in its existence, "data suggest there was
an impact cataclysm that affected the entire inner solar
system, resurfacing the terrestrial planets, and that
the source of the impacting debris was the asteroid
belt. Comets do not appear to have been important."1
study concludes that a chondritic asteroid about
170 km in diameter broke up much later, and that its
pieces fell on the terrestrial planets. That surge
of impacts included the catastrophic Chicxulub meteorite
at the Cretaceous/Tertiary (K/T) boundary.2
1. Kring, David A., Barbara A. Cohen.
2002. Cataclysmic bombardment throughout the inner solar
system 3.9--4.0 Ga. Journal of Geophysical Research,
Vol. 107, No. E2, pp. 4-1 to 4-6.
2. Bottke, William F.,
David Vokrouhlický, David Nesvorný. 6 September 2007.
An asteroid breakup 160 Myr ago as the probable source of the K/T impactor.
Nature, Vol. 449, pp. 48-53.
Pole-Aitken is the biggest basin on the Moon, at over
2,600 km across and 12 km deep. In the Solar System
it is second in size only to the Borealis basin on Mars.
"The chemical composition of material within
lunar craters, as well as their size distribution, matches
nicely with asteroids, not comets."
Hand, Eric. 26 June 2008. The hole at
the bottom of the Moon. Nature, Vol. 453, pp. 1160-1163.
their discovery in 1877, determining the origin of [Mars'
moons] Phobos and Deimos has remained problematic."
Some evidence "has led many investigators
to suggest that they are captured asteroids. However,
the orbits of both moons are extremely circular and
their Laplace plane is very close to the martian equatorial
plane. Captured objects would be expected to have
elongate orbits with randomly oriented orbital planes."
"An alternative hypothesis that is frequently
overlooked is the possibility that Phobos and Deimos
are the result of a giant impact." There
is also the fact that, "similar to the Earth-Moon
system, Mars has too much angular momentum" "to
be explained by the accretion of many small bodies." "A
planetesimal with 0.02 Mars masses must have collided
with that planet early in its history in order for Mars
to spin at its current rate." "The number
of impact basins on Mars support the idea that large
objects struck its surface early in its history, including
evidence that a giant impact formed the Borealis basin
and created the martian dichotomy."1
is a divided planet. Its southern highlands cover
about 2/3 of the planet and are on average about 4 km
higher than the northern plains, a difference that is
known as the hemispheric dichotomy."2
The light blue part of Mars in the image on the
right are the northern plains. Long thought to
have been a product of mantle circulation, evidence
now shows that the region is probably the result of
the largest impact in the Solar System (artist's conception
on the left). The elliptical shape, close to 10,650
km by 8,520 km, had made it seem unlikely to be an impact
crater since impact craters are usually round. However,
"Small impact craters are essentially formed on
a flat surface."2
But for an impact large enough to make the
hemispheric dichotomy, the curvature of the planet comes
into play. A simulation determined that the colliding
asteroid had a diameter in the range of 1,600-2,700
km, travelled at 6-10 km/s, and struck at an angle
between 30 and 60 degrees.4
Mars itself has a diameter of 6,780 km.2
The dark blue basin near the bottom
of the image on the right is the Hellas basin. It was also formed by an
impact, but is only 2,300 km across.2
crust of Mars likely formed in the same way Earth's
did, by surface cooling of a magma ocean. This
produces basalt, which is the oceanic crust of Earth.
A team studying the northern plains of Mars believe
"the northern lowlands crust, by contrast, probably
arose primarily from shock melting in the deep and previously
depleted martian mantle." "Impact melting
occurs because of decompression following the initial
shock". "The volume of crust missing
from the northern lowlands is about 1.5 x 109
"The total melt volume produced during the
impact is 6 x 108
impacts penetrate into the mantle. The resulting
rarefaction wave completely removes the surrounding
crust, which re-impacts elsewhere on the planet or is
ejected to space."3
Simulation results show that "depending on
impact angle, 50--70% of the melt stays inside the excavated
boundary, 25--30% is deposited outside the boundary,
and the remainder is ejected from the planet."3
impact energy and initial crustal thickness, a basin
may be retained or impact-induced crust may be topographically
of giant impacts on Mars indicate that impact energies
of 3 x 1028
to 3 x 1029
J are capable of excavating a basin with size comparable
to the northern lowlands without generating so much
melt that the crust is flooded and the basin erased."
