by John M. Saul
During a varied career in geology, I have observed two matters that seem to stand out as fundamental truths. First of all, there is a great deal that we do not understand about the workings of Earth. And secondly, it is clear that everything is connected. Not just the many aspects of geology — mineralogy, earthquake studies, water supplies, oil and gas exploration, and much else — but also planetary astronomy, biology, and even cancer studies. The connections are not necessarily evident in our usual interactions with the world around us, however, or even by plunging into specialized studies, because so many key events took place in the exceedingly distant past, and so much has changed since.
Geology, along with astronomy, turns out to be a good base from which to explore the great nexus of "things connected" because its subject matter includes events that set the stage for much of what was to come.
Earth and Moon, Mercury, Venus, and Mars all formed in hot regions of the Solar System, close to the Sun. To a large extent, these planetary bodies are composed of atoms, molecules, and minerals that condense at high temperatures, metals and silicates, for example. That is why they are “rocky.” Water and hydrocarbons condensed in the cold outer regions of the Solar System, hence the gaseous nature of Jupiter and other bodies farther away from the Sun.
How then, did Earth acquire its water and gas, and its oil? A key factor was “the Late Heavy Bombardment,” a long-ago episode whose vestiges include large impact craters on the scarred face of the Moon. Earth, too, must have undergone the same bombardment. For in the vastness of the Solar System, Earth and Moon are neighbors, and much like two people standing together in a field during a hailstorm, both must have suffered the same pelting. In decades past, it was thought that the bombardment came about when our planet and its moon swept up debris, the crumbs of creation, left in their region after the formation of the Solar System. This was a theory that initially made good scientific sense, but ultimately had to be discarded as new observations were made and additional information accumulated. A coherent and widely accepted theory, the “Nice Model” (the outlines of which were first set out in 2005 by a group of scientists based in Nice, France) has provided an attractive alternative explanation for the bombardment, an explanation that also accounts for other puzzling observations, including the distribution of certain asteroids with unusual orbits.
One consequence of the Late Heavy Bombardment was the transfer into the region of Earth and the Moon of low-density materials – water and hydrocarbons – that had originally solidified in the cold far-reaches of the Solar System. This greatly altered the chemistry of Earth’s crust and upper mantle, as well as its 3-D structure, which, as a consequence of the giant impacts, acquired great numbers of deep “craterform” scars.
Large circular scars visible on a shaded relief map of southern Arizona between 33°-35° N and 110°-112°W. The vertical dimension is exaggerated twofold and the illumination is at a low angle. J.M. Saul (1978): “Circular structures of large scale and great age on the Earth’s surface”, Nature, vol. 271, pp. 345-349.
The word “Late” in “Late Heavy Bombardment” refers to the timing of the bombardment, approximately 4100 to 3800 million years ago, as a final cosmological episode, after which geology can be said to have governed events here on Earth. Part, at least, of the famous observation by James Hutton (1726–1797) “that we find no vestige of a beginning — no prospect of an end” may require revision.
In 1963, Nobel Laureate Robert Robinson proposed that petroleum was a material of inorganic, "mineral," origin to which molecules of biological origin had been added. Robinson wrote of the “duplex origin of petroleum,” and it is likely he was correct, unless the origin was actually “triplex” or “multiplex.” For, as suggested by their widespread occurrence throughout the Solar System and beyond, hydrocarbons are easy to make.
An instructive occurrence of hydrocarbons involves the natural gas found in great quantities mixed with the water of Australia’s Great Artesian Basin. The water is generally believed to be replenished, albeit it inadequately, by run-off from far distant mountains (although precisely which mountains remains unclear). This already-unsatisfactory explanation is further confounded by the presence of the gas, which cannot possibly have had such an origin.
These days it is almost universally believed that traces of the Late Heavy Bombardment have been eliminated from the surface regions of our own planet by rain, wind, cycles of erosion and, most of all, by the workings of plate tectonics, with subduction and deep burial of much of Earth's crust over time. I argue differently, claiming that impact scars, now deeply eroded, remain, but that scientists have not established the correct criteria for recognizing them. In A Geologist Speculates, I argue that traces of these 3-D scars are all about us and that they provide the template and patterning of all later geology, including the arc-like geometries of many subduction zones where one tectonic plate passes below another.
