more challenging to classify. One notable
difference between lunar and terrestrial impact craters is the lower
diameter range for each morphological type on Earth. This difference is
due to the higher gravity on Earth.
On Earth, the basic types of impact structures are:
simple structures, up to 4 km in diameter, with uplifted and
overturned rim rocks, surrounding a bowl-shaped depression, partially
filled by breccia, and
complex impact structures and basins, generally 4 km or more in
diameter, with a distinct central uplift in the form of a peak and/or
ring, an annular trough, and a slumped rim. The interiors of these
structures are partially filled with breccia and rocks melted by the
Meteorite fragments are found only at the smallest craters and they are
quickly destroyed in the terrestrial environment. For impact events on
Earth that form craters larger than approximately 1 km across, the
pressures and temperatures produced upon impact are sufficient to
completely melt and even vaporize the impacting body and some of the
target rocks. For example, the peak pressure produced by the impact of a
stony (chondritic) body into a common terrestrial rock, such as granite,
at 25 km per second (a reasonable impact velocity for an asteroidal impact
on the Earth) is 900 GPa or 9 million times atmospheric pressure. In such
cases, the recognition of a characteristic suite of rock and mineral
deformations, termed "shock metamorphism", which are uniquely produced by
extreme shock pressures, is indicative of an impact origin.
Examples of shock effects include conical fractures known as shatter
cones, microscopic deformation features in minerals, particularly the
development of so-called planar deformation features in silicate minerals
such as quartz, the occurrence of various glasses and high pressure
minerals, and rocks melted by the intense heat of impact.
More Shatter Cones
Although the number of known impact craters on Earth is relatively small,
the preserved sample is an extremely important resource for understanding
impact phenomena. They provide the only ground-truth data currently
available and are amenable to extensive geological, geophysical and
geochemical study. Earth's impact craters also provide important data on
the structure of such landforms in all three dimensions. In some cases,
the large size of terrestrial impact craters, up to approximately 300 km
in diameter, requires orbital imagery and observation to provide an
overall view of their structure and large-scale context.
Impacts and extinctions
The tendency to discount impact processes as a factor in the Earth's
more recent geologic history was severely challenged by the interpretation
in 1980 that Cretaceous-Tertiary (K-T) boundary sediments world-wide were
due to a major impact event and that impact was the causal agent for a
mass extinction event. The acceptance of the K-T impact hypothesis by the
more general terrestrial geoscience community was not instantaneous and
considerable controversy and debate was generated. Today, there are few
workers who would deny that there is abundant diagnostic evidence that a
major impact event occurred at the K-T boundary. It is fair to say,
however, there is less consensus on the role of impact in the associated
mass extinction event, with some workers still having difficulty in
accepting impact-related processes as the cause.
Impacts and atmospheric blow-out
The impact signal of the K-T event is recognizable globally, because large
impact events have the capacity to blow out a hole in the atmosphere above
the impact site, permitting some impact materials to be dispersed globally
by the impact fireball, which rises above the atmosphere. These materials
do not require atmospheric winds for dispersal and have the capacity to
encircle the globe in relatively short-time periods, before eventually
returning to the surface. Model calculations indicate that it does not
require a K-T-sized event, which produced the buried 180 km diameter
Chicxulub impact structure in the Yucatan, Mexico, to result in
atmospheric blow-out. Relatively small impact events, resulting in impact
structures in the 20 km size-range can produce atmospheric blow-out. At
present, however, the K-T is the only biostratigraphic boundary with a
clear signal of the involvement of a large-scale impact event. The
involvement of impact in other boundary events in the terrestrial
stratigraphic record has been suggested but little evidence has been
From estimates of the terrestrial cratering rate, the frequency of
K-T-sized events on Earth is of the order of one every 50-100 million
years. Smaller, but still significant impact events, occur on shorter time
scales and will affect the terrestrial climate and biosphere to varying
degrees. The formation of impact craters as small as 20 km could produce
light reductions and temperature disruptions similar to a nuclear winter.
Such impacts occur on Earth with a frequency of two or three every million
years. The most recent known structure in this size range is Zhamanshin in
Kazaksthan, with a diameter of 15 km and an age of 1 million years.
Impacts of this scale are not likely to have a serious affect upon the
biosphere and cause mass extinctions. The most fragile component of the
present environment, however, is modern human civilization, which is
highly dependent on an organized and technologically complex
infrastructure for its survival. While we seldom think of civilization in
terms of million of years, there is little doubt that if civilization
lasts long enough, it will suffer severely or may even be destroyed by an
In marine impacts there is also another consideration; namely, the
generation of a tsunami. For example, an impact anywhere in the Atlantic
Ocean by a body 400 m in diameter would devastate the coasts on both sides
of the ocean with wave runups of over 60 m. The 1960 tsunami, generated by
a magnitude 8.6 Chilean earthquake, is thought to have been the largest
this century. An impact-generated tsunami 10 times more powerful will
occur with a typical recurrence time of a few 1000 years. Small impacting
bodies release their energy in the atmosphere, as an air burst. The
threshold size at which this is exceeded depends on the strength of the
impacting body. For example, iron impacting bodies up to 20 m will deposit
their energy in the atmosphere and not reach the surface; whereas, comets
as large as 200 m will deposit their energy in the atmosphere. Such air
burst explosions, fortunately, are not efficient at delivering their
energy to the ground, because some of initial energy is blown into space.
The Tunguska event in 1908 was due to the atmospheric explosion of a
relatively small, approximately few tens of metres, body at an altitude of
10 km. The energy released, has been estimated to be 10-100 megatons TNT
equivalent. Although the air blast resulted in the devastation of 2000 sq.
km of Siberian forest (Fig. 4), there was no loss of human life due to the
very sparse population. Events such as Tunguska occur on a time-scale of
100's of years. Devastation 8km from ground zero of the 1908 Tunguska
event, which was the result of a small asteroid exploding in the
atmosphere. Such events occur on time-scales of hundreds of years.
It must be remembered that impact is a random process not only in space
but also in time. The next large impact with the Earth could be an
"impact-winter"-producing event or even a K-T-sized event. To emphasize
this point, in March 1989 an asteroidal body named 1989 FC passed within
700,000 km of the Earth. This Earth-crossing body was not discovered until
it had passed the Earth. It is estimated to be in the 0.5 km size-range,
capable of producing a Zhamanshin-sized crater or a devastating tsunami.
Although 700,000 km is a considerable distance, it translates to a miss of
the Earth by only a few hours, when orbital velocities are considered. At
present, no systems or procedures are in place, specifically for
mitigating the effects of an impact.