"If post-impact crust is thinner than the
initial crust, a basin is retained".5
1. Craddock, Robert A. February 2011. Are Phobos
and Deimos the result of a giant impact? Icarus, Vol.
211, No. 2, pp. 1150-1161.
2. Kiefer, Walter S. 26 June 2008. Forming the martian
great divide. Nature, Vol. 453, pp. 1191-1192.
3. Marinova, Margarita M., Oded Aharonson, Erik Asphaug.
26 June 2008. Mega-impact formation of the Mars hemispheric
dichotomy. Nature, Vol. 453, pp. 1216-1219.
4. Nimmo, F., S.D. Hart, D.G. Korycansky, C.B. Agnor.
26 June 2008. Implications of an impact origin for the
martian hemispheric dichotomy. Nature, Vol. 453, pp.
5. Reese, C.C., C.P. Orth, V.S. Solomatov. 2011.
Impact megadomes and the origin of the martian crustal
dichotomy. Icarus, Vol. 213, pp. 433-442.
Earth, to form the Moon and continental crust
standard theory says the Moon formed from debris
kicked out into space when a planetesimal about the
size of Mars grazed the Earth.
large impact energy (for example, on the order of that
for the Moon-forming impact [on Earth] of around 1031
J), retention of an impact basin is unlikely."
Would a global magma ocean form? "For
impact energies that are not too large, global magma
ocean formation may be difficult due to rapid crystallization
of impact melt and further cooling during lateral spreading."4
of low-viscosity liquid silicates causes rapid cooling
and crystallization of the impact melt volume."
"After crystallization, isostatic adjustment
causes the partially molten region to rise up to the
surface and form a laterally spreading layer".
"Instead of forming a global layer, impact
melt may spread only partially around the planet."
"After spreading cessation, differentiation
results in crustal growth over an area centered on the
crust is thicker than the initial crust, a topographic
dome centered on the impact is formed." This
has tentatively been named " 'impact megadome'
to refer to topographically high-standing impact-induced
Before the collision, all
of Earth's crust was basalt, as our oceanic crust is
today. The Shock Dynamics theory adds that melt
in the collision area also formed Earth's elevated continental
crust, in the shape of the protocontinent in the image
below. It was this protocontinent
that was much later struck and shattered by the giant
meteorite of the Shock Dynamics event.
An earlier study
of the effects of giant impacts concluded that "the
primary shock wave of the canonical Moon-forming giant
impact melted about 30-55% of the planet [Earth], depending
on its initial temperature." "This melt
is likely to be rapidly extruded onto the surface before
it solidifies." In fact, continental crust covers 41% of Earth's
surface. Previously, some researchers had erroneously
proposed that such a collision would melt the whole
reasonable to propose
that a sufficiently
large impact would mix basalt crust with mantle to form
continental crust. This is the Shock Dynamics
position. On the other hand, making continental
crust through plate interaction remains a problem
for Plate Tectonics theory:
understanding of how continents grow and differentiate
still remains somewhat obscure."1
"A fundamental problem in the formation of
continental crust is that the majority of magmas erupted
on earth are basaltic and yet the continents do not
have a basaltic bulk composition."3
"The continental crust has an andesitic bulk
composition, which cannot have been produced by the
basaltic magmatism that dominates sites of present-day
"Continental crust overlies continental lithosphere
simply because it is made up of the lighter of the two
types of 'surface seeking' [or floating] materials."