Plate tectonics went into high gear approximately 700 to 550 million years ago in a great episode culminating in the continent-to-continent collision between the western and eastern portions of the supercontinent of Gondwana. This collision formed a “supermountain” 8000 kilometers long and 1000 kilometers wide, extending southwards from southernmost Israel through the gemstone-producing areas of eastern Africa all the way into eastern Antarctica, and including Sri Lanka and Madagascar, both of which are known as the “Isle of Gems.” The work of geochronologists during the last half-century has shown that, aside from some special cases (most notably involving diamonds, emeralds and quartz), the transparent gemstones associated with the supermountain are the world’s oldest: the world’s first transparent crystals of garnets, rubies, aquamarines, tourmalines and some fifty other gem species were formed in the root zones of the Gondwanan mountains during these times.
The supermountain was short-lived. Around 700 million years ago, land-based microbes capable of degrading rocks and producing soil had become abundant, but plants with roots that would retain the soil had not yet come into existence. This resulted in the extraordinarily rapid weathering of the supermountain and the dumping of great quantities of various materials into the seas.
Large multi-celled animals (or, rather, large multi-celled animals with active life-styles) first appeared in the world’s oceans approximately 542 million years ago. At that time, the seas already contained sufficient oxygen for the production of collagen, an oxygen-expensive group of molecules present in all multicellular animals and essential for the construction of their tissues. The seas also contained enough phosphorous – derived from erosion of the supermountain – for the abundant production of the energy-currency molecule ATP (in which the “P” stands for “phosphate”), needed to power sizable animals.
In short, I argue that an unprecedented type of gem-crystallization first occurred in the root-zone of an unprecedented supermountain, and that the unprecedented flourishing of active multi-cellular marine animals occurred when the chemical erosion of the supermountain washed unprecedented quantities of phosphorous into the seas. But why all the “unprecedented” changes? In contrast to most members of the geological fraternity, I believe that these times were marked by the first occurrences of modern-style plate tectonics, by which I mean plate tectonics with the deep subduction of plates. I further believe that the first deep-subduction events, and many others that followed, occurred along curved zones of weakness in the Earth’s crust, along the edges of great 3-D scars, vestiges of the Late Heavy Bombardment.
Single-celled creatures existed in enormous abundance before the Cambrian Explosion, and they still do. Such creatures can multiply and vary with relatively few constraints. But multi-celled animals are composed of tissues, and the rate at which their constituent cells (each of which has a specialized function) divide, and the degree to which they are allowed to vary, are tightly constrained by the “architecture” of the tissue itself.
When a tissue is sufficiently weakened (whether by a noxious chemical, infection, injury, an inadequate immune system, old age, unfortunate genes, or loss of access to adequate supplies of oxygen) it may lose control of some of its constituent cells. If these uncontrolled cells are not destroyed by the animal’s immune system or by other means, they may continue to multiply and vary with few constraints, and take up an independent existence as though they were single-celled creatures. A cell of this nature may eventually form a cancer, a pathology that does not exist among single-celled creatures.
And so a reasoned series of “things connected” is established. The production of cancer cells from poorly oxygenated or otherwise weakened tissues. Tissues held together by oxygen-expensive molecules of collagen family. Emergence of active multi-celled animals in oxygenated seas rich in phosphorous. Marine phosphorous in abundance as a consequence of chemical weathering of the supermountain. The supermountain as a consequence of plate tectonics with deep subduction. Deep subduction enabled along zones of weakness in the Earth’s crust. And deep zones of weakness in Earth’s crust as the consequence of the Late Heavy Bombardment, before which Earth’s fate belonged to the realm of cosmology.
I thank Jesse H. Ausubel for his encouragement and help on this project, and the Richard Lounsbery Foundation for its backing.