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Early 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 Another 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. Large Impacts On the Moon South 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.
"Mars 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."1 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."1 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°.3 Mars itself has a diameter of 6,780 km.1 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.1
The original 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 km3." "The total melt volume produced during the impact is 6 x 108 km3."3 "Planetary-scale 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."2 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."2 1. Kiefer, Walter S. 26 June 2008. Forming the martian great divide. Nature, Vol. 453, pp. 1191-1192. 2. Marinova, Margarita M., Oded Aharonson, Erik Asphaug. 26 June 2008. Mega-impact formation of the Mars hemispheric dichotomy. Nature, Vol. 453, pp. 1216-1219. 3. 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. 1220-1223.
"Before the Pathfinder mission there was a general consensus that the Martian surface is very mafic... like the Martian meteorites. But the first measurements by Pathfinder showed that the rocks are felsic (containing the whitish mineral feldspar), rather like the Earth's continental crust," says Heinrich Wänke, from the Max Planck Insitut für Chemie, Mainz. Mafic rocks are high in magnesium and iron and are thought to derive from pristine mantle material; felsic rocks are rich in silicates, potassium and sulphur but low in magnesium and are thought to derive from rock that has undergone subsequent processing since the planet's formation. Martian Surface: What is it made from? European Space Agency, Science and technology webpage. Two types of rock have been identified as covering the surface of Mars. "Calculated chemical compositions for Thermal Emission Spectrometer (TES) global surface units indicate that surface type 1 has basaltic andesite composition and surface type 2 has the composition of andesite." "Andesitic volcanism on Earth is mostly associated with thick, continental crust. Conversely, on Mars surface type 2 (possibly andesitic) materials overlie thin crust in the northern plains, whereas the thick southern highlands are overlain by surface type 1 (basaltic) materials." "Until recently, the Earth's silicic continental crust... was thought to be geochemically unique. However, the analysis of rocks having chemical compositions similar to andesite at the Mars Pathfinder landing site suggests that Martian crust contains silicic rocks, and the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) mapped an abundance of a global spectral unit interpreted to be andesitic." Andesitic rock was thought to occur only through Plate Tectonics, by melting of basalt in the presence of water at subduction zones, and so this was a surprise. The researchers offer the possibility that the type 2 material is just weathered type 1 material. Yet it is unlikely that the weathered material would reside only in the northern plains of Mars.1 A Mars lander has raised questions about andesitic volcanism on the planet. Two volcanic rocks in the crater Gusev have a surface rind apparently altered by a small amount of water. If the upper third of Mars, the lowlands, has a rind that was similarly altered by water, conclusions about its andesitic composition would be affected.2 However, the Mars Odyssey gamma ray spectrometer surveyed the surface of Mars up to 57 degrees north. While this barely reached the northern lowlands, and elements were assessed in sections that included highland crust, the northernmost sections were relatively enriched in Silicon and thus closer to andesite.3 Deeper sampling of the northern lowlands will be needed. 1. McSween, Harry Y. Jr., Timothy L. Grove, Michael B. Wyatt. 2003. Constraints on the composition and petrogenesis of the Martian crust. Journal of Geophysical Research, Vol. 108, No. E12, 5135, pp. 9-1 to 19. 2. Kerr, Richard A. 9 April 2004. Mars Rock Crud Gets in the Way. Science, Vol. 304, pp. 196-197. 3. Taylor, G. Jeffrey, Linda M.V. Martel, Suniti Karunatillake, Olivier Gasnault, William V. Boynton. February 2010. Mapping Mars geochemically. Geology, Vol. 38, No. 2, pp. 183-186. On early Earth, to form the Moon and continental crust Today, standard theory says the Moon formed from debris kicked out into space when a planetesimal about the size of Mars grazed the Earth. 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 continental crust, in the shape of the protocontinent in the image on the right. It was this protocontinent that was much later struck and shattered by the giant meteorite of the Shock Dynamics event. A 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 planet. 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. 5319-5333. The evidence from Mars, while still tentative, supports the idea that continental crust forms when a sufficiently large impact mixes basalt crust with mantle. Making continental crust through plate interaction remains a problem for Plate Tectonics theory.
"Our 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 crustal growth."4 "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 "Although 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."4 Experiments 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 times.4 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 The "formation 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 or restite."2 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 ago. "'Delamination' 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 lower-density mantle."3 "It appears that lithospheric thickening (such as occurs at sites of continental-scale collisions) is required to achieve delamination."4 However, "because subduction is a continuous process, the episodic pattern of crust formation ages is a strong argument against crustal growth at converging boundaries."1 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."4 1. Albarède, Francis. 1998. The growth of continental crust. Tectonophysics, Vol. 296, pp. 1-14. 2. Arndt, Nicholas T., Eric Lewin, Frances Albarède. 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. Rudnick, Roberta L. 7 December 1995. Making continental crust. Nature, Vol. 378, pp. 571-578. The ridge that was there long before Highlighted below is seafloor that was not overrun by continents or crustal waves. A spreading ridge extends from the southern 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 ridges,
and appears to have been run over by North America.
It may be a remnant of the collision that could have produced the Moon and the protocontinent. Evidence 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.
Samples 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. Another 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.
It 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.
A pair 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. Ingle,
Stephanie, Millard F. Coffin. 2004. Impact origin for
the greater Ontong Java Plateau? Earth and Planetary
Science Letters, Vol. 218, pp. 123-134.
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