"The origin of subcontinental lithosphere
is not well understood. Downward freezing of asthenosphere...
is not an acceptable explanation because this process
would produce lithosphere with about the same composition
as normal [oceanic] asthenosphere."2
the process is complicated, [continental] crust formation
boils down to the extraction of material of granitic
composition from a source of basaltic composition."2
An early idea, and one that remains popular, is
"that the continents form by accretion of island
arcs of andesitic composition." Island arcs
are lines of volcanos at subduction zones; the lava
is andesitic. "This 'andesite model' of crustal
growth appealed to uniformitarian sensibilities, in
that processes we see occurring today could account
for the formation of the continents. Subsequent
investigations of continental crust and island arcs,
however, have demonstrated the difficulties with this
simple model. The andesite model of crust formation
cannot account for the bulk-crust Cr and Ni contents
(average andesites have abundances that are too low)
nor its Th/U ratio. Furthermore, a large portion
of the continents probably formed during [ancient] Archaean
times and andesites are uncommon in Archaean volcanic
sequences." Perhaps most problematic for
the andesite model, however, is that intra-oceanic island
arcs are estimated to have basaltic, rather than andesitic,
bulk compositions." "Thus accretion
of modern island arcs produces basaltic crustal additions
and cannot account for the intermediate composition
of post-Archaean crust."5
have shown that it is possible to produce andesite from
material below the crust, peridotite, by adding high
heat and water, "leading to generation of mantle-derived
intermediate to silicic melts." Researchers
believe these conditions may have existed during Archaean
It should be noted that these conditions would
also likely have been present when the planetesimal
struck Earth, leading to the formation of the Moon.
The mixture of mantle and oceanic crust is fundamental,
yet is difficult to achieve by a slow series of actions
that Plate Tectonics requires: "Continental crust
consists of granitoid rocks that formed through a complex
series of events, which includes partial melting of
peridotite to form basalt, and reprocessing of basalt
in a subduction environment."2
of continental crust generates large volumes of residue."
For example, "the formation of 40 km-thick
crust generates a 200 km-thick layer of mafic cumulate
That is true for all continental crust, which
covers 41% of Earth. This dense material is missing
and must have fallen deep into the Earth. Yet
if continental crust formation has been ongoing, as
Plate Tectonics theory proposes, then this waste material
should be just below continental crust at various places
around the world. On the other hand, if it formed
early into a protocontinent, as Shock Dynamics theory
proposes, then all of it would have fallen away long
of the lower crust has been suggested as a possible
mechanism for the removal of the mafic residues of basalt
differentiation." Researchers have "proposed
that a mafic lower crust, if it is thickened and cooled
sufficiently, will convert to a high-density mineral
assemblage, leading to a gravitationally unstable configuration
in which the lower crust can sink into the underlying
"It appears that lithospheric thickening
(such as occurs at sites of continental-scale collisions)
is required to achieve delamination."5
However, "because subduction is a continuous
process, the episodic pattern of crust formation ages
is a strong argument against crustal growth at converging
Another version refers to "convective instabilities".
"This process is distinct from delamination
because the lower crust does not 'peel off', but rather
forms 'blobs' that drip off the base of the crust."3
But another element is necessary. "Calculation
of the instability times for a dense, lower crustal
layer to sink into the mantle show that high temperatures
(>700°C, or >500°C with an initial background
strain rate) are required for this process to occur
in ~10 million years. The high temperatures required...
suggest that this process is restricted to [island]
arcs, volcanic rifted margins, and continental regions
that are either undergoing extension, are underlain
by a mantle plume or have had part of the conductive
upper mantle removed."3
"Although delamination... provides a means
of explaining the non-basaltic composition of the crust,
it is a difficult process to document." And
"recognizing delamination in older regions remains
a difficult proposition."5
1. Albarčde, Francis. 1998. The growth
of continental crust. Tectonophysics, Vol. 296, pp.
2. Arndt, Nicholas T., Eric Lewin, Frances
Albarčde. 2002. Strange partners: formation and survival
of continental crust and lithospheric mantle. in
The Early Earth: Physical, Chemical and Biological Development.
Fowler, C.M.R., C.J. Ebinger, C.J. Hawkesworth, editors.
Geological Society, London, Special Publications, Vol.
199, pp. 91-103.
3. Jull, M., P.B. Kelemen. April 10,
2001. On the conditions for lower crustal convective
instability. Journal of Geophysical Research, Vol. 106,
No. B4, pp. 6423-6446.
4. Reese, C.C., C.P. Orth, V.S. Solomatov. 2011.
Impact megadomes and the origin of the martian crustal
dichotomy. Icarus, Vol. 213, pp. 433-442.
5. Rudnick, Roberta L. 7 December 1995.
Making continental crust. Nature, Vol. 378, pp. 571-578.
6. Tonks, W. Brian, H. Jay Melosh. March
25, 1993. Magma Ocean Formation Due to Giant Impacts.
Journal of Geophysical Research, Vol. 98, No. E3, pp.
ridge that was there long before
below is seafloor that was not overrun by continents or crustal
waves. A spreading ridge extends from the
Indian Ocean (Mid-Indian or Southwest Indian Ridge) to the eastern Pacific (East
Pacific Rise). As a "fast" spreading ridge,
it looks smoother on this digital elevation map than other spreading
of the Earth" Peter W. Sloss, NOAA/NGDC 1994. Shading
by J.M. Fischer
and appears to have been run over by North
a remnant of the collision that could have produced
the Moon and the protocontinent.
that the East Pacific Rise (EPR) and the Southwest Indian
Ridge (SWIR) existed prior to the Mid-Atlantic Ridge
(MAR) is found in their chemistry. Differences
in the level of silicon enrichment, measured in comparison
to magnesium (Mg/Si), are shown in the histogram below.
of the ridge rock (abyssal peridotite) were tested.
"Most samples on the MAR from drill cores
are normally distributed about zero, whereas those dredged
from the EPR and SWIR show consistently negative Mg/Si."
"These chemical shifts are well known in
peridotites from modern ocean basins as products of
marine weathering and hydrothermal alteration."
That indicates that the EPR/SWIR rocks are old
and weathered compared to the MAR. Yet plate tectonics
says that rocks at the center of all active ridges are
young, and should be distributed about zero. The
EPR is supposed to be spreading faster than the MAR,
so it should show even less weathering! The hump
in the EPR/SWIR histogram above zero is likely due to
samples from the new part of the SWIR, formed during
the Shock Dynamics event (see the shaded map above).
Canil, Dante, Cin-Ty A. Lee. July 2009.
Were deep cratonic mantle roots hydrated in Archean
oceans? Geology, Vol. 37, No. 7, pp. 667-670.
feature on the Pacific floor that appears to have been
there earlier and then overrun is the Marshall-Gilbert
Island chain to the north and the Louisville Ridge-Eltanin
Fracture Zone to the south. They are on the left
of the red arrows below. The crustal wave that
ended as the Tonga Trench ran over this line.
is likely that the Ontong Java Plateau, a huge flood
basalt feature, was in place prior to the event as well.
The northern edge of the crustal wave guided around
the southern side of the plateau as the wave rolled
east. In the above picture, Ontong Java is the
wispy white area to the left of the top arrow. It
is outlined in the picture below.
of researchers has even proposed that the Ontong Java
Plateau was formed by a large impact. They find
that "an object about 20 kilometers in diameter
impacting... Pacific lithosphere and penetrating into
the uppermost asthenosphere would have initiated massive
decompression melting in the upper mantle, and may have
resulted in emplacement of the greater Ontong Java Plateau",
including the other provinces shown above. "Geophysical,
geochemical, and geodynamic evidence from the [Ontong
Java] province are difficult to reconcile with mantle
plume models", the commonly accepted explanation
for its origin.
Stephanie, Millard F. Coffin. 2004. Impact origin for
the greater Ontong Java Plateau? Earth and Planetary
Science Letters, Vol. 218, pp. 123-134.
online presentation at: http://www.mantleplumes.org/OJ_Impact.html